Abstract: The tropics cover 8.2 billion hectares or approximately 40% of the world's land area. These regions are characterized by a large portion of the world's rapidly increasing population, high risks of soil and environmental degradation because of harsh climate and resource-poor farmers, and rapid decomposition of soil organic matter because of continuously high temperatures. Predominant soils of the tropics include Oxisols, Aridisols, Ultisols, and Alfisols. Soil and ecosystem degradation lead to emissions of greenhouse gases (e.g., carbon dioxide, methane, and nitrous oxide) into the atmosphere. Anthropogenic activities that exacerbate gaseous emissions include deforestation and biomass burning, low- or no-input subsistence agriculture, plowing, drainage of wetlands, and elimination or shortening of restorative fallows. Soils of the tropics contain about 496 Pg of soil organic carbon (SOC) or 32% of the global pool. The historic loss of the SOC pool, due to land-use change and cultivation, may be 17-39 Pg compared with the global loss of 66-90 Pg. If 60-80% of the SOC lost can be resequestered through land-use change and adoption of recommended management practices, the potential of SOC sequestration in the tropics is 12-27 Pg over a 25-to 50-year period. Important strategies of SOC sequestration include reduction in emission of greenhouse gases and sequestration of carbon (C) in biomass and soils. The potential of C sequestration in soils and biomass of the tropics is estimated at 10.0-25.0 Pg by effective erosion control, 5.7-10.8 Pg through restoration of degraded soils and ecosystems, 58-115 Pg through biofuel offset, 2.2-4.1 Pg through adoption of recommended practices on croplands, and 6.0-12.0 Pg through adoption of recommended practices on grazing lands. Of this, the potential of SOC sequestration is only 13.9-26.9 Pg over the 50-year period. Realization of this vast potential is a challenge for researchers, land managers, and policymakers.
Abstract: Savanna ecosystems are water limited and responsive to rainfall on short time scales, characteristics that can be exploited to estimate fractional cover of trees, grass, and bare soil over large-scale areas from synthesis of remote sensing and rainfall measurements. A method is presented to estimate fractional cover based upon the differing ways in which grasses and trees respond to rainfall, and implementation of this method is demonstrated along the Kalahari Transect (KT), an aridity gradient in southern Africa. Seasonally averaged normalized difference vegetation index (NDVI) and the sensitivity of the NDVI to interannual variations in wet season rainfall are used as state-space variables in a linear unmixing model. End members for this analysis were determined on the basis of best fit to the observed data. The realized end members were consistent with the qualitative characteristics of trees (high NDVI, low sensitivity of NDVI to interannual variations in rainfall), bare soil (low NDVI, low sensitivity), and the transient grass/ bare soil area (moderate NDVI, high sensitivity). Observed sensitivity of NDVI to rainfall was measured as the relationship between wet season NDVI and normalized rainfall over a 16-year period (1983-1998). The unmixing model yields a north-to-south decrease in tree fractional cover that corresponds to the decrease in mean wet season precipitation from 1600 to 300 mm along the KT (R2=.87). The fractional tree cover results compare favorably with available ground-based observations. The potential extent of grass cover is limited by the dominance of trees on the northern end of the transect, peaks at the location with approximately 450 mm of mean wet season rainfall, and is limited by rainfall in the arid southern portion of the transect. With mean NDVI for grass inferred from the data, yearly estimates of tree, grass, and bare soil fractional cover can be derived. These annual estimates, which are difficult to obtain from traditional unmixing procedures, are important parameters in fuel load and land-atmosphere exchange models. No calibration or training sets were required for this analysis, and this method has the additional capability to predict fractional-cover components for future rainfall scenarios.
Abstract: Nutrient intake and growth performance of 40 Malpura (M) and 32 Awassi×Malpura (AM) weaner (90 days old) lambs divided into four homogeneous groups were assessed. The groups grazed 8 h daily, receiving 10 g/kg (C10), 15 g/kg (C15), 20 g/kg (C20) and 25 g/kg (C25) body weight of concentrate. The pasture was dominated by Cenchrus ciliaris (33.7%) followed by Commelina forskalaei (11.4%), Elusine indica (9.1%), Erichola polystachya, Elusine aegyptica, Vigna sinesis, Sorghum halepense and other plant species. The biomass yield of C. ciliaris dominated pasture during the month of June was 2.084 t DM/ha. Daily dry matter intake (g/kg W0.75) was increased (P<0.01) along with the concentrate supplementation level averaging 70.7 (C10), 78.5 (C15), 82.8 (C20) and 94.9 (C25), respectively. Digestibility of dry matter, organic matter, crude protein and gross energy increased (P<0.05) with the increase in supplementation level of dietary concentrate, while the digestibility of NDF, ADF and cellulose decreased (P<0.05). Daily intake of digestible crude protein (DCP) and digestible energy (DE) per unit metabolic body weight differed between groups, decreasing from C25 to C10. Initial body weight of the lambs was similar in the four treatments and genetic groups while finishing body weights were higher (P<0.01) in high concentrate supplemented groups (C25>C20>C15>C10). Furthermore, the finishing body weight was higher in crossbred AM (23.4 kg) than native Malpura (21.0 kg) lambs. On average the growing lambs under hot semiarid environment consumed 7.5 g DCP and 239.7 kcal DE/kg W0.75 and had 115.7 g average daily gain. The results indicated that irrespective of the level of concentrate supplementation, the growth response and feed conversion efficiency was better in AM crosses than native Malpura lambs.
Abstract: The USA has about 336 Mha of grazing lands of which rangelands account for 48%. Changes in rangeland soil C can occur in response to a wide range of management and environmental factors. Grazing, fire, and fertilization have been shown to affect soil C storage in rangelands, as has converting marginal croplands into grasslands. Carbon losses due to soil erosion can influence soil C storage on rangelands both by reducing soil productivity in source areas and potentially increasing it in depositional areas, and by redistributing the C to areas where soil organic matter mineralization rates are different. Proper grazing management has been estimated to increase soil C storage on US rangelands from 0.1 to 0.3 Mg C ha-1year-1 and new grasslands have been shown to store as much as 0.6 Mg C ha-1year-1. Grazing lands are estimated to contain 10-30% of the world's soil organic carbon. Given the size of the C pool in grazing lands we need to better understand the current and potential effects of management on soil C storage.
Abstract: Pollen evidence suggests that grasslands were well established in southern Africa by the Late Tertiary. Evidence for grassland composition in the region during the Quaternary includes published accounts of isotopes, grass phytoliths and pollen of both grasses and woody plants from a wide range of different environments. Isotope data were derived from speleothems (stalagmites), fossil bones, and fossil tooth enamel and plant material in fossil hyrax dung. The different data types suggest that, with perhaps the exception of the dry southern Kalahari region, temperate grassland consisted of a relatively increased C3- to C4-grass ratios during the Last Glacial Maximum (LGM). Cold winter temperature extremes in the southern high latitude and altitude regions and a persistent winter rainfall pattern over the Cape region during the LGM probably limited the distribution of C4 grasses and canceled out any advantages gained from lowered CO2 concentrations in the atmosphere. In contrast, in the tropics where marked seasonal temperature fluctuations were lacking, C4-grass growth was favored.
Abstract: This study examined the short-term response of rangeland to annual (1993/94 to 1996/97 seasons) application of fertilizer [all combinations of 0; 10; 30 and 50
kgha-1nitrogen (N), with 0 and 10 kgha-1of phosphorus (P)] in terms of aboveground phytomass production, crude protein content, seasonal rain-use efficiency (S-RUE), nutrient use efficiency, botanical composition, basal cover and soil compaction. After 4 years of fertilization, the botanical composition of the high N-fertilized plots changed from a climax to subclimax vegetation.Elionurus muticus showed the greatest decrease in frequency (62%) due to N fertilization. Phosphorus fertilization only, had an insignificant (p>0·05) influence on species composition and basal cover over the 4 years. The production increased (p[less-than-or-equals, slant]0·01) with the application of N together with P, compared to applying only N (1074vs. 823 kgha-1). The more fertilizer applied, the more sensitive to drought the climax grasses became. Nitrogen and phosphorus fertilization significantly (p[less-than-or-equals, slant]0·01) affected S-RUE. The highest S-RUE of 6·64 kg phytomass ha-1mm-1and 0·23 kg crude protein ha-1mm-1were obtained with fertilizer application of 50 kgN plus 10 kgPha-1. On average over the 4 years, nutrient use efficiency with N+P application for all treatments was lower than without P. The increases in plant production per kg N applied, both with and without P were 28·00 and 29·81 kg respectively. Soil compaction increased with increase in N fertilization. These results confirm the vulnerability of native grasses in dry areas, following change in soil fertility.
Abstract: We evaluated the effects of livestock grazing on C content of the plant-soil system (to 60 cm) of two semi-arid grasslands: a mixed-grass prairie (grazed 12 years), and a short-grass steppe (grazed 56 years). Grazing treatments included season-long grazing at heavy and light stocking rates, and non-grazed exclosures. Significantly higher soil C (0-30cm) was measured in grazed pastures compared to non-grazed exclosures, although for the short-grass steppe higher soil C was observed with the heavy grazing treatment only. Excluding grazing caused an immobilization of C in excessive aboveground plant litter, and an increase in annual forbs and grasses which lack dense fibrous rooting systems conducive to soil organic matter formation and accumulation. Our data indicate that higher soil C with grazing was in part the result of more rapid annual shoot turnover, and redistribution of C within the plant-soil system as a result of changes in plant species composition.
Abstract: The tallgrass prairies of North America are among the most biologically diverse grasslands in the world. The way these lands are managed can have significant impacts on the biophysical and compositional structure of plant and animal communities. Soil stability and other hydrologic factors are also affected by grassland utilization practices. To better understand how changing grassland management practices are impacting their respective ecosystems, we must be able to map and monitor changing land use practices over large geographic areas. We examined the potential of multitemporal Landsat Thematic Mapper (TM) and ERS-2 Synthetic Aperture Radar (SAR) imagery, and the combination of these two data sources for discriminating among three commonly used tallgrass land management practices in areas dominated by cool- and warm-season grass species in eastern Kansas. Our results showed that cool- and warm-season grasses could be discriminated with 90.1% accuracy using the TM data and 73.2% using the SAR data. The three grassland management practices were correctly classified 70.4% of the time using TM data and 39.4% using SAR data. When TM and SAR data were combined, the information contribution by SAR data to the discrimination of grasslands was statistically insignificant. From our findings, we believe Landsat TM data can be used to discriminate among various grassland types at a level of accuracy suitable for land use change monitoring and assessing the impacts of changing government land use policies such as the US Department of Agriculture's Conservation Reserve Program (CRP).
