Abstract: P>1. Plant functional traits are dimensions of ecological strategy variation and provide insights into the assembly of plant communities. For woody rain forest vegetation of northern coastal New South Wales, Australia, we quantified four continuous traits (leaf size, seed size, wood density and maximum height) for 231 freestanding woody species and documented community composition for 216 plots. Using trait-gradient analysis, we partitioned species trait values between alpha (within-site) and beta (among-site) components. This allowed us to identify both trait shifts along gradients and variation among co-occurring species. 2. Alpha trait components consistently varied more widely than beta components, meaning that trait variation among species within plots was wider than variation in the mean trait values of plots where species typically grow. 3. Beta trait components covaried significantly among leaf area, seed size, wood density and maximum height. For example, species found in habitats with a large mean leaf size were consistently also found in plots with large mean seed size (r = 0.70). Beta correlations show that these leaf, wood and seed traits respond in parallel to the dominant abiotic gradients: soil types, topographic position, elevation and large-patch disturbances such as those caused by cyclones-storms, landslips or fires. 4. In contrast, the alpha components of traits were largely uncorrelated among species. Alpha leaf area was not associated with alpha larger seeds, meaning that leaf area and seed size act as independent axes of differentiation among coexisting species. 5. Synthesis. The different correlation structures for alpha and beta components of traits reflect community assembly processes at different scales. Within sites, assembly processes have not created strong linkages among these traits. But across different sites in the landscape, abiotic drivers have created strong linkages.
Abstract: P>Stem mechanical properties are critically linked to foliage deployment and growth strategy, yet variation in stem mechanics across species and habitats is poorly understood. Here, we compared 32 plant species growing across four sites of contrasting rainfall and soil nutrient availability in Australia. The modulus of elasticity (MOE) and modulus of rupture (MOR) were tightly correlated with dry sapwood density within sites, but species from low-rainfall environments had higher wood density for a given MOE and MOR compared with species growing in high-rainfall environments. The ratio of MOE to MOR was slightly lower for species at low-rainfall sites, suggesting that wood was stronger for a given elasticity. Most species had thick bark, but the mechanical contribution of bark to stem MOE was small. Our results suggest that arid-adapted species would need to deploy more dry mass to support stems. Our results also highlight the importance of understanding how the biomechanics-wood density relationship evolves under different environmental conditions to better understand plant growth across diverse habitats.
Abstract: Woody stems comprise a large biological carbon fraction and determine water transport between roots and leaves; their structure and function can influence both carbon and hydrological cycles. While angiosperm wood anatomy and density determine hydraulic conductivity and mechanical strength, little is known about interrelations across many species. We compiled a global data set comprising two anatomical traits for 3005 woody angiosperms: mean vessel lumen area ((A) over bar) and number per unit area (N). From these, we calculated vessel lumen fraction (F = (A) over barN) and size to number ratio (S = (A) over bar /N), a new vessel composition index. We examined the extent to which F and S influenced potential sapwood specific stem conductivity (K-S) and wood density (D; dry mass/fresh volume). F and S varied essentially independently across angiosperms. Variation in K-S was driven primarily by S, and variation in D was virtually unrelated to F and S. Tissue density outside vessel lumens (D-N) must predominantly influence D. High S should confer faster K-S but incur greater freeze-thaw embolism risk. F should also affect K-S, and both F and D-N should influence mechanical strength, capacitance, and construction costs. Improved theory and quantification are needed to better understand ecological costs and benefits of these three distinct dimensions.
Abstract: Current ecological thinking emphasizes that systems are complex, dynamic, and unpredictable across space and time. What is the diversity in interpretation of these ideas among today's ecologists, and what does this mean for environmental management? This study used a Policy Delphi survey of ecologists to explore their perspectives on a number of current topics in ecology. The results showed general concurrence with nonequilibrium views. There was agreement that disturbance is a widespread, normal feature of ecosystems with historically contingent responses. The importance of recognizing multiple levels of organization and the role of functional diversity in environmental change were also widely acknowledged. Views differed regarding the predictability of successional development, whether "patchiness" is a useful concept, and the benefits of shifting the focus from species to ecosystem processes. Because of their centrality to environmental management, these different views warrant special attention from both managers and ecologists. Such divergence is particularly problematic given widespread concerns regarding the poor linkages between science (here, ecology) and environmental policy and management, which have been attributed to scientific uncertainty and a lack of consensus among scientists, both jeopardizing the transfer of science into management. Several suggestions to help managers deal with these differences are provided, especially the need to interpret broader theory in the context of place-based assessments. The uncertainty created by these differences requires a proactive approach to environmental management, including clearly identifying environmental objectives, careful experimental design, and effective monitoring.
Abstract: 1. Rees & Venable (2007; Journal of Ecology, 95, 926-936) correctly identified scaling relations across species between offspring size at independence and adult size as patterns needing theoretical explanation. They also correctly identified that Charnov's (1993; Life History Invariants, Oxford University Press) model did not provide an adequate explanation. 2. Rees and Venable attacked several opinions which they attributed to us, but which we do not hold, and which we did not express in the papers they cited. Here we clarify the main points where we agree and where we disagree with Rees and Venable. 3. Rees and Venable claimed that we interpreted cross-species correlations between traits as constraints on the evolution of life histories. This claim is wrong. We interpret correlation between traits as arising from coordinated evolution, just as Rees and Venable do. Our papers cited by Rees and Venable consciously avoided the terminology of constraints, and passages in them show clearly that we see coordination between traits as arising from natural selection. 4. A model we have been developing independently (Falster et al. 2008; American Naturalist, 172, 299-317) agrees with Rees and Venable in concluding that longer times to adulthood are not, in themselves, sufficient to predict a positive relationship between offspring size and adult size. Falster et al. (2008) shows that density dependence later during growth, together with size advantage persisting from offspring size, can in principle provide an explanation for a positive relationship, and for differences in the slope of this relationship between plants and mammals.
Abstract: We compiled data from seed rain studies at 33 sites from around the world to determine whether the greater mean seed mass of tropical plants is associated with production of fewer seeds per square meter of ground. We found no significant linear relationship between latitude and annual seed rain density, but found some evidence for a mid-latitude peak in seed rain density (quadratic relationship, p=0.018; R-2=0.23). Combining seed rain data with seed mass data suggests that vegetation at the equator produces between 19 and 128 times more total mass of seed per year than does vegetation at 60 degrees. This gradient in seed production would far outweigh the doubling in net primary productivity (NPP) over the same range of latitudes. Thus, our (admittedly small) dataset suggests that tropical vegetation allocates a much greater proportion of NPP to reproduction. This raises two important questions for the future: 1) why might tropical vegetation commit more energy to seed production than vegetation further from the equator? 2) What aspect of plant growth might receive proportionally less energy in tropical ecosystems?.
Abstract: Height gain plays an important role in plant life-history strategies and species coexistence. Here main-stem costs of height gain of saplings across species within a rainforest community are compared. Scaling relationships of height to diameter at the sapling stage were compared among 75 woody rainforest plant species in subtropical eastern Australia using standardized major axis regression. Main-stem costs of height gain were then related to other functional traits that reflect aspects of species ecological strategies. Slopes (beta) for the height-diameter (H-D) scaling relationship were close to 1 center dot 3, in line with previous reports and with theory. Main-stem volume to achieve 5 m in height varied substantially between species, including between species within groups based on adult height and successional status. The variation was largely independent of other species traits, being uncorrelated with mature plant height (H-max) and with leaf size, and weakly negatively correlated with wood density and seed size. The relationship between volume to reach 5 m and wood density was too weak to be regarded as a trade-off. Estimated main-stem dry mass to achieve 5 m height varied almost three-fold across species, with wood density and stem volume contributing roughly equally to the variation. The wide range in economy of sapling height gain reported here is presumed to be associated with a trade-off between faster growth and higher mortality rates. It is suggested that wide diameters would have a stronger effect in preventing main-stem breakage in the short term, while high wood density would have a stronger effect in sustaining stem strength over time.
Abstract: Here, we evaluated how increased shading and declining net photosynthetic capacity regulate the decline in net carbon balance with increasing leaf age for 10 Australian woodland species. We also asked whether leaves at the age of their mean life-span have carbon balances that are positive, zero or negative. The net carbon balances of 2307 leaves on 53 branches of the 10 species were estimated. We assessed three-dimensional architecture, canopy openness, photosynthetic light response functions and dark respiration rate across leaf age sequences on all branches. We used YPLANT to estimate light interception and to model carbon balance along the leaf age sequences. As leaf age increased to the mean life-span, increasing shading and declining photosynthetic capacity each separately reduced daytime carbon gain by approximately 39% on average across species. Together, they reduced daytime carbon gain by 64% on average across species. At the age of their mean life-span, almost all leaves had positive daytime carbon balances. These per leaf carbon surpluses were of a similar magnitude to the estimated whole-plant respiratory costs per leaf. Thus, the results suggest that a whole-plant economic framework, including respiratory costs, may be useful in assessing controls on leaf longevity. New Phytologist (2009) 183: 153-166doi: 10.1111/j.1469-8137.2009.02824.x.
Abstract: How and why organisms are distributed as they are has long intrigued evolutionary biologists(1-4). The tendency for species to retain their ancestral ecology has been demonstrated in distributions on local and regional scales(5-7), but the extent of ecological conservatism over tens of millions of years and across continents has not been assessed(8-13). Here we show that biome stasis at speciation has outweighed biome shifts by a ratio of more than 25:1, by inferring ancestral biomes for an ecologically diverse sample of more than 11,000 plant species from around the Southern Hemisphere. Stasis was also prevalent in transocean colonizations. Availability of a suitable biome could have substantially influenced which lineages establish on more than one landmass, in addition to the influence of the rarity of the dispersal events themselves. Conversely, the taxonomic composition of biomes has probably been strongly influenced by the rarity of species' transitions between biomes. This study has implications for the future because if clades have inherently limited capacity to shift biomes(13), then their evolutionary potential could be strongly compromised by biome contraction as climate changes.
Abstract: Understanding evolutionary coordination among different life-history traits is a key challenge for ecology and evolution. Here we develop a general quantitative model predicting how offspring size should scale with adult size by combining a simple model for life-history evolution with a frequency-dependent survivorship model. The key innovation is that larger offspring are afforded three different advantages during ontogeny: higher survivorship per time, a shortened juvenile phase, and advantage during size-competitive growth. In this model, it turns out that size-asymmetric advantage during competition is the factor driving evolution toward larger offspring sizes. For simplified and limiting cases, the model is shown to produce the same predictions as the previously existing theory on which it is founded. The explicit treatment of different survival advantages has biologically important new effects, mainly through an interaction between total maternal investment in reproduction and the duration of competitive growth. This goes on to explain alternative allometries between log offspring size and log adult size, as observed in mammals (slope = 0.95) and plants (slope = 0.54). Further, it suggests how these differences relate quantitatively to specific biological processes during recruitment. In these ways, the model generalizes across previous theory and provides explanations for some differences between major taxa.
Abstract: Worldwide decomposition rates depend both on climate and the legacy of plant functional traits as litter quality. To quantify the degree to which functional differentiation among species affects their litter decomposition rates, we brought together leaf trait and litter mass loss data for 818 species from 66 decomposition experiments on six continents. We show that: (i) the magnitude of species-driven differences is much larger than previously thought and greater than climate-driven variation; (ii) the decomposability of a species' litter is consistently correlated with that species' ecological strategy within different ecosystems globally, representing a new connection between whole plant carbon strategy and biogeochemical cycling. This connection between plant strategies and decomposability is crucial for both understanding vegetation-soil feedbacks, and for improving forecasts of the global carbon cycle.
Abstract: Aim To provide the first global quantification of the slope and shape of the latitudinal gradient in seed mass, and to determine whether global patterns in seed mass are best explained by growth form, vegetation type, seed dispersal syndrome, or net primary productivity (NPP). Location Global. Methods We collected seed mass data for 11,481 species x site combinations from around the world. We used regression to describe the latitudinal gradient in seed mass, then applied general linear models to quantify the relative explanatory power of each of the variables hypothesized to underlie the latitudinal gradient in seed size. Results There is a 320-fold decline in geometric mean seed mass between the equator and 60 degrees. This decline is not linear. At the edge of the tropics, there is a sudden 7-fold drop in mean seed mass. The strongest correlates of the latitudinal gradient in seed mass are plant growth form, and vegetation type, followed by dispersal syndrome and NPP. A model including growth form, vegetation type, dispersal syndrome and NPP explains 51% of the variation in seed mass. Latitude explains just 0.2% of the residual variation from this model. Main Conclusions This is the first demonstration of a major decrease in seed size at the edge of the tropics. This drop in seed mass is most closely correlated with changes in plant growth form and vegetation type. This suggests that the drop in seed mass might be part of a sudden change in plant strategy at the edge of the tropics.
Abstract: Leaf mass per area (M-A) is a central ecological trait that is intercorrelated with leaf life span, photosynthetic rate, nutrient concentration, and palatability to herbivores. These coordinated variables form a globally convergent leaf economics spectrum, which represents a general continuum running from rapid resource acquisition to maximized resource retention. Leaf economics are little studied in ancient ecosystems because they cannot be directly measured from leaf fossils. Here we use a large extant data set (65 sites; 667 species-site pairs) to develop a new, easily measured scaling relationship between petiole width and leaf mass, normalized for leaf area; this enables M-A estimation for fossil leaves from petiole width and leaf area, two variables that are commonly measurable in leaf compression floras. The calibration data are restricted to woody angiosperms exclusive of monocots, but a preliminary data set (25 species) suggests that broad-leaved gymnosperms exhibit a similar scaling. Application to two well-studied, classic Eocene floras demonstrates that M-A can be quantified in fossil assemblages. First, our results are consistent with predictions from paleobotanical and paleoclimatic studies of these floras. We found exclusively low-M-A species from Republic (Washington, U.S.A., 49 Ma), a humid, warm-temperate flora with a strong deciduous component among the angiosperms, and a wide M-A range in a seasonally dry, warm-temperate flora from the Green River Formation at Bonanza (Utah, U.S.A, 47 Ma), presumed to comprise a mix of short and long leaf life spans. Second, reconstructed M-A in the fossil species is negatively correlated with levels of insect herbivory, whether measured as the proportion of leaves with insect damage, the proportion of leaf area removed by herbivores, or the diversity of insect-damage morphotypes. These correlations are consistent with herbivory observations in extant floras and they reflect fundamental trade-offs in plant-herbivore associations. Our results indicate that several key aspects of plant and plant-animal ecology can now be quantified in the fossil record and demonstrate that herbivory has helped shape the evolution of leaf structure for millions of years.
Abstract: Background and Aims When ecologically important plant traits are correlated they may be said to constitute an ecological 'strategy' dimension. Through identifying these dimensions and understanding their inter-relationships we gain insight into why particular trait combinations are favoured over others and into the implications of trait differences among species. Here we investigated relationships among several traits, and thus the strategy dimensions they represented, across 2134 woody species from seven Neotropical forests. Methods Six traits were studied: specific leaf area (SLA), the average size of leaves, seed and fruit, typical maximum plant height, and wood density (WD). Trait relationships were quantified across species at each individual forest as well as across the dataset as a whole. 'Phylogenetic' analyses were used to test for correlations among evolutionary trait-divergences and to ascertain whether interspecific relationships were biased by strong taxonomic patterning in the traits. Results The interspecific and phylogenetic analyses yielded congruent results. Seed and fruit size were expected, and confirmed, to be tightly related. As expected, plant height was correlated with each of seed and fruit size, albeit weakly. Weak support was found for an expected positive relationship between leaf and fruit size. The prediction that SLA and WD would be negatively correlated was not supported. Otherwise the traits were predicted to be largely unrelated, being representatives of putatively independent strategy dimensions. This was indeed the case, although WD was consistently, negatively related to leaf size. Conclusions The dimensions represented by SLA, seed/fruit size and leaf size were essentially independent and thus conveyed largely independent information about plant strategies. To a lesser extent the same was true for plant height and WD. Our tentative explanation for negative WD-leaf size relationships, now also known from other habitats, is that the traits are indirectly linked via plant hydraulics.
Abstract: There is considerable debate about whether community ecology will ever produce general principles. We suggest here that this can be achieved but that community ecology has lost its way by focusing on pairwise species interactions independent of the environment. We assert that community ecology should return to an emphasis on four themes that are tied together by a two-step process: how the fundamental niche is governed by functional traits within the context of abiotic environmental gradients; and how the interaction between traits and fundamental niches maps onto the realized niche in the context of a biotic interaction milieu. We suggest this approach can create a more quantitative and predictive science that can more readily address issues of global change.
Abstract: Light availability generally decreases vertically downwards through plant canopies. According to optimisation theory, in order to maximise canopy photosynthesis plants should allocate leaf nitrogen per area (N-area) in parallel with vertical light gradients, and leaf mass per area (LMA) and leaf angles should decrease down through the canopy also. Many species show trends consistent with these predictions, although these are never as steep as predicted. Most studies of canopy gradients in leaf traits have concerned tall herbaceous vegetation or forest trees. But do evergreen species from open habitats also show these patterns? We quantified gradients of light availability, LMA, leaf N and phosphorus ( P), and leaf angle along leaf age sequences and vertical canopy profiles, across 28 woody species from open habitats in eastern Australia. The observed trends in LMA, Narea and leaf angle largely conflicted with expectations from canopy optimisation models, whereas trends in leaf P were more consistent with optimal allocation. These discrepancies most likely relate to these species having rather open canopies with quite shallow light gradients, but also suggest that modelling the co-optimisation of resources other than nitrogen is required for understanding plant canopies.
Abstract: Plant species vary widely in their average leaf lifespan (LL) and specific leaf area (SLA, leaf area per dry mass). The negative LL-SLA relationship commonly seen among species represents an important evolutionary trade-off, with higher SLA indicating greater potential for fast growth (higher rate of return on a given investment), but longer LL indicating a longer duration of the revenue stream from that investment. We investigated how these leaf-economic traits related to aggregate properties of the plant crown. Across 14 Australian sclerophyll shrub species, those with long LL accumulated more leaf mass and leaf area per unit ground area. Light attenuation through their canopies was more severe. Leaf accumulation and light attenuation were more weakly related to SLA than to LL. The greater accumulation of foliage in species with longer LL and lower SLA may counterbalance their generally lower photosynthetic rates and light-capture areas per gram of leaf.
