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

Significance

We present new qualitative and statistical analyses of 59 palaeoecological records across Southeast Asia to show that, instead of swings between open savanna and dense rainforest ecosystems, the climatic changes of the Last Glacial Period (119–11.7 ka) and particularly the Last Glacial Maximum (conventionally ~23–19 ka) involved fluid transitions between lowland rainforest, more open canopy seasonally dry forest, and tropical montane forest. This “hybrid” open forest biome provides an alternative to the currently accepted binary ecologies for the region and yields new insights into ecological resilience for tropical forests in Southeast Asia and beyond. Additionally, it points to diversified rather than overturned resource availability for humans that were occupying and migrating through the region.

Abstract

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 δ ¹³C biomarker proxies indicate the expansion of C ₄-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.