Evaluating Ecohydrological Theories of Woody Root Distribution in the Kalahari

The contribution of savannas to global carbon storage is poorly understood, in part due to lack of knowledge of the amount of belowground biomass. In these ecosystems, the coexistence of woody and herbaceous life forms is often explained on the basis of belowground interactions among roots. However, the distribution of root biomass in savannas has seldom been investigated, and the dependence of root biomass on rainfall regime remains unclear, particularly for woody plants. Here we investigate patterns of belowground woody biomass along a rainfall gradient in the Kalahari of southern Africa, a region with consistent sandy soils. We test the hypotheses that (1) the root depth increases with mean annual precipitation (root optimality and plant hydrotropism hypothesis), and (2) the root-to-shoot ratio increases with decreasing mean annual rainfall (functional equilibrium hypothesis). Both hypotheses have been previously assessed for herbaceous vegetation using global root data sets. Our data do not support these hypotheses for the case of woody plants in savannas. We find that in the Kalahari, the root profiles of woody plants do not become deeper with increasing mean annual precipitation, whereas the root-to-shoot ratios decrease along a gradient of increasing aridity

Transnational agricultural land acquisitions threaten biodiversity in the Global South

Agricultural large-scale land acquisitions have been linked with enhanced deforestation and land use change. Yet the extent to which transnational agricultural large-scale land acquisitions (TALSLAs) contribute to—or merely correlate with—deforestation, and the expected biodiversity impacts of the intended land use changes across ecosystems, remains unclear. We examine 178 georeferenced TALSLA locations in 40 countries to address this gap. While forest cover within TALSLAs decreased by 17% between 2000 and 2018 and became more fragmented, the spatio-temporal patterns of deforestation varied substantially across regions. While deforestation rates within initially forested TALSLAs were 1.5 (Asia) to 2 times (Africa) higher than immediately surrounding areas, we detected no such difference in Europe and Latin America. Our findings suggest that, whereas TALSLAs may have accelerated forest loss in Asia, a different mechanism might emerge in Africa where TALSLAs target areas already experiencing elevated deforestation. Regarding biodiversity (here focused on vertebrate species), we find that nearly all (91%) studied deals will likely experience substantial losses in relative species richness (−14.1% on average within each deal)—with mixed outcomes for relative abundance—due to the intended land use transitions. We also find that 39% of TALSLAs fall at least partially within biodiversity hotspots, placing these areas at heightened risk of biodiversity loss. Taken together, these findings suggest distinct regional differences in the nature of the association between TALSLAs and forest loss and provide new evidence of TALSLAs as an emerging threat to biodiversity in the Global South

Local and distant impacts of water infrastructure on land use change in Africa.

The development of rural economies across the global south is often related to access to water and the development of water infrastructure. It has been argued that the drilling of wells and construction of new dams would unleash the agricultural potential of African nations that are exposed to seasonal water scarcity, strong interannual rainfall variability, and associated uncertainties in water availability. There are, however, some important environmental and social externalities that often remain overlooked, and whose impacts may be underestimated. The drilling of new wells has been reported to induce pastoralist communities to increase livestock density, often triggering overgrazing and land degradation, with important environmental impacts. For instance, overgrazing-induced loss of grass cover may allow for an increase in soil erosion and enhance the rate of dust emissions from denuded landscapes into the atmosphere with important impacts on local, regional, and global biogeochemical cycles. While this phenomenon is a known desertification pathway across subtropical Africa, its long-range impacts remain poorly understood. A region affected by overgrazing as a result the establishment of new boreholes and wells can be found in the Southern Kalahari (Southern Africa), where the loss in grass cover has been associated with the remobilization of linear dune systems. Here I use modeled pathways of air parcel trajectories starting from overgrazed regions of Southern Africa (as well as from other major dust sources in the Southern hemisphere) to investigate the potential impacts on the Southern Ocean. I found a positive relationship between ocean productivity and trajectory densities of dust emitted from Southern Africa. Water infrastructure can also have a more local impact on land use and rural livelihoods. For instance, dam construction for agriculture or other uses, is often seen as a solution to the water scarcity problem in Africa, a continent affected by high levels of economic water scarcity and a strong untapped irrigation potential. While water security is often presented as the pathway to poverty alleviation and invoked to justify large dam projects for irrigation, it is still unclear to what extent small holders will benefit from them. Are large dams built to the benefit of subsistence farmers or of large-scale commercial agriculture? Here I use remote sensing imagery in conjunction with advanced machine learning algorithms to map the irrigated areas (or ‘command areas’) that have appeared in the surroundings of 18 major dams built across the African continent between 2000 and 2015. I quantify the expansion of irrigation afforded by those dams, the associated changes in population density, forest cover, and farm size. I find that, while in the case of nine dams in the year 2000 there were no detectable farming patterns, in 2015 a substantial fraction of the command area (ranging between 8.5% and 96.7%) was taken by large-scale farms (i.e., parcels >200ha). Seven of the remaining 9 dams showed a significant increase in average farm size and number of farms between 2000 and 2015, with large-scale farming accounting for anywhere between 5.2% and 76.7% of the command area. Collectively, these results indicate that many recent dam projects in Africa are associated either with the establishment of large-scale farming or a transition from small-scale to mid-to-large scale agriculture. I analyze the value of water acquired by agribusiness investors by determining the value generated by irrigation through the enhancement of agricultural production in the command area of the same dams