Abstract: We compiled information from the international literature to quantify the relationships between seed mass and survival through each of the hazards plants face between seed production and maturity. We found that small-seeded species were more abundant in the seed rain than large-seeded species. However, this numerical advantage was lost by seedling emergence. The disadvantage of small-seeded species probably results from size-selective post-dispersal seed predation, or the longer time small-seeded species spend in the soil before germination. Seedlings from large-seeded species have higher survival through a given amount of time as seedlings. However, this advantage seems to be countered by the greater time taken for large-seeded species to reach reproductive maturity: our data suggested no relationship, or perhaps a weak negative relationship, between seed size and survival from seedling emergence through to adulthood. A previous compilation showed that the inverse relationship between seed mass and the number of seeds produced per unit canopy area per year is countered by positive relationships between seed mass, plant size and plant longevity. Taken together, these data show that our old understanding of a species' seed mass as the result of a trade-off between producing a few large offspring, each with high survival probability, versus producing many small offspring each with a lower chance of successfully establishing was incomplete. It seems more likely that seed size evolves as part of a spectrum of life history traits, including plant size, plant longevity, juvenile survival rate and time to reproduction.
Abstract: Fitting a line to a bivariate dataset can be a deceptively complex problem, and there has been much debate on this issue in the literature. In this review, we describe for the practitioner the essential features of fine-fitting methods for estimating the relationship between two variables: what methods are commonly used, which method should be used when, and how to make inferences from these lines to answer common research questions. A particularly important point for line-fitting in allometry is that usually, two sources of error are present (which we call measurement and equation error), and these have quite different implications for choice of line-fitting method. As a consequence, the approach in this review and the methods presented have subtle but important differences from previous reviews in the biology literature. Linear regression, major axis and standardised major axis are alternative methods that can be appropriate when there is no measurement error. When there is measurement error, this often needs to be estimated and used to adjust the variance terms in formulae for line-fitting. We also review fine-fitting methods for phylogenetic analyses. Methods of inference are described for the line-fitting techniques discussed in this paper. The types of inference considered here are testing if the slope or elevation equals a given value, constructing confidence intervals for the slope or elevation, comparing several slopes or elevations, and testing for shift along the axis amongst several groups. In some cases several methods have been proposed in the literature. These are discussed and compared. In other cases there is little or no previous guidance available in the literature. Simulations were conducted to check whether the methods of inference proposed have the intended coverage probability or Type I error. We identified the methods of inference that perform well and recommend the techniques that should be adopted in future work.
Abstract: Leaf dark respiration (R) is one of the most fundamental physiological processes in plants and is a major component of terrestrial CO2 input to the atmosphere. Still, it is unclear how predictably species vary in R along broad climate gradients. Data for R and other key leaf traits were compiled for 208 woody species from 20 sites around the world. We quantified relationships between R and site climate, and climate-related variation in relationships between R and other leaf traits. Species at higher-irradiance sites had higher mean R at a given leaf N concentration, specific leaf area (SLA), photosynthetic capacity (A(mass)) or leaf lifespan than species at lower-irradiance sites. Species at lower-rainfall sites had higher mean R at a given SLA or A(mass) than species at higher-rainfall sites. On average, estimated field rates of R were higher at warmer sites, while no trend with site temperature was seen when R was adjusted to a standard measurement temperature. Our findings should prove useful for modelling plant nutrient and carbon budgets, and for modelling vegetation shifts with climate change.
Abstract: The tissue traits and architectures of plant species are important for land-plant ecology in two ways. First, they control ecosystem processes and define habitat and resources for other taxa; thus, they are a high priority for understanding the ecosystem at a site. Second, knowledge of trait costs and benefits offers the most promising path to understanding how vegetation properties change along physical geography gradients. There exists an informal shortlist of plant traits that are thought to be most informative. Here, we summarize recent research on correlations and tradeoffs surrounding some traits that are prospects for the shortlist. By extending the list and by developing better models for how traits influence species distributions and interactions, a strong foundation of basic ecology can be established, with many practical applications.
Abstract: One fusion between ecology and evolution is well established, under the title of population biology. The years 2006-2020 will see a new fusion, likely to prove equally creative. Inputs from ecology to this second fusion will be worldwide data sets for ecological traits across many species. Inputs from evolution will be phylogenetic trees with well-resolved topology and with increasingly tight geological dates for each branch point. There will be unification of two aims: first to explain the spread of different ways of making a living, across the range of present-day species; and second, to narrate the evolutionary history that has led up to present-day ecology.
Abstract: 1. Wildfires strongly influence the biotic composition and carbon cycle of many ecosystems. Plant species provide the fuel for wildfires, but vary widely in their flammability. This study aimed to determine what plant characteristics control leaf litter flammability and to clarify how they are related to other functional traits. 2. Litter flammability varied across 14 tree species occurring in a mosaic of five floristic associations. 3. Differences in heat-release rate between species were driven by leaf size, from small-leaved conifers, casuarinas and acacias to large-leaved eucalypts and Brachychiton. 4. Large leaves created an open litter-bed structure that burned more rapidly because it was better ventilated. The results on heat-release rate were partitioned according to fundamental principles for the transport of oxygen through a packed fuel bed, showing that heat-release rate scaled linearly with estimated gas-flow rates, as expected in ventilation-controlled fires. 5. Species that were able to resprout after fire had litter that burned more intensely and was more likely to sustain a spreading fire than litter from obligate seeders, and were correspondingly larger-leaved. 6. Many fire-prone wooded ecosystems in the region consist of large-leaved resprouting tree species co-occurring with small-leaved obligate seeders.
Abstract: Pressure-volume (P-V) curves for leaves or terminal shoots summarize leaf-level responses to increasing water deficit. P-V curve traits and field-measured shoot xylem pressures were characterized across 62 species from four sites differing in rainfall and soil phosphorus. Within-species variation in the measured traits was small relative to differences among species and between environments. P-V curve traits tended to differ with site rainfall but not with soil phosphorus. Turgor loss points (TLPs) varied widely and averaged more negative in species from lower-rainfall sites. Differences between species in TLP were driven mainly by differences in solute potential, rather than by differences in cell wall elasticity. Among species at individual sites, species seemed to vary in leaf-response strategy reflected in TLP independently from water-uptake strategy reflected in predawn xylem pressures and in xylem pressure drop from predawn to midday.
Abstract: Through identifying and understanding ecologically important dimensions of plant trait variation we gain insight into why particular trait combinations are favoured and into the implications of trait differences among species. Here, we describe relationships among several poorly understood leaf and stem traits across species from several Australian vegetation types. Species with lower wood density (WD) consistently deployed more leaf area per unit shoot mass (LA/SM), as did the larger-leaved species within forested sites. Higher LA/SM is likely to lead to faster growth rates, implying a previously unrecognized implication to interspecific variation in leaf size and WD. Leaf : sapwood area ratio is one of several important traits contributing to a plant's water-use strategy, yet, we still only poorly understand how plants vary in the extent to which hydraulic properties and traits such as leaf size, WD and LM/SM are coordinated, and what the implications of this variation may be.
Abstract: Accessory costs of reproduction are those that are necessary to mature a seed, but that do not involve the direct cost of provisioning the seed itself. This study aims to quantify accessory costs in a range of species, and test whether they decrease as a proportion of total reproductive expenditure with increasing seed mass, as might be expected if economies of scale came into play at larger seed sizes. We also test whether accessory costs varied with growth form, pollination mode, and dispersal mode, with the expectation that biotic pollination and dispersal modes should incur greater costs. Reproductive allocation (dry biomass) over one season, was calculated for 14 diclinous angiosperm species. Accessory costs averaged 73% of total reproductive allocation, with the majority spent on packaging and dispersal. Total accessory costs, packaging and dispersal costs, and costs incurred prior to pollination were proportional to direct costs of reproduction in major axis regressions. However, larger seeded species incurred significantly greater costs associated with aborted seeds and fruits, and matured a smaller proportion of ovules. This is consistent with larger seeded species being more selective of the ovules/embryos matured than small-seeded species. Total accessory costs, and proportion of ovules aborted, were also significantly greater for biotically dispersed species, but only due to an association with larger seed masses. Costs associated with abortions were lower for biotically pollinated species, due to a general trend of more ovules per ovary, resulting in greater cost sharing. This study demonstrates that expenditure on items other than seeds accounts for the majority of reproductive allocation in flowering plants. Yet, far more literature exists on seed mass variation than on investment in accessory structures. We found a proportional relationship between accessory costs and seed mass that warrants further investigation within the context of selection on margin returns on investment.
Abstract: Global-scale quantification of relationships between plant traits gives insight into the evolution of the world's vegetation, and is crucial for parameterizing vegetation-climate models. A database was compiled, comprising data for hundreds to thousands of species for the core 'leaf economics' traits leaf lifespan, leaf mass per area, photosynthetic capacity, dark respiration, and leaf nitrogen and phosphorus concentrations, as well as leaf potassium, photosynthetic N-use efficiency (PNUE), and leaf N : P ratio. While mean trait values differed between plant functional types, the range found within groups was often larger than differences among them. Future vegetation-climate models could incorporate this knowledge. The core leaf traits were intercorrelated, both globally and within plant functional types, forming a 'leaf economics spectrum'. While these relationships are very general, they are not universal, as significant heterogeneity exists between relationships fitted to individual sites. Much, but not all, heterogeneity can be explained by variation in sample size alone. PNUE can also be considered as part of this trait spectrum, whereas leaf K and N : P ratios are only loosely related.
Abstract: Improved phylogenies and the accumulation of broad comparative data sets have opened the way for phylogenetic analyses to trace trait evolution in major groups of organisms. We arrayed seed mass data for 12,987 species on the seed plant phylogeny and show the history of seed size from the emergence of the angiosperms through to the present day. The largest single contributor to the present-day spread of seed mass was the divergence between angiosperms and gymnosperms, whereas the widest divergence was between Celastraceae and Parnassiaceae. Wide divergences in seed size were more often associated with divergences in growth form than with divergences in dispersal syndrome or latitude. Cross-species studies and evolutionary theory are consistent with this evidence that growth form and seed size evolve in a coordinated manner.
Abstract: 1 Potential height, which spans at least an order of magnitude across species, is considered an important indicator of light capture strategy. Still, it remains unclear how potential height is coordinated with other traits that influence height growth rate, stem persistence and performance in low light. We proposed that contrasting correlations between potential height and other plant attributes would be observed for sets of species selected to span two hypothetical axes of light availability within mature forest and time since disturbance. 2 We selected 45 perennial rain forest species in Australia's wet tropics to span gradients of light availability and successional status and measured potential height together with traits influencing light capture and regeneration strategy on mature individuals. The traits included leaf mass per area, leaf nitrogen, wood density, stem mass per length, branch mass fraction and seed mass. 3 Potential height was significantly correlated with numerous traits among species selected to span each of the two gradients. Height was positively correlated with leaf mass area(-1), leaf nitrogen and seed mass and negatively correlated with leaf area ratio at the branch tip along both light and successional gradients. Height was positively correlated with wood density along the successional axis, with the opposite relationship along the light gradient. 4 Trait relationships differed in either slope or intercept between the two gradients, reflecting different strategic trade-offs. At a given height, shorter species in the successional gradient were characterized by lower leaf mass area(-1), lighter wood, smaller seeds, lower leaf nitrogen and lower leaf area ratio at the branch tip than similar sized species along the light gradient. 5 The results of this study support the idea of two distinct, trait-mediated axes of coexistence among short and tall plant species within vegetation. In several cases, trait relationships were weak or non-significant when species groupings were merged, indicating the importance of separating out the two sets for comparative studies.
Abstract: 1. The leaf size-twig size spectrum is an important spectrum of variation between species, although the costs and benefits of larger vs smaller leaf size are poorly understood. This study quantified the dry mass costs of deploying leaf area in relation to leaf size, across 70 species from four sites contrasted on the basis of rainfall and soil nutrients in east-temperate Australia. 2. Leaf mass fraction beyond 10 mm(2) of conductive cross-section (LMF10) varied threefold and was strongly positively correlated with leaf size, both across all species and within each habitat. This and other key correlations were significant both across species and as evolutionary divergences. 3. An LMF10 advantage for larger-leaved species should translate into a proportional advantage in dry mass acquisition, yet species with small leaves persist and sustain populations. This raises the question as to what factors might counterbalance an LMF advantage associated with larger leaf size. 4. Within some sites, specific leaf area (SLA) decreased with leaf size, which counterbalanced the LMF10 advantage. Within other sites, and across all sites, SLA did not decline with leaf size. In these comparisons other factors, such as avoidance of overheating, must counterbalance the LMF10 advantage to larger-leaved species. 5. Wood density was negatively correlated with leaf size. However, the relationship was weaker within sites in the lower rainfall zone, and species in these sites had generally higher wood density. Sapwood cross-sectional area per unit leaf area (Huber value) varied approximately ninefold across all species, and was correlated with both leaf size and SLA across all species, but not within sites.
Abstract: Aim Our aim was to quantify climatic influences on key leaf traits and relationships at the global scale. This knowledge provides insight into how plants have adapted to different environmental pressures, and will lead to better calibration of future vegetation-climate models. Location The data set represents vegetation from 175 sites around the world. Methods For more than 2500 vascular plant species, we compiled data on leaf mass per area (LMA), leaf life span (LL), nitrogen concentration (N-mass) and photosynthetic capacity (A(mass)). Site climate was described with several standard indices. Correlation and regression analyses were used for quantifying relationships between single leaf traits and climate. Standardized major axis (SMA) analyses were used for assessing the effect of climate on bivariate relationships between leaf traits. Principal components analysis (PCA) was used to summarize multidimensional trait variation. Results At hotter, drier and higher irradiance sites, (1) mean LMA and leaf N per area were higher; (2) average LL was shorter at a given LMA, or the increase in LL was less for a given increase in LMA (LL-LMA relationships became less positive); and (3) A(mass) was lower at a given N-mass, or the increase in A(mass) was less for a given increase in N-mass. Considering all traits simultaneously, 18% of variation along the principal multivariate trait axis was explained by climate. Main conclusions Trait-shifts with climate were of sufficient magnitude to have major implications for plant dry mass and nutrient economics, and represent substantial selective pressures associated with adaptation to different climatic regimes.
Abstract: We used correlated divergence analysis to determine which factors have been most closely associated with changes in seed mass during seed plant evolution. We found that divergences in seed mass have been more consistently associated with divergences in growth form than with divergences in any other variable. This finding is consistent with the strong relationship between seed mass and growth form across present-day species and with the available data from the paleobotanical literature. Divergences in seed mass have also been associated with divergences in latitude, net primary productivity, temperature, precipitation, and leaf area index. However, these environmental variables had much less explanatory power than did plant traits such as seed dispersal syndrome and plant growth form.
Abstract: Background and Aims Species' 2C-values (mass of DNA in G(1) phase 2n nuclei) vary by at least four orders of magnitude among seed plants. The 2C-value has been shown to be co-ordinated with a number of other species traits, and with environmental variables. A prediction that species 2C-values are negatively related to leaf life span (LL) and leaf mass per area (LMA) is tested. These leaf traits are components of a major dimension of ecological variation among plant species. Methods Flow cytometry was used to measure the 2C-values for 41 Australian seed plant species, 40 of which were new to the literature. Where possible, LL and LMA data from the global literature were combined with 2C-values from our data set and online C-value databases. Key Results Across all species, weak positive relationships were found between 2C-values and both LL and LMA; however, these did not reflect the relationships within either angiosperms or gymnosperms. Across 59 angiosperm species, there were weak negative relationships between 2C-values and both LL (r(2) = 0(.)13, P = 0(.)005) and LMA (r(2) = 0(.)15, P = 0(.)002). These relationships were the result of shifts to longer LL and greater LMA in woody compared with herbaceous growth forms, with no relationships present within growth forms. It was not possible to explain a positive relationship between 2C-values and LMA (r(2) = 0(.)30, P = 0(.)024) across 17 gymnosperm species. The 2C-value was not related to LL or LMA either across species within orders (except for LMA among Pinales), or as radiation divergences in a model phylogeny. Conclusions Gymnosperms appear to vary along a spectrum different from angiosperms. Among angiosperms, weak negative cross-species relationships were associated with growth form differences, and traced to a few divergences deep in the model phylogeny. These results suggest that among angiosperms, nuclear DNA content and leaf strategy are unrelated.
Abstract: One way species of low maximum height can accrue sufficient light income to persist in vegetation is via rapid height growth immediately following disturbance. By surveying patches of known time since fire, we reconstructed height-growth trajectories for 19 post-fire recruiting species from fire-prone vegetation in south-eastern Australia. Cross-species patterns of height growth were compared to several plant traits thought to influence height strategy, including leaf mass per area, stem tissue density, stem diameter and capacity to resprout. Shorter species were found to temporarily outpace taller species, both as resprouters and within reseeders. Among reseeders, a single axis of variation summarised patterns of height-growth, time to onset of reproduction and longevity. This axis was tightly correlated with maximum height, leaf mass per area and stem diameter at a given height. These results illustrate how a range of height strategies can coexist in fire-prone vegetation, via the time-process initiated by disturbance.
