PHYTOLITHS - INDICATORS OF GRASSLAND DYNAMICS DURING THE LATE HOLOCENE IN INTERTROPICAL AFRICA

The reconstruction of African tropical grassland history during the la te Holocene can be carried out using phytolith analysis. Fossil phytol ith assemblages from Lake Guiers, in the Sahelian region of Senegal, a nd from Lake Sinnda, in the Guineo-Congolian region of Congo were inve stigated. The results are interpreted on the basis of modern phytolith assemblages from the same regions and compared to pollen data previou sly obtained. Tall or short grass associations are discriminated by th eir phytolith index Iph(%)=saddle/(cross + dumbel + saddle), while the density of shrubs and trees is indicated by relative proportions of t he dicotyledon phytoliths. The phytolith data emphasize that, in the G uineo-Congolian region around Lake Sinnda, the driest phase of the lat e Holocene occurred between 4000 and 1200 yr B.P., commencing with the opening of the dense forest and its replacement by a short grass sava nna. From ca. 1000 yr B.P., wetter climatic conditions developed, as r epresented by the setting up of a tall grass savanna woodland. The mod ern shrub and tall grass savanna was developed ca. 700 yr B.P. In the Sahelian region around Lake Guiers, the driest phase occurred after ab out 2000 yr B.P. and has not been followed by moister conditions. A ta ll grass savanna woodland was gradually replaced by a shrub and short grass savanna which still occurs. A short period of development of swa mpy vegetation, which can be related to a lake level change, interrupt ed the semi-arid adaptation of the vegetation, between about 2000 yr B .P. and the present. (C) 1997 Elsevier Science B.V.