The evolution of the vegetation in the Lake Victoria basin during the late holocene and its climatic implications
by
Immaculate Ssemmanda
Department of Geology, Makerere University, P.O.Box 7062, Kampala, Uganda.
Coauthors: Annie Vincens (CEREGE BP 80, 13545 Aix-en-Provence, France)

The pollen sequence of the core V95-2P (00 58.67'S, 033 27.32'E, 67 m depth) from Lake Victoria mirrors larger extension of forests between ca. 6500 and ca. 4100 yr. B.P., attesting to wetter climatic conditions in the region than during the later Holocene period. Highest humidity was experienced in the region prior to ca. 6500 yr. B.P., before the semideciduous forest formations became widespread in the neighborhood of the lake. From ca. 5000 yr. B.P., the forests around Lake Victoria were mainly of semideciduous character with increasing abundance of Celtis associated with Holoptelea grandis, mixed with some Guineo-Congolian elements such as Tetrorchidium. The period ca. 4100 to ca. 3000 yr. B.P. shows a progressive decline of semideciduous forest formations and the establishment of open vegetation with Capparidaceae and abundant Gramineae, attesting to a dry climate. After ca. 3000 yr. B.P., the pollen data, particularly that from high altitude, evidence an amelioration of climate. The dry montane forest with Podocarpus and Juniperus procera underwent significant extension which reached its maximum at ca. 1700 yr. B.P. At low altitude in the neighborhood of the lake, the extension of the semideciduous forests in relation to this wetter climate, is evidenced mainly until ca. 2200 yr. B.P. During this sub-humid climatic phase, either the amount of precipitation was inadequate or the dry seasons were too long to permit a large development of evergreen forests in the Lake Victoria region. From ca. 1700 yr. B.P., the significant decrease in the abundance of the regional taxa testifies to the decline of the Juniperus-Podocarpus dry montane forest. During this dry period, both the montane and the semideciduous forests decline at the profit of open grass dominated formations: open woodlands and probably savannas. The pollen data for the sequence V95-2P farther attests that the vegetation in the neighborhood of the lake responds slowly and progressively to climatic changes of small magnitudes such as the ones that occurred after ca. 4000 yr. B.P. This buffering capacity could be due to the large water catchment area of the Lake Victoria basin, its geomorphology (relatively gentle sloping tectonic sag), the good hydrological network of rivers and swamps and the possibly the nature of the vegetation.

Date received: May 18, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahr-08.