Role of topography in facilitating coexistence of trees and grasses within savannas

The factors and processes that may explain the observed coexistence of trees and grasses in savannas are not well understood. Here we propose a new hypothesis that addresses this issue. We hypothesize that variations in elevation at relatively short horizontal scales of similar to1 km force similar variations in soil moisture and thus create significantly different hydrologic niches within any large area. Under water-limited conditions the relatively wet valleys favor trees, while the relatively dry hills favor grasses. This coexistence of trees and grasses is only possible for a window of climatic conditions that are characteristic of savannas.'' To test this hypothesis, numerical simulations are performed for the region of West Africa using a model that simulates vegetation dynamics, the Integrated Biosphere Simulator ( IBIS), and a distributed hydrologic model, Systeme Hydrologique Europeen ( SHE). IBIS is modified to include the groundwater table (GWT) as a lower boundary. The spatial distribution of GWT is simulated by SHE. At 9degreesN the model simulates trees even when the GWT is assumed to be infinitely deep; at 13degreesN the model simulates grasses even when the capillary fringe of the GWT reaches the surface. However, for the transitional climate, at 11degreesN, trees are simulated when the GWT is at similar to2.5 m from the surface, but grasses are simulated when the GWT is deeper than 2.5 m. These results suggest that the variability of soil moisture forced by topography can be a determinant factor of vegetation distribution within savannas. Furthermore, they confirm that this role of topography can be significant only in a certain climatic window characteristic of savannas.