Multiscale modelling of the drivers of rainforest boundary dynamics in Kakadu National Park, northern Australia

Understanding the relative importance of factors driving savanna–forest dynamics is vital as changes in the extent of tropical forests can have major impacts on global climate and carbon balance, biodiversity, and human well-being. Comparison of aerial photographs for 50 rainforest patches in Kakadu National Park had previously revealed a landscape wide expansion of rainforest boundaries between 1964 and 2004. Here, we used generalized linear and mixed effects models to assess the role of fire, buffalo impact, and patch characteristics in determining the rate of boundary change. The analysis was conducted at both the patch scale, and within-patch (plot) scale, to capture the different processes operating. At the patch scale, the rate of change was best explained by rainforest type and historical buffalo impact. Fire activity, patch size, and fragmentation were not important predictors of the rate of change. At the plot scale, distance from rainforest edge was the most important predictor of the probability of change, while the fragmentation and aspect of the boundary were unimportant. Rainforest expansion has occurred through a process of margin extension rather than eruption of new patches. The rainforest expansion is consistent with having been driven primarily by shifts in global change phenomena, such as increased rainfall and atmospheric CO2, rather than changes in disturbance regime. It appears that the current fire regimes are not sufficiently destructive to limit the overall expansion of the rainforest patches. Rainforest type and historical buffalo impact have mediated the rate of change at the patch scale.