Tree–grass competition varies across select savanna tree species: a potential role for rooting depth

Abstract

Tree–grass competition is recognized as an important ecological process in savannas and woodlands, yet we only have a limited understanding of how it varies across tree species, particularly in relation to water use and root morphology. We grew seedlings of three African savanna tree species either with or without grasses in a greenhouse experiment over 8 months. We measured soil moisture content at three depths throughout the course of the experiment and measured seedling height, root depth, and root allocation as a function of depth at the end of the experiment. The tree species differed markedly in their response to grass competition, ranging between a reduction >50 % in relative growth rate to no effect. The species also differed in terms of relative root mass allocation with depth, despite a lack of maximum rooting depth differences. Against expectations, the species with the deepest rooting allocation was highly susceptible to competition from grasses. Grass presence led to a reduction of soil moisture in relation to controls and the reduction increased as a function of depth. Analysis of soil moisture reductions during dry periods indicated that grass uptake was dominant in the upper soil layers, suggesting that lower water availability at depth associated with the presence of grass was the result of reduced flow to deeper layers. We hypothesize that shallow-rooted tree species may be able to successfully engage in exploitative competition with grasses, but deep-rooted species may be exposed to interference competition when water is limiting.