Climate drove gradual rainforest transitions in Wallacea (Sulawesi, Indonesia) during the last glacial-interglacial transition (24,000 – 9,000 years BP)

The rainforests of Wallacea are one of the most remarkable biodiversity hotspots on Earth. Understanding how Wallacean rainforests have responded to climate change and wildfire disturbance in the recent geologic past has significant implications for understanding their capacity to resist and recover from ongoing environmental disturbances. Quaternary glacial-interglacial transitions have disrupted rainforest ecosystems across the globe in different ways, and reconstructing responses to deglaciation can serve as a natural experiment for understanding tropical forest resilience. Yet, in Wallacea it has been challenging to understand how hydroclimate change during the last glacial to interglacial transition (LGIT; 24 – 9 ka) shaped Holocene rainforests because few terrestrial paleoecological records extend continuously into the Last Glacial Maximum (LGM). Here, we present a pollen and microcharcoal record from Lake Towuti (318 m asl), Indonesia, which documents the dynamics of rainforest communities and fire regimes from ∼24 to 9 ka at centennial time-scales. The pollen record demonstrates a strong climatic control on the development of rainforests across the LGIT. At Lake Towuti, a linear, multi-millennial ecological transition occurred from a more spatially homogenous, disturbance-prone Saurauia-Fagaceae forest with no apparent modern analogues during the LGM (24 – 18 ka) into a more diverse lowland and montane wet forest complex along elevation gradients by the early Holocene. The development of ecologically modern rainforests began at 17 ka synchronous with rapid brGDGT-derived temperature increases of ∼4 °C. By 12.7 ka, modern analogues in rainforest pollen composition were reached, tracking substantial moisture increases inferred from leaf-wax hydrogen isotopes (δDwax). The paleoecological record from Lake Towuti demonstrates that forests up to ∼1000 m elevation in eastern Sulawesi have significantly reorganized since the LGM, but these forest transitions occurred linearly in sync with regional climate, without threshold responses into alternative stable states such as grasslands, showing resilience to past global climate change.