Abstract: 1. The Standardised Major Axis Tests and Routines (SMATR) software provides tools for estimation and inference about allometric lines, currently widely used in ecology and evolution.
2. This paper describes some significant improvements to the functionality of the package, now available on R in smatr version 3.
3. New inclusions in the package include sma and ma functions that accept formula input and perform the key inference tasks; multiple comparisons; graphical methods for visualising data and checking (S)MA assumptions; robust (S)MA estimation and inference tools.
Abstract: Sapwood cross-sectional area per unit leaf area (SA:LA) is an influential trait that plants coordinate with physical environment and with other traits. We develop theory for SA:LA and also for root surface area per leaf area (RA:LA) on the premise that plants maximizing the surplus of revenue over costs should have competitive advantage. SA:LA is predicted to increase in water-relations environments that reduce photosynthetic revenue, including low soil water potential, high water vapor pressure deficit (VPD), and low atmospheric CO2. Because sapwood has costs, SA:LA adjustment does not completely offset difficult water relations. Where sapwood costs are large, as in tall plants, optimal SA:LA may actually decline with (say) high VPD. Large soil-to-root resistance caps the benefits that can be obtained from increasing SA:LA. Where a plant can adjust water-absorbing surface area of root per leaf area (RA:LA) as well as SA:LA, optimal RA:SA is not affected by VPD, CO2 or plant height. If selection favours increased height more so than increased revenue-minus-cost, then height is predicted to rise substantially under improved water-relations environments such as high-CO2 atmospheres. Evolutionary-attractor theory for SA:LA and RA:LA complements models that take whole-plant conductivity per leaf area as a parameter.
Abstract: • Co-occurring species often differ in their leaf lifespan (LL) and it remains unclear how such variation is maintained in a competitive context. Here we test the hypothesis that leaves of long-LL species yield a greater return in carbon (C) fixed per unit C or nutrient invested by the plant than those of short-LL species.
• For 10 sympatric woodland species, we assessed three-dimensional shoot architecture, canopy openness, leaf photosynthetic light response, leaf dark respiration and leaf construction costs across leaf age sequences. We then used the YPLANT model to estimate light interception and C revenue along the measured leaf age sequences. This was done under a series of simulations that incorporated the potential covariates of LL in an additive fashion.
• Lifetime return in C fixed per unit C, N or P invested increased with LL in all simulations.
• In contrast to other recent studies, our results show that extended LL confers a fundamental economic advantage by increasing a plant’s return on investment in leaves. This suggests that time-discounting effects, that is, the compounding of income that arises from quick reinvestment of C revenue, are key in allowing short-LL species to succeed in the face of this economic handicap.
Abstract: * Plant light interception efficiency is a crucial determinant of carbon uptake by individual plants and by vegetation. Our aim was to identify whole-plant variables that summarize complex crown architecture, which can be used to predict light interception efficiency. *We gathered the largest database of digitized plants to date (1831 plants of 124 species), and estimated a measure of light interception efficiency with a detailed three-dimensional model. Light interception efficiency was defined as the ratio of the hemispherically averaged displayed to total leaf area. A simple model was developed that uses only two variables, crown density (the ratio of leaf area to total crown surface area) and leaf dispersion (a measure of the degree of aggregation of leaves). *The model explained 85% of variation in the observed light interception efficiency across the digitized plants. Both whole-plant variables varied across species, with differences in leaf dispersion related to leaf size. Within species, light interception efficiency decreased with total leaf number. This was a result of changes in leaf dispersion, while crown density remained constant. *These results provide the basis for a more general understanding of the role of plant architecture in determining the efficiency of light harvesting.
Abstract: 1. Numerous plant traits are known to influence aspects of individual performance, including rates of carbon uptake, tissue turnover, mortality and fecundity. These traits are bound to influence emergent properties of vegetation because quantities such as leaf-area cover, average height, primary productivity and density of standing biomass result from the collective behaviour of individuals. Yet, little is known about the influence of individual traits on these emergent properties, despite the widespread use in current vegetation models of plant functional types, each of which is defined by a constellation of traits.
