Kalahari Transect Research Articles

Effects of precipitation and soil water potential on drought deciduous phenology in the Kalahari

AbstractWe utilized an ecosystem process model to investigate the influence of precipitation and soil water potential on vegetation phenology in the semi‐arid, drought‐deciduous ecosystems in the Kalahari region of South Africa. The timing of leaf flush was assumed to be the first day during which a rainfall event exceeded that day's estimate of potential evapotranspiration after a defined dry season. Leaf senescence was assumed to be a dynamic feedback between soil water potential and net plant carbon gain and was determined by dynamically modeling the effects of concomitant trends in soil water potential and net primary production on leaf area index (LAI). Model predictions of LAI were compared with satellite‐derived normalized difference vegetation indices (NDVI) for 3 years at two sites along the Kalahari transect. The mean absolute error for the prediction of modeled leaf flush date compared with leaf flush dates estimated from NDVI were 10.0 days for the Maun site and 39.3 days for the Tshane site. Correlations between model predicted 10‐day average LAI and 10‐day composite NDVI for both Maun and Tshane were high (ρ=0.67 and 0.74, respectively, P<0.001), suggesting that this method adequately predicts intra‐annual leaf area dynamics in these dry tropical ecosystems.

Inferred controls on tree/grass composition in a savanna ecosystem: Combining 16‐year normalized difference vegetation index data with a dynamic soil moisture model

A two‐layer soil moisture model was developed to infer the controls on the tree/grass coexistence in a savanna ecosystem. The model was applied along a mean annual precipitation gradient in southern Africa, known as the Kalahari Transect (KT), which follows a north‐south decline in mean annual rainfall from ∼1600 mm/yr to ∼250 mm/yr between the latitudes 12°–26°S. Satellite‐derived fractional covers for trees, grass, and bare soil were used as input to the model, with the fractional grass cover responsive to interannual variability in rainfall, as defined by observed statistics. Other inputs to the model included satellite‐based radiation budget measurements and interpolated, ground‐based rainfall measurements. The soil moisture model structure allows both trees and grass to have access to the upper zone, while trees alone can extract water from the lower zone. We focus our analysis on the wet season months of November–March, in which 87% of the annual rainfall is received along the KT. Simulations were performed on daily time steps for 16 years, representing a range of interannual rainfall variability, at 196 equally spaced positions along the transect. The results indicate that two distinct areas of the KT exist in terms of vegetation‐rainfall relationships. North of 18°S, the trees and grass are rarely water stressed during the wet season and a large portion of the water balance is accounted for by leakage through the bottom of the root zone in sandy soils. The vegetation cover in the northern end of the transect does not reach its potential in terms of water exploitation and is most likely nutrient limited. In the southern portion of the KT, tree fractional cover is such that trees become water stressed in drier than average wet seasons and suffer no water stress during wetter than average wet seasons. The extent of the fractional grass cover for individual years is controlled by the water demand stress. We find that observed tree/grass fractional cover in the water‐limited region of the transect is best explained by considering, on an individual basis, an allowable range of stress for trees and grass, while minimizing the amount of water that is unexploited by the vegetation and lost as leakage from the root zone.

Prospects for subsistence livelihood and environmental sustainability along the Kalahari Transect: The case of Matsheng in Botswana's Kalahari rangelands

Botswana's Kalahari rangelands have historically supported a diversity of wildlife and a number of subsistence livelihood activities, such as agropastoralism, hunting and gathering. Recently, however, concerns about environmental and livelihood sustainability in the Kalahari have been raised. This paper demonstrates the validity of these concerns using data from research undertaken in the Matsheng area of southwest Botswana. It is illustrated that: (a) Matsheng soils are too infertile to sustain productive arable farming, (b) herbaceous vegetation cover decreases in a reversed decay function towards settlements, (c) the herbaceous vegetation cover trend is inversely reflected by woody-thorny vegetation, (d) despite being a major rangeland activity, livestock production benefits only a minority of inhabitants, and (e) rangelands are no longer the major source of livelihood for the majority.

Tree spacing along the Kalahari transect in southern Africa

Spatial pattern in tree distribution is examined at 10 sites along the IGBP Kalahari Transect in southern Africa in order to examine the patterns of community structure across a large average annual rainfall gradient. Analysis indicates aggregation among all individuals in the vegetation communities at most sites, with no occurrence of aggregation in individuals at the most southern arid site. The spatial distribution for the largest 25% of individuals is predominantly random. Uniform pattern is only observed in the distribution of canopy dominants at one site. A comparison between expected and observed densities of understory vegetation beneath large trees shows significant patterns at six of ten sites, with lower than expected values at both the northern and southernmost sites in the transect, and significantly higher than expected values at intermediate sites. Variation in the spatial distribution of understory individuals with respect to larger canopy individuals suggests that distribution of suitable regeneration sites and subsequent patterns of establishment may be critical phenomena in determining the spatial pattern of vegetation.

