BIOMASS BURNING AND ECOSYSTEM DYNAMICS OF THE SAHULIAN RAINFORESTS

The Sahulian rainforests of New Guinea and northern Australia contain 5% of global plant diversity on less than 1% of its surface and includes the third largest area of rainforest in the world. The region is characterized by high climate variability over annual to decadal timescales, which is driven by the ENSO (El Niño-Southern Oscillation) phenomenon. In 1997-98 an exceptionally strong ENSO event occurred and was associated with severe drought and devastating fires that caused extensive damage to vast stands of normally wet tropical forests. Despite the devastating impact that extreme disturbance events such as this have had on biotic communities in recent times, there is little information on the long-term consequences of these events on forest ecosystems. An analysis of microscopic charcoal preserved in sediments from ten wetlands in the Sahulian region provides a proxy record of regional ENSO events during the last 20,000 years. The record shows that the frequency and intensity of ENSO events has fluctuated over time with peaks in ENSO-related climate variability occurring between 17,000-9000 yr B.P. and 5000 yr B.P. to the present. A high-resolution analysis of charcoal and pollen from a lake in the rainforest of northern Australia (Lake Euramoo) provides a detailed reconstruction of vegetation response to fluctuating levels of fire-related disturbance. The frequency of disturbance events associated with fire increased in the last 5000 years, resulting in fragmentation of rainforest habitat, a shift towards competitive dominance of taxa adapted to drought/fire or frost, and a tendency towards higher species turnover rates within the rainforest community. Periodic episodes of high climate variability and associated disturbance, such as occurred during the late glacial transition and the mid to late Holocene, may represent significant episodes for promoting and maintaining diversity of tropical rain forests due to the evolutionary possibilities provided by newly create gaps and accelerated regeneration dynamics.