Kalahari Transect Research Articles

An Assessment of Multiple Drivers Determining Woody Species Composition and Structure: A Case Study from the Kalahari, Botswana

Savannas are extremely important socio-economic landscapes, with pastoralist societies relying on these ecosystems to sustain their livelihoods and economy. Globally, there is an increase of woody vegetation in these ecosystems, degrading the potential of these multi-functional landscapes to sustain societies and wildlife. Several mechanisms have been invoked to explain the processes responsible for woody vegetation composition; however, these are often investigated separately at scales not best suited to land-managers, thereby impeding the evaluation of their relative importance. We ran six transects at 15 sites along the Kalahari transect, collecting data on species identity, diversity, and abundance. We used Poisson and Tobit regression models to investigate the relationship among woody vegetation, precipitation, grazing, borehole density, and fire. We identified 44 species across 78 transects, with the highest species richness and abundance occurring at Kuke (middle of the rainfall gradient). Precipitation was the most important environmental variable across all species and various morphological groups, while increased borehole density and livestock resulted in lower bipinnate species abundance, contradicting the consensus that these managed features increase the presence of such species. Rotating cattle between boreholes subsequently reduces the impact of trampling and grazing on the soil and maintains and/or reduces woody vegetation abundance.

Analysis of temperature data over semi-arid Botswana: trends and break points

Climate change is a global challenge which impacts negatively on sustainable rural livelihoods, public health and economic development, more especially for communities in Southern Africa. Assessment of indices that signify climate change can inform formulation of relevant adaptation strategies and policies for the communities. Diurnal temperature range (DTR) is acknowledged as an expedient measure of the scourge as it is sensitive to variations in radiative energy balance. In this study, a long-term (1961–2010) daily temperature data obtained from nine (9) synoptic stations in Botswana were analyzed for monotonic trends and epochal changes in annual maximum (T max), minimum (T min) temperatures and DTR time series. Most of the considered stations were along the Kalahari Transect, a region which is at high risk of extensive environmental change due to climate change. Mann–Kendall trend and Lepage tests were applied for trend and change point analysis, respectively. The statistical analysis shows that stations in the southern part of the country experienced significant negative trends in annual DTR at the rate of −0.09 to −0.30 °C per decade due to steeper warming rates in annual T min than annual T max trends. On the contrary, stations in the northern part of the country experienced positive trends in annual DTR brought about by either a decreasing annual T min trend which outstripped annual T max or annual T max which outpaced annual T min. The increasing trends in DTR varied from 0.25 to 0.67 °C per decade. For most of the stations, the most significant annual DTR trends change point was in 1982 which coincided with the reversal of atmospheric circulation patterns.

The Effects of Interannual Rainfall Variability on Tree–Grass Composition Along Kalahari Rainfall Gradient

Precipitation variability has been predicted to increase in a global warmer climate, and is expected to greatly affect plant growth, interspecies interactions, plant community composition, and other ecosystem processes. Although previous studies have investigated the effect of intra-annual rainfall variability on plant growth and ecosystem dynamics, the impacts of interannual rainfall variability remain understudied. This paper uses satellite data and develops a new mechanistic model to investigate the response of tree–grass composition to increasing interannual rainfall variability in arid to sub-humid ecosystems along the Kalahari Transect in Southern Africa. Both satellite data and model results show that increasing interannual rainfall fluctuations favor deep-rooted trees over shallow-rooted grasses in drier environments (that is, mean annual rainfall, MAP 900–1000 mm). Trees have a competitive advantage over grasses in dry environments because their generally deeper root systems allow them to have exclusive access to the increased deep soil water resources expected to occur in wet years as a result of the stronger interannual rainfall fluctuations. In relatively wet environments, grasses are favored because of their high growth rate that allows them to take advantage of the window of opportunity existing in years with above average precipitation and thus increase fire-induced tree mortality. Thus, under increasing interannual rainfall fluctuations both direct effects on soil water availability and indirect effects mediated by tree–grass interactions and fire dynamics are expected to play an important role in determining changes in plant community composition.