TrajectoryFiles.zip

These are raw 168hour forward trajectory files used in the paper titled "Mapping areas of the Southern Ocean where productivity likely depends on dust delivered iron"<div><br></div><div>The "Analyses.zip" contains results used in the paper.</div

Transnational agricultural land acquisitions threaten biodiversity in the Global South

Agricultural large-scale land acquisitions have been linked with enhanced deforestation and land use change. Yet the extent to which transnational agricultural large-scale land acquisitions (TALSLAs) contribute to—or merely correlate with—deforestation, and the expected biodiversity impacts of the intended land use changes across ecosystems, remains unclear. We examine 178 georeferenced TALSLA locations in 40 countries to address this gap. While forest cover within TALSLAs decreased by 17% between 2000 and 2018 and became more fragmented, the spatio-temporal patterns of deforestation varied substantially across regions. While deforestation rates within initially forested TALSLAs were 1.5 (Asia) to 2 times (Africa) higher than immediately surrounding areas, we detected no such difference in Europe and Latin America. Our findings suggest that, whereas TALSLAs may have accelerated forest loss in Asia, a different mechanism might emerge in Africa where TALSLAs target areas already experiencing elevated deforestation. Regarding biodiversity (here focused on vertebrate species), we find that nearly all (91%) studied deals will likely experience substantial losses in relative species richness (−14.1% on average within each deal)—with mixed outcomes for relative abundance—due to the intended land use transitions. We also find that 39% of TALSLAs fall at least partially within biodiversity hotspots, placing these areas at heightened risk of biodiversity loss. Taken together, these findings suggest distinct regional differences in the nature of the association between TALSLAs and forest loss and provide new evidence of TALSLAs as an emerging threat to biodiversity in the Global South

Coordinates, mean annual precipitation (MAP), and plant community composition of the study sites.

<p>Information on precipitation includes mean annual precipitation (MAP) ± the standard deviation of annual precipitation and (in parentheses) the minimum annual precipitation recorded in 1971–2006. Because rainfall data from Kuke do not exist, the values reported are from Ghanzi.</p

Grain size analysis of the soil samples was conducted using a particle size analyzer (LS 13 320, Beckman Coulter®).

<p>Soil samples were collected from depths 0–0.1 m, 0.1–0.3 m, 0.3–0.7 m, and 0.7–120 m at all sites except for the Kuke/Ghanzi area, where we have used only soils from the top 10 cm.</p

Left: Fire frequency from 2000 to 2011 calculated from the MODIS Burned Area Product (MCD45) [<b>72</b>] in yr⁻¹.

<p>White areas experienced no fires during this period. Right: Average fire frequency (in yr⁻¹) calculated along a longitudinal transect (21.3°) using a moving box of approximately 100×100 km.</p

Dry biomass density of roots in the four sites across the Kalahari aridity gradient.

<p>The error bars indicate the minimum and maximum data values, unless outliers are present (shown as circles). The black line indicates the median, while the box boundaries are the lower and upper quartiles. Based on a set of 60 soil profiles sampled at each site.</p

Measured average woody plant biomass (above and belowground) per unit area (top 1.1 m) across the Kalahari transect aridity gradient.

<p>Inset: ratio of below to above ground biomass. The error bars represent ± standard deviation calculated for a set of 3 plot replicates at each site.</p