Abstract: 1. Large-seeded species have long been known to have higher survivorship during establishment than small-seeded species. Here, we assessed the size of this advantage by compiling published data on survival through seedling emergence, seedling establishment and sapling establishment. 2. We found no relationship between seed mass and survival through the transition from viable seed in or on the soil to newly emerged seedlings (P = 0.47, n = 33 species). 3. Synthesis of data from experimental studies on the advantages of large-seeded species establishing under particular hazards (such as shade, drought or herbivory) confirmed that seedlings of large-seeded species perform better than those of small-seeded species in most situations. However, the magnitude of this advantage was not sufficient to counterbalance the greater number of seeds produced by small-seeded species m(-2) of canopy outline year(-1). 4. Synthesis of data from field studies of populations under natural conditions also showed that large-seeded species have higher survival through early seedling establishment than small-seeded species (P = 0.006, n = 112 species). However, the magnitude of this advantage would only be sufficient to counterbalance the greater number of seeds produced by small-seeded species m(-2) of canopy outline year(-1) if mortality continued at the same rate for some time. 5. The time required for a species with 10-fold larger seeds to recoup the advantage gained by a smaller-seeded species during seed production ranged from 8.8 weeks for the smallest seeded species in the data set, up to an implausible 4.2 years for the largest-seeded species. Thus, while large-seeded species do have a survival advantage over small-seeded species during seedling establishment, the available evidence suggests that advantages must also accrue during other stages in the life cycle. One possibility is that the greater seed production of small-seeded species (m(-2) of canopy outline year(-1)) is partly offset by larger canopies and longer reproductive life spans in large-seeded species.
Abstract: Retaining a reserve of buds or meristems for recovery from occasional damage is widespread among plants. Yet possession of a bud bank is not ubiquitous. Presumably there are costs to maintaining buds and for some species these costs outweigh the benefits. This paper has two themes. We estimate carbon costs of constructing and maintaining buds, and review other costs and benefits of buds in different situations. Second, we develop a framework for thinking quantitatively about the costs and benefits. Given available data and some reasonable assumptions, the absolute carbon costs of buds seem small. So the fact that many species do not maintain bud banks suggests that the benefits of buds may be negligible for them. Alternatively, buds may carry other costs. Costs of bud protection or of reserves that are coordinated with buds seem likely candidates.
Abstract: 1. The trade-off between seed mass and the number of seeds a plant can make for a given amount of energy underpins our understanding of seed ecology. However, there is little information on the magnitude of the fecundity advantage of small-seeded species over an entire plant lifetime. 2. We compiled data from the literature to quantify the relationships between: (i) seed mass and plant size (because the photosynthetic area of a plant determines how much energy is available for allocation to seed production); (ii) seed mass and plant reproductive lifetime (the number of years a plant has to produce seeds); and (iii) seed mass and the number of seeds produced per individual per year, and per lifetime. 3. Seed mass was positively related to all measures of plant size (canopy area, plant height, stem diameter, plant mass and canopy volume). There were also positive correlations between seed mass and time to first reproduction, plant life span, and reproductive life span. Thus, although small-seeded species produce more seeds per unit canopy area per year than large-seeded species, large-seeded species tend to have larger canopies and more reproductive years. 4. These patterns accord well with independently gathered data on annual and lifetime seed production. The negative relationship between seed mass and the number of seeds produced per year was much shallower on a per individual basis than on a per unit canopy basis. Seed mass was not significantly related to the total number of seeds produced by an individual plant throughout its lifetime. 5. Our previous understanding of seed mass as a spectrum from production of many small seeds, each with low establishment probability, to a few large seeds each with higher establishment probability, was missing some important elements. To understand the forces shaping the evolution of seed mass, we will need to consider plant size and longevity, as well as seedling survival rates and the number of seeds that can be produced for a given amount of energy.
Abstract: 1. Plant species composition and species response to grazing are fundamental to the management of grazing lands. Reliable prediction of grazing responses from species' traits would be a major step forward in generalizing knowledge and management beyond the locality. Recently results have been presented showing that plant species' responses to grazing in subhumid grasslands in Argentina and Israel could be predicted from simple traits: height, leaf size, life history and specific leaf area (SLA). This study assessed whether those relationships between traits and grazing responses hold for Australian semi-arid and arid shrublands and woodlands. 2. Eleven lists of grazing responses from five published grazing studies were matched with a plant trait data set. Trait distributions were compared between response groups pooled across studies. Relationships between traits and grazing response were also assessed, both within and across studies, using meta-analysis. 3. Overall there was little evidence for predictability of grazing responses with simple traits in the semi-arid and arid shrublands and woodlands. There were relationships between grazing response and life history and growth form, and some weak evidence for grazing increasers having high SLA. 4. Synthesis and applications. It was concluded that prediction of grazing responses with simple traits is less clear in semi-arid and arid rangelands, which are characterized by openness at ground level and high diversity of growth forms, compared with subhumid grasslands that have structurally simple, continuous swards. The finding that species' traits may have differing predictive capacity for grazing responses in different situations means that we need more empirical studies in different situations. The most important contrasts between situations for investigation are likely to be on axes of rainfall/productivity and evolutionary history of grazing.
Abstract: Bringing together leaf trait data spanning 2,548 species and 175 sites we describe, for the first time at global scale, a universal spectrum of leaf economics consisting of key chemical, structural and physiological properties. The spectrum runs from quick to slow return on investments of nutrients and dry mass in leaves, and operates largely independently of growth form, plant functional type or biome. Categories along the spectrum would, in general, describe leaf economic variation at the global scale better than plant functional types, because functional types overlap substantially in their leaf traits. Overall, modulation of leaf traits and trait relationships by climate is surprisingly modest, although some striking and significant patterns can be seen. Reliable quantification of the leaf economics spectrum and its interaction with climate will prove valuable for modelling nutrient fluxes and vegetation boundaries under changing land-use and climate.
Abstract: 1 A widely used classification of plant response to fire divides species into two groups, sprouters and non-sprouters. In contrast, regeneration responses to catastrophic wind throw and small gap disturbance are more often considered a continuum. 2 We determined general patterns in the distribution of sprouting ability across species with respect to disturbance type and intensity, vegetation type and phylogeny and assessed the adequacy of a dichotomy for describing species' sprouting responses. These are important steps if sprouting is to be adopted widely and consistently as a functional trait. 3 Quantitative data were compiled from the literature and differences in species' sprouting proportions between disturbance classes were assessed using simple sprouting categorizations, visually using histograms and with mixture models. 4 The sprouter/non-sprouter dichotomy effectively characterized intense disturbances, such as fires resulting in stem-kill (peaks at 13%, 79% probability of sprouting). But there was a continuum of responses following less intense disturbances. Where substantial above-ground tissue was retained, as for wind throw, localized gap disturbances and low intensity fires, there were fewer non-sprouters and more intermediate sprouters. 5 Comparisons across diverse vegetation types and disturbances require quantitative records of sprouting, although the simple sprouter/non-sprouter dichotomy was sufficient for comparisons within fire. Patterns appeared consistent across broad vegetation types. Sprouting ability showed little phylogenetic conservatism.
Abstract: 1. Plant survival following disturbance was modelled simply as the probability that at least one of n stems sprouts, each stem having an independent probability of sprouting, s. This first-order model with any stem on any plant in any species having probability s = 0.18 of sprouting after clipping (s = 0.09 after burning) explained nearly half of the deviance associated with species' mortality in a field experiment on 43 species from a range of growth forms. 2. Allowing species to take either a low or high per-stem sprouting probability (SSP) improved statistical explanation substantially. Fitting growth form SSP was less effective, showing that much of the apparent among-species variation in SSP was within growth forms. 3. Allowing each species to have a different SSP essentially provided a saturated model. The estimated species-specific probabilities were positively related to the depth from which sprouts could emerge after disturbance. Predicting species' bud depth from some simply measured morphological trait would be a considerable advance. Limited evidence suggested that sprout depth was associated with thick or dense leaves (low specific leaf area and leaf water content). 4. Depicting plants as a collection of independent stems with equal probability of sprouting appears a reasonable first-order model for whole-plant sprouting, despite being morphologically simplified.
Abstract: Leaf trait data were compiled for 258 Australian plant species from several habitat types dominated by woody perennials. Specific leaf area (SLA), photosynthetic capacity, dark respiration rate and leaf nitrogen (N) and phosphorus ( P) concentrations were positively correlated with one another and negatively correlated with average leaf lifespan. These trait relationships were consistent with previous results from global datasets. Together, these traits form a spectrum of variation running from species with cheap but frequently replaced leaves to those with strategies more attuned to a nutrient-conserving lifestyle. Australian species tended to have SLAs at the lower end of the spectrum, as expected in a dataset dominated by sclerophyllous species from low fertility or low rainfall sites. The existence of broad-scale, 'global' relationships does not imply that the same trait relationships will always be observed in small datasets. In particular, the probability of observing concordant patterns depends on the range of trait variation in a dataset, which, itself, may vary with sample size or species-sampling properties such as the range of growth forms, plant functional 'types', or taxa included in a particular study. The considerable scatter seen in these broad-scale trait relationships may be associated with climate, physiology and phylogeny.
Abstract: The mean size of seeds produced by plants at the equator is two to three orders of magnitude higher than the mean size of seeds produced by plants at 60degrees. We compiled data from the literature to assess the possibility that this latitudinal gradient in seed size allows species to cope with more difficult seedling establishment conditions in tropical environments. We found no relationship between latitude and seedling survival through 1 week (P = 0.27, n = 112 species). There was also no evidence that a larger seed mass is required to gain a given level of seedling survival in tropical environments (P = 0.37, n = 112 species), and no relationship between latitude and the duration of the juvenile period (P = 0.57, n = 132 species). Thus, our results are not compatible with the idea that seedling establishment is more difficult in the tropics.
Abstract: Patterns in the spatial distribution of organisms provide important information about mechanisms that regulate the diversity of life and the complexity of ecosystems(1,2). Although microorganisms may comprise much of the Earth's biodiversity(3,4) and have critical roles in biogeochemistry and ecosystem functioning(5-7), little is known about their spatial diversification. Here we present quantitative estimates of microbial community turnover at local and regional scales using the largest spatially explicit microbial diversity data set available (> 10(6) sample pairs). Turnover rates were small across large geographical distances, of similar magnitude when measured within distinct habitats, and did not increase going from one vegetation type to another. The taxa - area relationship of these terrestrial microbial eukaryotes was relatively flat (slope z = 0.074) and consistent with those reported in aquatic habitats(8,9). This suggests that despite high local diversity, microorganisms may have only moderate regional diversity. We show how turnover patterns can be used to project taxa - area relationships up to whole continents. Taxa dissimilarities across continents and between them would strengthen these projections. Such data do not yet exist, but would be feasible to collect.
Abstract: A widely-used description of vegetation response to fire is that species can be clearly classified as sprouters or non-sprouters. We aimed to assess: (1) how well this dichotomous classification (sprouter/non-sprouter) described the responses of a semi-arid flora to experimental disturbance; (2) how similar were sprouting responses to treatments mimicking intense herbivory and fire; (3) how well easily-measured traits could predict sprouting. Sprouting was assessed for 45 species from a range of growth forms (grasses, forbs, sub-shrubs, woody shrubs and trees) from semi-arid south-eastern Australia. We used two treatments: clipping at stem base, and clipping followed by burning with a blowtorch. A dichotomy accounted for >60% of deviance explained by species identity. Models with three or four groups were not substantially better. The dichotomy was not between 0% and 100% sprouting, rather between 'weak' and 'strong' sprouters. Probabilities of sprouting for weak sprouters were 23% after clipping and 6% after burning, while strong sprouters had sprouting probabilities of 90% after clipping and 79% after burning. While sprouting varied in space and time, the dichotomy was robust to this variation. Sprouting ability increased with size in most of the species with variable sprouting. Sprouting was partially related to growth form; grasses sprouted strongly, chenopods weakly, and forb and woody species covered the range of sprouting. Strong sprouters were likely to have more stems per plant, greater basal area, shorter potential height and deeper buds than weak sprouters. A hierarchical model that used growth form and then stems-per-plant provided a simple, robust predictor of sprouting. Four-fifths of species responded consistently to clipping and burning while one-fifth of species were strong sprouters after clipping but weak sprouters after burning. Burning reduced sprouting most in intermediate sprouters. Differences between sprouting after clipping and burning reflected increased intensity and were related to the depth of buds below ground.
Abstract: Relationships between seed mass and several aspects of plant regeneration ecology were investigated in a post-fire environment in Ku-ring-gai Chase National Park near Sydney, Australia. We found a significant positive relationship between seed mass and time to seedling emergence (P < 0.001) and a strong negative relationship between seed mass and time between emergence and production of the first true leaf (P < 0.001). Surprisingly, we found no relationship between seed mass and seedling establishment (P = 0.21). It seems most likely that this lack of relationship is a result of the many stochastic factors affecting seedling establishment during any given recruitment episode at any given site. A cause of mortality was assigned to 56% of the 781 seedlings that died during the present study. There was no relationship between cause of death and seed mass (P = 0.28). Of the seedlings for which the cause of death was known, 57% were killed by herbivory and 21% were killed by drought. Seedling-seedling competition affected only one species.
Abstract: 1. Ecologists have long recognized that plants occurring in areas of low rainfall or soil nutrients tend to have smaller leaves than those in more favourable regions. 2. Working with a large data set (690 species at 47 sites spread widely through south-east Australia) for which this reduction has been described previously, we investigated the morphology of leaf size reduction, asking whether any patterns observed were consistent across evolutionary lineages or between environmental gradients. 3. Leaf length, width and surface areas were measured; leaf traits such as pubescence or lobing were also scored qualitatively. There was no correlation between soil phosphorus and rainfall across sites. Further, there was no evidence that pubescence, lobing or other traits assessed served as alternatives to reduction of leaf size at the low ends of either environmental gradient. 4. Leaf size reduction occurred through many combinations of change in leaf width and length, even within lineages. Thus consistent patterns in the method of leaf size reduction were not found, although broad similarities between rainfall and soil P gradients were apparent.
Abstract: In microeconomics, a standard framework is used for determining the optimal input mix for a two-input production process. Here we adapt this framework for understanding the way plants use water and nitrogen (N) in photosynthesis. The least-cost input mixture for generating a given output depends on the relative cost of procuring and using nitrogen versus water. This way of considering the issue integrates concepts such as water-use efficiency and photosynthetic nitrogen-use efficiency into the more inclusive objective of optimizing the input mix for a given situation. We explore the implications of deploying alternative combinations of leaf nitrogen concentration and stomatal conductance to water, focusing on comparing hypothetical species occurring in low-versus high-humidity habitats. We then present data from sites in both the United States and Australia and show that low-rainfall species operate with substantially higher leaf N concentration per unit leaf area. The extra protein reflected in higher leaf N concentration is associated with a greater drawdown of internal CO2, such that low-rainfall species achieve higher photosynthetic rates at a given stomatal conductance. This restraint of transpirational water use apparently counterbalances the multiple costs of deploying high-nitrogen leaves.
Abstract: 1. Most plants withdraw nutrients from leaves as they age, and redeploy them elsewhere in the plant. The proportion of nutrients resorbed and the residual nutrient concentration in senesced leaves are different but complementary indices of nutrient conservation via this process. A major spectrum of strategic variation runs from plant species with typically long leaf lifespan (LL), high leaf mass per area (LMA), low leaf nutrient concentrations, and low photosynthetic capacity, to species with the opposite characteristics. It is unknown to what extent either facet of resorption covaries with the LL-LMA spectrum. 2. Green-leaf and senesced-leaf N and P concentrations were quantified for 73 evergreen species from four sites in eastern Australia (nutrient-rich and nutrient-poor sites in each of two rainfall zones). Leaf nutrient concentrations in green and senesced leaves were negatively correlated with LL across all species and at most sites, especially if N-2-fixing species were excluded from analyses involving leaf N. 3. Proportional resorption did not differ with soil nutrients, as has been found elsewhere, nor was it correlated with LL. Green-leaf and senesced-leaf nutrient concentrations were lower for species on poorer soils. A simple model was described in which the proportion of resorbed vs soil-derived nutrients deployed in new leaves is set by the relative cost of nutrients from the two sources. The model provides a prospective explanation for the observed differences between species from nutrient-rich and nutrient-poor habitats. 4. The results from this study provide support for the argument that selection to minimize nutrient losses has affected the residual nutrient concentration in senesced leaves, rather than proportional resorption per se. Further, variation among species in residual nutrient concentration was correlated with one of the key spectra of strategic variation between plant species, the leaf lifespan-LMA axis of variation.
Abstract: Aim We set out to test the hypothesis that rates of pre- and post-dispersal seed predation would be higher towards the tropics, across a broad range of species from around the world. We also aimed to quantify the slope and predictive power of the relationship between seed mass and latitude both within and across species. Methods Seed mass, pre-dispersal seed predation and post-dispersal seed removal data were compiled from the literature. Wherever possible, these data were combined with information regarding the latitude at which the data were collected. Analyses were performed using both cross-species and phylogenetic regressions. Results Contrary to expectations, we found no significant relationship between seed predation and latitude (log(10) proportion of seeds surviving predispersal seed predation vs. latitude, P = 0.63; R (2) = 0.02; n = 122 species: log(10) proportion of seeds remaining after postdispersal seed removal vs. latitude, P = 0.54; R (2) = 0.02; n = 205 species). These relationships remained non-significant after variation because of seed mass was accounted for. We also found a very substantial (R (2) = 0.21) relationship between seed mass and latitude across 2706 species, with seed mass being significantly higher towards the tropics. Within-species seed mass decline with latitude was significant, but only about two-sevenths, as rapid as the cross-species decline with latitude. Results of phylogenetic analyses were very similar to cross-species analyses. We also demonstrated a positive relationship between seed mass and development time across ten species from dry sclerophyll woodland in Sydney (P < 0.001; R (2) = 0.77; Standardized Major Axis slope = 0.14). These data lend support to the hypothesis that growing period might affect the maximum attainable seed mass in a given environment. Main conclusions There was no evidence that seed predation is higher towards the tropics. The strong relationship between seed mass and latitude shown here had been observed in previous studies, but had not previously been quantified at a global scale. There was a tenfold reduction in mean seed mass for every c . 23degrees moved towards the poles, despite a wide range of seed mass within each latitude.
