[Paper] Forest mosaics, not savanna corridors, dominated in Southeast Asia during the Last Glacial Maximum

via PNAS, 26 December 2023: Recent research challenges the long-held belief of a savanna corridor in Southeast Asia during the Last Glacial Maximum, proposing a more complex landscape of forest mosaics. Analyzing 59 paleoecological records, the study reveals a dynamic transition between different forest types, including lowland rainforest, seasonally dry forest, and montane forest. This finding not only redefines our understanding of Southeast Asia’s ecological past but also highlights the region’s forest resilience to climatic changes, providing insights into human adaptations and resource availability during this period.

The dominant paradigm is that large tracts of Southeast Asia’s lowland rainforests were replaced with a “savanna corridor” during the cooler, more seasonal climates of the Last Glacial Maximum (LGM) (23,000 to 19,000 y ago). This interpretation has implications for understanding the resilience of Asia’s tropical forests to projected climate change, implying a vulnerability to “savannization”. A savanna corridor is also an important foundation for archaeological interpretations of how humans moved through and settled insular Southeast Asia and Australia. Yet an up-to-date, multiproxy, and empirical examination of the palaeoecological evidence for this corridor is lacking. We conducted qualitative and statistical analyses of 59 palaeoecological records across Southeast Asia to test the evidence for LGM savannization and clarify the relationships between methods, biogeography, and ecological change in the region from the start of Late Glacial Period (119,000 y ago) to the present. The pollen records typically show montane forest persistence during the LGM, while δ¹³C biomarker proxies indicate the expansion of C₄-rich grasslands. We reconcile this discrepancy by hypothesizing the expansion of montane forest in the uplands and replacement of rainforest with seasonally dry tropical forest in the lowlands. We also find that smooth forest transitions between 34,000 and 2,000 y ago point to the capacity of Southeast Asia’s ecosystems both to resist and recover from climate stressors, suggesting resilience to savannization. Finally, the timing of ecological change observed in our combined datasets indicates an ‘early’ onset of the LGM in Southeast Asia from ~30,000 y ago.

Source: Forest mosaics, not savanna corridors, dominated in Southeast Asia during the Last Glacial Maximum