Vegetation and Fire at the Last Glacial Maximum in Tropical South America

Abstract

This chapter aims to review current knowledge of the key vegetation\rtypes, and their composition, structure, distribution, and fire regime across the South\rAmerican tropics during the global Last Glacial Maximum ca. 21,000 cal yr BP (calendar\ryears before present). We do this by synthesising previously published Last\rGlacial Maximum fossil pollen and charcoal data as well as Last Glacial Maximum\rvegetation model simulations, in comparison with ecoregion/biome maps of\rpresent day vegetation. Both model simulations and empirical data suggest that\rthere were no large-scale differences in major biome distributions between the\rLast Glacial Maximum and present (notwithstanding the Atlantic forests of SE\rBrazil), with biome shifts largely associated with ecotonal areas – downslope expansion\rof montane grasslands in the Andes at the expense of montane forest, and\rsavanna expansion at the expense of rainforest and gallery forest at the Amazon\rbasin margins. However, species composition and structure of these Last Glacial\rMaximum forests was quite different from those of today. At the Last Glacial\rMaximum, pollen data show that montane Andean taxa descended into the lowlands\rto form novel non-analogue forest communities with lowland Amazonian\rtaxa, whilst vegetation model simulations show that carbon limitation caused by\rlow atmospheric CO2 likely produced forest communities with reduced canopy\rdensity and hence lower biomass than present-day forests. These pollen datamodel\rcomparisons show that although Amazonia was probably still dominated\rby closed forest at the Last Glacial Maximum, its carbon store may have been\ronly 50% of present. Most charcoal records show reduced burning during the Last\rGlacial Maximum compared with today, most likely due to the significantly colder\rtemperatures.