Root-niche separation between savanna trees and grasses is greater on sandier soils

In savannas, partitioning of below-ground resources by depth could facilitate tree-grass coexistence and shape vegetation responses to changing rainfall patterns. However, most studies assessing tree versus grass root-niche partitioning have focused on one or two sites, limiting generalization about how rainfall and soil conditions influence the degree of rooting overlap across environmental gradients. We used two complementary stable isotope techniques to quantify variation (a) in water uptake depths and (b) in fine-root biomass distributions among dominant trees and grasses at eight semi-arid savanna sites in Kruger National Park, South Africa. Sites were located on contrasting soil textures (clayey basaltic soils vs. sandy granitic soils) and paired along a gradient of mean annual rainfall. Soil texture predicted variation in mean water uptake depths and fine-root allocation. While grasses maintained roots close to the surface and consistently used shallow water, trees on sandy soils distributed roots more evenly across soil depths and used deeper soil water, resulting in greater divergence between tree and grass rooting on sandy soils. Mean annual rainfall predicted some variation among sites in tree water uptake depth, but had a weaker influence on fine-root allocation. Synthesis. Savanna trees overlapped more with shallow-rooted grasses on clayey soils and were more distinct in their use of deeper soil layers on sandy soils, consistent with expected differences in infiltration and percolation. These differences, which could allow trees to escape grass competition more effectively on sandy soils, may explain observed differences in tree densities and rates of woody encroachment with soil texture. Differences in the degree of root-niche separation could also drive heterogeneous responses of savanna vegetation to predicted shifts in the frequency and intensity of rainfall.