Abstract: The Kinangop highland grasslands, the world stronghold of the endangered Sharpe's longclaw (Macronyx sharpei), consist almost entirely of privately owned land. As human population in the grasslands increases, the mean acreage of land holdings decreases and more grassland is being converted to other uses. Land subdivision information, existing human population data, and current land use data from a subset of 162 farms were used to extrapolate the past and future extent of Sharpe's longclaw habitat throughout the landscape on Kinangop Plateau. It was estimated that grasslands covered 50% of the plateau in 2000, but only 58% of the grassland area was likely to be tussock grasslands, the preferred habitat of Sharpe's longclaw. Large farms are being subdivided rapidly, and more than half of the remaining tussock grasslands are likely to be found on farms that are not large enough to support large longclaw territories. We predict that by 2010 tussock grasslands will cover only one-fifth of Kinangop, and all farms that are large enough to act as longclaw reserves are likely to have been subdivided. We propose urgent prioritisation of a network of large farms (>30 ha) for conservation action.
Abstract: Mosaics of natural forest and grassland tracts in sub-Saharan Africa provide differences in woody species biodiversity. These mosaics are of considerable interest as they are a major biodiversity bank. Their richness is felt to be threatened, for example by local burning. This study focuses on the impact of burning on biodiversity in the Budongo Forest Reserve in Uganda. Woody species at different development stages are compared between a forest stratum and the adjacent grassland stratum. Spatial variability of biodiversity indices is analysed within and between these strata, using statistical and geostatistical analysis methods. The forest stratum shows a higher woody species diversity on a per plot basis. With overall species diversity calculations, however, the grassland stratum has a higher woody species variability than the forest. For the long-term preservation of woody species diversity in this landscape, it is necessary to reconsider and adjust current management practices to maintain (fire) disturbances.
Abstract: Montane grassland is a severely threatened habitat in Kenya. Despite a high level of faunal endemism, it has received very little conservation attention. We investigated habitat selection in a threatened grassland endemic bird species, Sharpe's longclaw (Macronyx sharpei) (family Motacillidae), in order to understand its likely response to land-use changes. Between November 1995 and May 1996, we studied 41 territories of this species on the Kinangop Plateau in central Kenya. With an overall density of 0.4 birds ha-1, longclaws lived in permanent groups of two to seven individuals. They were sedentary and territorial, with a mean home range size of 0.5 ha. The species avoided non-grassland areas entirely, and within grassland showed a strong preference for short grass with tussocks. Territory sizes and foraging ranges were smaller, and rates of pecking for food higher, in this grassland type than in open short grass or long grass. Land use changes, in particular conversion to cultivation or woodlots and ploughing up of grassland to remove tussocks, pose a serious and immediate threat to this species. Dairy farming is potentially compatible with grassland conservation, and conservation-friendly livestock rearing needs to be promoted through economic incentives, awareness-raising and technical advice.
Abstract: The study evaluated the synergism between wheat bran and tanniferous feeds (dry meal or silage from pods of Acacia sieberiana) on intake and digestibility of various nutrients as well as weight gain in sheep fed a basal diet of veld hay (Cynodon dactylon). Dietary treatments comprised veld hay given ad libitum and supplemented with: 270 g of wheat bran (WB) alone (T1), 203 g of WB plus 83 g of silage (T2), 135 g of WB plus 165 g of silage (T3), 330 g silage without WB (T4) or 306 g of dry pods without WB (T5). The silage was prepared by mixing ground pods with water in a ratio of 40:60 (weight of pod:weight of water) and ensiling in 200-l drums for 35-45 days. Thirty young South African Merino sheep (15 males and 15 females), averaging 24 kg liveweight and 8 months of age, were blocked by sex and weight and within block, randomly distributed to the dietary treatments. A growth study was followed by a metabolism phase. All animals were slaughtered at the end of the trial to evaluate the weight of the dressed carcasses and some body organs. Faecal samples were examined at the beginning and at the end of the trial in order to evaluate the effect of tannins on endoparasites. Dry matter intakes for treatments 1, 2, 3, 4 and 5 were 1017, 1087, 1169, 1238 and 1214 g, respectively. Average liveweight gains (ADG) were 97, 114,114, 132 and 123 g in the same order. The addition of silage increased (P<0.01) hay and DM intake but had no effect (P>0.05) on liveweight gain and feed conversion efficiency. Increasing the levels of silage in the diet linearly (P<0.01) increased ADG. Sex did not influence intake or ADG. The digestibilities of DM, OM and detergent fibres as well as excretion of purine derivatives were similar (P>0.05) among treatments but the WB diet promoted higher N digestibility. The pod and silage diets increased (P<0.01) faecal N and lowered (P<0.05) urinary N excretion by sheep. N retention was similar among treatments. Carcass, liver and kidney weights were similar (P>0.05) among treatments and the organs had no lesions. The results show that combining WB and silage did not show significant synergism but condensed tannins present in the silage and pod supplements produced beneficial effects probably by forming tannin-protein complexes which by-passed the rumen and were digested at the lower segments of the gastro-intestinal tract. The results of the study have shown that the pods of Acacia sieberiana can constitute an important source of nitrogen to livestock and the liveweight gains are indications that, if legume pods are well managed, they can reduce weight loss in animals grazing low quality rangelands and enhance production.
Abstract: Biogenic volatile organic carbon (BVOC) emissions are an important component of the global BVOC budget, contributing more than 90%. Emissions vary with species and vegetation type; therefore to produce accurate global budgets data is required from different vegetation types. This study investigates BVOC emissions from savannas, Kalahari woodlands, Kalahari open shrublands and Mopane woodlands in southern Africa. BVOC emission samples from individual species were collected using leaf cuvettes and the BVOC concentrations were determined by GC-FID/MS. Ten of the 14 woodland species measured were high isoprene emitters, while two showed high monoterpene emission capacities. Landscape average isoprene emission capacities were estimated to be as high as 9, 8 and 1Â mg CÂ m-2Â h-1 for savannas, woodlands and shrub lands, respectively. The monoterpene emission capacity for Mopane woodlands were estimated at almost 3Â mg CÂ m-2Â h-1, while for other landscapes it varied between 0.04 and 3Â mg CÂ m-2Â h-1. Isoprene and monoterpene emissions at a savanna site in South Africa showed a seasonal variation, which is more pronounced for isoprene. During the winter (June-September) estimated emissions were <10Â mg CÂ m-2Â d-1, with peak emissions (ranging between 20 and 80Â mg CÂ m-2Â d-1) occurring during the summer months (December-March) when foliar density peaked. The total BVOC emission from southern African (south of the Equator) savannas was estimated to be in the range of 18-74Â Tg CÂ yr-1.
Abstract: Fossil plants and terrestrial gastropods collected by the Uganda Palaeontology Expedition at Bukwa site I in 1997 and 1998, throw light on the palaeoenvironmental conditions that characterised the area during the end of the Early Miocene, some 17.5 Ma ago. Some of the evidence indicates the presence of grasslands in situ at the locality, but part of the fossil gastropod assemblage and some of the plants suggest the presence of woodland to forest nearby.
Abstract: The metaphysiological modelling approach relates aggregated population dynamics to biomass gained from resources consumed, relative to physiological attrition and mortality losses. A GMM (Growth, Metabolism and Mortality) formulation was applied to the interactive dynamics of herbivore-vegetation systems, taking into account seasonality in resource production, heterogeneity in resource quality, adaptive responses by consumers to this variability, and nutritional influences on mortality losses. In transforming the intake or functional response to changing resource availability to a biomass gain response, allowance was made for constraints on daily digestive capacity, and for changing diet quality as resources became depleted over the seasonal cycle. Because of the steeply saturating form of the intake response, the output dynamics was inherently unstable in a constant environment with a single, uniform resource, for realistic parameter values. Stabilization resulted when resources were heterogeneous and herbivores adjusted their resource selection adaptively over the course of the year. This mechanism caused the gain response to diverge from the intake response, due to changing diet quality over the annual cycle. In seasonally varying environments, no equilibrium between resource production and consumption persists. Nevertheless, dynamic stability can emerge from the adaptive responses of consumers to spatial and temporal variability in resource availability and quality.
Abstract: The terrestrial biosphere model Carbon Assimilation in the Biosphere (CARAIB) was improved by introducing two vegetation storeys and implementing a new module which simulates the equilibrium distribution of the vegetation inferred from physiological processes and climatic constraints. In this fourth version of CARAIB, we differentiate ground-level grasses from tree canopies, which allows us to determine the light available to grasses as a direct function of the leaf area index (LAI) of the forest canopy. Both of these storeys are potentially composed of several plant functional types (PFT). The cover fraction of each PFT within each storey is estimated according to its respective net primary productivity (NPP). A biome is assigned to each grid cell on the basis of three physiological criteria: (1) the cover fraction, (2) the NPP, and (3) the LAI; and two climatic constraints: (1) the growing degree-days (GDD) and (2) the lowest temperature reached during the cold season (Tmin), which are well-known indices of vegetation expansion boundaries. Total biospheric carbon stocks (vegetation+soil) are reconstructed by forcing the model with eight climatic scenarios of the Last Glacial Maximum (LGM, 21 ka BP), which were obtained from the Paleo-Modelling Intercomparison Project (PMIP) from four general circulation models (MRI2, UGAMP, LMD4, and GEN2) using prescribed and computed sea surface temperatures (SSTs). The model was also forced with a current climate together with a preindustrial atmospheric CO2 level of 280 ppm as reference simulation. To validate the model, current biome distribution is reconstructed and compared, for the modern climate, with two distributions of potential vegetation and, for the LGM, with pollen data. The model simulations are in good agreement with broad-scale patterns of vegetation distribution. The results indicate an increase in the total biospheric carbon stock of 827.8-1106.1 Gt C since the LGM. Sensitivity analyses were performed to discriminate the relative effects of the atmospheric CO2 level ("fertilization effect"), the climate (present or LGM), and the sea level. Our results suggest that the CO2 fertilization effect is mostly responsible for the total increase in vegetation and soil carbon stocks. The four GCMs diverged in their predicted responses of continental climate to calculated SSTs. Only one of them, i.e., MRI2, predicted a marked decline of the continental temperatures in response to lower calculated SSTs. For this GCM, the effect of reduced SSTs on continental biospheric carbon stocks was a decrease of 544.1 Gt for the soil carbon stock and of 283.7 for the vegetation carbon stock, which means a decrease in the total biopsheric carbon stock of 827.8 Gt.
