Modelling the effects of rainfall variability and fire on tree populations in an Australian tropical savanna with the Flames simulation model

The tropical savanna landscape of northern Australia is perhaps the most extensive and flammable ecosystem in the world. Like savannas worldwide, there still remains uncertainty as to the functioning of much of this system and the mechanisms maintaining a state between open grassland and woodland. We use the development of a process-based model to examine the processes operating on the eucalypt dominated tree component of the northern Australian savannas, focussing on the interaction of water availability and fire.A process-based, tree population dynamics model (Flames) has been developed to understand the interactive effects of fire and rainfall variability on trees in Australian savanna ecosystems where there has been limited modelling to date. This paper shows that the model is capable of simulating the range of tree populations found in northern Australia by considering soil properties (texture and depth) and rainfall distribution. The model indicates that seasonal variability and inter-annual variability in rainfall is critical in defining the structure and density of tree stands in northern Australia. We also show that fire plays an important role in defining tree structure by removing the most sensitive (the very young and old) trees and that frequent fronting fires will lead to ongoing reductions in tree populations. While fire is an important factor in northern Australia, the model indicates its effects on tree populations at a sub-continental scale are secondary to the impact of rainfall variability.