Tropical savannas are characterized by the coexistence of trees and grasses, and the future of savanna vegetation will depend on how these two plant types respond to changing climate. One major determinant of savanna responses to shifting rainfall patterns will be tree and grass functional rooting depths, which determine the extent of tree-grass competition and whether plants can capitalize on moisture at varying soil depths. Savanna ecologists have long debated whether trees and grasses differentiate rooting niches, with some studies indicating that trees have deeper functional rooting zones, while others have suggested tree and grass water uptake largely overlap. However, most have evaluated this question at a single site, limiting generalization across environmental gradients. Here, we took a comparative approach to understand how tree-grass root-niche differentiation varies with soil texture and rainfall – two factors that influence infiltration and could thus shape rooting strategies. We examined tree and grass functional rooting depths at eight sites on sandy and clay soils across a rainfall gradient within Kruger National Park, South Africa. By analyzing the stable isotope composition of soil and stem water samples, we inferred functional rooting profiles of the dominant species at each site and compared within and across sites.
Results/Conclusions
At all eight sites, trees used deeper soil water than grasses. Isotopic data indicated that grass water uptake was primarily from the shallowest depth sampled (5 cm), while average tree water uptake ranged from 15 to 50 cm depths. However, these varied with soil texture and mean annual rainfall. Trees and grasses both had deeper roots on sandy soils than clay soils, consistent with expectations based on infiltration dynamics. The deepest roots and the greatest differences between trees and grasses were found at higher rainfall on sandy soils, while on clay soils rooting depths were more consistent across the rainfall gradient. These findings indicate that trees are better able to utilize deeper soil moisture on sandy soils, corroborating a proposed mechanism for several broader-scale patterns observed in tropical savannas: higher tree densities and faster rates of woody encroachment on sandier soils, as well as divergent responses of tree cover to rainfall intensity depending on soil texture. Variation in tree and grass functional rooting depths could produce heterogeneous responses of savanna vegetation to a changing climate. Considering the joint influences of edaphic and climatic context on rooting depths, as we do here, will generate more reliable model predictions of vegetation change.