Abstract: Aqueous, ethanolic, dichloromethane and n-hexane extracts of Scilla natalensis Planch. and Ledebouria ovatifolia (Bak.) Jessop bulbs (Hyacinthaceae) were screened for antibacterial, anti-inflammatory, antischistosomic, anticancer and anthelmintic activity. Poor antibacterial activity against both Gram-positive and Gram-negative bacteria was shown with S. natalensis extracts. Good antibacterial activity was shown by the ethanolic and dichloromethane extracts of L. ovatifolia against Gram-positive bacteria. In the anti-inflammatory screening, the dichloromethane and hexane extracts of S. natalensis resulted in good inhibition against both COX-1 and COX-2. Ethanolic extracts had the highest inhibitory effect against nematodes in the anthelmintic assays. Poor anti-inflammatory and anthelmintic activity was found with L. ovatifolia. Aqueous extracts of S. natalensis had good activity against Schistosoma haematobium, with a minimum inhibitory concentration of 0.4 mg ml-1. Aqueous extracts of fresh L. ovatifolia bulb material were found to be lethal to S. haematobium at a concentration of 1.6 mg ml-1. The phytochemical screening of S. natalensis bulbs revealed the presence of saponins and bufadienolides within the bulbs. Bulbs of L. ovatifolia contained bufadienolides.
Abstract: Sediment samples were collected in northwestern Nebraska from the following lithologic units: (1) the upper Oligocene Monroe Creek Formation, the lower Miocene Harrison Formation and [`]upper Harrison' beds of the Arikaree Group; and (2) the lower Miocene Runningwater Formation and the Dawes Clay Member (Box Butte Formation) of the Ogallala Group. The samples were processed using heavy liquid extraction and analyzed for siliceous plant microfossils (phytoliths). All samples yielded well preserved and diverse assemblages dominated by grass phytoliths, but included phytoliths produced by woody and herbaceous dicotyledons, palms, and sedges. The abundance of woody dicotyledons and palms decreased up section. The grass assemblages consisted mainly of Festucoid (C3) phytolith morphotypes. An index assessing the amount of tree cover (d:p, the ratio of dicotyledon phytoliths to grass phytoliths) was employed to interpret the data. According to this analysis, there was open C3-dominated grassland in the study area between 25 and 17 Ma, at least 7 Ma before supposed adaptations to grasslands in ungulates (hypsodonty) originate.
Abstract: Pollen evidence suggests that grasslands were well established in southern Africa by the Late Tertiary. Evidence for grassland composition in the region during the Quaternary includes published accounts of isotopes, grass phytoliths and pollen of both grasses and woody plants from a wide range of different environments. Isotope data were derived from speleothems (stalagmites), fossil bones, and fossil tooth enamel and plant material in fossil hyrax dung. The different data types suggest that, with perhaps the exception of the dry southern Kalahari region, temperate grassland consisted of a relatively increased C3- to C4-grass ratios during the Last Glacial Maximum (LGM). Cold winter temperature extremes in the southern high latitude and altitude regions and a persistent winter rainfall pattern over the Cape region during the LGM probably limited the distribution of C4 grasses and canceled out any advantages gained from lowered CO2 concentrations in the atmosphere. In contrast, in the tropics where marked seasonal temperature fluctuations were lacking, C4-grass growth was favored.
Keywords
Abstract: Nebkha dunes have been proposed as a reliable rapid indicator of aeolian erosion and dryland degradation. This paper tests the applicability of these links for the Molopo Basin, southern Africa where it is shown that nebkha sediments are largely locally derived from interdune areas and are significantly enriched in available inorganic nutrients. Feedbacks with the bush canopy are the most likely cause of the enrichment, such that this can occur without associated declines in nutrient availability in surrounding source areas. Thus, although the nebkhas indicate aeolian transport of sediment, the immediate association with soil degradation is over-simplistic.
Abstract: Plant and bird species richness and abundance in two patches of native Acacia karroo and two patches of alien Prosopis species drainage line woodland in the southern Kalahari, South Africa, were compared using 37 plot (100 m2) samples for the plants and 12 line transects (ca. 1 km long) for the birds. Vertical distribution of canopy cover differed among the four sites in all four strata. The densest Prosopis woodland had less herbaceous understorey cover, and more cover between 1 and 5 m above-ground than Acacia woodlands or the relatively openProsopis woodland. The tallest Acacia woodland differed from other sites in having 40% canopy above 5 m. The densities of all trees >50 mm basal diameter (mainly A. karroo and Prosopis, but including a few A. hebeclada and Ziziphus mucronata) did not differ among sites, but abundance of A. karroo andProsopis species differed greatly among sites. Mean plant species density per 100 m2 plot differed among sites, being greater in the Acacia woodlands than in Prosopis -dominated vegetation. The densest Prosopis woodland was the least diverse site botanically, and was also least similar to the tallest Acacia woodland, sharing only 11 plant species. The ratio of fleshy fruited plant species to all others encountered differed between woodland types being lower in Prosopis than in Acacia woodland. Individual A. karroo trees differed from size-matchedProsopis trees in being less multi-stemmed and having fewer branches in contact with the soil when mature. The numbers of species of sub-canopy plants and fleshy fruited plant species increased with the size of the trees, but tended to be lower for large Prosopis than for large A. karroo. Sorenson's similarity index for bird species assemblages forAcacia sites compared with both Prosopis sites was 0·61. All species that occurred in Prosopis, with the exception of Red-eyed Bulbul Pycnonotus nigricans, were a subset of species that occurred in Acacia woodland. Bird species assemblages were least similar between the densest Acacia site and the most open Prosopis site. The Shannon indices for the Acacia and Prosopis communities showed some variation between patches and between counts. Indices for the Acacia sites were significantly higher than indices for the Prosopis sites, indicating that the bird communities in the native woodland sites are consistently more species-rich and more diverse. In terms of guilds, raptors were absent from Prosopis woodlands, in general frugivores were sparse in Prosopis woodlands, and insectivores were about half as abundant, but there were no large differences between nectarivores, mixed feeders (seeds and insects) and granivores in both types of woodland.
Abstract: The origin of the grassy habit during the Eocene and the development of C4 grasses during the Miocene/Pliocene boundary are discussed before the origin of primary and secondary grassland in Eurasia and North America are discussed. A comparison shows that both Northern and Southern hemisphere primary grassland originated due to climatic changes to drier conditions during the end of the Eocene, and that modern grassland vegetation types can be traced back to the Oligocene. The Eurasian steppes becomes more fragmented towards the west and south and relicts of primary grassland exists only in the most xerothermic localised habitats in central and western Europe. Secondary grassland clearly due to manmade deforestation, started with the spread of Neolithic husbandry. Southern African grasslands were however not only determined by droughty conditions, but cooler conditions at high altitudes are one of the major driving forces that prevent colonisation by trees of a generally tropical origin.
Abstract: Given current population trends and projections in sub-Saharan Africa, it is anticipated that substantial intensification of agricultural cropland is certain within the next decades. In the absence of adoption of improved technologies poor rural populations in this region will continue to degrade and mine the natural resources to ensure their survival. All these actions will have far-reaching implications for environmental quality and human health. However, only through the integration of environment and development concerns with greater attention to these link can we achieve the goal of fulfilling the basic needs, improved living standards for all, better protected and managed eco-systems and a safer, more prosperous future. The paper reviews case studies and provides examples of the integration, analysis, and visualization of information from remotely sensed, biophysical and socioeconomic information to assess the present situation hindering agricultural development in sub-Saharan Africa. These studies show the interactions between socio-economic and environmental factors that can help governments and policy-makers assess the scope of the problems, examine alternatives and decide on a course of action. Sound decisions depend on accurate information, yet most African countries face severe competing demands for the financial and human commitments necessary to staff an information system equal to its policy-making requirements. The role of international data centers is reviewed in terms of their abilities to develop and maintain information systems that bring together available accumulated knowledge and data. This permits comparative studies, which make it possible to develop a better understanding of the relationships among demographic dynamics, technology, cultural behavioral norms, and land resources and hence better decision making for sustainable development.
Abstract: Environmentalists, scientists, and land managers have long debated the role of ranching in landscape conservation with some contending that ranching represents the major threat to ecological systems, while others believe it is key to long-term conservation. We contrast the impacts of livestock grazing with those of the major alternative land use at this time, suburban and ex-urban development, on the semi-arid Chihuahuan Desert grasslands and savannas of southern Arizona and New Mexico, USA, and northern Chihuahua and Sonora, Mexico. Because landscape change has resulted from complex interactions among natural and anthropogenic disturbances, attempts to identify simple causal relationships resulting from livestock are of limited ecological significance. Far more important is long-term conservation of basic ecological processes at large spatial scales, which in turn requires that certain social conditions be maintained. In the face of rapid, extensive suburban and ex-urban development in the region, conservation of functioning ranch units represents the most viable means of sustaining ecological function. Examples of community-based adaptive management illustrate the potential of coalitions of ranchers, agencies, scientists, and environmentalists to conserve the biodiversity of these landscapes, protecting a matrix of publicly and privately owned land through an extension of UNESCOs biosphere reserve model.
Abstract: Remote sensing digital image analysis is applied to monitor desertification processes in central San Luis Province (Argentina), where signs of severe landscape degradation have been observed in the last decades. Two Landsat images, acquired in 1982 and 1992 were used to evaluate the potential of using remote sensing analysis in desertification monitoring. After geometric and radiometric correction of both images, multitemporal comparison techniques were utilized to emphasize areas of greater degradation. Spectral unmixing of the vegetation, water and sand components facilitated the analysis of areas of heterogeneous cover from satellite images. Simple differences between unmixed images of sand or water revealed dune movement, re-vegetation trends and variations in water bodies, as a result of changing rainfall and land use patterns.
Abstract: This paper provides a theoretical analysis of the effects of temporal controls on the stochastic behavior of renewable natural resources such as fisheries and rangelands. First, we show that temporal resource management is characterized by the existence of a threshold effect. Second, we point out that in the presence of environmental uncertainty, the ability of a manager to move a resource to a desirable state depends on the magnitude of the effect that the temporal control has on the state of the resource. Third, we show that if this magnitude is below a critical threshold then it will be impossible for the manger to reach any desirable state of the resource in a finite amount of time. Finally, we solve an optimization problem from the standpoint of a resource manager. Our analysis of this problem shows that the proper course of action involves choosing the temporal control in a way so that the resource is able to provide society with a flow of consumptive and non-consumptive services, and the persistence of the undesirable state is minimized.
Abstract: Range condition at sites of differing land-use intensity at a communal farm was assessed. Vegetation, soil and termite parameters were tested for their potential as indicators. The vegetation indicators did not discriminate between two sites of high and low land-use intensity. However, the soil fertility parameters provided interesting results. The phosphorus (P), nitrogen (N), organic carbon (OC), light fraction (LF), C:N and C:LF data indicate that nitrogen limitation is more critical under high land-use and grazing pressure. Organic carbon levels are generally low but reduced to critical levels under high land-use intensity; nutrients such as phosphorus and nitrogen are conserved in the system, whereas organic carbon losses are high and inputs low. The conversion of the [`]active carbon pool' into the [`]slow pool', a prerequisite for long-term soil resilience, is not taking place under high land-use intensity. Termites might play an important role in maintaining range condition.
