Regional patterns and controls of leaf decomposition in Australian tropical rainforests

Future climates have the potential to alter decomposition rates in tropical forest with implications for carbon emissions, nutrient cycling and retention of standing litter. However, our ability to predict impacts, particularly for seasonally wet forests in the old world, is limited by a paucity of data, a limited understanding of the relative importance of different aspects of climate and the extent to which decomposition rates are constrained by factors other than climate (e.g. soil, vegetation composition). We used the litterbag method to determine leaf litter decay rates at 18 sites distributed throughout the Australian wet tropics bioregion over a 14-month period. Specifically, we investigated regional controls on litter decay including climate, soil and litter chemical quality. We used both in situ litter collected from litterfall on site and a standardized control leaf litter substrate. The control litter removed the effect of litter chemical quality and the in situ study quantified decomposition specific to the site. Decomposition was generally slower than for other tropical rainforests globally except in our wet and nutrient-richer sites. This is most likely attributable to the higher latitude, often highly seasonal rainfall and very poor soils in our system. Decomposition rates were best explained by a combination of climate, soil and litter quality. For in situ litter (native to the site) this included: average leaf wetness in the dry season (LWDS; i.e. moisture condensation) and the initial P content of the leaves, or LWDS and initial C. For control litter (no litter quality effect) this included: rainfall seasonality (% dry season days with 0-mm rainfall), soil P and mean annual temperature. These results suggest that the impact of climate change on decomposition rates within Australian tropical rainforests will be critically dependent on the trajectory of dry season moisture inputs over the coming decades.