Hydraulic Lift As a Determinant of Tree-Grass Coexistence on Savannas

Abstract:

The coexistence of woody plants and grasses in savannas is determined by a complex set of interacting factors, including resource availability and disturbance. Existing theories explaining coexistence focus on competitive relations or disturbances preventing the system from attaining a state with complete grass or tree dominance. The effect of hydraulic lift on interactions between woody plants and grasses and the dynamics of savanna ecosystems remains poorly understood. Here, we develop a mechanistic model to investigate the role of hydraulic lift on the stability of savannas. The model accounts both for competition for soil water in the shallow soil layer and fire-induced disturbance. We find that hydraulic lift expands the parameter range in which savannas are stable at the expense of woodlands. Our study shows that hydraulic lift can be an important mechanism responsible for coexistence of woody plants and grasses in savannas.