Abstract: Architecture can vary widely across species. Both steeper leaf angles and increased self-shading are thought to reduce potential carbon gain by decreasing total light interception. An alternative hypothesis is that steeper leaf angles have evolved to improve day-long carbon gain by emphasising light interception from low angles. Here we relate variation in architectural properties (leaf angle and leaf size) to cross-species patterns of leaf display, light capture and simulated carbon gain in branching-units of 38 perennial species occurring at two sites in Australian forest. Architectural comparison was made possible by combining 3D-digitising with the architecture model YPLANT. Species with shallow angled leaves had greater daily light interception and potentially greater carbon gain. Self-shading, rather than leaf angle, explained most variance between species in light capture and potential carbon gain. Species average leaf size was the most important determinant of self-shading. Our results provide the first cross-species evidence that steeper leaf angles function to reduce exposure to excess light levels during the middle of the day, more than to maximise carbon gain.
Abstract: We investigated the relationship between seed size and seed survival in the soil in 67 species from arid Australia. There was a very weak, marginally significant positive relationship between the viability of fresh seeds and diaspore mass. However, by the time seeds had been buried in the soil for 1 year in nylon mesh bags, there was a highly significant positive relationship between diaspore mass and diaspore viability. Over the range of seed masses observed, a tenfold increase in diaspore mass was associated with a threefold increase in the odds of surviving 1 year of burial in the soil. Thus, large-seeded species were favoured over small-seeded species during this important selective process. However, the magnitude of this advantage was small compared with the advantage experienced by small-seeded species during seed production. We also investigated aspects of diaspore morphology in relation to viability retention during burial. We found no relationship between seed survival and the amount of protective tissue per unit diaspore surface area. Diaspore mass was a better predictor of survival than was diaspore surface area.
Abstract: Seed ecologists have often stated that they expect larger-seeded species to have lower survivorship through postdispersal seed predation than smaller-seeded species. Similar predictions can be made for the relationship between survivorship through predispersal seed predation and seed mass. In order to test these predictions, we gathered data regarding survivorship through 24 hours of exposure to postdispersal seed predators for 81 Australian species, and survivorship through. predispersal seed predation for 170 Australian species. These species came from an and environment, a subalpine environment, and a temperate coastal environment. We also gathered data from the published literature (global) on survivorship through postdispersal seed predation for 280 species and survivorship through predispersal seed predation for 174 species. We found a weak positive correlation between seed mass and the percentage of seeds remaining after 24 hours of exposure to postdispersal seed predators at two of three field sites in Australia, and no significant relationship across 280 species from the global literature, or at the remaining field site. There was no significant relationship between seed mass and survivorship through predispersal seed predation either cross-species or across phylogenetic divergences in any of the vegetation types, or in the compilation of data from the literature. Postdispersal seed removal was responsible for a greater percentage of seed loss in our field studies than was predispersal seed predation. On average, 83% of diaspores remained after 24 hours of exposure to postdispersal seed removers, whereas 87% of seeds survived all predispersal seed predation that occurred between seed formation and seed maturity. Mean seed survival was higher in the field studies than in the literature compilations, and species showing 100% survival were heavily underrepresented in the literature. These differences may be due to biases in species selection or publication bias. Seed defensive tissue mass increased isometrically with seed mass, but there was no significant relationship between the amount of defensive tissue per gram of seed reserve mass and survivorship through postdispersal seed predation.
Abstract: In plants, investment in height improves access to light, but incurs costs in construction and maintenance of the stem. Because the benefits of plant height depend on which other height strategies are present, competition for light can usefully be framed as a game-theoretic problem. The vertical structure of the world's vegetation, which is inefficient for plant growth, can then be understood as the outcome of evolutionary and ecological arms races. In addition, game-theoretic models predict taller vegetation on sites of higher leaf area index, and allocation to reproduction only after an initial period of height growth. However, of 14 game-theoretic models for height reviewed here, only one predicts coexistence of a mix of height strategies, a conspicuous feature of most vegetation. We suggest that game-theoretic models could help account for observed mixtures of height strategies if they incorporated processes for coexistence along spectra of light income and time since disturbance.
Abstract: Variation in plant functional traits results from evolutionary and environmental drivers that operate at a variety of different scales, which makes it a challenge to differentiate among them. In this article we describe patterns of functional trait variation and trait correlations within and among habitats in relation to several environmental and trade-off axes. We then ask whether such patterns reflect natural selection and can be considered plant strategies. In so doing we highlight evidence that demonstrates that (1) patterns of trait variation across resource and environmental gradients (light, water, nutrients, and temperature) probably reflect adaptation, (2) plant trait variation typically involves multiple-correlated traits that arise because of inevitable trade-offs among traits and across levels of whole-plant integration and that must be understood from a whole-plant perspective, and (3) such adaptation may be globally generalizable for like conditions; i. e., the set of traits (collections of traits in syndromes) of taxa can be considered as "plant strategies."
Abstract: There is a spectrum from species with narrow, frequently branched twigs carrying small leaves and other appendages, to species with thick twigs carrying large leaves and appendages. Here we investigate the allometry of this spectrum and its relationship to two other important spectra of ecological variation between species, the seed mass-seed output spectrum and the specific leaf area-leaf lifespan spectrum. Our main dataset covered 33 woody dicotyledonous species in sclerophyll fire-prone vegetation on low nutrient soil at 1,200 mm annual rainfall near Sydney, Australia. These were phylogenetically selected to contribute 32 evolutionary divergences. Two smaller datasets, from 390 mm annual rainfall, were also examined to assess generality of cross-species patterns. There was two to three orders of magnitude variation in twig cross-sectional area, individual leaf size and total leaf area supported on a twig across the study species. As expected, species with thicker twigs had larger leaves and branched less often than species with thin twigs. Total leaf area supported on a twig was mainly driven by leaf size rather than by the number of leaves. Total leaf area was strongly correlated with twig cross-section area, both across present-day species and across evolutionary divergences. The common log-log slope of 1.45 was significantly steeper than 1. Thus on average, species with tenfold larger leaves supported about threefold more leaf area per twig cross-section, which must have considerable implications for other aspects of water relations. Species at the low rainfall site on loamy sand supported about half as much leaf area, at a given twig cross-section, as species at the low rainfall site on light clay, or at the high rainfall site. Within sites, leaf and twig size were positively correlated with seed mass, and negatively correlated with specific leaf area. Identifying and understanding leading spectra of ecological variation among species is an important challenge for plant ecology. The seed mass-seed output and specific leaf area-leaf lifespan spectra are each underpinned by a single, comprehensible trade-off and their consequences are fairly well understood. The leaf-size-twig-size spectrum has obvious consequences for the texture of canopies, but we are only just beginning to understand the costs and benefits of large versus small leaf and twig size.
Abstract: 1 Leaf life span (LL) and leaf mass per area (LMA) are fundamental traits in the carbon economy of plants, representing the investment required per unit leaf area (LMA) and the duration of the resulting benefit (LL). Species on dry and infertile soils converge towards higher LMA. It has been generally assumed that this allows species from low-resource habitats to achieve longer average leaf life spans, as LMA and LL are often correlated. 2 Leaf fife span and LMA were measured for 75 perennial species from eastern Australia. Species were sampled from nutrient-rich and nutrient-poor sites within high and low rainfall regions. LL and LMA were positively correlated across species within each site. In addition, evolutionary divergences in LL and LMA were correlated within each site, indicating that cross-species relationships were not simply driven by differences between higher taxonomic groups. 3 Within a rainfall zone, LL-LMA combinations shifted as expected along common axes of variation such that species on poorer soils had higher LMA and longer LL, but significantly so only at high rainfall. 4 Low rainfall species were expected to have shorter LL at a given LMA or, equally, require higher LMA to achieve a given LL, i.e. shift to a parallel axis of variation, and this was observed on both nutrient-rich and nutrient-poor soils. On average, 30% higher LMA was seemingly required at dry sites to achieve a given LL. Thus, convergence towards higher LMA has different consequences for leaf life span in dry and nutrient-poor habitats. 5 The broad shifts in LL-LMA combinations between site types were also seen when comparing closely related species-pairs (phylogenetically independent contrasts) occurring on nutrient-rich and nutrient-poor soils (within each rainfall zone), and at high- and low-rainfall sites (at each soil nutrient level).
Abstract: We examined patterns of seedling root architecture, morphology and anatomy in Australian perennial plants chosen as phylogenetically independent contrasts (PICs) for rainfall in the areas they inhabit. Our objective was to assess whether there are consistent evolutionary patterns in structure of seedling root systems in species from different rainfall environments when examined across multiple evolutionary lineages. Seedlings were grown to a standardised developmental stage under controlled conditions. We found that seedling root systems of species restricted to low rainfall environments are characterised by greater proportional allocation to main root axis and have proportionally smaller main root axis diameter and areas of stele and xylem. Species of low rainfall environments also had higher specific root length (SRL) of the main axis, but lower SRL when the entire root system was considered. Seedling root system elongation rates were higher in species of high rainfall relative to those of low rainfall environments, paralleling expected differences in relative growth rate. The higher root system elongation rates in species of high rainfall environments were associated with greater numbers of growing tips in the root system, but not with differences in elongation rates of individual tips, relative to species of low rainfall environments.
Abstract: Across species, leaf lifespan (LL) tends to be correlated with leaf mass per area (LMA). Previously we found that Australian perennial species from low-rainfall sites had c. 40% shorter LL at a given LMA than high-rainfall species. Here we relate indices of leaf strength (work to shear, W-shear , and tissue toughness) to LL and LMA across the same suite of species. W-shear is the work required to cut a leaf with a blade; W-shear divided by leaf thickness gives tissue toughness. Low- and high-rainfall species did not differ in their LL at a given W-shear, but dry-site species had lower W-shear at a given LMA, leading to the observed LL - LMA shift with rainfall. These patterns were driven by 50% lower tissue toughness in dry-site species. The lower toughness was linked with high leaf N concentration, which is known to enhance water conservation during photosynthesis in low-rainfall species. Our results suggest that a significant cost of this strategy is reduced LL for a given investment in leaf tissue (LMA).
Abstract: An important aim of plant ecology is to identify leading dimensions of ecological variation among species and to understand the basis for them. Dimensions that can readily be measured would be especially useful, because they might offer a path towards improved worldwide synthesis across the thousands of field experiments and ecophysiological studies that use just a few species each. Four dimensions are reviewed here. The leaf mass per area-leaf lifespan (LMA-LL) dimension expresses slow turnover of plant parts (at high LMA and long LL), long nutrient residence times, and slow response to favorable growth conditions. The seed mass-seed output (SM-SO) dimension is an important predictor of dispersal to establishment opportunities (seed output) and of establishment success in the face of hazards (seed mass). The LMA-LL and SM-SO dimensions are each underpinned by a single, comprehensible tradeoff, and their consequences are fairly well understood. The leaf size-twig size (LS-TS) spectrum has obvious consequences for the texture of canopies, but the costs and benefits of large versus small leaf and twig size are poorly understood. The height dimension has universally been seen as ecologically important and included in ecological strategy schemes. Nevertheless, height includes several tradeoffs and adaptive elements, which ideally should be treated separately. Each of these four dimensions varies at the scales of climate zones and of site types within landscapes. This variation can be interpreted as adaptation to the physical environment. Each dimension also varies widely among coexisting species. Most likely this within-site variation arises because the ecological opportunities for each species depend strongly on which other species are present, in other words, because the set of species at a site is a stable mixture of strategies.
Abstract: Colonisation theory predicts that large-seeded species are more likely to show increased seedling recruitment in response to seed addition than are small-seeded species. This is both because their seedlings tend to have better survivorship potential, and because their background density of germinating seedlings tends to be lower. We tested this hypothesis by combining data from a recent review of seed addition studies with seed mass data. Logistic regressions showed positive relationships between seed mass and propensity to increase seedling establishment in response to seed addition in experiments in which establishment success was assessed within 6 months or a year, but not in experiments in which establishment success was assessed more than a year after seed addition. When data for all time periods were combined, a generalised linear model including terms for seed mass, time and an interaction term showed a significant positive relationship between seed mass and species response to seed addition. Thus, knowing a species' seed mass significantly increased our ability to predict its response to seed supplementation.
Abstract: 1. It has been suggested that leaf size may represent a foraging scale, with smaller-leaved species exploiting and requiring higher resource concentrations that are available in smaller patches. 2. Among 26 shrub species from a sclerophyll woodland community in New South Wales, Australia, species with smaller leaves tended to occur in better light environments, after controlling for height. The dark respiration rates of small-leaved species tended to exceed those of larger-leaved species. 3. However, the higher-light environments where smaller-leaved species tended to occur had a patch scale larger than whole plants. There would not have been any foraging-scale impediment to large-leaved species occupying these higher-light patches. An alternative explanation for small-leaved species being more successful in higher-light patches, in this vegetation with moderate shading, might be that they were less prone to leaf overheating. 4. Such relationships of leaf size to light across species at a given height may be important contributors to the wide spread of leaf sizes among species within a vegetation type, along with patterns down the light profile of the canopy, and effects associated with architecture and ramification strategy.
Abstract: In patch-occupancy models for vegetation, propagule output per area occupied is a key species trait, influencing the potential to colonize vacant patches, and hence species dynamics and coexistence. We estimated seed output across a range of species and quantified its relationship to seed dry mass, seed N and P content, and accessory costs in fruiting structures. Fruiting and seed production data were obtained for 47 woody perennial species, spanning an almost 3000-fold range of seed mass, over a period of one year in Ku-ring-gai Chase National Park, New South Wales, Australia. Seed output was measured as numbers per ml canopy outline and per m(2) leaf area. Of cross-species variation in seed output per m(2) canopy outline per year, 72% could be predicted from seed mass alone, with a directly inverse relationship (log-log slope not significantly different from - 1). Seed output per m(2) leaf area could be predicted somewhat more tightly (75%), indicating leaf area per canopy outline area accounted for some cross-species variation. Reproductive production per m(2) occupied per year varied much less than seed mass and accounted for the remaining variation in seed output. Although accessory costs were about equal in magnitude to seed mass as a component of aggregate investment per seed, they were strongly correlated with seed mass, and consequently did not add substantially to the predictive power. Total mass of N or P per seed were found to be slightly but significantly better predictors of seed output variation than dry seed mass (83% and 78%, respectively). This supports the idea that mineral nutrients are a more fundamental currency for seed production than dry mass. Seed mass, whether measured as dry mass or as N or P, appears to be the principal driver of variation in seed output per m(2) occupied, and consequently is among the most important dimensions of ecological variation across coexisting species.
Abstract: Seedling relative growth rate (RGR) achieved under favourable growth conditions can be thought of as a useful bioassay of the potential ability of species to take advantage of favourable growth opportunities; that is, of a species' growth strategy. The consistency of relationships between RGR and its component attributes leaf nitrogen productivity (LNP), leaf N per area (LNCa), specific leaf area (SLA) and leaf mass ratio (LMR) was assessed across 12 datasets comprising three growth forms (grasses, herbaceous dicots and woody plants; 250 species in total). These relationships were characterised in terms of scaling slopes (regressions on log-log axes, the slopes giving the proportional relationship between the variables). Mathematically, the expected scaling slope between RGR and each component is 1.0, giving an appropriate null hypothesis to test against (whereas the widely used null hypothesis of zero correlation is in fact inappropriate for this situation). Deviations below 1:1 scaling slopes indicate negative covariance between the components. Consequently, the correlation structure between the components of RGR should also be investigated. Biologically, RGR should scale 1:1 with SLA at a given LNCa and somewhat more weakly with LNCa at a given SLA. SLA and LNCa should themselves scale with a slope of between 0 and -1, with the actual slope indicating the extent to which between-species variation in SLA dilutes leaf N on an area basis versus the ability of species to maintain LNCa at a given growth irradiance. On average, across the 12 datasets RGR scaled close-to-proportionally with SLA, and 1:1 with SLA at a given LNCa. RGR scaled with LNCa with null or negative slopes, since SLA and LNCa scaled negatively (with slopes generally shallower than -1); however, RGR scaled positively (but less than proportionally) with LNCa at a given SLA. For these key relationships there were no qualitatively different conclusions with respect to the growth form under consideration or the growth irradiance at which the seedlings were grown. RGR also scaled close-to-proportionally with LNP, while LNP and LNCa were negatively associated. These relationships involving LNP are difficult to interpret since it can be shown that they are, at least potentially, the result of the interactions between RGR, SLA and LNCa, as well as reflecting intrinsic differences in the efficiency of nitrogen use in the growth process.
Abstract: 1. The aim of this study was to identify whether plant species show consistent responses to livestock grazing. The analyses were based on 35 published studies from Australian rangelands providing 55 species response lists. The primary data set comprised 1554 responses from 829 species. 2. Eight-hundred and twenty-nine species were categorized as increasers, decreasers or neutral under grazing. Of 324 species that occurred in at least two response lists, 133 (41%) responded inconsistently, increasing at least once and decreasing at least once. While 59% of species responded consistently, these results suggest that our ability to predict vegetation change under grazing is limited. 3. Particular species were not inherently more or less consistent. Rather, as species occurred in more trials, the likelihood of at least one opposite response increased; no species that occurred at least eight times was wholly consistent. A binomial model indicated that the probability of an opposite response, across all species, was 0.275. 4. Contrary responses within species must result from context rather than from species' traits. Species were more likely to decrease in response to grazing at lower rainfall than at higher rainfall. Forbs tended to increase under grazing at sites where wet seasons were cooler. Changing the grazing animal was weakly correlated with change in response direction, although not enough for it to be useful for manipulating pasture composition. We found little support for ideas that different responses within species are due to differences in alternative forage available, or due to non-linearity of response to grazing intensity. 5. At present it appears we can predict species response direction about three-quarters of the time,, at a continental scale. This represents an upper limit of the reliability of prediction based on species' traits alone. Presently we do not know what aspects of the context might allow us to predict reliably the remaining one-quarter of responses.