Vegetation cover trends along the Botswana Kalahari transect

Vegetative response aspects of climate change studies include the determination of vegetation cover changes across climatic gradients. Vegetation characteristics and soil moisture measurements were obtained from four locations with decreasing rainfall along the Botswana Kalahari transect. These are referred to as Pandamatenga, (698 mm year −1), Maun (460 mm year −1), Okwa (407 mm year −1) and Tshane (365 mm year −1). Trends in major vegetative cover and soil components included species types and richness assessments which reflected certain changes southward but also showed interesting degrees of variability. This occurred despite the apparent homogeneity of the Kalahari sands and predominantly semi-arid savanna shrub–woodland vegetation cover. Despite linear decreases, both in rainfall and soil moisture content, results indicated high soil moisture variability at the Okwa location which relates to unique climatic and geological factors. Also many species are unique to specific locations for instance Pandametanga is characterized by Zambesian species while the Maun location is characterized by Colophosphermum mopane. This changes southwards as Acacias become more dominant and significantly co-occur with Grewia flava. While the average total numbers of plants decreased, total numbers of different species varied little from the wetter to the drier end of the moisture gradient. The association between rainfall and woody cover is negative implying that canopy cover extents (in terms of vegetation density) increase along the gradient due to increases in bush encroachment in drier areas. Therefore, while representing a continuum in terms of species numbers, in other respects (e.g. species types and ground cover components) vegetation zones in the Kalahari may be regarded as discontinuous units. Differences between mapped vegetation zones may be accounted for in terms of relatively unique ecosystem factors which function partly in response to geological conditions and partly in response to local (as opposed to regional) climatic factors. This appears to have led to ecosystem adaptive measures (in terms of species responses) rarely mirrored elsewhere along the gradient. Hence adaptability in terms of species migrations in response to relatively rapid climate changes may not readily take place over the Kalahari in Botswana.

Soil moisture and plant stress dynamics along the Kalahari precipitation gradient

We present an analysis of water balance and plant water stress along the Kalahari precipitation gradient using a probabilistic model of soil moisture. The rainfall statistical characteristics, obtained from daily data of four stations along the transect, show that the rainfall gradient (from 950 to 300 mm/year) is mostly due to a decrease in the mean rate of storm arrivals (from 0.38 to 0.09 1/day) rather than to a change in the mean storm depth (practically constant at 10 mm). Using this information and typical vegetation and soil parameters, the analysis relates the vegetation properties along the transect with those of climate and soil. It is shown that differences in water balance and plant water stress between trees and grasses generate varying preferences for vegetation types along the transect, with deeper‐rooted trees favored in the more mesic regions of the northern Kalahari and grasses favored in the drier zones of the southern Kalahari. The point of equal plant water stress is found at about 420 mm of rainfall during the growing season (October–April), indicating the possibility of tree‐grass coexistence in the central sector of the Kalahari. These findings are consistent with patterns of vegetation distribution across the Kalahari transect.

Early spatial and temporal validation of MODIS LAI product in the Southern Africa Kalahari

We evaluate the operational MODIS Leaf Area Index (LAI) product using field-sampled data collected at five sites in southern Africa in March 2000. One site (Mongu, Zambia) was sampled monthly throughout the year. All sites were along the International Geosphere Biosphere Programme's (IGBP) Kalahari Transect, which features progressively lower annual precipitation, and hence, lower vegetation productivity, from north to south. The soils are consistently sandy. At each site, we sampled the vegetation overstory along three 750-m transects using the Tracing Radiation and Architecture in Canopies (TRAC) instrument. The resulting plant area index values were adjusted with ancillary stem area data to estimate LAI. Despite some instrument characterization and production issues in the first year of MODIS operations, our results suggest the first-year MODIS LAI algorithm correctly accommodates structural and phenological variability in semiarid woodlands and savannas, and is accurate to within the uncertainty of the validation approach used here. Limitations of this study and its conclusions are also discussed.

Determining land surface fractional cover from NDVI and rainfall time series for a savanna ecosystem

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 ( R 2=.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.