Abstract: Stomatal size and density were measured from graminoid cuticular fragments extracted from dated sediments in two tropical-montane crater lakes on Mount Kenya. The sediments had been dated in other studies and spanned 1500-37[punctuation space]000 calibrated years BP. Changes in the mean size and density of the graminoid stomata were found. Using a coarse signal analysis the two lakes gave fairly similar results, although there was some divergence at the start and end of the time period analyzed. There is some correspondence between the atmospheric CO2 concentration and graminoid stomatal density during the transition from the LGM to the start of the Holocene, where stomatal density decreased while CO2 concentrations increased. All the changes observed may have been plastic responses within existing species at the site or competitive replacements of grass floras. We argue that higher stomatal density may have been a response to falling CO2 levels during the last glaciation, accompanying the replacement of a C3 flora by C4 species. The stomatal size changes exhibited over this time period may have adapted plants to changes in soil water availability. That stomatal morphology changes in a sample flora (not a single taxon) over millennia is a novel finding, and one that may have implications for paleoecological interpretation and the prediction of grass behavior in the future.
Abstract: This paper explores the long-standing interest in the introduction of fodder legumes as an essential component of mixed farming systems in Africa. The central argument is that because of certain biological characteristics and their association with historic changes in European agricultural, fodder legumes have become endowed with a mantle of absolute goodness. However, while the screening of fodder legumes has been an important theme in agricultural research for over 70 yr, the actual level of use by farmers and livestock keepers in Africa is still insignificant. This analysis leads to a call for a more realistic approach to legume research and promotion. Particular attention is placed on the idea that the bio-physical and socio-economic factors that have previously been seen as constraints to legume adoption should now be viewed as system properties and incorporated into the design specification of technology. Without such an approach the [`]logic of fodder legumes' will continue to limit the contribution that agricultural research can make in Africa.
Abstract: The evolution of two decision support systems are traced from their roots in academia to deployment to technical advisors in USDA Natural Resource Conservation Service. The Grazing Lands Application (GLA) decision support system (DSS) was designed to provide forage inventories for grazing management of ranches. The other tool, NUTBAL, evolved as a stand alone DSS, emerging as a component of GLA when a supporting monitoring technology for nutritional profiling of free-ranging animals provided the user rapid estimates of diet quality from fecal scans with near infrared reflectance spectroscopy (NIRS). The adoption pattern of GLA and NUTBAL were quite different, with GLA experiencing less widespread adoption in USDA NRCS. The primary causes were (1) limited adoption rate of GLA within NRCS associated with changing culture in the information technology development group, (2) time overloading and staff reassignments for new programs, (3) changing software/hardware development environments imposed by the client disrupting development and system design and (4) large up front conversion of a largely paper-based system to a digital form. GLA was transformed to web-based delivery and streamlined to gain greater acceptance by users and ease time constraints on use of spatial tools. The NUTBAL system experienced more of a user driven evolution since it emerged from the GLA suite of tools and was supported by on-ranch monitoring systems capability of directly linking the livestock producer's animals with the software. NUTBAL's linkage to animal monitoring systems seems to have accelerated adoption rates. Ease of access to supporting input data coupled with early involvement of the target user and extensive analysis of the decision environment were critical to future success of these systems. Targeting technical advisors instead of livestock producers appears to be a more viable development track unless new innovations in DSS delivery systems can emerge using the internet.
Abstract: SPUR (Simulation of Production and Utilization of Rangelands) is a grassland ecosystem simulation model. SPUR2.4 output was compared with 29 years of cow-calf data from a field experiment conducted in north-central Texas, USA. Simulated primary productivity for C4 shortgrass was good, adequate for C4 mid-grass and live C3 wintergrass but inadequate for total wintergrass. The productivity of C3 annual grass predicted by the model was of the same order of magnitude as productivity measured in the field and appears to be adequate in terms of predicting secondary production. Between-season herbage standing crop and long-term persistence were simulated adequately for individual soils but not for areas containing more than one soil series. The model gave accurate simulations of weaning weight per hectare for both the purebred Hereford and Charolais-cross animals for all grazing treatments and intensities. However, the lack of ability to simulate accurately with more than one soil per grazing unit must be corrected before the model will adequately simulate secondary productivity for landscapes that contain different soil series.
Abstract: The eastern margins of the Great Karoo, South Africa, represent a transition zone between the semi-arid shrublands of the Karoo to the west and the moister grasslands of the east. For this reason, the surficial sediments and younger geomorphic features of the area might reliably be expected to record evidence for environmental change, particularly with regard to the precipitation record and phases of greater aridity as the boundary of the Karoo migrated in response to climate change during the Late Quaternary. The high altitude catchment on which we report here is situated on the inland flank of the upwarped South African Great Escarpment, which here forms the Stormberg range. The catchment is unique in that a number of geomorphic features have, within their structure, preserved evidence of aeolian, fluvial and colluvial processes, which were active in the past. We present evidence from a variety of depositional landforms and sedimentary deposits that suggests that Late Quaternary climates in the eastern Karoo have been both moister and significantly more arid than at present, as well as displaying a greater seasonality in rainfall.
Abstract: Countries of semi-arid West Africa are experiencing growing populations, expansion of cultivated land and intensification of crop and livestock production an ever-increasing burden on the region's limited natural resources, consequently increasing degradation rates. A broad range of technologies combating degradation currently exist. This paper presents and discusses both traditional techniques as well as modern techniques which are derived from the traditional ones. Various methods of mulching and application of manure or mineral fertilizers are used to maintain or increase soil fertility. The use of mulch as well as various methods of integrating shrubs, trees and herbaceous vegetation into the cultivation systems are the technologies currently used to decrease the effects of wind erosion. Similarly, water erosion can be prevented with lines of stones, mulch and grasses. The most promising methods which impede decreasing natural vegetation are promotion of natural regeneration and to some extent the implementation of agricultural parks. In contrast, community-based management of grazing land or forested areas are difficult to establish. Most conservation strategies are limited by the availability of mulch, fertilizer or manure to fertilize fields or to protect sufficient land areas from wind and water erosion. Household constraints of individual farmers also play a crucial role in increasing such protection. Great efforts are needed to distribute the knowledge of the various conservation strategies throughout the different regions of semi-arid West Africa, and to develop new technologies, preferably with farmers' participation, to enable adoption. As degradation continues and populations increase, this must be accomplished in the near future in order to hinder devastation of land resources.
Abstract: It is claimed that high returns can be achieved from hunting and ecotourism operations. As a result wildlife production is a rapidly growing form of land-use in South Africa. Lately, rural African communities have approached regional conservation agencies for aid to establish small game reserves so that they too may benefit from wildlife production. However, wildlife operations have high input costs relative to domestic stock operations and no attempt has been made to determine the effect of property size on the costs and revenue generated by wildlife. This paper attempts to develop a method for identifying the relevant economic variables of wildlife production, subsistence production and commercial beef production and the revenues that these separate land-uses generate. Thence to observe their relationship with property size by means of an illustrative example. In this way the size ranges for which each of the three land-uses is most appropriate can be determined. Finally, the method is evaluated against the results of the example to identify future refinement. The results of this example indicate that the profit curve of wildlife rises far more steeply than those of either subsistence production or commercial beef production. However, due to the effect of high input costs associated with wildlife, both commercial and subsistence beef production are more profitable at small land sizes. This indicates that investing large sums of money into small game reserves of the order of 3000 ha or less may not be justified on the basis of profits alone.
Abstract: In South Tunisia, as part of a native seed production program, a collection of 52 accessions ofCenchrus ciliaris was assessed for 12 different traits related to seed and spike. Possible relations between these, eco-geographic origin, and 4 adaptive traits of agronomical value were investigated. We could not establish reliable correlations between traits, nor between traits and eco-geographic origin. However, accessions were very distinct and homogeneous. Our data suggest that native seed production can consider South Tunisia as a single source and target region, but should define sound criteria to select a limited number of accessions for multiplication.
Abstract: Soil degradation due to tillage has been reported Africa-wide. Other main causes of soil degradation are overgrazing, extensive cultivation of marginal lands, widespread clearing of vegetation for agriculture, deforestation, exploitation of unsuitable agricultural technologies, mis-management of arable lands, and frequent drought. Hence, declining soil fertility and increasing population pressure on lands are fragile bases on which to build expectations for improved crop production. This paper recognizes conservation tillage systems as one means for preventing food shortages and natural resources degradation throughout the continent. Conservation tillage has the potential for increasing soil organic matter content and enhancing soil aggregation. Conservation tillage systems can create an aggregated, fertile surface layer that is important from a soil erosion reduction perspective and thus for a sustainable agriculture in Africa. Some indigenous tillage systems in Africa can be adapted to meet objectives of conservation tillage systems. Further, recent technological developments in tillage and seeding machinery will certainly enhance the rate of farmer's acceptance and adoption of conservation tillage.
Abstract: The Nama Karoo region of South Africa is characterized by low (~200 mm) and variable annual rainfall, which results in grass and shrub biomass production, which is low and highly variable in space and time. These characteristics of Nama Karoo rangelands challenge the ability of the region's livestock farmers to make a sustainable living. In this paper we model a farming system, which attempts to create an environmental buffer of forage reserves by restricting access of livestock within numerous small camps. This is achieved by using a multi-camp infrastructure, which forces the livestock to remove non-selectively most of the available forage within a camp. Non-selective grazing in small camps allows for long rest periods of each camp, and these rest periods build up forage reserves for the dry years. A computer model of a 7000 ha farm was used to simulate rainfall and above-ground plant biomass accumulation, and to test the economic merits of investing large sums of money in multi-camp infrastructure. The model shows that 60 camps or more allows time for forage reserves to build up, but that more than 150 camps becomes too costly. Our simulations suggest that given 250 mm yr-1 rainfall and the agriculturally recommended stocking rate, camp numbers of 60-80 provide higher profits than other camp numbers investigated. However, with higher rainfall and more animals, increasing camp numbers up to 150 is economically viable and more ecologically desirable. At low rainfall (<200 mm yr-1) production is too low to warrant investment in multi-camp infrastructure.
Abstract: Developments in and around game reserves and ranches in South Africa have led to controlled burning becoming a necessary and regular activity. The management objectives of these fires are well-defined, and thus predicting the duration and extent of a burn is vitally important. Testing scenarios via computer simulation is desirable since this removes the potential risks associated with fire, whilst at the same time ensuring that management policies are attained. There are various approaches to developing a spatial simulation fire model. In this article we present a cellular automaton (CA) model that is capable of predicting fire spread in spatially heterogeneous Savanna systems. The physical basis of Rothermel's fire spread model (1972) was modified to a spatial context and used to improve the CA model introduced by Karafyllidis and Thanailakis (1997). The proposed fire model was verified using data for three human-induced fires in the Mkuze Game Reserve, South Africa, and was found to satisfactorily predict spatial fire behaviour.
