Holocene boundary dynamics of a northern Australian monsoon rainforest patch inferred from isotopic analysis of carbon (14C and δ13C) and nitrogen (δ15N) in soil organic matter

Soil organic matter (SOM) was sampled from lateritic soil profiles across an abrupt eucalypt savanna-monsoon rainforest boundary on the north coast of Croker Island, northern Australia. Accelerator mass spectrometry dating revealed that SOM that had accumulated at the base of these 1.5 m profiles had a radiocarbon age of about 5000 years. The mean carbon and nitrogen stable isotope composition of SOM from 10 cm deep layers from the surface, middle and base of three monsoon rainforest soil profiles was significantly different from the means for these layers in three adjacent savanna soil profiles, suggesting the isotopic 'footprint' of the vegetation boundary has been stable since the mid Holocene. Although there were no obvious environmental discontinuities associated with the boundary, the monsoon rainforest was found to occur on significantly more clay rich soils than the surrounding savanna. Tiny fragments of monsoon rainforest and abandoned 'nests' (large earthen mounds) of the orange-footed scrubfowl, an obligate monsoon rainforest species, occurred in the savanna, signalling that the rainforest was once more extensive. Despite episodic disturbances, such as tropical storm damage and fires, the stability of the boundary is probably maintained because clay rich soils enable monsoon rainforest tree species to grow rapidly and achieve canopy closure, thereby excluding grass and reducing the risk of fire. Conversely, slower tree growth rates, grass competition and fire on the savanna soils would impede the expansion of the rainforest although high rainfall periods with shorter dry seasons may enable rainforest trees to grow sufficiently quickly to colonize the savanna successfully.