Abstract: 1. Relationships were examined among photosynthetic capacity (A(mass) and A(area)), foliar dark respiration rate (Rd-mass and Rd-area ), stomatal conductance to water (G(s)), specific leaf area (SLA), and leaf nitrogen (N) and phosphorus (P) across 79 perennial species occurring at four sites with contrasting rainfall levels and soil nutrients in eastern Australia. We hypothesized that the slope of log-log 'scaling' relationships between these traits would be positive and would not differ between sites, although slope elevations might shift between habitat types. 2. A(mass), Rd-mass SLA N-mass and P-mas, were positively associated in common slopes fitted across sites or rainfall zones, although rather weakly within individual sites in some cases. The relationships between A(mass) (and Rd-mass) with each of N-mass and SLA were partially independent of each other, with A(mass) (or Rd-mass) increasing with SLA at a given N-mass or with N as, at a given SLA (only weakly in the case of A(mass)). These results improve the quantification and extend the generalization of reported patterns to floras largely unlike those studied previously, with the additional contribution of including phosphorus data. 3. Species from drier sites differed in several important respects. They had (i) higher leaf N and P (per dry mass or area); (ii) lower photosynthetic capacity at a given leaf N or P; (iii) higher Rd-mass at a given SLA or A(mass); and (iv) lower G(s) at a given A(area) (implying lower internal CO2 concentration). 4. These trends can be interpreted as part of a previously undocumented water conservation strategy in species from dry habitats. By investing heavily in photosynthetic enzymes, a larger drawdown of internal CO2 concentration is achieved, and a given photosynthetic rate is possible at a lower stomatal conductance. Transpirational water use is similar, however, due to the lower-humidity air in dry sites. The benefit of the strategy is that dry-site species reduce water loss at a given A(area), down to levels similar to wet-site species, despite occurring in lower-humidity environments. The cost of high leaf N is reflected in higher dark respiration rates and, presumably, additional costs incurred by N acquisition and increased herbivory risk.
Abstract: When a plant invests in construction of a leaf, the revenue-stream that accrues is shaped by three variables: first, the light-capture area per milligram dry mass invested, analogous to a potential rate of return on investment; second, the longevity of the leaf, analogous to the expected duration of the revenue stream; and third, a time-discount rate, quantifying the fact that light-capture area deployed in the immediate future is more valuable to the plant than the same area deployed at some later time. Recent comparative data make it possible to quantify the cross-species trade-off between the first variable and the second variable. Here we develop an approach through which the consequences of the third variable, the time-discount rate, can be related to the trade-off between the first variable and the second variable. The approach involves an equal-benefit set, the cross-species equivalent of a fitness set. A wide spread of strategies is actually observed to coexist in vegetation, from low to high light capture area per gram and, correspondingly, from high to low leaf longevity. The coexistence suggests that the different observed strategies do not have a clear-cut advantage over the other. The equal-benefit set can be used to investigate what levels of time discount would make it the case that neither the highest-longevity nor the highest light-capture area per milligram strategies would have a clear advantage over the other, with regard to the time-discounted value of the revenue stream generated per milligram invested in leaf.
Abstract: Seedling traits have been described across 1744 species of Australian plants; Six traits were coded as binary alternatives: (1) phanerocotyly vs cryptocotyly; (2) first leaf scale-like vs leaflike; (3) first leaf single vs paired; (4) cotyledons hairy vs, glabrous; (5) hypocotyl hairy vs glabrous, and (6) embryo green vs non-green at maturity. Seed volume was calculated from measurements of seed dimensions. Three approaches were used in analysing the data: (1) the taxonomic distribution of binary character states was described; (2) the strength of bivariate character associations was quantified at species level ('cross-species' correlations and regressions); (3) the data were arrayed on a phylogenetic tree in order to analyse by 'phylogenetic regression' for correlated evolutionary shifts in trait pairs. AU the traits appeared evolutionarily malleable. For example, while cryptocotyly was the minority condition (22% of species), it occurred in many different Laxa (124 genera, 40 families, 24 orders) with high levels of polymorphy (16%, 25% and 42% for genera, families and orders). Similarly, the less common attribute states for first leaf type, hypocotyl texture, cotyledon texture and embryo colour occurred in species From right across the dicotyledon phylogeny, as did independent evolutionary divergences in each of these characters. These patterns indicate that debate over which conditions are primitive and which are advanced will not have any general answer, only an answer for a specific branch-step in the phylogenetic tree. In nearly all cases, correlated-divergence analyses showed the same patterns as cross-species analyses. The strongest associations were between seed volume and cryptocotyly, seed volume and scale-like first leaf, and seed volume and presence of green embryo. In addition, cryptocotyly and scale-like first leaf, and cotyledon and hypocotyl type, were strongly associated in both correlated-divergence and cross-species analyses. Interpretation of results was mostly presented with respect to seed size, a trait which we consider to be pivotal in a species' seedling establishment strategy. AU possible pairwise combinations of binary seedling traits were found in our study species. Taken together, the various lines of evidence presented here suggest that the traits have assorted more or less independently of each other and provide no evidence of functional groups based on these attributes. Thus, the several existing seedling typologies should be regarded as classifications of convenience rather than as reflecting fundamental types. (C) 2000 The Linnean Society of London.
Abstract: 1. Seedling relative growth rate (RGR) is often decomposed into the product of specific leaf area (leaf area per leaf mass, SLA), net assimilation rate (rate of mass increase per unit leaf area per unit time, NARa) and leaf mass ratio (ratio of leaf to total dry mass, LMR). Commonly, most cross-species variation in RGR is accounted for by variation in SLA, while no general relationships occur between RGR and either NARa or LMR. NARa can be factored into the product of leaf nitrogen productivity (rate of mass increase per unit leaf nitrogen per unit time, LNP) and leaf nitrogen concentration (area basis, LNCa). In this way the influence on RGR of leaf nitrogen - how it is displayed, and how it is utilized - can be investigated. 2. Seedlings of 28 Australian woody dicot species were grown under controlled, favourable conditions. Variation in SLA, LNP, LNCa and LMR explained c. 44%, 22%, 19% and 15% of variation in RGR, respectively. SLA and LNP were positively associated with RGR, while LNCa was negatively associated with RGR. LNP and LNCa were negatively correlated, the counteracting trends between RGR and each of these attributes resulting in no relationship between RGR and NARa. It is argued that this phenomenon may be widespread and may contribute to the inconsistency of reported relationships between NARa and RGR. 3. The functional balance between leaves and roots can be described in terms of mass allocation and morphology (static ratios or allometric coefficients) or, alternatively, in terms of leaf 'activity' (NAR) and root 'activity' (nitrogen uptake rate, NUR). At any given time most species allocated greater mass to leaves than to roots, but species with low RGR tended to be partitioning a greater ongoing proportion of new biomass to the roots rather than to the leaves, resulting in a proportionally greater increase in root surface compared with leaf surface over time. Nitrogen uptake rate was correlated with leaf and whole-plant nitrogen concentration, but not with other attributes. While it is clear that root and leaf functions must be co-ordinated (and thus in balance) for growth to occur, there is little evidence that this balance varies systematically with RGR across all species.
Abstract: It has recently been shown that most low-abundance species at a location are substantially more abundant somewhere else within their geographical range (somewhere-abundant). Fewer than 10% are everywhere-sparse Here, two everywhere-sparse species from dry sclerophyll woodland were compared with phylogenetically contrasted somewhere-abundant species, at sites where both were at low abundance. In each pair, everywhere-sparse species produced approximately ten-fold fewer seeds per area of canopy cover than the somewhere-abundant species, consistently across replicate sites. Around individuals, a significantly larger proportion of the immediately colonizable neighbourhood was already occupied by the same species, for the everywhere-sparse compared to the somewhere-abundant species, in each pair and across replicate sites. Together, these differences amount to a much lower potential rate of increase in everywhere-sparse species compared with somewhere-abundant species, and are consistent with their having low capacity for opportunistic increase to high abundance. Our findings suggest novel differences between species that differ in local abundance patterns across geographical ranges, and provide a basis for further exploration of life-history and demographic differences between everywhere-sparse and somewhere-abundant species.
Abstract: Several experiments have shown that seedlings from larger-seeded species are better able to survive various hazards during establishment. Previous work has suggested a general mechanism might underpin this outcome. Larger-seeded species might tend to mobilize their metabolic resources over a longer period into the autotrophically functioning structures of the seedling. Consequently relatively more resources would remain uncommitted at any given time during the early period of the seedling's growth, and available to support respiration during carbon deficit. An important aspect of this larger-seed-later-commitment mechanism would be that at a given time, larger-seeded species would hold more resources uncommitted not just absolutely, but relative to the functional seedling structures that needed to be supported. Here we quantify, across a wide range of phanerocotylar species, an allometric pattern that supports the generality of a larger-seed-later-commitment mechanism as an explanation for superior performance by larger-seeded species in face of the hazards of seedling establishment. Larger-seeded species allocate relatively less to cotyledon area, reflecting the initial functional size of the seedling, and relatively more to dry mass per unit area of cotyledon, reflecting stored metabolic reserves. The shift in relative allocation is progressive, rather than seedlings falling into discrete morphological types. The allometry is similar whether considered as correlated evolutionary divergences (phylogenetically independent contrasts) or as correlation across present-day species.
Abstract: Leaves are most vulnerable to herbivory during expansion. We hypothesised that one factor favouring small leaves could be that smaller-leaved species have shorter expansion times and are therefore exposed to high levels of herbivory for a shorter period than large leaves. In order to test this hypothesis, leaf expansion time and leaf area loss were measured for 51 species from Sydney, Australia. Strong positive correlations were found between leaf length and area and leaf expansion time, confirming that small leaves do expand in a shorter time than large leaves. The amount of leaf area lost was highly variable (from 0.5 to 90% of total leaf area), but was significantly related to both leaf expansion lime and log leaf area. The amount of leaf area lost was not significantly correlated with specific leaf area nor with the presence of distasteful substances in the leaves, but was lower on species with hairy expanding leaves.
Abstract: 1 If different factors inhibiting plant growth, e.g. low rainfall or low soil nutrients, were to select for species that have similar constellations of traits, then the unfavourable factors might usefully be grouped together as 'stress'. 2 We offer a method for assessing this issue. A species mixture at a site is described by a point on a plane with two traits as axes. Change along an environmental gradient is then represented as a trajectory across the trait-plane. Trajectories along different environmental gradients are compared. 3 We measured leaf width, specific leaf area (SLA) and mature canopy height for the 386 perennial species found at 46 sites spread along rainfall and soil total phosphorus gradients in south-eastern Australia. Each trait was lognormally distributed across species within sites, hence the mean of log(10)(trait) satisfactorily described the species mixture at each site. 4 Combinations of assemblage-mean leaf width with SLA followed similar trajectories as rainfall and soil total P decreased. For these traits in this setting, the method indicated that low rainfall and low soil P favour similar trait-combinations. 5 Mature plant height also decreased along both rainfall and soil P gradients, and thus was positively correlated with leaf width and SLA at the level of assemblage means. The rainfall trajectories involving height behaved differently from the soil P trajectories, especially at rainfalls below c. 400 mm year(-1), where assemblage-mean height declined much further than at low soil P. 6 Across all species, traits were only very loosely correlated (r(2) from 0.04 to 0.17). For leaf width and SLA, evolutionary divergences were positively correlated, both before and after cross-correlation with divergence in rainfall and soil P was removed. This latter measures evolutionary divergence correlation. within habitat. For height the picture was more complicated. Considering these within-habitat divergence correlations, species that were taller at maturity tended to have lower SLA and leaf width. This pattern is the reverse of the broad geographical correlation of assemblage means, showing that the patterns across assemblages result from species being selectively sifted from the regional flora into sites, not from evolutionary or cross-species correlations. 7 The trait-combination trajectory approach showed some commonalities between low soil nutrient and low rainfall habitats with regard to traits favoured in species occurring there, but also some differences. The approach has potential for clarifying which environmental factors can usefully be grouped together as 'stress', and which trait combinations can usefully be regarded as part of a syndrome favoured by stress.
Abstract: Aim In this study we compare the incidence of rarity within lineages between three floras: Australia, North America and New Zealand. Methods We used published data for absolute numbers of species and for numbers of rare species to ask three questions: 1. Do families have similar proportions of rare species in different floras? 2. Can proportions of rare species within families be interpreted with reference to biogeographic history of particular floras? and 3. Is the proportion of rare species consistent between families and genera within each flora? Results Within families in each flora, genera usually had similar proportions of rare species; indicating processes determining rarity are in general not operating differentially within families. However, in comparisons within-family between floras, approximately 62% of comparisons showed significant differences in the proportion of rare species. Main conclusions These results imply that there are no general rules whereby the particular traits a family possesses are conducive to rarity wherever they occur. Rather, such attributes must exert influence conditionally, in conjunction with the biogeographical and competitive setting where speciation and extinction or persistence has occurred.
Abstract: 1 Species-pairs from woody dicot lineages were chosen as phylogenetically independent contrasts (PICs) to represent evolutionary divergences along gradients of rainfall and nutrient stress, and within particular habitat types, in New South Wales, Australia. Seedlings were grown under controlled, favourable conditions and measurements were made for various growth, morphological and allocation traits. 2 Trait correlations across all species were identified, particularly with respect to seedling relative growth rate (RGR) and specific leaf area (SLA), a fundamental measure of allocation strategy that reflects the light-capture area deployed per unit of photosynthate invested in leaves. 3 Across all species, SLA, specific root length (SRL) and seed reserve mass were the strongest predictors of seedling RGR. That is, a syndrome of leaf and root surface maximization and low seed mass was typical of high RGR plants. This may be a high-risk strategy for individual seedlings, but one presumably mitigated by a larger number of seedlings being produced, increasing the chance that at least one will find itself in a favourable situation. 4 Syndromes of repeated attribute divergence were identified in the two sets of gradient PICs. Species from lower resource habitats generally had lower SLA. Thus, in this important respect the two gradients appeared to be variants of a more general 'stress' gradient. 5 However, trends in biomass allocation, tissue density, root morphology and seed reserve mass differed between gradients. While SLA and RGR tended to shift together along gradients and in within-habitat PICs, no single attribute emerged as the common, primary factor driving RGR divergences within contrasts. Within-habitat attribute shifts were of similar magnitude to those along gradients.
Abstract: Hummock grasses in the genus Triodia R. Pr. were examined at 116 recently burned locations across mainland Australia, and were recorded as regenerating by resprouting, or by seed only. Prior to this survey it was known that the capacity to resprout varies considerably in Triodia, but it was unclear whether some species might be entirely resprouting and other species entirely fire-killed; whether this might be associated with particular traits of species or particular landscape types; or whether resprouting might be more common in particular regions such as the northern monsoonal grasslands. Seven species had both fire-killed and resprouting populations represented among the sites examined. Three species [T. basedowii. E. Pritzel, T.marginata N.T. Burbidge and T. melvillei (C.E. Hubbard) Lazarides] were recorded as fire-killed at all sites examined, and five species [T. bynoei (C.E. Hubbard) Lazarides, T. bunicola (S.W.L. Jacobs) Lazarides, T. intermedia Cheel, T. molesta N.T. Burbidge and T. spicata N.T. Burbidge] were recorded as resprouting at all sites. Another seven species were recorded at one sire only. There was no clear overall trend for fire-killed populations to be more prevalent in south rather than north, or east vs west, or coastal vs inland parts of the continent. No clear relationships were found between regeneration strategy and site attributes. Considered as a group, resprouting species were more likely to have an open growth form than a dense growth form, whereas the opposite was true for fire-killed species. Given that no trait either of species or of landscape was found to be a consistent correlate of resprouting, a likely inference is that resprouting might vary between different fires, even for a given population at a given sire. Against this inference, fire-killed populations had substantial densities of seedlings much more often. Longitudinal studies would be needed to demonstrate directly whether resprouting varied between different fires at a sire, and to investigate the implications of this for maintenance or extinction of local populations.
Abstract: At focal sites within dry sclerophyll woodland and temperate rain forest, species were identified that were of low local abundance and hence in the tail of the rank-abundance curve. We then asked the question: What proportion of tail species within a given community are constitutive members of the tail everywhere throughout their geographical range, versus what proportion are found as substantially more abundant somewhere within their range? Out of 55 tail species identified from dry sclerophyll woodland and 116 tail species identified from temperate rain forest, 91% and 95%, respectively, were significantly more abundant at other locations ("somewherc-abundant" species), versus 9% and 5% "everywhere-sparse'' species. Among eight attributes in dry sclerophyll woodland and nine attributes in temperate rain forest compared between somewhere-abundant and everywhere-sparse species, none discriminated consistently between the two groups of species. The size and dispersal morphology of seeds, flowering and fruiting durations and seasons, regeneration strategy after fire, size of geographical ranges, maximum plant height, and size class revealed no consistent distinctions. For the small minority of species that are everywhere-sparse, some general explanation may exist as to why they are in the tail of rank-abundance curves, though none was located among the attributes compared in this paper. For the majority of tail species that are somewhere-abundant, any explanation as to why they are in the tail will need to account for different outcomes in different places.
Abstract: Leaf structural and chemical characteristics were compared in pairs of species that were phylogenetically independently contrasted along rainfall gradients (10 pairs) or soil nutrient gradients (9 pairs), using perennial plants in New South Wales, Australia. The objective was to test hypotheses regarding leaf attributes that should be successful in sustaining populations in vegetation at lower vs. higher rainfall or lower vs. higher soil nutrients, and especially to assess the proposition that lower rainfall and lower soil nutrients favor similar shifts in leaf attributes, and in this sense can be grouped together as sources of "stress." Some evolutionary divergences in leaf structure occurred repeatedly toward the lower end of both rainfall and soil nutrient gradients. These include narrower leaves, lower specific leaf area (SLA), thicker lamina, and denser leaf tissue. In other respects, the different resource gradients appeared to favor different leaf attributes. In 8 of 10 contrasts, plants of low-water environments had more sclerified vasculature than their relatives on higher rainfall sites. This may be an adaptation to resist wilting and so mitigate cell damage. In 9 of 10 contrasts, plants of low-water environments had more nitrogen per unit leaf area than their relatives in high-rainfall sites. This may be to exploit higher light availability in arid communities. In 7 of 9 contrasts, plants from low soil nutrient environments had thicker epidermis than their relatives on higher nutrient soils. In 8 of 9 soil nutrient contrasts, leaf nitrogen concentration was lower in the low-nutrient species, and consequently, the ratios of phenol/N and tannin/N were higher. Thicker epidermis and increased defense/N ratios may reduce herbivory and thereby reduce demand on soil nutrients required to replace leaves.
