Determinants of savanna-fire dynamics in the eastern Lake Victoria catchment (western Kenya) during the last 1200 years

Abstract

In tropical ecosystems, climate and human impact are generally considered the main determinants of savanna fire distribution, but their relative roles in controlling fire dynamics over the long-term are difficult to assess, due to scarcity of multi-proxy, long-term data. We combined pollen, charcoal and sedimentological data obtained from the 1200-year sediment record of Lake Simbi, a small crater lake near Lake Victoria in western Kenya, to assess the main drivers of century-scale variability in biomass burning, and its long-term effects on local vegetation dynamics. The effect of climate variability was inferred through comparison of the Simbi charcoal time series with an independent reconstruction of the region's moisture-balance history, from the sediment record of nearby Lake Victoria. Our data show that decade-scale climate variability exerted a primary control over burning, with intermediate levels of moisture maximizing the occurrence of fire, in accordance with both spatial and temporal patterns of biomass burning at regional to sub-continental scales. Despite the challenge to identify pre-colonial human impact unambiguously, our data also suggest that human disturbances between 1000 and 1200 AD likely amplified the effect of climate on biomass burning, in contrast to today's situation, where fire prevalence is mostly suppressed due to agricultural landscape fragmentation. Finally, combination of the pollen and charcoal data allowed to quantitatively estimate the effect of fire on vegetation, with fire explaining up to one third of the reconstructed vegetation changes through time. This variability is mostly related to the expansion/contraction of savanna ecosystems, showing that before the 20th century (i.e., the colonial and post-independence periods), fire mediated this region's vegetation changes at century and millennial time scales. Our results produce a long-term historical perspective on the variability of biomass burning in tropical savanna ecosystems, with relevance for assessing future trends resulting from ongoing climate and land-use changes.