Vegetation and climate in lowland southeast Asia at the last glacial maximum

The regional geography of southeast Asia, and global climate forcing, were very different to today during the Last Glacial Maximum (LGM). In order to determine how these factors influenced the lowland vegetation and climate of the region, pollen data for the LGM are examined from ten recently constructed late Quaternary pollen records derived from terrestrial and marine cores within the southeast Asian region. Interpretation is facilitated by comparison of LGM pollen spectra with those extracted from the same records for the mid-Holocene period, prior to a major influence of people on the vegetation landscape. It is found that precipitation was lower (probably by c. 30-50%) than today, while mean temperatures may have been reduced by as much as 6-7degreesC, assuming little influence of reduced atmospheric carbon dioxide levels. Rainforest was replaced by grassland to some extent in areas presently experiencing some seasonality in climate but rainfall reduction was insufficient to have a major influence on the extent of evergreeen forest in the core area of tropical rainforest. Montane forest elements descended to low altitudes but augmented rather than replaced existing lowland rainforest plants. Continental shelves, exposed by sea levels at least 120 in lower than today, appear to have been covered largely by rainforest in wetter areas and by grassland, open woodland and sedgeland in drier areas, except for the colonisation by Artemisia steppe of the exposed northern part of the South China Sea. Apart from this steppe invasion, there is no other evidence of large scale regional movements of vegetation types. Despite the lower rainfall, the extent of continental shelf exposure within the Indonesian region probably resulted in the presence of a greater area of tropical evergreen rainforest than exists today. The reconstruction has implications for debates over LGM tropical sea surface temperature, rainforest refugia, and the pattern and timing of human colonisation of the southwest Pacific region.