Abstract: The influence of seed mass on the evolution of seedling foraging strategies for light acquisition under deep shade was assessed in a comparative study of etiolation behavior. This was done across 50 Australian species varying in seed reserve mass by eight orders of magnitude during the first week after germination. Proportional increase in hypocotyl length in shade compared to light was similar across the range of seed reserve mass. Etiolation did not lead to increase in hypocotyl length per wet mass, in other words, etiolated hypocotyls were not thinner. However, hypocotyl length per dry mass did increase, more so in smaller-seeded species. Thus, part of the hypocotyl elongation was because of increased water content, which would increase vulnerability to loss of turgor. There was also reallocation of dry matter from root to hypocotyl, again more so in smaller-seeded species, which would decrease anchorage strength and increase vulnerability to soil drying. Results were very similar when considered as correlated evolutionary divergences, compared to the cross-species patterns. The higher-risk etiolation behavior of smaller-seeded species can be understood through their having little to lose. Because they hold less reserve resource uncommitted and attempt a faster initial growth rate, their chances of sustained longevity in shade below the compensation point are very low.
Abstract: A leaf-height-seed (LHS) plant ecology strategy scheme is proposed. The axes would be specific leaf area SLA (light-capturing area deployed per dry mass allocated), height of the plant's canopy at maturity, and seed mass. All axes would be log-scaled. The strategy of a species would be described by its position in the volume formed by the three axes. The advantages of the LHS scheme can be understood by comparing it to Grime's CSR scheme, which has Competitors, Stress-tolerators and Ruderals at the corners of a triangle. The CSR triangle is widely cited as expressing important strategic variation between species. The C-S axis reflects variation in responsiveness to opportunities for rapid growth; in the LHS scheme, SLA reflects the same type of variation. The R axis reflects coping with disturbance; in the LHS scheme, height and seed mass reflect separate aspects of coping with disturbance. A plant ecology strategy scheme that permitted any species worldwide to be readily positioned within the scheme could bring substantial benefits for improved meta-analysis of experimental results, for placing detailed ecophysiology in context, and for coping with questions posed by global change. In the CSR triangle the axes are defined by reference to concepts, there is no simple protocol for positioning species beyond the reference datasets within the scheme, and consequently benefits of worldwide comparison have not materialized. LHS does permit any vascular land plant species to be positioned within the scheme, without time-consuming measurement of metabolic rates or of field performance relative to other species. The merits of the LHS scheme reside (it is argued) in this potential for worldwide comparison, more than in superior explanatory power within any particular vegetation region. The LHS scheme avoids also two other difficulties with the CSR scheme: (a) It does not prejudge that there are no viable strategies under high stress and high disturbance (the missing quadrant in the CSR triangle compared to a two-axis rectangle); (b) It separates out two distinct aspects of the response to disturbance, height at maturity expressing the amount of growth attempted between disturbances, and seed mass (inverse of seed output per unit reproductive effort) expressing the capacity to colonize growth opportunities at a distance. The advantage of LHS axes defined through a single readily-measured variable needs to be weighed against the disadvantage that single plant traits may not capture as much strategy variation as CSR's multi-trait axes. It is argued that the benefits of potential worldwide comparison do actually outweigh any decrease in the proportion of meaningful variation between species that is captured. Further, the LHS scheme opens the path to quantifying what proportion of variation in any other ecologically-relevant trait is correlated with the LHS axes. This quantification could help us to move forward from unprofitable debates of the past 30 years, where CSR opponents have emphasized patterns that were not accommodated within the scheme, while CSR proponents have emphasized patterns that the scheme did account for.
Abstract: 1. Previous studies have shown that among British species, seeds that persist in the soil tend to be small and compact compared with non-persistent seeds. We tested whether or not this pattern is repeated among 101 Australian species, from a range of habitats. 2. Seed mass was plotted against variance of seed dimensions, across all species. Species with persistent seeds were found across the whole range of seed mass (0.217-648.9 mg) and variance (0.0000-0.2497), providing no evidence for a critical mass or variance which separated persistent from transient seeds. 3. We tested whether or not divergence within individual clades between persistent and transient seeds was associated with increased seed mass or seed dimension variance, using phylogenetically independent contrasts (PICs). Their was no consistent tendency found. 4. Thus for Australian species, persistent seeds were not smaller or more compact than transient seeds when compared across all species or when compared using PICs. Presumably the natural history of burial and disturbance operates differently in British and Australian habitats.
Abstract: 1. The Australian continent provides an important test of macroecological patterns given its unique biota and long-term geographical isolation. However, macroecological contributions from the Australian continent are rare. We explored the relationship between abundance and geographical range for Australian frogs (Order Anura) across complete geographical ranges, and investigated how adult body size relates to both abundance and the size of geographical ranges. 2. Our analyses followed two complementary paths. First, we employed cross-species correlations at a number of taxonomic levels to analyse present-day ecological associations among the macroecological variables. Secondly, we tested whether there were correlated evolutionary divergences among the variables by use of the phylogenetic regression. In the event, cross-species patterns and correlated divergence patterns proved quite similar for this dataset. 3. For Australian frogs there is a strong, significant positive correlation between abundance and geographical range, that is not mediated through body size, and that is found for both cross-species correlations and as correlated evolutionary divergences. Among biological mechanisms that have been proposed, some can be rejected. Feasible explanations would invoke resource use or intrinsic rates of increase or both. 4. There is also a significant positive correlation between adult body size and geographical range. We were able to discard four of the five proposed mechanisms describing this relationship. The remaining mechanism relates to homeostasis and environmental variability. 5. Previous studies of the relationship between adult body size and abundance have revealed a strong negative correlation when studies are performed over areas that encompass most or all of the geographical ranges of the species in question. Contrary to these studies, a negative relationship was not found for Australian frogs. None of the eight explanations postulated from other studies can account for this discrepancy, leading to the conclusion that Australian frogs provide a contrast to macroecological patterns prevailing elsewhere. 6. Body size could not explain directly the relationship between abundance and geographical range. While there was a significant positive correlation between body size and geographical range, there was no relationship between body size and abundance. If a mechanism incorporating the intrinsic rates of increase of species is to explain the positive relationship between abundance and geographical range, then ecological factors other than body size must be determining differences in intrinsic rates of increase.
Abstract: Within plant communities seed mass often varies over 3 to 5 orders of magnitude, yet simple evolutionary models predict a single optimum seed mass. Here we explore a class of models where seed mass determines 1) the number of seeds produced via a size-number trade-off and 2) competitive ability - plants arising from large seeds are assumed to have a competitive advantage over those derived from small seeds. In this setting the existence of a single-species global ESS seed mass requires the competitive advantage of large seeds over small ones to be unbounded. If there is a limit on the competitive advantage that large seeds obtain then it is always possible to find a smaller seed mass that will successfully invade. In such circumstances there might be a multi-species coevolutionarily stable coalition of several species each with a different seed mass. In this way a wide range of seed masses could be promoted by evolution. In general the adaptive landscape generated by these models is extremely flat leading to slow evolutionary dynamics. The implications of these results for the interpretation of observational, comparative and experimental studies are discussed.
Abstract: Many species of tropical moist forests have large seeds compared to those found in temperate floras. This could be attributed to a prevalence of woody growth forms, or adaptation to dispersal by vertebrates, or the dense shade of a closed canopy, rather than to an intrinsic tropical v. temperate difference. We compared tropical v. temperate seed mass data at two geographic scales; firstly within Australia between tropical, subtropical and temperate open woodlands, then between six tropical and temperate datasets from live continents. Within Australia we found that seed mass increased with decreasing latitude in ten out of eleven growth form-dispersal mode combinations: only wind-dispersed graminoids showed no significant effect. While the pattern of generally larger seeds in the tropical flora was associated with tropical families tending to have larger seeds than temperate families, we also found that tropical species had larger seeds than temperate cofamilials. As a preliminary test of the generality of these findings we included a further four tropical and temperate data sets in a second analysis. Average tropical seed masses were larger than temperate seed masses in all testable growth form-dispersal mode combinations, and statistically larger in five out of;seven cases: unassisted, vertebrate- and wind-dispersed woody species, and unassisted and vertebrate-dispersed forbs. No difference was found for wind-dispersed forbs or for graminoids lacking dispersal structures. That an effect was found in these combinations rules out explanations based solely on characteristics of tropical v. temperate dispersers, although this may be a contributing factor. Instead we suggest that some aspect of tropical systems, for example higher metabolic costs of seedling production, has selected for a general increase in seed mass among tropical taxa.
Abstract: 1 Time-varying matrix models were constructed for the population dynamics of two species of arid zone shrub (Eremophila maitlandii and E. forrestii) using parameters estimated from a long-term sheep grazing study (1983-93). 2 The models were used to assess the relative contributions of episodic versus continuous mortality and recruitment processes to total population change over 48-year periods. In 'baseline' models that used best estimates of return times for droughts and wet episodes, 15-40% of total deaths and 50-70% of total recruitment occurred during episodes, In none of the models, using a range of event return times, did event-driven mortality exceed 60% or event-driven recruitment exceed 85%. 3 Because these conclusions could not easily be accommodated within the current event-driven world view, we compiled other published quantifications of arid zone shrub dynamics. For long-lived species, drought episodes have not generally been found to cause population decline of more than about 20%. There are no documented examples of recurring recruitment episodes, Single recruitment episodes reported have been highly variable in magnitude and often patchy in space. 4 Continuous processes of recruitment and mortality have been recorded as background in most arid zone shrub populations, but the contribution of this low level change to overall dynamics is often perceived as small in comparison to episodic events, We conclude that population dynamics of arid zone shrubs should be perceived as driven both by continuous processes and by episodes of more substantial mortality or recruitment.
Abstract: 1 The demographies of two long-lived arid zone shrubs, Eremophila maitlandii. and Eremophila forrestii, were characterized using data from a grazing trial (1983-93) in arid Western Australia. Two sites were used, one dominated by annual pasture, the other by shrubs. Recruitment, mortality and size change were described from 11 annual samplings, encompassing conditions of prolonged drought and 1 year of unusually high rainfall. 2 The dynamics of these two species can be described by a combination of event-driven and continuous processes, Both recruitment and mortality were observed in all years for both species. Highest rates of recruitment were observed during the wet year and in the two subsequent years. Highest rates of mortality were observed in the 2 years following the year of lowest rainfall. 3 Stocking rate had no effect on the mortality rate of the unpalatable E. forrestii but the mortality rate of the palatable E. maitlandii was greater under high stocking. The effect of high stocking and low rainfall was additive, rather than excessive stocking being especially damaging in drought years, 4 Higher mortality rates were observed in the shorter and younger stages of both species. Tall individuals had very low mortality rates, Shrubs decreased in height in the year(s) before death. 5 Individuals of both species increased in height during the wet period and decreased during the drought. Net change in height over the trial period was small. High stocking led to higher rates of size decline during the drought. There was large height variation within all cohorts and many plants present at the start of the trial remained short, Taken together, these results suggest that caution must be exercised if size frequency distributions are used to infer population dynamics.
Abstract: We present data on the geographic, latitudinal and climatic ranges of 819 species in the large Australian tree genus Eucalyptus L'Herit. The distribution of geographic range sizes is highly skewed, with 68% of species having ranges that cover less than 1% of the continent and only 3% of species having ranges covering more than 10%. The mean range size is 1.5% of the continent and the median 0.5%. At each latitude there is a wide scatter of geographic and latitudinal range sizes. Rapoport's rule, that range size increases with increasing latitude, was not supported. Mean range size peaks in the 20-25 degrees S latitudinal band; this pattern is affected by the shape of the continent but is not simply a consequence of it. Climatic range size, expressed in terms of mean annual temperature and rainfall, also peaks at about 25 degrees S. The mean, median, and frequency distribution of range sizes of arid zone species is similar to that of mesic species. Eucalypt species diversity is highest in the 30-35 degrees S latitudinal band; once again, this pattern is not simply a consequence of the shape of the continent. The frequency distribution of range sizes in the two main eucalypt suballiances, Symphyomyrtus and Monocalyptus, are not significantly different, although Symphyomyrtus contains all the species with range sizes covering more than 6% of the continent. Symphomyrtus species tend to occupy drier sites than Monocalyptus.
Abstract: 1. The relationship between seedling potential relative growth rate RGR and seed mass was investigated by growing in the glasshouse 26 species selected to form 13 phylogenetically independent contrasts (PICs). The two species in each PIC diverged at least 10-fold in seed mass and each PIC represented a phylogenetic branching independent of the other PICs in the study. 2. During the process of deployment following germination, seedlings undergo an initial tial dry mass decline before autographic growth overtakes respiration and mass begins to increase exponentially. This initial decline tended to be earlier and deeper in the smaller-seeded species of each PIC. Mass began to increase earlier in the smaller-seeded species in eight of the 13 PICs, there being no difference in the other five; the percentage mass decline from day 0 to day 2 was significantly greater in the smaller-seeded species in four of the 13 PICs, almost significantly greater in two further PICs and not different in seven PICs. 3. In all species exponential growth was under way by day 6 and continued through day 40. RGR was significantly different between the members of 12 of the 13 PICs. In 10 of these the larger-seeded member had slower RGR, vs the reverse relationship in two PICs, and there was also an overall negative correlation between seed mass and RGR considered across all 26 species. Thus the association reported in several previous studies between larger seed mass and slower potential seedling RGR occurs repeatedly within genera and families, as well as across older divergences between families and orders. 4. RGR differences within PICs were most often owing to differences in net assimilation rate (leaf mass basis) NARw, rather than to differences in allocation to leaf mass as a proportion of whole plant mass. 5. There were interesting unexplained patterns in the relationship of apical meristem volumes, measured at day 26, to seed mass. Root meristems were larger in the larger-seeded species in nine of 13 PICs (as perhaps might be expected, though there was little overall relationship between seed mass and meristem volume) but shoot meristems were larger in the smaller-seeded species in 11 of 13 PICs. Differences in meristem volume within PICs were considerable, often 10-fold.
Abstract: Variation between species in seedling potential relative growth rate (RGR) is among the most important spectra of plant adaptation. Investigations are reported into the components responsible for this variation, using phylogenetically independent contrasts (PICs). The two species for each PIC were selected to diverge in seed mass at least four-fold, seed mass being a known correlate of RGR. Consistent with previous reports, the main influence on RGR differences between species was leaf area per unit leaf mass rather than net assimilation (NAR(a)). The PIC design showed that SLA differences both underpinned old RGR divergences between orders and families, and also were repeatedly responsible for more recent RGR divergences between genera and species.
Abstract: We present data on the climatic ranges of 819 species of Eucalyptus L'Herit in Australia, in terms of mean annual temperature and rainfall. 53% of species currently have ranges spanning less than 3 degrees C of mean annual temperature, with 41% having a range of less than 2 degrees C, and 25% with less than 1 degrees C. 23% of species have ranges of mean annual rainfall that span less than 20% variation. Unless current projections greatly overestimate future climate change in Australia, within the next few decades many eucalypt species will have their entire present day populations exposed to temperatures and rainfalls under which no individuals currently exist. While we recognize that the actual climatic tolerances of many species are wider than the climatic envelope they currently occupy, the data indicate that if even a moderate proportion of present day boundaries actually reflect thermal or rainfall tolerances, substantial changes in the tree flora of Australia may be expected.
Abstract: This field study was designed to test whether the taxonomic group and geographic range size of a host plant species, usually found to influence insect species richness in other parts of the world, affected the number of gall species on Australian eucalypts. We assessed the local and regional species richness of gall-forming insects on five pairs of closely related eucalypt species. One pair belonged to the subgenus Corymbia, one to Monocalyptus, and three to different sections of Symphyomyrtus. Each eucalypt pair comprised a large and a small geographic range species. Species pairs were from coastal or inland regions of eastern Australia. The total number of gall species on eucalypt species with large geographic ranges was greater than on eucalypt species with small ranges, but only after the strong effect of eucalypt taxonomic grouping was taken into account. There was no relationship between the geographic range size of eucalypt species and the size of local assemblages of gall species, but the variation in insect species composition between local sites was higher on eucalypt species with large ranges than on those with small ranges. Thus the effect of host plant range size on insect species richness was due to greater differentiation between more widespread locations, rather than to greater local species richness. This study confirms the role of the geographic range size of a host plant in the determination of insect species richness and provides evidence for the importance of the taxon of a host plant.
Abstract: It is hypothesized that plant species that produce vertebrate-dispersed seeds (fleshy fruits and brightly coloured arillate seeds) would not be common in fire-prone vegetation because seeds are deposited on the soil surface and are unlikely to survive fires. They have not previously been known to have any means of burial that would enable them to await the next fire as a buried soil seed-bank. We studied vertebrate-dispersed species in fire-prone sclerophyll vegetation to look at mechanisms used by vertebrate-dispersed plants for persisting in these environments. Seeds do not survive heating to 150 degrees C, although species from fire-prone environments are more likely to tolerate moderate heating (80 degrees C) without affecting viability than vertebrate-dispersed species from environments where fire is rare. Most vertebrate-dispersed species have the capacity to regenerate vegetatively, although they are not more likely to have this strategy than ant-dispersed species. Finally, ants do take vertebrate-dispersed seeds and fruits (particularly small ones) indicating the potential for the formation of a soil seed-bank. Some species only persist in fire-prone environments by recolonizing from unburnt areas.
Abstract: 1 Seedlings were grown in dense shade, below the compensation point. At cotyledon stage (i.e. when placed in dense shade from germination onwards), species with larger seed mass tended to survive longer. This was true within 11 phylogenetically independent contrast (PIG) pairs of plant species, as well as across all 22 species. 2 RGR of early seedlings in full light (to day 10 after germination), and cotyledon dark respiration, were measured as potential indicators of the speed with which metabolic resources in the seed were used during early seedling development. These indicators proved not to be such good predictors of longevity under dense shade as was seed mass itself. 3 Seedlings were also grown in the light until five days after true leaves appeared, before being transferred to dense shade. The different behaviour of these true-leaf seedlings was consistent with the idea that extra resources in seeds are the direct cause of longer dense-shade survival. Longevity in dense shade at leaf stage was often shorter than for cotyledon-stage seedlings, especially for larger-seeded species: Longevity in dense shade at leaf stage was best correlated with a slow potential RGR in full light measured over days 10-30 of growth. Correlation with seed mass was marginal and probably an indirect correlation via RGR, and there was no correlation with leaf dark respiration.
