The lowland tropics of Wallacea in south-east Asia represent some of the most diverse, biogeographically significant forests globally. Long-term ecological data for this region are however scarce, making it difficult to predict vegetation response to future drivers of change. These drivers include the increased intensity and frequency of precipitation extremes expected for south-east Asia in the 21st century. Emerging palaeoclimatic data suggest that marine isotope stage 2 (MIS 2) within Wallacea was significantly drier and more seasonal than either MIS 3 or the Holocene, and that the Holocene was characterised by a series of lower-amplitude hydroclimatic shifts. Examining floristic response to these shifts can provide insight into the acclimation thresholds of forest taxa to hydroclimatic drivers of different magnitudes, and illuminate the dynamism of the tropical landscape over millennia.
This talk presents results from palynological (pollen and charcoal) reconstructions from two sites – Lake Lantoa (16,500 yrs BP to the present) and Lake Towuti (~1,000,000 yrs BP to the present) – with a focus on local and regional floristic change from the start of MIS 2 to the present. Our results show that closed forest has persisted in the local and regional landscape over the assessed time period in spite of being subject to drier climates and heightened fire activity prior to the Holocene. However, there are notable intra-forest compositional shifts evident in the records in response to glacial scale climatic forcing and changing patterns of landscape disturbance (including burning). These include 1) contraction of tropical coniferous forest between 31,000 and 17,000 yrs BP in response to heightened seasonality; 2) the replacement of seasonal forest taxa with rainforest species under increasingly humid conditions at the MIS 2–Holocene boundary), and; 3) a slight return of seasonal/secondary forest types in response to late-Holocene anthropogenic activity. These findings demonstrate the stability of the tropical forest biome in Wallacea to key drivers of change, but highlight intraforest compositional dynamism in the landscape in response to these drivers.