role of water and fire in driving tree dynamics in Australian savannas

Ecologists rely on models to explore tree compositional changes especially when occurring over decades or centuries. In this article, we construct a theoretical, mathematical model to investigate the long‐term relationships between savanna stand structure, water resource availability and fire disturbance. We show how dry season length, rather than mean annual precipitation, leads to savanna stability, and how the soil properties and variation in annual rainfall distribution determines a climatic equilibrium for woody total basal area, with fire disturbance acting as a perturbation away from this state. This leads to our premise that rainfall and tree population dynamics drives the savanna state leading to grasses and fire, rather than grasses promoting fire to drive the savannas. The model predicts that savanna tree stands undergo cyclic variation in tree populations as a result of long‐term population cycles and germination events. This outcome is true regardless of the presence of fire, however, fire does introduce a more heterogeneous stand size structure. Synthesis. Using a mathematically transparent model for water resource availability and stand structure in savannas, we demonstrate how seasonal rainfall distribution, specifically seasonal drought, acts as the primary determinant for stand structure through stand water dynamics, with frequent fire disturbance able to reduce the populations from the climatically induced state.