Abstract: Seed mass is correlated with a number of other plant traits, including dispersal mode, growth form and specific leaf area. Specific leaf area is the main determinant of potential relative growth rate and an indicator of the site quality to which a species is adapted. The relationships with dispersal mode and growth form have consistent form in five datasets from three continents, and each account for about 20-30% of variation in log seed mass. Thus, there is also very substantial variation within growth form and dispersal categories. Much, but not all, of the 20-30% is associated with shifted family composition between growth forms or dispersal modes. Experiments have shown that seedlings of larger-seeded species are better able to survive hazards including deep shade, drought, physical damage and the presence of competing vegetation. If there is a common mechanism under these different hazards, it seemingly must be a 'reserve effect', whereby during deployment and early growth larger-seeded species hold a bigger percentage of seed reserves uncommitted to seedling structure and available to support respiration or repair damage. A reserve effect has not yet been demonstrated directly. It remains possible that different mechanisms operate under different hazards. Under a reserve effect, advantages of larger seed size should be temporary, and temporary advantage has indeed been observed with regard to seedling survival under dense shade. Although larger seed mass confers benefits on seedlings, larger seeds must necessarily be produced in smaller numbers per unit of resource allocated. Seed mass is presumed to have evolved as a compromise between these counterposed pressures. Yet there has proved to be surprisingly little difference in average seed mass between very different vegetation regions, at least in temperate climates. Rather, there is startlingly wide variation in seed mass among species growing interspersed with each other. Recent applications of game theory may be capable of accounting for this wide variation between coexisting species, but at present these models are driven by competition among seedling species (as opposed to between seedlings and adults). It remains unclear whether competition among seedlings is a decisive influence on species composition in most of the world's vegetation types.
Abstract: Recent studies have reported conflicting evidence about correlations between seed size and plant species geographic range sizes. Using phylogenetically independent contrasts (PICs) within genera, we found no consistent differences in reserve mass between species with similar dispersal morphology and 'wide' versus 'narrow' geographic ranges. There was also no tendency within genera for broad ranged species to be those that allocate a larger percentage of the resources invested in each diaspora to dispersal structures. PICs were also constructed between species having a tenfold difference in seed size. In these PICs, the larger seeded species often occupied a greater number of regions than species with smaller seed sizes. This result was generated primarily through the comparison of species from different genera, families or higher level taxa which differed not only in seed mass but also in dispersal modes and growth forms. Finally, comparing species within Acacia and Eucalyptus having similar seed size but different dispersal modes, we found that bird dispersal (in Acacia) and possession of a wing for wind dispersal (in Eucalyptus) was associated with wider geographic range compared to lower-investment dispersal modes. Taken together, these comparisons indicate that seed size is not itself important as a factor influencing breadth of geographic range. Dispersal mode and growth form may have an influence, however, and seed size differences may be associated with contrasts in dispersal mode or growth form.
Abstract: 1 Five temperate floras were studied to assess to what extent seed size correlations with other plant attributes are consistent across floras. The floras were from three continents: Australia (semiarid woodlands of western New South Wales, arid woodlands of Central Australia, and the Sydney region), North America (Indiana Dunes) and Europe (Sheffield region, UK). The plant attributes used were growth form, plant height, perenniality and dispersal mode. We used general linear models to consider not only the primary correlations between seed size and each other attribute, but also the overlap patterns among correlations to determine if each correlation could be interpreted as a secondary effect via a third variable. 2 Plant height and growth form were consistently correlated with the largest proportion of log seed mass variation (up to 37% in Central Australia). Although there was strong overlap in the amount of log seed mass variation explained by the two attributes (6-22%), each could explain small but significant variation after the other in all floras. The strong association between growth form/plant height and seed size was found not only among unassisted and wind-adapted species, but also among species dispersed by other means. 3 In all floras, dispersal mode was also able to account for significant variation in log seed mass independently of growth form and plant height. The association between plant perenniality and seed size could be explained as a secondary correlation of growth form and plant height with both seed size and perenniality. 4 There were significant differences in log seed mass among the five floras. However, seed size ranged over at least five orders of magnitude in each flora. Differences between floras could account for relatively little (4%) of the variation in seed size between species, compared to the attributes growth form (20%), plant height (20%) and dispersal mode (29%), despite the quite different soils and climates of the five floras. This suggests that seed size is more strongly associated with other plant attributes than with the environmental conditions for establishment. It appears that within any one community, plants have found a diversity of possible solutions to the problems of seedling establishment, resulting in a wide range of log seed mass.
Abstract: Seed size is correlated with a number of other plant attributes, and such correlations are commonly interpreted as adaptations to different life histories. However, seed size is also strongly related to species phylogeny. In the current comparative literature, correlations between trait variation and taxonomic membership are often interpreted as evidence for phylogenetic constraints, and such phylogenetic explanations of variation are presented as alternatives to adaptive explanations. However, phylogenetic patterns of trait variation should not be discussed exclusively in terms of constraints. Neither should phylogenetic and adaptive explanations for Variation be automatically presented as alternatives. Another explanation for similarity among related taxa is phylogenetic niche conservatism (PNC). This process is well known to exist-indeed, it is an inevitable consequence of evolution by natural selection-and is explicitly adaptive. Under PNC, radiation within a lineage is shaped by phylogeny, by the environment, and by competition from other lineages present in the environment. These factors may inhibit divergence within a lineage (hence ''conservatism''), but there are no insuperable obstacles (constraints) to divergence. We have included multiple data sets within one analysis, permitting an assessment of how a given lineage has diverged under different physical conditions or in the presence of different competing lineages. We found that taxonomic membership statistically accounted for the majority of seed size variation within data sets. Differences among data sets were also largely associated with shifts in taxonomic composition. These two findings do not distinguish between phylogenetic constraints and PNC; however, three other features of the data indicate that phylogeny has its effect through niche conservatism at least as much as through constraint. First, within many lineages, divergence of taxa from their lineage means was greater than divergence between different lineages, showing that phylogeny does not absolutely constrain variation. Second, much seed size variation correlated with taxonomic membership was also correlated with growth form and dispersal mode, suggesting a role of niche adaptation in the conservation of sets of traits within lineages. Third, patterns of divergence within some lineages were not always consistent across data sets, pointing to the role of niche divergence in trait divergence, which is the logical complement of PNC. These results indicate that a PNC model of trait evolution provides a better explanation for seed size variation than phylogenetic constraint.
Abstract: 1. Large seeds are thought to provide an adaptive advantage to seedlings growing in shaded environments. Four hypotheses that could account for the possible mechanisms underlying this advantage were tested using 23 species ranging in seed size from 0.04 to 22.2 mg. The hypotheses were: (i) seedlings from large seeds are able to survive longer in shade than seedlings from small seeds; (ii) seedlings from large seeds divert relatively more resources to shoot development when grown under shaded conditions compared to seedlings from small seeds; (iii) seedlings from large seeds always have larger shoots than small-seeded seedlings growing under shaded conditions due to the large initial reserve size of large seeds and the reduction in growth rate differences between small and large seeds under growth-limiting conditions; and (iv) seedlings from large seeds can support a stronger etiolation response than seedlings from small seeds. 2. There was no difference in survival from emergence with seed size under the treatments no shade, 50% shade, 80% shade and 95% shade. However, large-seeded species survived significantly longer than small-seeded species under 99% shade. 3. The hypothesis that seedlings from large seeds divert relatively more resources to shoot development than seedlings from small seeds was rejected as there was no significant difference in allocation to shoots with seed size under any shade treatment. 4. Seedlings from large seeds were significantly heavier and had significantly longer shoots than small-seeded species after 6 weeks under 95% shade only. Under the other shade treatments of no shade, 50% shade and 80% shade there was no relationship between seed size and seedling size after 6 weeks growth. Small-seeded species showed a greater reduction in relative growth rate under shade compared to full light than large-seeded species. 5. Seedlings from large seeds had a stronger etiolation response to 95% shade than seedlings from small seeds. We suggest that large seed size provides additional resources to support etiolation compared to small seed size. Consequently the longer shoots of large-seeded species under 95% shade can be attributed to a larger initial seedling size and greater stem elongation. Seedlings from large seeds are able to maintain a height advantage over small-seeded species due to the suppressed relative growth rates of small-seeded species. 6. We suggest that: (i) large seeds enable seedlings to tolerate shade for longer by providing a larger initial energy reserve and that this may be an advantage in habitats where gaps in the canopy are regularly created; and (ii) large seeds provide seedlings with increased height relative to small-seeded species and that this height may be an advantage in habitats where there is a steep gradient of light such as in herbaceous vegetation, or for seeds germinating below litter.
Abstract: The indigenous arid zone shrub Acacia victoriae grows rapidly and is relatively shortlived. The relationship between population structure, growth and mortality was examined by annual monitoring of over 1200 plants at twenty sites from 1978 to 1981. Sites were reassessed in 1992. Plants of similar size varied greatly in growth rate. Annual mortality ranged up to 16%. Of 752 plants alive in 1981, 206 (27%) survived until 1992. A transition matrix model, based on growth and mortality over the 3 year period, predicted a size-class frequency distribution after 12 years that was significantly different from the one observed in 1992. The proportion of large plants present in 1992 was higher than predicted. The model's prediction that a large proportion of plants would remain in the lower size-classes was reliable. We concluded that size is not a good indicator of age and that it is unreliable to identify cohorts of this species by examining size-class frequency distributions.
Abstract: Seed size varies over several orders of magnitude in any one community. We outline a number of hypotheses that could account for this variation, briefly discuss the reasoning and evidence underlying each of these hypotheses, and then test each hypothesis with a database of 248 species from the semiarid woodlands of western New South Wales, Australia. Information on seed weight, growth form, plant longevity, height, dispersal mode, dormancy, and germination season was used. We considered not only pairwise relationships between seed weight and each other variable, but also alternative hypotheses whereby relationships arose as a result of indirect correlations through other variables. The strongest associations of seed size were with plant height and growth form. The seed-size variation accounted for by growth form largely overlapped with that accounted for by plant height, but each also accounted for some further variation independently of the other. Of the five hypotheses tested, the correlative patterns were inconsistent for two. Two others showed the predicted pattern, but these patterns could alternatively be interpreted as arising from secondary correlation via the combination of plant height and growth form. Only plant height, growth form, and dispersal mode had significant associations with seed size independent of the other attributes measured.
Abstract: 1 The hypothesis that large seed size provides an advantage for seedlings establishing in low soil moisture was tested under both field and glasshouse conditions, using a range of sympatric species of different seed sizes and growth forms from the semiarid woodland flora of western New South Wales, The hypothesis predicted that the relative advantage of large seeds should be greater under dry-soil than better-watered conditions. 2 In the field, seedlings from large seeds had higher percentage emergence and survival than small seeded seedlings, but there was no evidence of a relatively greater advantage under drier soil conditions. Daytime temperatures were very high during the field experiment and even the most favourable watering treatment showed quite low rates of emergence and survival. This low survival may have reduced the ability to detect a greater advantage of large seeds over small seeds under drier than under better- watered conditions. 3 In the glasshouse, the difference in survival from emergence between seedlings from small and large seeds increased under increasingly dry soil conditions, thus supporting the hypothesis. In addition, survival time of seedlings under drying soil conditions was positively associated with seed size. 4 The difference in the field results compared with the glasshouse experiment suggests that although large seeds do provide an advantage for seedlings establishing in low soil moisture conditions, this may be realized only under particular field conditions, depending on the characteristics of the rainfall event and the soil conditions. Thus, only a subset of establishment events may provide selection pressure for large seed size.
Abstract: Hummocks of Triodia basedowii E. Pritz, T. pungens R.Br. and Plectrachne schinzii Henr. were treated with ash and with standard and high applications of slow-release fertiliser. Following rain and the ensuing growth and flowering, the soils and the hummocks were sampled for biomass and for nutrient content, and compared to controls. In a similar but separate experiment, seed set in inflorescences was assessed for hummocks treated with standard fertiliser application, for hummocks treated by removal of neighbouring hummocks, and for controls. In the absence of nutrient addition, total topsoil nitrogen concentration was higher under hummocks than outside, whereas total topsoil phosphorus concentration was lower under hummocks than outside. The absolute amount of phosphorus in the hummock itself was insufficient to account for the depletion of phosphorus in topsoil under the hummock. Available soil phosphorus was about 1-4% of total phosphorus, and was generally highest towards the edge of hummocks. Additions of ash did not significantly affect soil nutrients, while addition of fertiliser produced substantial increases in total nitrogen and in available and total phosphorus. The increases did not extend to the area surrounding the hummock. The increases in soil nutrients did not result in significant increases in new green growth, or in inflorescence biomass or number. However, nutrients appear to have been taken up into the plants, including into old tissue, because there were increases in nutrient concentration in plant tissues, which sometimes were statistically significant and sometimes not. In the second experiment, neither increased soil nutrients nor removal of neighbours increased the number of inflorescences produced, or the proportion of florets that gave rise to filled grain.
Abstract: 1 The dispersal mode adopted by a plant species is frequently associated with other attributes of the plant and its habitat. In this paper we review these associations and present a set of hypotheses which, when considered together, make a probabilistic prediction of the dispersal mode adopted by a plant species. When applied to a species list, the hypotheses can be used to generate a prediction of its dispersal spectrum, i.e. the percentages of different dispersal modes that have been adopted. 2 The formulation of such a set of hypotheses has several purposes: (i) to summarize existing knowledge about dispersal adaptations and their interrelations with other attributes of plants and their habitats; (ii) to couch that knowledge in such a way that falsifiable predictions can be made; (iii) to arrive at provisional conclusions about which factors are the most important in shaping the evolution of dispersal mode in different plants or different environments. 3 The review of relationships between dispersal mode and other attributes of plants and their habitats lead to the following provisional conclusions; (i) seeds larger than 100 mg tend to be adapted for dispersal by vertebrates while those smaller than 0.1 mg tend to be unassisted; most seeds, however, are between 0.1 and 100 mg, and in this range all of the dispersal modes are feasible; (ii) plant growth form and stature (sometimes in relation to the canopy height of the vegetation) seem to exclude certain dispersal modes; (iii) the availability of specific dispersal vectors seems rarely to be an important determinant of dispersal mode; (iv) attributes of the physical environment also seem rarely to be important, except indirectly through their influence on plant stature and seed size.
Abstract: 1. Elaiosomes are lipid-rich appendages on the seeds of some plant species that promote dispersal of the seeds by ants. In this study we investigated the hypothesis that elaiosomes on seeds have converged in composition and attractiveness with the insect prey of ants. 2. The fatty acid compositions of 12 species of elaiosomes were compared to seeds of the same species and to seven orders of insects. The fatty acid compositions of the elaiosomes were more like those of insects than seeds; the levels of palmitic, palmitoleic, stearic and oleic acids in elaiosomes and insects were particularly similar. 3. The response of six ant species to the diglyceride 1,2-diolein was compared. This lipid is purported to be the principal attractant in elaiosomes and a review of the literature revealed that it is also an important component of insect haemolymph. The ant species tested include different amounts of elaiosomes and insect prey in their diets. 4. Response to pith impregnated with 1,2-diolein was greatest in the three species that are the most active elaiosome collectors, Aphaenogaster longiceps, Rhytidoponera metallica and Pheidole sp. 1. These species all include large amounts of insect material in their diets and two of them, Aphaenogaster longiceps and R. metallica, rarely collect any plant material other than elaiosomes. A fourth species, Iridomyrmex 'anceps', a scavenger which is an occasional elaiosome collector, was attracted to diolein pith but did not distinguish between it and control pith. The two nectar-collecting species that do not normally collect elaiosomes, Polyrhachis sp. 1 and Camponotus consobrinus, did not respond to diolein. 5. The results of this study are consistent with the hypothesis that the chemical composition and the behavioural releaser in elaiosomes have converged with the invertebrate prey of ants. The attraction of carnivorous and omnivorous ant species may be an important adaptive advantage of elaiosomes.
Abstract: 1. In Australia, plant species with fleshy fruits adapted for dispersal by vertebrates are concentrated on fertile soils and species adapted for dispersal by ants on infertile soils. 2. We tested some predictions of a hypothesis that availability of a specific mineral nutrient or nutrients may limit the production of fleshy fruit and so tend to confine frugivory to relatively fertile soils. 3. Nutrient concentrations were compared in the dispersal structures of 22 elaiosome-bearing, 29 fleshy-fruited and five arillate plant species. Discriminant function analysis revealed that K and N were the only nutrients that significantly separated the different dispersal structures. Potassium concentration in the fleshy pulp of vertebrate-dispersed diaspores was significantly higher than in elaisomes of ant-dispersed diaspores. Nitrogen content of elaiosomes was significantly higher than of flesh. Nitrogen and K content of arils was similar to that of elaiosomes but was significantly different from fleshy pulp. Plant species with different dispersal modes, growing at the same site, showed the same differences in K and N concentrations. 4. The hypothesis that fleshy fruit production is limited by the availability of a specific nutrient or nutrients was therefore rejected for all nutrients except K. An examination of dispersal spectra in relation to K availability, rather than to general soil fertility may help explain the observed correlation between dispersal mode and soil fertility in Australia and South Africa.