Abstract: To study the carbon storage increase of major forest ecosystems in tropical South America, such as Amazon rain forest, Atlantic rain forest, semideciduous forest, and Araucaria forest, the Last Glacial Maximum (LGM) and the early Holocene vegetation cover were reconstructed by pollen records. Marked forest expansion points to a significant total carbon storage increase by tropical forests in South America since the LGM and the early Holocene. The Amazon rain forest expansion, about 39% in area, had 28.3×109 tC (+20%), the highest carbon storage increase since the LGM. The expansion of the other much smaller forest areas also had a significant carbon storage increase since the LGM, the Atlantic rain forest with 4.9×109 tC (+55%), the semideciduous forest of eastern Brazil with 6.3×109 tC (+46%), the Araucaria forest with 3.4×109 tC (+108%). The estimated carbon storage increase of the four forest biomes since the early Holocene is also remarkable. The extensive deforestation in the last century strongly affected the carbon storage by tropical forests.
Abstract: Charred modern grass epidermis preserves the carbon isotopic composition of the parent plant photosynthetic pathway. Fifty-nine modern grasses and sedges were collected in lowland western Uganda. All charred epidermal samples from C4 grasses or sedges preserve a carbon isotopic value within the range typical for C4 plants (-17 to -10[per mille sign]), and charred epidermal fragments from C3 plants have carbon isotopic values between -30 and -26[per mille sign]. The process of charring results in a slightly enriched carbon isotopic signature (-11.9[per mille sign] mean charred value as compared to -12.8[per mille sign] mean unaltered grass tissue value). [delta]13C measurements of replicate samples from the same plant vary within 1-2[per mille sign], yet all values for the same plant stay within the expected values for the photosynthetic pathway of the plant. [delta]13C measurements on >180-[mu]m charred grass epidermal fragments extracted from surface sediment samples from three lakes on the lowland western Ugandan landscape confirm the predominant lowland C4 grass input ([delta]13C=-16 to -19[per mille sign]). These results demonstrate the utility of using carbon isotopic analysis of charred grass epidermis to reconstruct C3 vs. C4 grassland assemblages on the landscape. Furthermore, such downcore [delta]13C profiles can be used to highlight key zones of C3 vs. C4 grass change for which taxonomic analysis of fossil grass epidermis could provide more detailed information regarding grassland community composition.
Abstract: In the Otjiwarongo region (Northern Namibia), Vertisol-Kastanozem-Calcisol soil associations occur as patches of several hundred hectares in extent. They have formed in fine-grained Mid-Holocene sediments which accumulated on both sides of the subcontinental watershed between the Ugab River draining into the South Atlantic and the Omatako Omuramba draining into the Kalahari Basin. Kastanozem formation cannot be explained by the environments that exist at present. The humification suggests open savanna environments in the past and does not accord with the shrublands and thornbush savanna at present. Using AMS 14C and OSL data, it is possible to distinguish two periods of soil degradation during the recent past. Initially, most of the Kastanozems and Vertisols were buried by slope wash sediments to a depth of several decimetres. This process started in the mid-19th century at the latest. In a second phase, the soils were affected by rill and gully erosion, indicating increased runoff. This occurred during the last decades of the 19th and the first decades of the 20th century, probably as a result of intensified cattle farming. In contrast to other parts of Namibia, the prominent river channels of the Otjiwarongo region, most of them up to 20 m wide and 3-4 m deep, are a result of recent erosion. Degradation of vegetation and soils, and river channel formation, seem to be the main causes of farmland aridification.
Abstract: Abstract Species diversity and community structure indices (richness, diversity and evenness) are typically derived from contemporary biological survey atlases to identify conservation areas or to determine macroecological-environment relationships. Surprisingly, there are few studies based on national taxa surveys that describe and measure the influence human disturbance patterns may have on these ecological measures. This study uses various spatial statistical methods to examine and model large-scale spatial structure in diversity, structure indices derived from a comprehensive bird atlas, and composite variables of environment and human land transformation. Data were derived from an atlas of South African birds and segregated into five vegetation biomes. For each sample location, environmental and land-use data were used to calculate composite environmental gradients in climate, topography and human land transformation. Semivariograms were used to detect large-scale trends and spatial scale, and Moran's I statistic correlograms to test for large-scale spatial autocorrelation in detrended diversity indices. Ordinary least squares regressions for all biomes indicated a significant positive relationship between high levels of human transformation and species diversity, whereas high transformation had a significant negative influence on evenness. These same relationships held for the majority of biomes after the effects of environmental gradients were removed. The analysis also shows that in areas with favourable environmental resource levels, diversity for birds increases. These areas also tend to contain a large proportion of highly transformed land, and evenness decreases in the woodland, grassland, Karoo and fynbos biomes. This suggests that high-intensity transformation may bring in novel resources for birds not normally found in some of the biomes, with some species adept at exploiting these changes and reaching high densities with a tendency for assemblage structure to drift towards single species dominance. The results imply that species data derived from contemporary atlases may begin to demonstrate the effects of human influence on ecological measures rather than only indicating the effects of environmental variation on community structure.
Abstract: A simulation model for the African tick Boophilus decoloratus is presented. This model is based on the use of a dynamic life table that is driven by abiotic variables (temperature and vegetation status) remotely sensed (AVHRR sensor of the NOAA series of satellites) over time. The model incorporates temperature-dependent rates of egg production and development, climate-driven density-independent mortality rates, and density-dependent regulation of on-host stages. The model successfully describes both the seasonality and annual variation in the numbers of questing larvae and engorging females observed in eight sites throughout sub-Saharan Africa. Climate data from 1983 to 1999 in 10-day intervals are used as the basic input for modelling the dynamic patterns of activity at four different sites in Africa and to understand how abiotic factors can modulate the long-term life cycle of B. decoloratus.
Abstract: In arid and semi-arid rangeland regions, landscapes that trap and retain resources, such as rain water, soil particles, and organic matter, provide more favorable habitats for vegetation and fauna, and are considered more functional than landscapes that lose, or leak, these essential resources. The cover and arrangement of perennial vegetation patches is an important indicator of whether landscapes retain or leak resources. Patchiness attributes, as descriptors of resource retention potential in landscapes, can be obtained from remotely-sensed imagery, such as aerial videography and high-resolution satellites where this imagery has been classified into perennial vegetation patch and open interpatch pixels. In this paper, we compare four landscape patchiness metrics on their ability to indicate how well landscapes potentially function to retain resources. Landscape patch attributes (e.g. patch cover and spacing) and on-ground inspection of soil and vegetation attributes were used to rate and rank four sites relative to their potential to retain resources. A directional leakiness index (DLI) that is highly sensitive to patch cover, shape, and configuration correctly and adequately ranked sites in the same order as our field ratings. The lacunarity index also correctly ranked sites, but showed little separation amongst sites with reduced potential to retain resources. The weighted mean patch size (WMPS) index and proximity index failed to correctly rank sites. The directional leakiness and lacunarity indices can be calculated for any remotely-sensed imagery that is of sufficient resolution to measure landscape patchiness at scales where processes of resource conservation are operating. For example, imagery of 0.2-1Â m pixel sizes from arid and semi-arid rangelands can be classified into flow-obstructing patches and open non-obstructing interpatches. Such classified imagery and leakiness or lacunarity indicators can then be used to monitor changes in the resource retention potential of these landscapes. However, the applicability of these indicators for monitoring more humid vegetation types, and for assessing larger landscape areas (i.e. at coarser scales), needs to be evaluated.
Abstract: A pattern of seasonal mobility between the coast and the hinterland by Khoekhoe pastoralists in the south-western Cape of South Africa was documented at the time of European contact. Because the region is characterized by a mosaic of geological substrates of various ages and vegetation types with different proportions of C3 and C4 plants, this mobility model for prehistoric herders can be tested by analysis of carbon, oxygen and strontium isotope ratios in domestic bovid tooth enamel. Isotopic analysis was performed on archaeological sheep, cattle, eland and steenbok teeth from the Late Stone Age site of Kasteelberg on the Vredenburg Peninsula. Sequential sampling of enamel provided a chronological record of the isotopic composition of diet during tooth formation. Results from carbon and oxygen isotope analysis of archaeological and modern steenbok teeth show seasonal variation in the [delta]13C of local pastures in the vicinity of the site (up to 2·9[per mille sign] in a single individual), which could be due to seasonal change in proportions of C3 and C4 plants and/or seasonal variation in the [delta]13C of C3 plants. A pattern of seasonal change of 0·9-3·5[per mille sign] in the [delta]13C of tooth enamel is also observed in the sheep teeth. This amplitude of variation could be due to local seasonal changes, thus it is not possible, from the [delta]13C values, to infer whether the sheep were moving seasonally. Results from 87Sr/86Sr analysis of two sheep suggest that one individual stayed all year round on the coast, whereas the other had grazed in the interior for part of its life. Both [delta]13C and 87Sr/86Sr values measured in a cow tooth suggest that this animal spent part of its life in the interior, even though it died at Kasteelberg. Results on this cow also suggest longer residence within habitats rather than seasonal mobility. A clear pattern of seasonal mobility between the coast and the interior has not been shown for prehistoric pastoralists. However, the pattern of mobility attested in historical times might have developed only later in prehistory, when cattle pastoralism developed in the region.
Abstract: During the Pleistocene the vegetation changes in the high Colombian Andes included changes from C3 to C4 plants. This is inferred from [delta]13C values of the C31 n-alkane from the Funza-2 sedimentary record taken from the high plain of Bogotá at 2550 m elevation. The environmental factors thought to be responsible for these changes were investigated using a single point simulation of the BIOME3 vegetation model, including changes in precipitation, temperature and atmospheric CO2 concentrations. The model shows that changes are for a major part caused by these latter two factors. The isotopic signature of the n-alkanes of several extant C3 and C4 grasses from the area were determined to calibrate the interpretation of the isotopic record. From the geochemical record, we estimated the altitudinal distribution of C3 and C4 plants, using present grass distribution patterns based on floristic data as a template. This information, in combination with palaeotemperature estimates, enabled the reconstruction of atmospheric CO2 concentrations. The reconstructed CO2 concentrations follow the trends of the Vostok Antarctic ice core through three glacial and two interglacial stages. The lowest calculated CO2 concentration is ca. 210 ppmV for the glacial maxima and within the range of lowest values from Vostok, our highest value (310 ppmV) is for interglacial MIS 7. This represents a new method to reconstruct palaeoatmospheric pCO2. It is less accurate than measurements from ice cores, but has potential to be used for sediments that are much older than the ice cores.