Analysis of soil organic carbon and vegetation cover trends along the Botswana Kalahari Transect

Determination of trends in soil organic carbon (SOC) and vegetation cover along savanna ecosystem moisture gradients is critical to the understanding of ecosystem functioning and global change. Field results from 57 sites along the Botswana Kalahari Transect (BKT) showed general increases in both SOC and vegetation cover components along the temperature/moisture gradient. However, details in both SOC and woody cover trends revealed an area of relatively declining values in the central Kalahari. Image classification using TM single band data as input values confirmed the existence of an anomalously low woody cover area in the north-central Kalahari, within an area mapped as northern Kalahari tree and shrub savanna. It is postulated that the occurrence of such a zone of low shrubs in a gradient of otherwise increasing tree height and structural complexity may result mainly from edaphic factors, because of the co-incidence of near surface calcrete. However, the effects of fire and heavy grazing also influence the extent of low above-ground biomass, at least locally. Attempts were made to examine the extent to which soil organic carbon and vegetation components can be predicted using Thematic Mapper (TM) imagery, to increase potentially the number of sites in the BKT database. As a result of correlation and regression analyses, it was determined that the imagery-based SOC indexes of the literature were not useful in the Kalahari environment because of the extremely low SOC values encountered. Results of multiple regression analysis confirmed that woody vegetation cover could be predicted mainly through the use of TM3 single band radiance values throughout the BKT even during a non-drought period. Woody vegetation cover could be predicted to a lesser extent by the Transformed Normalised Vegetation Index. As high correlation coefficients were found between woody vegetation cover and SOC, the density of woody cover as derived from TM3 data may be taken as a surrogate for relative SOC abundance. Because this method is indirect, field checking will be required to verify results.

THE CLIMATE OF THE KALAHARI TRANSECT

SUMMARY The climate of the Kalahari Transect is defined by the nature of the general circulation of the atmosphere over Africa south of the equator and the regional circulation variations that determine local weather. Individual synoptic systems and the transport of moisture into the region by converging and diverging airstreams in both summer and winter govern conditions favouring or inhibiting the major rainfall-producing process of convective precipitation. The climate is illustrated by the spatial variation of moisture, rainfall, temperature, evaporation and other fields. It is shown that the climate of the transect varies from the subtropical in the south to the tropical in the north with characteristic gradients of most parameters between the two regions in both summer and winter. The Kalahari Transect is a multi-disciplinary regional project having as its aim the advancement of the understanding of the effects of human and climatically induced global change over a large area of southern Africa. It has a near-unique advantage of being a study where the underlying regional geology, comprising Kalahari sand, is uniform over a considerable range of latitude. This offers the prospect of reducing the uncertainties associated with regional manifestations of global change. Climate and climatic change remain major determinants of all facets of the Kalahari Transect study. In this paper, the nature of the regional climate of the transect and how it may change in the future are discussed. The Kalahari Transect has a latitudinal extent from 14°S to 28°S and a longitudinal equivalent from 21°E to 28°E. The delimited area covers much of central southern Africa, ranging from shrubland and grassy desert in the south to evergreen dense woodland in the north. The whole region is dominated by the southern hemisphere descending limb of the Hadley cell of the general atmospheric circulation, the southern part of the transect more so than the northern part. This means that anticyclonic circulation patterns are the dominant circulation type for most of the year. The attendant atmospheric conditions are characterised by subsiding, stable air masses, hazy lower tropospheric conditions and generally cloud-free skies. South of the Tropic of Capricorn disturbances in the mid-latitude westerlies assume an increasingly important role in determining rainfall, whereas to the north seasonal migrations of the Inter-Tropical Convergence Zone (ITCZ) become the major determinant of the annual rainfall cycle. Over the whole region cumulus convective activity, enhanced by atmospheric disturbances associated with negative pressure departures and suppressed by those with positive departures, is the major determinant of rainfall, which everywhere exhibits a strong seasonal cycle with a summer maximum. In general, it is difficult to acquire a dense coverage of continuous, long-period, modern meteorological records for Africa south of the Sahara. The same is true of the region covered by the Kalahari transect. Good general descriptions of the climate of the region are provided in the atlases of Jackson (1961) and Thompson (1965). The reviews of Torrance (1972), Schulze (1972, 1984). Hastenrath (1984) and Tyson (1986) provide additional and more up-to-date material. These sources and others have been used to compile a climatology of the Kalahari transect region. After a brief summary of the regional atmospheric circulation, the main parameters to be considered are rainfall, cloud cover, energy budget components, temperature, atmospheric moisture, evaporation and potential evapotranspiration. The summary of the transect climate concludes with estimates of possible future regional conditions that may follow from global greenhouse warming.