Abstract: The characteristic seed size of a plant species may reflect evolution toward an optimal compromise between size and number of seeds produced. The extent to which large seed size conveys an advantage to a seedling probably varies according to circumstances, and this may explain at least some of the great variation in seed size among species. Here we test whether large-seeded species are better able than small-seeded species to withstand defoliation at the seedling stage. We compared growth and survival of seedlings of 40 species of Australian angiosperms after removing 95% of their photosynthetic tissue shortly after emergence. We accounted for effects of phylogeny by selecting pairs of species that were ''phylogenetically independent contrasts.'' That is, the phylogenetic path connecting the two species of any pair was independent of the path connecting the two species of any other pair we used. We also selected the species so that the two members of each pair differed at least 10-fold in seed mass. Therefore, each pair provided an independent case for examining the consequences of evolutionary divergence in seed size. We found that the larger seeded species survived loss of photosynthetic tissue better than the smaller seeded species in 14 of the 16 pairs for which there was a significant difference in survival. The extent to which growth of survivors was reduced by loss of photosynthetic tissue also differed in 8 pairs, with the large-seeded species being less affected in 6 of these cases. We conclude that large seed size is generally associated with greater ability of seedlings to cope with loss of photosynthetic tissue. Consequently, large seed size may be favored in any circumstances in which seedlings are likely to experience carbon deficit early in development.
Abstract: We investigated the relationship of geographic range to growth form, dispersal morphology and diaspore weight within three comparative databases of 219-325 native species from arid central Australia, semi-arid western NSW and maritime Sydney. Geographic range was measured as the number of regions occupied. In the arid and semi-arid floras, but not in the maritime flora, species adapted for dispersal by vertebrates (mainly birds) tended to occupy more regions than species adapted for dispersal by other means. The same pattern occurred within four major families (Chenopodiaceae, Myoporaceae, Mimosaceae and Solanaceae), and thus was not a product of differences between families. We suggest that lineages with adaptations for long-range dispersal may have undergone a lesser incidence of geographic speciation, within the arid and semi-arid flora. Shrubs and trees were more likely than herbaceous species to be adapted for dispersal by vertebrates, but tended to have narrower geographic ranges. Thus the tendency for vertebrate-adapted species to have broader geographic ranges was stronger when considered within each growth form than when considered across all growth forms. There was a weak tendency for species with larger diaspores to have narrower geographic ranges, an effect in the opposite direction from that reported for North American oaks. Much of this can be interpreted as a result of taller growth forms tending to have both larger diaspores and narrower ranges.
Abstract: Risk spreading of germination may be particularly common in environments with unpredictable climates. Germinability, propensity to germinate at different temperatures and germination speed were classified for seeds of 105 species from the central Australian arid zone, and related to plant growth form, perenniality, seed size and seed dispersal mode. Almost all species had at least some seeds which were dormant, consistent with the idea that risk spreading is important in arid zones. Dispersal mode and plant perenniality were not found to be associated with germinability. Seeds of most species germinated rapidly relative to what has been recorded from higher-rainfall environments, as might be expected in an environment where wet soils are usually temporary. Faster germination tended to be associated with low germinability, suggesting a spectrum of strategies from species that risk a small number of their seeds in many rainfall events, to those that germinate only in large rainfall events but then risk large numbers of seeds.
Abstract: Influences of ant behavior on short- and long-term fate of elaiosome-bearing seeds were investigated in fire-prone sclerophyll vegetation near Sydney, New South Wales. The short-term fate of seeds was assessed by measuring dispersal distances, seed predation within ant nests, and seed burial. Most seeds were taken directly to ant nests. The mean dispersal distance was 1.1 m (range 0.06-11 m). Seed predation by three ant species was measured by feeding known quantities of seeds and inedible seed mimics, made from plastic beads, into nests. Nests were excavated 2, 7, and 30 d later to recover seeds and beads. The recovery rate of beads estimated the disappearance rate of seeds from nests due to factors other than predation. Pheidole sp. 1 appeared to eat most seeds soon after collection but also stored some seeds in small caches within the nest. Both Rhytidoponera "metallica" and Aphaenogaster longiceps removed most seeds from the nest intact (the elaiosome having been consumed) and discarded them on the surface, but left some within nests at depths < 12 cm. Seeds discarded on the soil surface may undergo secondary dispersal by other ants. A transition matrix analysis was used to assess the importance of this secondary dispersal route and to estimate the probabilities that seeds initially taken by a particular ant species would be either eaten or finally buried within a nest. The analysis indicated firstly that a significant proportion of seeds initially removed by A. longiceps and R. "metallica" will eventually be eaten by Pheidole spp. Secondly, some seeds will be discarded in the nests of species other than the original seed remover. The analysis also indicated that seeds initially taken by A. longiceps and R. "metallica" would be more likely to survive than those taken by Pheidole sp. 1. As many Australian ant-dispersed seeds are stimulated to germinate by fire, long-term seed fate was investigated by monitoring seedling emergence after fire from ant nests that had been fed a known quantity of seeds. Overall emergence was almost-equal-to 1% of seeds fed into nests. The greatest emergence was from nests of A. longiceps. This result is qualitatively consistent with those of the short-term experiments. However, the low overall emergence rate meant the power of this experiment to detect statistical differences between ant species was limited. The spatial pattern of emerging seedlings differed between nests of the different ant species. Seedlings from the nests of Pheidole sp. 1 were considerably more clumped than those from nests of the other three species. The seed fate experiments collectively indicated that seed-removing ants have different effects on seed fate, with A. longiceps and R. "metallica" being more beneficial than Pheidole.
Abstract: We investigated the role of ant behavior as a selective influence on the presentation and morphology of elaiosome-bearing seeds in Australian sclerophyll vegetation by manipulating characteristics of natural and artificial diaspores. We compared the response to these diaspores of three seed-removing ant species, two of which (Rhytidoponera "metallica" and Aphaenogaster longiceps), produce more favorable seed fates than the third species (Pheidole sp. 1). We measured the effect on removal rate of diaspore arrangement (degree of clumping), elaiosome removal, diaspore size (i.e., mass), elaiosome/load ratio, and composition of both the elaiosome and seed. Clumped seeds were removed significantly faster than single seeds by Pheidole sp. 1, a group forager. A. longiceps, a solitary forager, removed single seeds faster while R. "metallica" removed single and clumped seeds at the same rate. The presence of elaiosomes on seeds increased removal rates, especially by A. longiceps and R. "metallica." Both A. longiceps and R. "metallica" responded positively to elaiosome/load ratio while Pheidole sp. 1 responded mainly to diaspore size. These species-specific responses were such that diaspores with high elaiosome/seed ratios are more likely to be removed by A. longiceps or R. "metallica" than by Pheidole sp. 1. Diaspore characteristics other than size had inconsistent effects on removal rates. The differential response of the ant species to such characteristics as seed arrangement and elaiosome/seed ratios demonstrates the way in which ant behavior may have been an important selective force in the evolution and maintenance of myrmecochory.
Abstract: The herbivorous arthropod fauna of bracken fern (Pteridium aquilinum (L.) Khun), at a site near Sydney, Australia, is described and compared with previously reported bracken faunas in other geographic regions. Monthly sampling over 18 months found 17 species of herbivorous arthropods (15 insect and two mite species) from five orders. At the ordinal level, the mixture differed substantially from the bracken faunas of sites in Britain and Papua New Guinea. Notable was the presence of Thysanoptera and Acari, and the absence of Coleoptera and Hymenoptera. The mixtures of orders/families represented in the site bracken faunas in Britain, and less so in Australia, resembled those in the pool of herbivorous arthropods in those regions. Further, the mixture of orders on bracken was more similar to the mixture of orders on other ferns than to the mixture of orders among herbivorous insects on all plants; such similarity was not evident at family level. Compared with sites in other regions, the Sydney site had an abundance of pinna-sucking species and a dearth of mining species. Differences between regions in feeding niches most occupied tended to correspond with the differences in orders represented. Not all features of the fauna of bracken near Sydney reflected differences in the general herbivorous arthropod fauna of Australia compared with other regions, or differences between the herbivore faunas of ferns and seed plants. Its composition must be attributed in part to stochastic aspects of the speciation of herbivorous arthropods onto host plants.
Abstract: The hypothesis that more plant species with vertebrate-dispersed fruits occur on fertile soils because there is a greater probability of fruit removal from the parent plant was tested at 16 sites around Sydney, Australia. Removal rates from artificial fruit spikes were two and a half times greater on fertile than infertile soil sites, although this was not quite statistically significant. High variability in removal rate between sites was evident irrespective of fertility. Most removal occurred during the day indicating that birds were important consumers, rather than nocturnal mammals. Bird abundance and diversity did not differ between soil types. More frugivorous species were found in plant communities growing on fertile soil. Two models could explain the patterns observed. Firstly, plants with vertebrate-dispersed fruits could be favoured on fertile soils because of a high abundance of frugivorous birds accomplishing seed dispersal. Alternatively, plants with vertebrate-dispersed fruits could be favoured on fertile soil sites for some other reason and frugivorous birds could be attracted to these areas of abundant food. The correlation between soil fertility and the percentage of vertebrate-dispersed fruits was stronger than the correlation between soil fertility and removal rates and suggests that the second model is more likely to be true. Frugivorous birds are unlikely to be responsible for the high percentage of species with vertebrate-dispersed fruits in fertile soil environments.
Abstract: 1. The eggs of many stick insect species (Order Phasmatodea) bear a striking resemblance to seeds. These eggs are not only similar in size, shape, colour and texture to seeds, but in many species bear a knob-like structure known as a capitulum. This structure resembles an elaiosome, a lipid-rich appendage on some seeds known to be an adaptation for burial by ants. 2. We present observations and experiments demonstrating that capitula and elaiosomes are convergent in function as well as in appearance. 3. Capitula, like elaiosomes, promote removal of eggs to ant nests and buried eggs suffer reduced rates of parasitism by wasps. 4. Phasmatid nymphs are capable of emerging from eggs buried under 6 cm of soil. 5. Capitula are found only in those phasmatid species which drop eggs freely on to litter and not in species which bury eggs or glue them to vegetation. 6. Elaiosomes and capitula are both adaptations to use ant mutualists for burial, a striking example of evolutionary convergence between the plant and animal kingdoms.
Abstract: Fourteen sites occupied by Lagorchestes conspicillatus in the Northern Territory were examined from July to September 1986. The abundance of possible food items at each site was estimated by measuring the percentage cover of herbs, grasses and shrubs with foliage below one metre. Also recorded was the abundance of potential shelters, including grass tussocks, spinifex hummocks (Plectrachne and Triodia spp.) and low shrubs and trees. All sites possessed trees (more than 10 per ha) and shrubs (more than 5 per ha) together with more than 0-5% cover of green grass foliage. A model of suitable habitat was constructed using the 1986 results, and this was tested within the species' range in the Pilbara, Western Australia, and near Daly Waters in the Northern Territory in 1987. The abundance of L. conspicillatus was scored at control sites (sites with all features considered essential on the basis of 1986 data) and test sites (sites lacking 1-2 'essential' features). These data indicated that L. conspicillatus can occur at sites with as little as 0-2% cover of green grass provided there is more than 1.5% herb cover. Sites with no green grass (less than 0.2%) were usually not occupied. All sites occupied by L. conspicillatus had potentially suitable shelters, either shrubs, grass tussocks or spinifex hummocks within 50 m of the feeding areas. Diet selection by L. conspicillatus during the 1986 and 1987 dry seasons was as follows: (a) herbs were eaten in preference to other food items except certain seeds; (b) grass formed at least 10% of faecal contents irrespective of herb cover; and (c) at sites with less than 0.5% herb cover the levels of grass in the faeces increased to compensate for the low availability of herbs.
Abstract: The distribution of many plant species will change with global climate change, depending on their ability to disperse into, and establish in, new communities. Past migrations of species under climate change have been an order of magnitude slower than the rate of predicted climate change for the next century. The limited evidence available suggests that chance long distance dispersal events will be critically important in determining migration rates. We examine the JABOWA-derived gap replacement models and vital attributes/FATE models and ask: what do we need to know about dispersal and establishment to make improved projections of vegetation dynamics under climate change using these models? The minimal modifications of these models required to incorporate directional migration of species are described. To predict establishment success of species, we suggest that a more fundamental understanding is needed of how establishment ability under different conditions relates to seed and seedling attributes and how this may be affected by elevated CO2. Finally, we examine whether plant functional types based on vegetative attributes (used to model the response of adult plants) are correlated with functional types based on seed and seedling attributes. Available evidence suggests that the two sets of attributes are not strongly correlated; consequently, models of vegetation dynamics will need to incorporate seed biology explicitly.
Abstract: A seedling's chances of establishing successfully are likely to be affected by the quantity of metabolic reserves in the seed. Seed size is thought to evolve as a compromise between producing numerous smaller seeds, each with few resources, and fewer larger seeds, each with more resources. Seed size varies 10(11)-fold across plant species, so the compromise has been struck at very different levels. These basic ideas have been accepted for 50 years, and many studies have interpreted seed size differences between species by reference to larger seed size being adaptive under a variety of hazards. However, experimental tests of the benefits of large seed size in relation to particular hazards have been rare. More experiments are now being reported, but a consistent picture has yet to emerge. There is typically at least a 10(5)-fold range of seed mass between species even within a single area, suggesting thal much seed size variation is evolutionarily associated with other plant attributes.
Abstract: 1. A multivariate analysis of 43 traits of 300 species from semi-arid woodlands in western New South Wales. Australia is described. The 43 traits encompass vegetative, life-history, phenological and seed-biology characters. 2. Five main groups of species were produced which corresponded largely to growth form. These groups were (i) perennial forbs and C3 grasses, (ii) subshrubs of the family Chenopodiaceae, (iii) perennial C4 grasses, (iv) trees and shrubs and (v) annual forbs and grasses. The traits associated with these groups were vegetative, life-history and phenological. Seed-biology traits were poorly associated with the groups. 3. There were higher percentages of associations within and between vegetative, life-history and phenological traits, than within seed-biology traits. This has important consequences for models of vegetation dynamics in the context of species migrations under global climate change, as these models group species largely on the basis of their vegetative and life-history attributes.
Abstract: 1. It has been reported that species from environments where seedlings are prone to drought during establishment tend to have larger seeds. One proposed reason for this is that larger-seeded species might be able to allocate a larger proportion of mass to roots rather than shoots during early growth. 2. Seedlings of 32 species from arid central Australia were grown in coarse sand under standard conditions in a glasshouse, and harvested 10 days after germination. Seedlings from heavier-seeded species did not allocate relatively more resources to roots than lighter-seeded species. Complementary experiments confirmed this result for plants grown oil sandy loam, in drying as compared to well-watered soil. and under shaded conditions. 3. Seedlings of heavier-seeded species tended to survive longer than seedlings from lighter-seeded species when grown in the absence of any mineral nutrients other than those in the seed. 4. Seedlings of heavier-seeded species tended to be able to emerge from greater depths in the soil than lighter-seeded species. 5. Seedlings from heavier-seeded species had a slower relative growth rate than lighter-seeded species during the first 10 days after germination. Nevertheless, among these species, seed size was more important than relative growth rate or germination speed in determining seedling size 10 days after seeds were wetted.
Abstract: Myrmecochores are plant species that bear seeds with food-bodies adapting them for dispersal by ants. Within Australia and South Africa, myrmecochores account for a large percentage of the flora in sclerophyll vegetation on very infertile soils. On fertile soils, there are few myrmecochores and more species with fleshy fruits adapted for dispersal by vertebrates. This effect of very infertile soils may also account for the high incidence of myrmecochores in the floras of Australia and South Africa compared with other continents. The taxonomic distribution of myrmecochory shows it has evolved many times within the Australian flora. Evidence has been collected in relation to eight hypotheses that might account for the prevalence of myremecochory on very infertile soils: (1) Myrmecochore seeds do not appear to be relocated to nutrient-enriched microsites. Seed-removing ant species relocate their nest entrances frequently; active ant nests are not nutrient-enriched; and seedlings emerging after fire are not located in nutrient-enriched soil. (2) Traffic of seed-removing ants is not greater on infertile than on fertile soils. (3) Burial to avoid predation by small mammals is not a significant factor for seeds in Australian vegetation. (4) Experiments in which diaspore traits are manipulated support the idea that a larger food body increases the likelihood that a diaspore will be taken by a seed-disperser ant species that will not eat the embryo, rather than by a seed-predator ant. However, evidence so far does not support the hypothesis that, on very infertile soils, seed-disperser ants are consistently more available than seed-predator ants. (5) Removal rates of fruits adapted for dispersal by vertebrates were, on average, two and a half times greater in vegetation on fertile soils. Birds were the main removers. Our current opinion is that the fruit-removing bird assemblage is more likely to be a consequence of the high incidence of species with vertebrate-adapted fruits, rather than the reverse. (6) Fleshy fruits are more expensive in potassium than food bodies for ants, but not in nitrogen or phosphorus. This might be a reason why fleshy fruits are uneconomic on very infertile soils. (7) Species with smaller embryo-plus-endosperm weight are relatively more likely to be adapted for dispersal by ants rather than vertebrates. However, differences in the frequency distribution of embryo-plus-endosperm weights between infertile and fertile soils are moderate in the Sydney area. An indirect association between soil fertility and dispersal mode via embryo-plus-endosperm weight cannot account for much of the difference in dispersal spectrum. (8) Species growing to less than 2 m tall are relatively more likely to be adapted for dispersal by ants rather than vertebrates, and also account for more of the flora on infertile than on fertile soils. This indirect correlation is capable of accounting for as much as 80% of the higher incidence of myrmecochory on infertile soils, but the residual difference remains significant.
Abstract: (1) Survival was recorded at intervals in five cohorts of Atriplex vesicaria seedlings germinating between February 1979 and August 1985. This paper considers survivorship in relation to microtopography, soil surface conditions and proximity to mature shrubs. (2) For various cohorts and times, survival of seedlings varied with soil surface, microtopography and proximity to large shrubs. There was no consistency, however, between cohorts as to which microsites supported greatest survival.
Abstract: (1) Diaspore weight and dispersal were measured in 229 plant species of the central Australian arid zone. The frequency distribution of log diaspore sizes was unimodal, as has been observed for other habitats. The quantity of reserve (embryo plus endosperm) stored in the seed was related to the weight of the dispersal unit. (2) Reserve weight was greater for woody species than for herbaceous species. Within each growth form, there was no clear relationship between perenniality and reserve weight. (3) Most graminoids were wind dispersed, and elaiosomes for ant dispersal were more common among the woody species. The number of species with unassisted seeds was low in central Australia, when compared with other arid communities.