Abstract: To measure and manage for C sequestration in heterogeneous rangeland systems, we need to more fully understand spatial patterns of soil resources. Spatial distributions of aggregate stability and soil carbon were investigated in a semiarid rangeland in New Mexico, USA. Soil was analyzed from plant interspaces, black grama (Bouteloua eriopoda (Torr.) Torr.), and mesquite (Prosopis glandulosa Torr.) in a landscape-replicated study. Aggregate stability at the 250 [mu]m scale, carbonate C, organic C and N, C:N ratio, and glomalin, were all highest under mesquite. Soil C:N ratio was the best predictor of aggregate stability. Estimates of metric tons of C per hectare in the top 10 cm were highly variable at patch and landscape scales, varying from 4.2 to 10.5 under mesquite and from 3.0 to 7.0 in interspaces. High variability of aggregate stability and soil C has important implications for C sequestration. We argue that this multi-scale soil heterogeneity must be considered when measuring and managing for C sequestration.
Abstract: Although the distribution and structure of pinyon-juniper woodlands in the southwestern United States are thought to be the result of historic fluctuations in regional climatic conditions, more recent increases in the areal extent, tree density, soil erosion rates and loss of understory plant diversity are attributed to heavy grazing by domestic livestock and interruption of the natural fire regime. Prior to 1850, many areas currently occupied by high-density pinyon-juniper woodlands, with their degraded soils and depauperate understories, were very likely savannas dominated by native grasses and forbs and containing sparse tree cover scattered across the landscape. The purpose of this study was to evaluate the effectiveness of mechanical overstory reduction and three slash treatment alternatives (removal, clustering and scattering) followed by prescribed fire as techniques for restoring grassland savannas from degraded woodlands. Plant cover, diversity, biomass and nutrient status, litter cover and soil chemistry and erosion rates were measured prior to and for two years following experimental treatment in a degraded pinyon-juniper woodland in central New Mexico. Treatment resulted in a significant increase in the cover of native grasses and, to a lesser degree, forbs and shrubs. Plant species richness and diversity increased most on sites where slash was either completely removed or scattered to serve as a mulch. Although no changes in soil chemistry or plant nutrient status were observed, understory biomass increased over 200% for all harvest treatments and was significantly greater than controls. While treatment increased litter cover and decreased soil exposure, this improvement did not significantly affect soil loss rates. Even though all slash treatment alternatives increased the cover and biomass of native grasses, scattering slash across the site to serve as a mulch appears most beneficial to improving plant species diversity and conserving site resources.
Abstract: To support grazing and fire management of communal semi-arid to sub-humid African rangelands, the assessment of grazing capacity is illustrated for cattle. The forage dry matter intake (DMI) was divided into four components: 1) the mature standing crop, 2) the green part of the standing crop, 3) the regrowth after range fires, and 4) browse. The relationship between the potential DMI and the crude protein (CP) concentration of the forage was modelled. The DMI ranged from 5 to 50% for forage with low to high CP concentrations, respectively. The DMI of browse was fixed at 0 and 10% of the DMI of unburnt vegetation for plain and woodland systems, respectively. The potential DMI of the four forage components combined, based on data of the early- to mid-dry season, was matched with livestock requirements to calculate grazing capacities. Possible combinations of and changes in the forage components, representing grazing capacities for different seasons of the year, were calculated and visualised. Lastly, it was demonstrated how to assess the proportion of the rangeland that should be burnt to make optimal use of the low quality forage available in abundance and the small amount of high quality regrowth.
Abstract: The objectives of this study were to understand the role of fires on land-cover changes, and conversely the role of vegetation cover as a controlling factor of fires. The study, which was conducted in a region at the savannah/forest transition in the southwestern part of the Central African Republic, explores the differential impact on land cover of early- and late-season fires and analyses burning regimes as a function of human use of the land. This was addressed using multivariate regression models between maps of land-cover change derived from remote sensing data, maps of burnt areas and a detailed map of ecotypes. In dense forests, burning is strongly associated with land-cover changes, while in savannahs the occurrence of (mostly) early fires does not lead to land-cover change. Fires associated with continuous and fragmented burnt patches have similar impacts on vegetation cover. Dense semi-humid forests in the study area were affected by a high level of burning due to land uses at their peripheries. The results confirm recent findings concerning human control on the timing of burning in savannahs. Early fires fragment the landscape and prevent the spatial diffusion of later damaging fires. Where no human settlements are present, late fires become more prevalent. Finally, the study measured an increase in vegetation cover in a few areas affected by very early burning. Using burnt area rather than active fire data allowed a better analysis of the spatial association between landscape attributes and burning events.
Abstract: Temperate grasslands are vast terrestrial ecosystems that may be an important component of the global carbon (C) cycle; however, annual C flux data for these grasslands are limited. The Bowen ratio/energy balance (BREB) technique was used to measure CO2 fluxes over a grazed mixed-grass prairie and a seeded western wheatgrass [Pascopyrum smithii (Rybd) Löve] site at Mandan, ND from 24 April to 26 October in 1996, 1997, and 1998. Above-ground biomass and leaf area index (LAI) were measured about every 21 days throughout the season. Root biomass and soil organic C and N content were determined to 110 cm depth in selected increments about mid-July each year. Peak above-ground biomass and LAI coincided with peak fluxes and occurred between mid-July to early August. Biomass averaged 1227 and 1726 kg ha-1 and LAI 0.44 and 0.59, for prairie and western wheatgrass, respectively. Average CO2 flux for the growing season was 279 g CO2 m-2 for prairie and 218 g CO2 m-2 for western wheatgrass (positive flux is CO2 uptake and negative flux is CO2 loss to the atmosphere). Using prior measured dormant season CO2 fluxes from the prairie sites gave annual flux estimates that ranged from -131 to 128 g CO2 m-2 for western wheatgrass and from -70 to 189 g CO2 m-2 for the prairie. This wide range in calculated annual fluxes suggests that additional research is required concerning dormant season flux measurements to obtain accurate estimates of annual CO2 fluxes. These results suggest Northern Great Plains mixed-grass prairie grasslands can either be a sink or a source for atmospheric CO2 or near equilibrium, depending on the magnitude of the dormant season flux.
Abstract: The primary approach used for reducing "weeds" in the native grasslands of the North American Great Plains is the application of a broadleaf-selective herbicide, which could have important implications to native plant and arthropod diversity. The objectives of this study were to identify the influence of herbicides on the forb and arthropod community composition, richness, and density, and determine relationships among the forb and arthropod communities in a tallgrass prairie of the North American Great Plains. In 1994, arthropod and forb communities were evaluated in eight treatment units and then a broadleaf-selective herbicide was applied to four of these units. Sampling of arthropod and forb communities were sampled under similar conditions in 1995 for post-treatment effects. These communities were highly variable across years regardless of treatment (herbicide and no herbicide). The herbicide treatment caused a reduction in overall forb dominance the year after treatment. Species richness increased from 1994-1995 in both treatments but the increase was less in the herbicide treatment. The herbicide application had no overall effect on forb species composition. The lack of effect of herbicide on the forb community composition coupled with a significant effect on species richness suggests that an important effect of herbicide application was a reduction of rare forbs. Analysis of these tallgrass communities did not yield significant differences in arthropod abundance or richness between grasslands treated with a herbicide and grasslands not treated with a herbicide. The arthropod community was defined by extreme variability across years reflecting extreme fluctuations regardless of herbicide application.
Abstract: Most soils in the Midwestern USA have lost 30 to 50% of their original pool, or 25 to 40 Mg C/ha, upon conversion from natural to agricultural ecosystems. About 60 to 70% of the C thus depleted can be resequestered through adoption of recommended soil and crop management practices. These practices include conversion from plow till to no till, frequent use of winter cover crops in the rotation cycle, elimination of summer fallow, integrated nutrient management along with liberal use of biosolids and biological nitrogen fixation, precision farming to minimize losses and enhance fertilizer use efficiency, and use of improved varieties with ability to produce large root biomass with high content of lignin and suberin. The gross rate of soil organic carbon (SOC) sequestration ranges from 500 to 800 kg/ha/year in cold and humid regions and 100 to 300 kg/ha/year in dry and warm regions. The rate of SOC sequestration can be measured with procedures that are cost effective and credible at soil pedon level, landscape level, regional or national scale. In addition to SOC, there is also a large potential to sequester soil inorganic carbon (SIC) in arid and semi-arid regions. Soil C sequestration has numerous ancillary benefits. It is truly a win-win situation: extremely cost-effective, and a bridge to the future until alternative energy options take effect.
Abstract: We examined the relationship between a Chihuahuan Desert grassland keystone species (banner tailed kangaroo rat, Dipodomys spectabilis) and several vegetation and soil indicators of rangeland health in order to define a threshold indicator value for irreversible change in ecosystem structure and function. The abundance of occupied and/or abandoned D. spectabilis burrow-mounds was assessed at 117 sites in south-central New Mexico where previous studies had reported vegetation cover and composition. The most robust indicator for presence/absence of D. spectabilis was shrub cover. D. spectabilis did not occur at sites with shrub cover >20%. It was concluded that a threshold value of 20% shrub cover could be applied to assessment and monitoring of Chihuahuan Desert rangelands because higher shrub cover results in the local extinction of this keystone species. The combination of data on the presence/absence of a keystone species with vegetation and soil indicators provides a method for identifying thresholds of degradation that may be irreversible.
Abstract: The North American grassland biome first appeared around 18 Ma in the mid Miocene. The familiar story of the Neogene evolution of this biome is of the replacement of ungulates (hoofed mammals) having a primarily browsing diet by the more derived grazing ungulates. However, new data show a more complicated pattern of faunal succession. There was a maximum taxonomic diversity of ungulates at 16-14 Ma, including a large number of grazers, and the subsequent decline in overall diversity was largely due to the decline of the browsers, with little corresponding increase in the grazers. Additionally the mid Miocene faunas (~18-12 Ma) contained a much greater number of browsers than any comparable present-day habitat. We discuss possible explanations for these non-analogous grassland faunas, including the possibility that the primary productivity of the vegetation was greater in the early to middle Miocene than it is today. One possible explanation for increased primary productivity is higher Miocene levels of atmospheric carbon dioxide than in the present day. The proposed difference in vegetational productivity also may explain why horses radiated as the main grazers in North America, in contrast to the radiation of antelope in the Plio-Pleistocene African grasslands.
