Water‐Limited Equilibrium of Savanna Vegetation Systems

The average annual water balance of savanna vegetation systems is modeled as an interactive competition between trees and grass for water and energy. Ecological optimality hypotheses are introduced which allow specification of the woodland canopy density, the grass canopy density, and the plant (water use) coefficients of both tree and grass under conditions of natural equilibrium. Only one tree‐grass equilibrium state is found and is shown to agree well with the observed state of savanna in Sudan and the Transvaal. This state is stable with respect to perturbations of vegetation canopy density, but is metastable with respect to shifts in climate. Two other equilibrium states exist as vegetation monocultures, i.e., grassland and forest, but both are shown to be unstable with respect to perturbations of vegetation density.