A quantitative description of the interspecies diversity of belowground structure in savanna woody plants

The relative importance of resource competition and disturbance in limiting woody cover is one of the most basic questions in savanna ecology. Modeling approaches that seek to address this question are limited in their ability to accurately represent resource competition, which occurs belowground, by the limited detail of existing data on root system structure. Using compressed air, we excavated individual trees and shrubs and mapped their coarse roots on a three‐dimensional grid system up to 1.5 m depth. We excavated four woody savanna species at three sites spanning a climate gradient on the Kalahari Transect in southern Africa and determined functions to describe the distribution of root biomass within the root systems. Overall, most of the variation in both large‐scale and small‐scale characteristics of root system structure was related to species. The species excavated fell into two groups that coexisted across the climate gradient. Acacia mellifera and Terminalia sericea had straight roots in a laterally extensive system that was shallow relative to the aggregate root profile for woody plants at the sites. Boscia albitrunca and Ochna pulchra had sinuous roots that were mostly concentrated beneath the canopy and were more prevalent in deep than near‐surface soil layers, departing from the conventional model of decreasing root abundance with depth. Based on our results, we suggest a reframing of the theoretical characterization of the root zone used in modeling approaches to consider diversity within the woody plant community.

Point rainfall statistics for ecohydrological analyses derived from satellite integrated rainfall measurements

AbstractSatellite rainfall measurements, nowadays commonly available, provide valuable information about the spatial structure of rainfall. In areas with low‐density rain gage networks, or where these networks are nonexistent, satellite rainfall measurements can also provide useful estimates to be used as virtual rain gages. However, satellite and rain gage measurements are statistically different in nature and cannot be directly compared to one another. In the present paper, we develop a methodology to downscale satellite rainfall measurements to generate rain‐gage‐equivalent statistics. We apply the methodology to four locations along a strong rainfall gradient in the Kalahari transect, southern Africa, to validate the methodology. We show that the method allows the estimation of point rainfall statistics where only satellite measurements exist. Point rainfall statistics are key descriptors for ecohydrologic studies linking the response of vegetation to rainfall dynamics.

Rainfall regime changes and trends in Botswana Kalahari Transect’s late summer precipitation

Sixty-year-long January–March (JFM) rainfall data from four (4) stations along the Kalahari Transect were analyzed for long-term trends and abrupt changes in rainfall regimes. On average, JFM rainfall accounts for more than 50 % of annual rainfall in the region. Mann–Kendall trend test has shown an insignificant heterogeneous trend in the seasonal rainfall of −1.097, 0.029, −0.407, and −1.327 mm/year for Maun, Ghanzi, Tsabong, and Tshane, respectively. An abrupt change in rainfall regimes in these areas was investigated and was found to occur in the year 1982 for all stations. The change is related to large-scale atmospheric circulations. Analysis of large-scale atmospheric circulations before 1982 over the region has shown the formation of a tropical low pressure convective system over the Kalahari Transect. The tropical system shifted eastward after 1982 to be centered over southeastern southern Africa with a significant reduction in rainfall over the Kalahari. A direct impact of this is the livestock-induced overgrazing which has lead to excessive removal of palatable herbaceous species thereby giving woody species, such as Accacia mellifera and Grewia flava, a competitive edge for dominance in the ecosystems. Seed production of A. mellifera depends on rainfall; therefore, abrupt changes in rainfall regimes impact livelihood and eco-tourism industry. The correlation studies between rainfall anomalies and NINO3.4 indices show a reduction in the influence of El-Nino Southern Oscillation on the Kalahari Transect rainfall after 1982.

Soil respiration at five sites along the Kalahari Transect: Effects of temperature, precipitation pulses and biological soil crust cover

There are increasing concerns that climatic and land use changes will enhance soil respiration rates and soil organic carbon loss, compromising agricultural productivity and further elevating atmospheric CO 2. Current understanding of dryland respiration is, however, insufficient to enable prediction of the consequences of these changes for dryland soils and CO 2 fluxes. The objectives of this paper are to present in- situ respiration data from five remote sites along a climatic gradient in the Kalahari of Botswana and to determine the effects of temperature, moisture and biological crust cover on soil CO 2 fluxes. Moisture was the primary limiting factor to efflux which increased with amount of simulated rainfall. On dry soils, mean CO 2 efflux was between 1.5 and 5.9 mg C m − 2 h − 1 . After 2 mm and 50 mm simulated wetting, mean rates increased to 4.0 to 21.8 and 8.6 to 41.5 mg C m − 2 h − 1 respectively. Once wet, soil CO 2 efflux increases with temperature, and sites at the hotter northern end of the transect lost more CO 2 than cooler southerly sites. Net respiration rates are, however, muted by autotrophic organisms in biological soil crusts which photosynthesise and take up CO 2. The temperature sensitivity of soil CO 2 efflux increased with moisture. Dry, 2 mm and 50 mm treated soils had a Q 10 of 1.1, 1.5 and 1.95 respectively. Our findings indicate that higher temperatures and a loss of biological crust cover will lead to greater soil C loss through respiration.