Abstract: Invasive alien plants are consumptive water-users, and may have reduced river flows in South Africa by about 6.7% according to a broad-scale study. An effective programme to bring the invasions under control would cost about US$ 92 million per year for the next 20 years. This paper reports on studies of four representative catchments (the Sonderend, Keurbooms, Upper Wilge and Sabie-Sand) to assess the impacts and costs of invasions at a scale that is more relevant to managers. Several alien plant species have invaded the catchments. Non-riverine invasions are mainly Pinus and Hakea species in Sonderend and Keurbooms, eucalypts in the Upper Wilge, and pines and scramblers (e.g. Lantana camara) in the Sabie-Sand catchment. Riverine invasions are dominated by Acacia mearnsii and, to a lesser extent, A. dealbata, except in the Sabie-Sand and the lower Sonderend River where Eucalyptus species are important. About 44% of the Sonderend, 54% of the Keurbooms, 2% of the Upper Wilge and 23% of the Sabie-Sand catchments has been invaded to some degree. The corresponding reductions in the natural river flows attributed to these invasions are about 7.2, 22.1, 6.0 and 9.4%. If the invasions are not controlled they could potentially spread, and occupy 51, 77, 70%, respectively, of the first three catchments. At an annual expansion rate of 10-15% this would take about 13, 26 and 63 years, respectively. The invadable areas in the Sabie-Sand catchment are already invaded so invasions will only increase in density. It would take about 26-30 years to reach 100% canopy cover. The projected flow reductions for the four catchments would increase to 41.5, 95.5, 25.1 and 22.3%, respectively. The estimated cost of the control programmes to prevent these losses would be about US$ 13.2, 9.9, 4.1 and 6.6 million for the Sonderend, Keurbooms, Upper Wilge and Sabie-Sand catchments, respectively. Should the catchments be allowed to become fully invaded before control operations were started, then the costs would rise to US$ 86.5, 20.5, 278.0 and 11.1 million, respectively. The impacts and costs are significant and are comparable with those calculated independently for other South African catchments. Water is acknowledged to be a key constraint to economic growth in South Africa and there is considerable pressure for efficient and sustainable use of the limited water resources. The projected impacts would justify control programmes aimed at clearing alien invaders for water conservation.
Abstract: Natural products are becoming more important in modern-day society as man is moving away from synthetic products, which can be detrimental to the environment and human health. Scientific research on the healing properties and bioactivity of natural compounds, especially of plant origin, has been extensive particularly in the Western world. However, a rich heritage of floral biodiversity is found in developing countries. South Africa, a country with a strong history of traditional healing, hosts a variety of around 30[punctuation space]000 plant species. Indigenous bulbous plants of importance to South African traditional healers mainly belong to the Amaryllidaceae and Hyacinthaceae families. A number of these plants have particular uses as disinfectants and anti-inflammatory agents, although there is still a lack of scientific research regarding their unique pharmacological compounds.
Abstract: Bush encroachment is a serious environmental and economic problem in Namibia, but little is known about impacts on native reptile diversity. Area-confined visual surveys were used to examine a diurnal lizard assemblage in central Namibian commercial ranchlands. Surveys were conducted in plots of open savanna habitat and proximal bush-encroached habitat. The following four species comprised 97.5% of all lizard observations: Pedioplanis undata, Mabuya varia, M. striata, and Lygodactylus bradfieldi. Pedioplanis undata was terrestrial, and the remaining three species were largely arboreal in our study plots. Mabuya varia was found in all savanna plots but was absent from all bush-encroached plots. Two species (P. undata and L. bradfieldi) were less abundant in bush-encroached plots. One species (M. striata) was more abundant in bush-encroached plots than in open savanna plots. Arboreal lizards demonstrated an avoidance to invasive woody plant species. Decreased diversity of habitat structure in bush-encroached habitats appears to influence native savanna lizard assemblages. Our results are consistent with accumulating evidence suggesting that bush encroachment and its associated ecological impacts are reorganizing savanna ecosystems throughout southern Africa.
Abstract: Carbon (C) and associated nutrient budgets related to land use in agro-ecosystems in West African savannas (WAS) are a matter of both local (sustainability of farming systems) and global (C balance) concern. In a mixed-farming system in southern Senegal, patterns of C, nitrogen (N) and phosphorus (P) allocation in the plant-soil system (down to a 40Â cm soil depth) were compared at harvest in 14 plots, six being under semi-permanent cultivation with groundnut (Arachis hypogaea L.), others being under continuous cultivation with millet (Pennisetum glaucum L.), maize (Zea mays L.) or rice (Oryza sativa L.). Carbon stored in the plant-soil system amounted to 25.0, 27.4, 34.9 and 71.9Â t per ha, respectively, in groundnut, millet, maize and rice fields. Ninety percent of C and P (total in plant Pt, available P in soil (POD)) and 95% of N of the whole ecosystem were stored in the soil. The high C and nutrient amounts found in rice plots were attributed to the clayey texture of the soil and to seasonal flooding. The lower values for C, N and POD found in soils in the bush ring (groundnut crops) compared to those of the compound ring (millet and maize crops) stemmed from land management. Higher values for C, N and POD in soils in the compound ring were maintained under continuous cultivation thanks to higher organic and nutrient inputs originating from crop residue recycling, manuring and, in the maize plots, spreading of household wastes. In the compound ring, the amount of C stored seemed to depend as much on the amount of C input as on the chemical richness of organic inflow. The effect of land management (bush versus compound ring) on soil properties was generally restricted to the 0-20Â cm layer (except for P, cations and pH), and the better soil status in the compound ring was linked to nutrient depletion of the bush ring. From the perspective of global change, the estimated potential of the WAS for C sequestration under continuous cultivation was found to be low. From a methodological point of view, soil carbon status may be considered as a relevant indicator for the fertility of agro-ecosystems in the WAS belt, provided that its biotic components are included, and that both the quality and dynamics of soil organic matter (assessment of seasonal variations, and C flows) and soil texture are characterised.
Abstract: North American leafy spurge (Euphorbia esula L.) is genetically diverse and composed of multiple genotypes introduced from several areas of Europe and Asia. Five species of leafy spurge flea beetle (Aphthona spp.) have been introduced as biological control agents for leafy spurge, but were collected in a relatively small region of Europe. Greenhouse and field experiments were conducted to determine if observed variation in feeding preference and reproduction of Aphthona spp. on North American leafy spurge may be due in part to leafy spurge genotype. Leafy spurge genotypes were collected from Austria; Manitoba in Canada; and the states of Montana, Nebraska, North Dakota, South Dakota, and Wyoming in the United States. Leafy spurge genotype affected feeding but not egg laying by Aphthona spp. adults. Aphthona czwalinae/lacertosa fed slightly less in a free-choice test on a genotype from Manitoba (7%) compared to genotypes from Nebraska and North Dakota (14%). Aphthona flava tended to feed less on the Nebraska genotype than any other genotype evaluated. Reproduction of Aphthona spp. was greatly affected by leafy spurge genotype. For instance, A. czwalinae/lacertosa produced 72 adults per plant from a Nebraska genotype compared to 11 to 32 adults per plant from all other genotypes evaluated. Thus, some observed variation in establishment and reproduction of Aphthona spp. can be attributed to leafy spurge genotype.
Abstract: Across nearly 100 sampling locations widely distributed within the arid shrubland of Western Australia, we demonstrated a general relationship between landscape function, primary productivity and rainfall-use efficiency. Sampling locations included landscapes that had been severely [`]degraded' by more than 100 years of grazing, mainly by sheep. There was generally less phytomass and poorer rainfall-use efficiency on dysfunctional or degraded landscapes than on functional or non-degraded landscapes. Relationships were stronger at broader spatial scales of patch-mosaics than at the scale of individual patches and are likely to be more readily interpreted over decadal rather than yearly time-scales. A-priori assessment of landscape [`]resilience' provided few insights into the capacity of landscapes to respond to rainfall. Contrary to expectations, herb mass increased on both resilient and non-resilient landscapes as proportional areas occupied by vegetated patches declined.
Abstract: The physiography of North Africa is not greatly changed from that of the Messinian. With the drawdown of the Mediterranean in the late Messinian the then existing river systems were incised into the landscape and emptied into Lake Cyrenaica, which occupied the substantially drained Eastern Mediterranean Basin. The record of incision provides a record of the magnitude of the Messinian river systems. An analysis of these river systems demonstrates that Messinian rainfall, a consequence of the Zeit Wet Phase, was greatest in east and south-central North Africa, in keeping with the derivation of the water mainly from the Indian Ocean. In central North Africa the Eosahabi River flowed from Messinian Lake Chad eroding the East Tibesti Valley and cutting a channel which is especially well preserved near the coast of the Gulf of Sirt. The penetrated sediments of Lake Cyrenaica and the marginal basins of the Messinian transgression are known as the Upper Evaporites and are generally fine clastics, gypsarenites and gypsum/anhydrite. They were rapidly deposited. Underlying them is the halite of the Lower Evaporites. This sequence of Upper and Lower Evaporites is much like the lower halite of the Tortonian South Gharib Formation and the overlying clastics and anhydrite of the Messinian Zeit Formation in the Gulf of Suez/Red Sea area. This type of sequence is termed an evaporitic couplet and, as has been demonstrated for the Gulf of Suez/Red Sea, is a consequence of a transition from a dry to a more humid climate in a restricted or semirestricted basin. This transition occurred at about 7.5 Ma in the Gulf of Suez/Red Sea and at about 5.8 Ma in the Mediterranean. The Zeit Wet Phase manifest in the above events developed in association with the initiation and development of the Asian monsoon and the drying of the Mediterranean. It can be understood in terms of the development of an evolving monsoon/desert system. The Asian monsoon was initiated 8-7 Ma in association with the uplift of Tibet; at this time the North African desert zone was displaced northwards to be over the Mediterranean and central and eastern North Africa became seasonally humid. A concept is advanced in which the early stage of the development of the Asian monsoon is seen as having two phases alternating at the precessional ca 21 ka cycle. One phase transferred moisture from the Indian Ocean mainly to southern Asia, the other phase transferred moisture mainly to North Africa. With the drawdown of the Mediterranean at about 5.8 Ma the Zeit Wet Phase intensified. A further northward movement of the humid and desert zones occurred at the time of drawdown. With the Messinian and early Pliocene transgressions the wet phase ameliorated but a substantial river system still crossed central North Africa. At about 4.6 Ma North Africa became drier probably in response to the developing dominance of the features of the monsoon that transferred moisture mainly to southern Asia, features of the monsoon that are well recognised today.
Abstract: Information on the geographical distribution of plant invasion has been recorded in detail in some areas of the world; however, in large regions such as South America there are a few, if any, records of the spread of alien plants and invasive species and even less information about their effects on ecosystems at different levels of organization. This study examines the extent to which woody species introduced during the last centuries are invading the Rolling Pampa (which is typical of the entire region of the Argentina pampas) and discusses whether this invasion is related to the species' genetics or to environmental factors. All woody species were surveyed along landscape corridors (highways and intersecting secondary dirt roads and streams), as well as in farmed fields under three different tillage systems: zero tillage in the entire field for all crops in the rotation (where tillage was replaced by a presowing herbicide application), zero tillage for selected crops, and conventional tillage. Landscape corridors along the roads had been invaded by 40 woody species (mostly trees). On the farmed land, fields under the zero tillage farming system were invaded by seven woody species (three tree species and four shrubs). With zero tillage for select crops only, woody species richness was reduced to three (one tree and two shrub species). In the conventional tillage, there were only three invading species, all shrubs. In both the roadside and riparian corridors, the species with the highest constancy values were Gleditsia triacanthos L., Morus alba L., and Melia azedarach L. In both types of zero tillage farmed fields, M. alba was absent, but G. triacanthos and M. azedarach remained the species with the highest constancy values. Both genetic and ecological factors were important determinants for the invasion of the pampas by woody species. The woody invasion process has reached a point at which the pampean grasslands on the better-drained soils will no longer be restored to a grassland biome without human intervention.