2. We examine the influence of four key traits (leaf economic strategy, height at maturation, wood density, and seed size) on four emergent vegetation properties (average height of leaf area, leaf-area index, net primary productivity and biomass density). We employ a trait-, size- and patch-structured model (TSPM) of vegetation dynamics that allows scaling up from individual-level growth processes and probabilistic disturbances to landscape-level predictions. A physiological growth model incorporating relevant trade-offs was designed and calibrated based on known empirical patterns. The resulting vegetation model naturally exhibits a range of phenomena commonly observed in vegetation dynamics.
3. We modelled single-species stands, varying each trait over its known empirical range. Seed size had only a small effect on vegetation properties, primarily because our metapopulations were not seed-limited. The remaining traits all had larger effects on vegetation properties, especially on biomass density. Leaf economic strategy influenced minimum light requirement, and thus total leaf area and basal area. Wood density and height at maturation influenced vegetation mainly by modifying individual stem mass. These effects of traits were maintained, and sometimes amplified, across stands differing in productivity and mean disturbance interval.
4. Synthesis: Natural trait variation can cause large differences in emergent properties of vegetation, the magnitudes of which approach those arising through changes to site productivity and disturbance frequency. Our results therefore underscore the need for next-generation vegetation models that incorporate functional traits together with their effects on the patch and size structure of vegetation.
Abstract: The question of whether natural selection favors genetic stability or genetic variability is a fundamental problem in evolutionary biology. Bioinformatic analyses demonstrate that selection favors genetic stability by avoiding unstable nucleotide sequences in protein encoding DNA. Yet, such unstable sequences are maintained in several DNA repair genes, thereby promoting breakdown of repair and destabilizing the genome. Several studies have therefore argued that selection favors genetic variability at the expense of stability. Here we propose a new evolutionary mechanism, with supporting bioinformatic evidence, that resolves this paradox. Combining the concepts of gene-dependent mutation biases and meiotic recombination, we argue that unstable sequences in the DNA mismatch repair (MMR) genes are maintained by their own phenotype. In particular, we predict that human MMR maintains an overrepresentation of mononucleotide repeats (monorepeats) within and around the MMR genes. In support of this hypothesis, we report a 31% excess in monorepeats in 250 kb regions surrounding the seven MMR genes compared to all other RefSeq genes (1.75 vs. 1.34%, P = 0.0047), with a particularly high content in PMS2 (2.41%, P = 0.0047) and MSH6 (2.07%, P = 0.043). Based on a mathematical model of monorepeat frequency, we argue that the proposed mechanism may suffice to explain the observed excess of repeats around MMR genes. Our findings thus indicate that unstable sequences in MMR genes are maintained through evolution by the MMR mechanism. The evolutionary paradox of genetically unstable DNA repair genes may thus be explained by an equilibrium in which the phenotype acts back on its own genotype.
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: 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.
Notes: Commentary on this paper by Anten & Poorter: http://dx.doi.org/10.1111/j.1469-8137.2009.02888.x
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: 1. Although shade tolerance is often assumed to be a fixed trait of species, recent work has reported size-related changes in the relative and absolute light requirements of woody taxa. We hypothesized that, in evergreen forests, light requirements of shade-tolerant species that accumulate multiple foliage cohorts will be more stable during juvenile ontogeny than those of intolerant species with short leaf lifetimes.
2. We quantified the light environments occupied by three size classes of 13 coexisting evergreens in a temperate rainforest, to determine how size influenced their relative shade tolerance. Minimum light requirements (MLRs) of species were estimated by computing the 10th percentile of the distribution of juveniles in relation to percentage canopy openness, for each size class. Leaf life span in low light (2%-5% canopy openness) was estimated by recording survival of marked leaves over 12 months, or retrospectively on species with clearly discernible foliage cohorts.