Nutrient limitations on aboveground grass production in four savanna types along the Kalahari Transect

The Kalahari Transect (KT) is an International Geosphere–Biosphere Programme mega-transect designed to examine hydrological and ecological patterns and processes throughout the savannas of southern Africa. The KT traverses a precipitation gradient ranging from ∼920 mm rain/year in the north to ∼260 mm rain/year in the south. Previous research shows a positive correlation between canopy cover and precipitation suggesting a water limitation on productivity. However, there has been minimal research on other possible sources of limitations, such as soil Nitrogen (N) and/or Phosphorus (P). We used a factorial in-situ experimental design to test for increased aboveground grass production (measured as peak season standing stock) under elevated soil P and P + N levels. Four sites along the KT precipitation gradient were used in this study: Mongu (Zambia), Pandamatenga (Botswana), Ghanzi (Botswana) and Tshane (Botswana). Soils at each site were amended with N and P fertilizers during the dry season. We extracted soil samples during the following growing season to analyze for plant available soil P. Vegetation samples were harvested from which we measured foliar P and aboveground grass biomass production. We saw differences in foliar P at the treatment and site level but not for the interaction between treatment and site. There were individual effects from site on biomass but not for any interactions with nutrient treatments. Despite higher levels of foliar P, we did not detect an increase in aboveground biomass. This may be explained by luxury uptake or allocation to belowground resources.

Combined effects of soil moisture and nitrogen availability variations on grass productivity in African savannas

Savannas cover about 20% of the Earth’s land area and 50% of Africa. As an indispensable component of savanna, grasses play an important role in these ecosystems. A better understanding of grass productivity and its controlling factors in savanna ecosystems could therefore be a key to understand the functioning of savannas and predict savanna responses to future climatic changes. In this study, a stable isotope fertilization experiment was conducted to determine how factors limiting grass production in savannas differ across regional climate gradients. The study was conducted on the geomorphically homogenous Kalahari Transect (KT), which offers an ideal setting to study nutrient and vegetation dynamics independently of confounding soil effects. The results show that the grasses assimilated the added fertilizer at all the sites but they did not respond to nitrogen fertilization for both dry and wet years, and at both dry and wet ends of the Transect. Although prior studies have proposed a switch between water and nitrogen limitations between arid and mesic savannas, our results suggest that nitrogen availability may not limit grass productivity across the whole KT. Thus, although the traditional classifications as nutrient poor (broad-leaf) and nutrient rich (fine-leaf) savanna ecosystems may still be useful, it does not necessarily imply the existence of nitrogen limitation in the nutrient poor area; in fact, it is more likely that the herbaceous species found in the more humid sites (nutrient poor sites) are already adapted to lower nitrogen availability.

Quantification of leaf gas exchange characteristics of dominant C 3/C 4 plants at the Kalahari transect

Leaf gas exchange characteristics play a key role in carbon sequestration and water balance from site to regional scales, yet show differences among plant species with alternative adaptations to the environment. To investigate the influences of climate regimes on leaf gas exchange characteristics and the underlying mechanisms and adaptations, we analyzed the photosynthesis and stomatal conductance characteristics of dominant C 3 and C 4 species at wet and dry ends of Kalahari transect in southern Africa by fitting empirical and mechanistic models to field measurements of leaf gas exchange rates. Adaptations to different climatic moisture regimes greatly influenced the leaf gas exchange characteristics of the species with different photosynthetic pathways along the Kalahari transect via a variety of mechanisms. We found that the leaf of C 3 species with higher leaf nitrogen concentration at the dry site had higher photosynthesis rates than those at the wet sites. Our analysis also indicated that Stipagrostis uniplumis had high sensitivities to both radiation and vapor pressure deficit ( VPD), indicating that the species may have soft guard cell structure to conserve water. We also found that the relatively high soil-to-leaf conductance allowing efficient water supply from soil to plant leaves might contribute to the small sensitivities of stomatal conductance to VPD for the grasses of S. ciliata and S. obtusa. The results of leaf gas exchange characteristics and underlying mechanisms provide basic but crucial parameters for simulation studies of carbon sequestration and water balance at site to regional scales in the Kalahari region.

