Has global environmental change caused monsoon rainforests to expand in the Australian monsoon tropics?

Abstract

A large research program in the Australian monsoon tropics has concluded that monsoon rainforests have expanded within the savanna matrix, a trend that has been emulated throughout the tropics worldwide. The driver of the northern Australian trend was not resolved, but it was suggested to be linked to a long-term trend towards wetter climates, atmospheric CO2 enrichment, and changed fire regimes. We review these findings with particular consideration of its analytical and evidentiary basis and plausibility of the global change hypothesis. Field validation has largely demonstrated that the aerial photographic technique that underpinned the previous research is reliable enough to detect rainforest expansion. Statistical modelling demonstrated that the expansion is related to sites with regionally low fire activity, although models are of low explanatory power reflecting the sketchy historical records of fire and feral animal impacts. Field studies show that current fire regimes adjacent to expanding rainforest patches are causing populations of the native conifer Callitris intratropica, an obligate seeder, to crash. Therefore, it is unlikely that changes in fire regimes, which have been deleterious to other fire-sensitive taxa and plant communities in the region, are responsible for the rainforest expansion. We conclude that the expansion of monsoon rainforests is most plausibly linked to the current wetting trend or elevated CO2 concentration. Increases in either water availability or CO2 concentration can potentially overwhelm the negative feedback between fire and rainforest cover that is responsible for the meta-stability of monsoon rainforest boundaries. However, further research at the continental scale, using aerial photography, tree rings and other proxies, is required to evaluate this hypothesis.