3. Agreement of ranks of species' MLR across size classes was significant, although not strong (Kendall's W = 0.159, P = 0.02). MLRs of the most shade-tolerant species changed little between size-classes, whereas those of most of the less-tolerant species rose with increasing size.
4. Shift in MLR across size-classes was negatively correlated with leaf life span, possibly because of the effects of leaf life span on biomass distribution and whole-plant carbon balance. Survival of light-demanding species with short leaf lifetimes may thus depend on their encountering increasing light levels as they grow taller, whereas progressive accumulation of an extensive leaf area by late-successional taxa enables them to continue to tolerate low light despite increasing size.
5. Results suggest that shade-tolerance differences between evergreens become increasingly apparent with increasing size. In identifying a relationship with leaf life span, this work also provides a basis for predicting changes in species' light requirements during juvenile ontogeny.
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: 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: One of the factors thought to contribute to ontogenetic declines in plant growth rates is diminishing light interception efficiency, as a result of the difficulties of avoiding self-shading among a growing number of leaves, and by stems. The effects of plant size on self-shading and light interception have rarely been quantified, however. We used a three-dimensional digitising system to construct virtual models of the architecture of Araucaria araucana seedlings 71 to 358 mm tall, and modelled their light interception in the forest understorey using the program YPLANT. We also analyzed seedling allometry, to determine the combined effects of biomass distribution and self-shading on total light interception. Average light interception efficiencies calculated for A. araucana (29 %) were the lowest reported for rainforest tree seedlings, reflecting the limitations imposed by short leaves, lack of petioles and an inability to develop planar foliage geometry on branches. Total light interception was related to seedling leaf area by an exponent of 0.735, reflecting increasing self-shading as seedlings grew bigger. However, because leaf area was related to seedling mass by an exponent of 1.24, light interception scaled nearly isometrically (0.91 power) with seedling mass. This resulted from taller plants having proportionally thinner stems, and a smaller fraction of their biomass in roots. Thus, an ontogenetic increase in self-shading in A. araucana is largely offset by allocation changes which increase leaf area ratio as seedlings grow bigger. These mechanisms conserving the relationship of light interception with plant mass seem likely to be restricted to species with long-lived leaves, growing in humid situations protected from wind stress. In open habitats, where wind and drought stress likely make such allocation patterns less feasible, the role of self-shading in ontogenetic declines in relative growth rate may be more evident.
Notes: Invited commentary. Featured in a special virtual issue of the journal: “Plant ecological strategy axes in leaf and wood traits” (July 2008) : http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1469-8137/homepage/virtual_special_issue_on_plant_ecological_strategy_axes_in_leaf_and_wood_traits.htm
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: 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: 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: 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: 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. 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: 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: 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: 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: Aims We implemented a neutral model of a positive relationship between abundance and distribution (occupancy) to examine how spatial structure influences abundance-occupancy relationships. The spatially explicit neutral model distributes individuals of species randomly and independently of one another in space to produce a positive abundance-occupancy relationship. Using empirical data, we tested whether abundance-occupancy relationships diverged significantly from the theoretical neutral model, and determined whether significant divergences emerged through intraspecific aggregation or over-dispersion of individuals. Location Field work was conducted in open-forest vegetation of the Black Mountain region in south-eastern Australia.
Methods At eight floristically similar sites in open-forest vegetation, we established a 20 x 20 m census plot and spatially mapped all individuals of each woody species. The abundance and distribution of each species was determined at each site at three spatial scales within the census plot. Observed abundance-occupancy relationships were compared with the spatially explicit neutral model using linear regression techniques. Monte-Carlo methods using a two dimensional Poisson process were then used to classify the spatial structure of species as random, aggregated or over-dispersed.
Results We found consistent evidence among the eight sites for abundance-occupancy relationships to diverge significantly from the neutral model at the three spatial scales within each community. The direction that the slopes of relationships diverged from the neutral model provided consistent evidence that aggregation of individuals within species was responsible for modifying the form of abundance-occupancy associations in this vegetation, a feature most evident with increasing scale.