Soil carbon and nitrogen dynamics in southern African savannas: the effect of vegetation-induced patch-scale heterogeneities and large scale rainfall gradients

Savanna ecosystems are mixed plant communities in which trees and grasses co-exist thereby providing a heterogeneous landscape with a mosaic of tree-dominated and grass-dominated soil patches. Despite the important role that nutrient availability plays in these systems, detailed knowledge of differences in carbon and nitrogen cycling in soil patches predominantly covered by tree canopies or by grasses is still lacking. In this study, a process-based model was used to investigate the carbon and nitrogen dynamics in soil plots located in grass-dominated and tree/shrub-dominated soil patches along the Kalahari Transect (KT). The KT in southern Africa traverses a dramatic aridity gradient, across relatively homogenous soils, providing an ideal setting for global change studies. Here we show that there are distinctly different dynamics for soil moisture, decomposition and nitrogen mineralization between soil plots located under tree canopies and in open canopy areas, especially at the dryer end of the KT. Such differences diminished when approaching the wetter end of this transect. This study shows that in savanna ecosystems, water availability determines the patterns and rates of nutrient cycling at large scales, while at the local scales, vegetation patchiness plays an important role in nutrient cycling. Savannas are relatively stable ecosystems, resilient to small rainfall modifications, although irreversible changes may occur with stronger shifts in climate conditions.

Spatial heterogeneity and sources of soil carbon in southern African savannas

Knowledge of the southern Africa soil carbon pool, its heterogeneity, sources (from trees or grasses), and potential response to climate is extremely limited. In this study the Kalahari Transect (KT) was used as a representative savanna ecosystem to quantitatively evaluate the spatial heterogeneity of the soil carbon pool and its contributing sources. The KT encompasses a dramatic aridity gradient on relatively homogenous soils. Two sites were chosen along the KT, representing dry and wet conditions. In February–March 2005, soil samples were collected at each site along a 300 m transect. Stable carbon isotope (δ 13C) and organic carbon content (%C) of the soils were utilized in the assessment in conjunction with geostatistical analysis of the spatial patterns of soil δ 13C and %C. At the dry savanna site, well-defined patterns in both δ 13C and %C were observed that were related to the distribution of woody vegetation. At the wet savanna site, the spatial patterns of δ 13C and %C were somewhat less pronounced, but still were impacted by the distribution of woody vegetation. The relative contributions from C 3 and C 4 vegetation to the soil carbon pool at the wet site were independent of tree locations, but dependent on woody plant locations at the dry site. At the dry site, ~ 40% of the soil carbon was derived from C 3 vegetation, whereas at the wet site ~ 90% of the soil carbon originated from C 3 vegetation. These results represent an important step in understanding the impact of regional climate change (e.g., rainfall variations) on carbon sequestration in southern Africa by providing quantitative information on soil carbon spatial distributions and sources under different climatic conditions (e.g., different rainfall regimes).

Isotope composition and anion chemistry of soil profiles along the Kalahari Transect

Savannas cover about 20% of the Earth's land area across a wide range of climatic conditions. As an important and distinct biome, savannas produce approximately 29% of global terrestrial net primary productivity. In these ecosystems the distribution of belowground resources remains poorly investigated and the relationship to the climatic conditions remains unclear. In the present study, vertical profiles of soil nutrients (chloride, nitrate, phosphate and sulfate) and nitrogen stable isotopes were analyzed at four sites along the Kalahari mega-transect, where a distinct rainfall gradient exists on a homogeneous soil substrate. The results show clear differences in nutrients and δ 15N vertical distributions between wet and dry seasons. The results also show how the formation of “fertility islands” (i.e., the concentration of soil nutrients in the soils beneath tree canopies) is not necessarily coupled with belowground processes in that the distribution of soil nutrients at the surface does not match belowground patterns. The results also indicate that phosphorus may be a limiting nutrient in these savanna ecosystems with seasonal dynamics in its cycling.

Carbon and nitrogen parasitism by a xylem‐tapping mistletoe (Tapinanthus oleifolius) along the Kalahari Transect: a stable isotope study

AbstractThe present study explores the xylem‐tapping parasitism by mistletoe (Tapinanthus oleifolius) on native tree species along the Kalahari Transect (KT) using the stable isotopes of carbon and nitrogen. Mistletoe‐host pairs were collected at three geographical locations along the KT rainfall gradient in the 2005 and 2006 wet seasons. Foliar total carbon, total nitrogen and their stable isotope compositions (δ13C and δ15N) were measured. Heterotrophy (H) was calculated using foliar δ13C values of mistletoes and their hosts as an indicator of proportion of carbon in the mistletoes derived from host photosynthate. Based on the mistletoe H‐value and relationship between the mistletoe foliar δ15N and their host foliar δ15N, the results showed that mistletoes along the KT derived both nitrogen and carbon from their hosts. Mistletoes may regulate water use in relation to nitrogen supply. The proportion of carbon in the mistletoes derived from host photosynthate was between 35% and 78%, and the degree of heterotrophy was species‐specific with only limited annual variation. The study emphasizes the importance of incorporating parasitic associations in future studies on studying carbon, water and nutrient cycling along the Kalahari.

