Vegetation History and Climate Changes in Africa North and South of the Equator (10° S to 10° N) During the Last Glacial Maximum

Abstract

This work presents vegetation maps and paleoenvironmental data for central Africa, covering both the last glacial maximum (LGM) and present times. The most striking feature is that the overall environmental conditions during the LGM have been much drier in central Africa than present-day conditions. Investigations focused on reconstructing former vegetation patterns for the Sudanian, Zambezian, and Guinean savannas, as well as tropical semi-evergreen and evergreen rainforests. A number of research projects concerning changes of vegetation cover have shown that even tropical regions have been affected by considerable climatic oscillations during the last 20,000 years. The increasing effect of human influence on appearance of the African vegetation cover imposes a significant problem for the study of past vegetations. Thus, all investigations studying temporal change in tropical ecosystems are confronted with the fact that areas that have not been influenced by man can rarely be found. The primary question is which vegetation forms allow us to draw conclusions applicable to a nearly natural vegetation. A first step is the estimation of potential quasi-natural vegetation formations under present climatic conditions. Derivation of West African paleovegetation is based on published paleoclimate and paleovegetation information, including palynology, deep-sea cores and isotope analysis. Using numerical relations between natural vegetation and climate under present conditions, models linking analogous vegetation and climate items permit subsequent assessment of paleoclimatic conditions. The present distribution of quasi-natural vegetation is linked closely to the climatic water-budget of the African continent. The amount of precipitation, as well as duration and intra- annual distribution of rain, also depends on the condition of important water-vapor sources for Africa, the SE Atlantic on Africa's west coast and the Indian Ocean on the continent's east side. The availability of well-resolved and dated profiles from the West African coast enables a reconstruction of sea-surface temperatures (SSTs) for the LGM. As there are significant connections between SSTs of oceans and the precipitation system today, such connections must have also existed during the LGM. The relations between the two allow establishment of transfer functions. These permit reconstructing the continent precipitation system for the LGM on the basis of SSTs. At first, the precipitation amount with its annual distribution is reconstructed. On the basis of reconstructed climatic water budget and in relation to present-day climate and vegetation conditions for the LGM, the paleovegetation is reconstructed and compiled in a map. Finally, the reconstructed vegetation is critically compared with respect to paleoenvironmental conditions for the LGM.