Vegetation response to rainfall variation and human impact in central Kenya during the past 1100 years
by
Iain Darbyshire
University of Wales, Aberystwyth
Coauthors: Henry Lamb (University of Wales, Aberystwyth), Dirk Verschuren (Ghent University)

High-resolution pollen data from a 625cm sediment core from Crescent Island Crater, a sub-basin of Lake Naivasha, Kenya, provide an 1100-year record of vegetational response to rainfall variation and human impact. The same core preserves a record of lake depth and rainfall variations in the form of changing sedimentary facies, and diatom and chironomid assemblages (Verschuren et al., 2000, Nature 403, 410-414). From about AD 1000 to 1270, the lake experienced a long saline lowstand, reflecting low rainfall during the Medieval Warm Period. One brief highstand (AD 1200-1240) interrupted this dry period. The Little Ice Age, from AD 1270 to 1850, was a period of generally wet conditions in the Naivasha catchment, although three severe droughts occurred. Highest historical lake levels were recorded in 1894. For much of the twentieth century, the region received rainfall lower than the long-term average. Prior to the eighteenth century, a clear positive correlation between lake depth and the abundance of regional forest and woodland pollen indicates that rainfall changes control pollen input. However, rather than being a direct vegetation response to climate, this correlation may have been a result of changes in the relative abundances of local herbaceous taxa and regional arboreal taxa, controlled by shoreline variation of Lake Naivasha, and by changes in the volume of river inflow. During the eighteenth century, the correlation between lake depth and Afromontane pollen abundance becomes less clear, probably because of human disturbance of the vegetation. Maasai immigration, and population expansion in response to favourable climate conditions, probably resulted in strong grazing pressure on the lower montane forest. This human impact on the terrestrial vegetation, although muted in the pollen record, largely masks the impact of climate change. Increases in ruderal herbaceous pollen and arrival of exotic trees during the twentieth century indicate increased local human disturbance during the colonial period. However, there is little direct evidence for the widespread clearance of montane forest over recent decades.

Date received: April 24, 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 # cahi-10.