Spatial patterns of soil nutrients in two southern African savannas

Savannas are extensive, seemingly stable mixtures of grasses and trees that are thought to result from a combination of facilitation, competition, and disturbance. Here, we study the spatial distribution in surface soils of soil organic matter and macronutrients at two savanna sites along the Kalahari Transect in southern Africa: a dry site with approximately 350 mm of precipitation annually and a wet site with approximately 950 mm of precipitation annually. The concentrations of total organic carbon, total nitrogen (TN), chloride, sulfate, phosphate, and (nitrate + nitrite) were analyzed in surface soils from these sites using a variety of traditional and geostatistical analyses. Soil nutrient content is generally higher at the wet site (e.g., 0.039% TN versus 0.023% TN, 0.67 μg/g PO4−3 versus 0.30 μg/g PO4−3, and 3.44 μg/g (NO3− + NO2−) versus 1.79 μg/g (NO3− + NO2−)), though there is considerable variation among sample populations from different vegetation covers. Our results indicate that the dry site is also more spatially heterogeneous than the wet site. At the dry site, where the range of spatial autocorrelation for TN, SO4−2, PO4−3, and (NO3− + NO2−) is 8.5 m, 8.5 m, 8.7 m, and 7.3 m, respectively, the spacing of all woody vegetation (both shrubs (woody plants ≤ 2 m) and trees (woody plants > 2 m)) contributes to the patterning of soil resources. At the wet site, where the range of spatial autocorrelation for TN, SO4−2, and PO4−3 is 4.6 m, 6.1 m, and 10.2 m, respectively, only trees contribute to the patterning of soil resources, though (NO3− + NO2−) displays a purely random distribution. We also find weak evidence that nitrogen may not be the, or the only, limiting nutrient in these ecosystems.

Dynamic response of grass cover to rainfall variability: implications for the function and persistence of savanna ecosystems

Savanna grass cover is dynamic and its annual extent resonates with wet season rainfall, as shown by satellite observations of normalized difference vegetation index (NDVI) time series for the Kalahari Transect (KT) in southern Africa. We explore the hydrological significance of the dynamic grass cover by applying a soil moisture model to the water-limited portion of the KT, which spans a north-south gradient in mean wet season rainfall, r ¯ , from approximately 700 to 300 mm. Satellite-derived tree fractional cover, x t, is shown to be highly correlated with ground meteorological measurements of r ¯ ( R 2 = 0.94 ) in this region. By implementing a simple expression for grass growth and decay in the model that factored in only x t and near-surface soil moisture, we were able to effectively reproduce the satellite-derived fractional grass cover, x g, along the transect over a 16-year period (1983–1998). We compared the results from dynamic grass model with those yielded by a static grass cover model in which x g was set to its 16-year average for each simulation. The dynamic quality of the grass was found to be important for reducing tree stress during dry years and for reducing the amount of water that is lost from the overall root zone during the wet years, relative to the static grass case. We find that the dynamic grass cover acts as a buffer against variability in wet season precipitation, and in doing so helps to maximize ecosystem water use. The model results indicate that mixed tree/grass savanna ecosystems are ideally suited to reach a dynamic equilibrium with respect to the use of a fluctuating limiting resource (water) by having functional components that respond to variability in rainfall over long timescales (trees) and short timescales (grasses).

The SAFARI 2000 – Kalahari Transect Wet Season Campaign of year 2000

AbstractOver 50 scientists from eight different countries coordinated research efforts in the Kalahari sand mass in Zambia and Botswana during the 2000 wet season as a part of the Southern African Regional Science Initiative – Kalahari Transect Wet Season Campaign (S2K‐KT Wet Season Campaign). The work focused on change in ecological processes along the International Geosphere–Biosphere Programme‐designated Kalahari Transect (KT). Topics included ecosystem structure, function, biogeochemistry, and modeling at the patch, landscape and regional scale. The KT of southern and central Africa follows a sharp precipitation gradient within an otherwise climatically and geographically similar region that contains a widely distributed, physically uniform soil and relatively little variation in elevation. This paper outlines the focus of the SAFARI 2000 research campaign as it relates to this study area and provides references to archived data sets generated during the study. It also describes vegetation patterns, climate, and 2000 wet season meteorological conditions for the region.