Abstract: The pollen-climate and pollen-vegetation relation in Africa, Madagascar and Arabia has been investigated using 1170 modern pollen samples presently available in the African Pollen Database. Data are collected from different sedimentary environments (soil, lake, river, traps, midden, etc.). In spite of an irregular spatial distribution, there are representatives in all of the major vegetation and climate zones. Accounting for the different trapping efficiency of the different sediment types, large-scale patterns can be determined from the pollen assemblages. Maps and response-surfaces depicting the distribution in geographic and climate space of the main pollen types distinguish the major phytogeographic regions. Computing dissimilarities among the African pollen spectra using squared chord distance demonstrates that pollen spectra within a particular vegetation zone are more similar to one another that they are to pollen samples from other vegetation zones. Using the method of modern analogs, we find significant correlations between modern temperature and precipitation reconstructed using the best modern analog, which suggests the potential of this database for quantitative climate reconstructions. The study of individual pollen types provides an alternative method to the biomization to characterize and quantify the land-surface conditions at a continental scale.
Abstract: Comparative geomorphological and pedological studies on Late Holocene landscape development in West African dry Savanna environments reveal several phases of morphodynamic activity and stability resulting in typical relief forms and related sediments. Although significant similarities in the geomorphological development of the various research areas are found, fieldwork and laboratory analysis show small-scale variations in sedimentary structure and soil formation. This is not only due to climatic fluctuations, because sediment datings and archaeological findings prove an early human impact on dry Savanna ecosystem. Nowadays, most of the study areas are affected by severe degradation processes, attributed to increasing population growth. Interpretations of rainfall data refer to specific climatic factors of Savanna environments, like precipitation structure, which promotes landscape degradation. Actual geomorphodynamics in West African dry Savanna can therefore have both anthropogenic and climatic causes.
Abstract: The Waite-Nicolson rangeland management method for semi-arid chenopod shrublands predicts that smaller paddocks with medium to moderate stocking rates help to preserve the native vegetation. Vegetation cover around waterpoints in three small paddocks (<2000 ha) from Middleback Station, South Australia was studied using multivariate analysis. Data from quadrats sampled along radiating transects were tested for correlations with a number of site features and grazing history factors. Two significant associations were detected: quadrats with an abundance of Rhagodia parabolica and less palatable species such as Maireana pyramidata, and Atriplex stipitata were correlated positively with proximity to water points, paddock age and stocking rate, and negatively with paddock size. In contrast, quadrats with species such as Rhagodia ulicina and the more palatable M. sedifolia were correlated with increasing distance from the water points and paddock size, but negatively with age and stocking rates. Transect direction was not correlated with either group. Twelve of the 20 species examined, including the important forage species A. vesicaria, also were not correlated with those paddock and grazing features included here. These results suggest that the distribution of some chenopod shrub species in fenced paddocks is still possibly affected by a combination of these factors in the long term by grazing at densities of 6 ha sheep-1 and that the method, although maintaining the fodder species, may not be preserving biodiversity at these grazing levels, although further study is needed.
Abstract: The area occupied by vegetated patches in comparison with area of the inter-patch, is becoming an accepted and useful standard against which to assess ecosystem function in arid and semi-arid environments. Proportional areas of vegetated patches were found to be positively correlated with oxidizable C and total N in surface soil in low-woodland, but not in low-shrubland communities. Suggested indices of soil-surface condition and derived ratings of landscape function were positively related to soil fertility but less clearly with proportional areas of vegetated patches. These relationships were community-type specific. It is unclear whether the suggested approach provides more information on landscape function than simple measures of patch heterogeneity. Patch-structure of low-woodland, compared to low-shrubland communities, may be more resistant to disturbance, however once disturbed, low-woodland is less resilient in that nutrient losses are likely to be greater.
Abstract: The objective of this study was to investigate the relationship among species richness, functional group composition, and climate for three sites representing the shortgrass steppe, the Chihuahuan desert grasslands and their biome transition zone. We found that perennial species richness increased as the climate became more favorable for plant growth. The biome transition zone was more similar to the Chihuahuan desert grassland site in most climate and vegetation characteristics, partly because of the shorter biogeographic distance between the two sites. This study clarified the ecological position of the biome transition zone site with respect to the adjacent biomes.
Abstract: Over the past century, C3 woody plants and trees have increased in abundance in many semiarid ecosystems, displacing native C4 grasses. Livestock grazing, climatic fluctuations, and fire suppression are several reasons proposed for this shift. Soil carbon isotopic signatures are an ideal technique to evaluate carbon turnover rates in such ecosystems. On the gunnery ranges of Fort Huachuca in southeastern Arizona, study sites were established on homogeneous granitic alluvium to investigate the effects of fire frequency on [delta]13C values in surface soil organic matter (SOM). These ranges have had no livestock grazing for 50 years and a well-documented history of fires. Prosopis velutina Woot. (mesquite) trees have altered SOM [delta]13C pools by the concentration of plant nutrients and the addition of isotopically light litter. These soil carbon changes do not extend beyond canopy margins. Elevated total organic carbon (TOC), plant nutrient (N and P) concentrations, and depleted SOM [delta]13C values are associated with C3 Prosopis on an unburned plot, which enables recognition of former Prosopis-occupied sites on plots with recent fire histories. Elevated nutrient concentrations associated with former Prosopis are retained in SOM for many decades. Surface SOM [delta]13C values indicate the estimated minimum turnover time of C4-derived carbon beneath large mature Prosopis is about 100-300 years. In contrast, complete turnover of original C3 carbon to C4 carbon under grasslands is estimated to take a minimum of 150-500 years. Our study confirms that C4 grass cover has declined over the past 100 years, although isolated C3 trees or shrubs were not uncommon on the historic C4-dominated grasslands. We find evidence in surface soil layers for a modern C3 plant expansion reflected in the substantial shift of SOM [delta]13C values from C4 grasses to C3 shrublands.
Abstract: Chromolaena odorata (L.) King and Robinson is an alien invasive weed to most of the Old World tropical regions of the earth, including South Africa where it is morphologically distinct from most other C. odorata plants examined from both its native and invasive range. It is thought that these morphological differences are related to difficulties encountered in successful establishment of biological control agents on the South African population of C. odorata. It has been postulated that the source population of the South African population will harbour potential biocontrol agents that will be suited to successful establishment on the South African plants. Several morphological, cytological and isozyme studies have been attempted to identify the source population of the South African population, but these have failed to identify the origin of the South African population. In this dissertation two PCR-based methods were attempted, in an investigation into whether the morphological differences and difficulties in establishment of biocontrol agents have a genetic basis. The two techniques attempted were: Inter Simple Sequence Repeat (ISSR) amplification, and DNA sequencing. Results could not be obtained using the ISSR method, and the reason for this was not discovered despite extensive trials. The internal transcribed spacer region and the external transcribed spacer region sequences were obtained from five samples, and compared. It was found that the ETS region gave more phylogenetic signal at the intraspecific level than the ITS region. However, due to difficulties in amplification of the external transcribed spacer region, work here focussed on obtaining Internal Transcribed Spacer sequences for 61 samples. Each of the samples sequenced had a unique ITS sequence, displaying a high level of intraspecific genetic diversity. The degree of this diversity is discussed with reference to the possible influences of polyploidy and concerted evolution on genetic structure. The ITS data indicated that some of the physical traits used to define ‘morphotypes’ of C. odorata were not correlated to genotype. From discussion and comparison of morphological character distributions and the ITS-based phylogeography it is suggested that the geographical origin of the South African population is Greater Antilelan, rather than from the continents of North and South America, which is where the Australasian, West African and Mauritian infestations are suggested to have originated.
Abstract: Lantana camara (lantana), a thicket-forming shrub, a number of different varieties of which were introduced into South Africa as ornamental plants but which has become a serious invasive weed. Conventional control measures for lantana are expensive and ineffective and it has therefore been targeted for biological control since 1961. To date, eleven biological control agent species have become established on lantana in South Africa. However, most agents persist at low densities and only occasionally impact pla nt populations. Three species regularly cause significant damage, but only reach sufficiently high numbers by midsummer after populations crash during the winter. Overall, the impact of the biological control programme on the weed is negligible and this has been ascribed to the poor selection of agents for release, the accumulation of native parasitoids, differences in insect preference for different varieties of the weed and variable climatic conditions over the weed’s range. This study suggests that the importance of varietal preferences has been over-estimated. A predictive bioclimatic modelling technique showed that most of the agents established in South Africa have a wide climatic tolerance and that the redistribution and importation of new climatypes of these agents will not improve the level of control. Additional agents are required to improve the biocontrol in the temperate conditions, and also to increase damage in the sub-tropical areas where most of the agents are established and where the weed retains its leaves year round. New candidate agents that possess biological attributes that favour a high intrinsic rate of increase, a high impact per individual and that improve the synchrony between the weed and the agent in climatic conditions that promote the seasonal leaflessness of plants should receive prior consideration. A survey in Jamaica indicated that additional biological control agents are available in the region of origin but that care should be taken to prioritise the most effective agents. The various selection systems currently available in weed biocontrol produce contradictory results in the priority assigned to candidate agents and a new selection system is proposed. The biology and host range of two new candidate natural enemies, the leaf-galling weevil, Coelocephalapion camarae and the leaf-sucking mirid, Falconia intermedia were investigated for the biocontrol of lantana. The studies indicated that these have considerable biocontrol potential, in that the weevil has a wide climatic tolerance and has the potential to survive the host leaflessness typical of temperate conditions, while the mirid has a high intrinsic rate of increase, and the potential for several generations a year. Both agents caused a high level of damage to the leaves, with the weevil galling the vascular tissue in the leaf-petiole and the mirid causing chlorotic speckling of the leaves. During laboratory trials both agents accepted indigenous species in the genus Lippia. However, under multiple choice conditions these agents showed a significant and strong oviposition preference for lantana. A risk assessment and post release field trials indicated that F. intermedia is likely to attack some Lippia species in the presence of lantana, but the levels of damage are predicted to be relatively low. A possible low incidence of damage to indigenous species was considered a justifiable ‘trade-off’ for the potentially marked impact on L. camara. Preference and performance studies on the two candidate agents suggested that most of the South African lantana varieties are suitable host plants. The mirid preferred certain varieties in multiple choice experiments, but this is unlikely to affect its impact under field conditions. Permission for release was accordingly sought fo r both species. Finally, the challenges facing the biological control programme and the potentialfor improving the control of L. camara in South Africa are considered.