Main conclusions Aggregation is not a mechanism that causes positive abundance-occupancy relationships, tinder the neutral model of a positive abundance-occupancy relationship, aggregation should be viewed as a mechanism which modifies a pre-existing relationship, rather than causing a relationship which would not have otherwise existed. In other words, in the absence of aggregation a positive abundance-occupancy relationship would still exist.
Abstract: Tool for extracting climate data (precipitation, wet-day frequency, temperature, diurnal temperature range, relative humidity, vapor pressure, sunshine duration, ground frost frequency and windspeed) from a global set of high-resolution climate grids, based on weather stations records taken between 1961-1990. This tool allows you to extract data for a list of site localities, specified by latitude and longitude.
Abstract: This protocol outlines techniques for mapping the 3D architecture of plants, and for simulating light capture and carbon gain of the digitised specimens using the Yplant software.
Abstract: The coexistence of species exhibiting a range of ecologically significant traits is a defining feature of plant communities worldwide. In this thesis, I develop theory that seeks to both explain the origins of this diversity and predict the mixture of strategies favoured under different sorts of environmental conditions. Since all plants require the same basic resources of light, water, and nutrients, any general theory must explain how different strategies persist in the face of strong competition. Here I focus on competition for light. By combining detailed mechanistic models describing the development of size-structured populations with an adaptive dynamics approach that tracks changes in phenotype mixture based on ecological success, I show how competitive shading influences the distribution of several key traits in plant communities.
Chapter one introduces the work. In chapter two, I show how size-asymmetric competition for resources leads to the evolutionary coordination of offspring size with adult size across species, using a simplified population model. Model predictions correspond well with the relationship observed among 322 plant species and suggest how differences among taxa relate quantitatively to specific processes during recruitment.
In chapters three to five I develop and analyse a more general model of trait-, size- and patch-structured vegetation. In chapter three, I outline how four different traits influence population dynamics and use this knowledge to quantify the influence these traits on emergent properties of vegetation. Chapter four outlines how fitness is assessed in a size-structured metapopulation model and addresses some of the numerical issues that arise when analysing such a model. In chapter five, I use the refined model to predict combinations of leaf economic strategy and height at maturation favoured under different disturbance regimes and site productivities. These two traits capture some of the most prominent life-history differences among plant species. In contrast to previous evolutionary models, I found evolutionarily stable, multi-strategy communities to be a generic outcome for vegetation structured by competition for light. Further, the predicted mix of strategies varied with abiotic factors, reflecting known empirical patterns. These results show how three basic features of plant communities - simple trade-offs, competition for light, and disturbance - lead routinely to multi-species assemblages.
Abstract: Potential height spans at least an order of magnitude across species and is considered an important indicator of light capture strategy. It is currently unclear, though, whether potential height is consistently coordinated with other aspects of light capture strategy such as pace of height growth, stem persistence and performance in low light. Theory and data suggest that available opportunities for light capture can be arrayed conceptually along at least two distinct gradients: one of time since disturbance and one of potential light income. I proposed that contrasting correlations between height and other plant attributes would be observed among sets of species selected to span these two gradients,reflecting different strategic trade-offs facilitating oexistence within each set. This speciesselection design was the main innovation in the current work. Traits were measured for 71 dicot species spread across three sites contrasted on mean height and vegetation cover. Traits chosen included leaf mass per area, stem tissue density, biomass expenditure per stem length and seed mass. They reflected differences in potential stem extension rate and recruitment strategy. I found strong evidence for contrasting trait correlations with height among the two sets of species sampled within sites, and among equivalent sets compared across sites. Additional support was gathered by assessing the consistency of relationships reported in the literature. These results may help to explain the coexistence of taller and shorter strategies, the diversity of light capture strategies in any given form of vegetation and also the limits to that diversity.