Vegetation structure characteristics and relationships of Kalahari woodlands and savannas

AbstractThe Kalahari Transect is one of several International Geosphere–Biosphere Programme (IGBP) transects designed to address global change questions at the regional scale, in particular by exploiting natural parameter gradients (Koch et al., 1995). In March 2000, we collected near‐synoptic vegetation structural data at five sites spanning the Kalahari's large precipitation gradient (about 300–1000 mm yr−1) from southern Botswana (∼24°S) to Zambia (∼15°S). All sites were within the expansive Kalahari sandsheet. Common parameters, including plant area index (PAI), leaf area index (LAI) and canopy cover (CC), were measured or derived using several indirect instruments and at multiple spatial scales. Results show that CC and PAI increase with increasing mean annual precipitation. Canopy clumping, defined by the deviation of the gap size distribution from that of randomly distributed foliage, was fairly constant along the gradient. We provide empirical relationships relating these parameters to each other and to precipitation. These results, combined with those in companion Kalahari Transect studies, provide a unique and coherent test bed for ecological modeling. The data may be used to parameterize process models, as well as test internally predicted parameters and their variability in response to well‐characterized climatological differences.

Simulating vegetation processes along the Kalahari transect

AbstractThe Sheffield Dynamic Global Vegetation Model has simulated the structure and net carbon exchange of vegetation at five sites along the Kalahari transect where there is a strong gradient in precipitation from 299 to 918 mm yr−1. There has been a decline in precipitation of 8 mm yr−1 along the whole of the transect since about 1970. Simulations of vegetation dynamics and structure indicate that this decline has exerted a notable effect on the vegetation, with reductions in woody plant cover at the dry end of the transect and reductions in tree density at the wetter end. These changes were driven primarily by reductions in the net primary production and increased rates of mortality, with rather small impacts of fire.

Canopy scale measurements of CO2 and water vapor exchange along a precipitation gradient in southern Africa

AbstractShort‐term measurements of carbon dioxide, water, and energy fluxes were collected at four locations along a mean annual precipitation gradient in southern Africa during the wet (growing) season with the purpose of determining how the observed vegetation–atmosphere exchange properties are functionally related to the long‐term climatic conditions. This research was conducted along the Kalahari Transect (KT), one in the global set of International Geosphere‐Biosphere Program transects, which covers a north–south aridity gradient, all on a homogenous sand formation. Eddy covariance instruments were deployed on a permanent tower in Mongu, Zambia (879 mm of rainfall per year), as well as on a portable tower in Maun (460 mm yr−1), Okwa River Crossing (407 mm yr−1), and Tshane (365 mm yr−1), Botswana for several days at each site. The relationships between CO2 flux, Fc, and photosynthetically active radiation were described well by a hyperbolic fit to the data at all locations except for Mongu, the wettest site. Here, there appeared to be an air temperature effect on Fc. While daytime values of Fc routinely approached or exceeded −20 μmol m−2 s−1 at Mongu, the magnitude of Fc remained less than −10 μmol m−2 s−1 when the air temperature was above 27°C. Canopy resistances to water vapor transfer, rc, displayed an overall decline from the wetter sites to the more arid sites, but the differences in rc could be almost exclusively accounted for by the decrease in leaf area index (LAI) from north to south along the KT. Ecosystem water use efficiency (WUE), defined as the ratio of net carbon flux to evapotranspiration, showed a general decrease with increasing vapor pressure deficit, D, for all of the sites. The magnitudes of WUE at a given D, however, were dissimilar for the individual sites and were found to be stratified according to the position of the sites along the long‐term aridity gradient. For example, Mongu, which has the wettest climate, has a much lower WUE for like levels of D than Tshane, which historically has the most arid climate. Given the similar inferred stomatal resistances between the sites, the disparate carbon uptake behavior for the grass vs. woody vegetation is the likely cause for the observed differences in WUE along the aridity gradient. The short‐term flux measurements provide a framework for evaluating the vegetation's functional adaptation to the long‐term climate and provide information that may be useful for predicting the dynamic response of the vegetation to future climate change.