POLLEN RECORDS, POSTGLACIAL | Africa

Our current knowledge of the vegetation response to climate changes in tropical Africa and particularly the reconstitution of stand dynamics is severely limited by the scarcity of long and complete, well-dated pollen sequences. Additional difficulties arise from the specific characters of pollen analysis in tropical ecosystems linked to the large number of the tree species, an order of magnitude more than in temperate ecosystems, their low pollen productivity, and the small number of grains that are currently determined at a specific level. This results in extreme under-representation or absence of many tree species in pollen diagrams. However, thanks to recent developments in database management, more than 580 fossil pollen sites have been identified in Africa and surrounding areas. Raw data are available for 350 of these sites. These records show that the shift from dry and cold to wet and warm conditions at the last glacial–interglacial transition led to the widespread increase in tree cover throughout the tropics. Tropical trees migrated north and penetrated into the Saharan desert during the Holocene, whereas forest communities expanded upslope in the East African highlands and included plant species nowadays present at low to mid-altitudes. This demonstrates that climate changes have not only caused migrations of plant communities but have also profoundly influenced relationships between species. During the Holocene, the environmental response to short-term climate events was strongly dependent upon local hydrological conditions, as illustrated by the record of vegetation changes in the equatorial lowlands to the dry climate event recorded around 4000 cal yr BP.