Past Climate variability in South America and Surrounding regions: From the Last Glacial Maximum to the Holocene
F. Vimeux, F. Sylvestre and M. Khodri
This chapter aims to review current knowledge of the key vegetation types, and their composition, structure, distribution, and fire regime across the South American tropics during the global Last Glacial Maximum ca. 21,000 cal yr BP (calendar years before present). We do this by synthesising previously published Last Glacial Maximum fossil pollen and charcoal data as well as Last Glacial Maximum vegetation model simulations, in comparison with ecoregion/biome maps of present day vegetation. Both model simulations and empirical data suggest that there were no large-scale differences in major biome distributions between the Last Glacial Maximum and present (notwithstanding the Atlantic forests of SE Brazil), with biome shifts largely associated with ecotonal areas – downslope expansion of montane grasslands in the Andes at the expense of montane forest, and savanna expansion at the expense of rainforest and gallery forest at the Amazon basin margins. However, species composition and structure of these Last Glacial Maximum forests was quite different from those of today. At the Last Glacial Maximum, pollen data show that montane Andean taxa descended into the lowlands to form novel non-analogue forest communities with lowland Amazonian taxa, whilst vegetation model simulations show that carbon limitation caused by low atmospheric CO2 likely produced forest communities with reduced canopy density and hence lower biomass than present-day forests. These pollen data-model comparisons show that although Amazonia was probably still dominated by closed forest at the Last Glacial Maximum, its carbon store may have been only 50% of present. Most charcoal records show reduced burning during the Last Glacial Maximum compared with today, most likely due to the significantly colder temperatures.