Climate, herbivory, and fire controls on tropical African forest for the last 60ka

Vegetation history in Africa is generally assumed to be strongly related to climate. However, disturbance by fire, herbivory, and human land use is also important to maintaining vegetation structure and may interact with climate to create tipping points for ecosystems. During the last 60ka, the transition from glacial aridity to increased moisture in much of northern and equatorial Africa led to widespread forest expansion; however, forests in southeastern Africa do not appear to have changed dramatically during arid periods associated with high-latitude cooling, suggesting a more complex biogeographic history. Here we present analyses of fossil pollen, charcoal, and Sporormiella (dung fungus) along with multiproxy climate reconstructions from a 60kyr record from central Lake Tanganyika, southeast Africa, which illustrate the interplay of climate and disturbance regimes in shaping vegetation composition and structure. We observe that forests dominated the region during the last glacial period despite decreased rainfall. At the end of the glacial, forest opening at 17.5 ka followed warming temperatures but preceded wetting, suggesting that water stress and disturbance from fire and herbivory affected initial landscape transformation. Our Sporormiella record indicates that mega-herbivore populations increased in the early Holocene. This higher animal density increased plant species richness and encouraged landscape heterogeneity until the mid-Holocene. At this time, regional drying followed by the onset of the Iron Age in the late Holocene resulted in expansion of thicket, more open woodland, and disturbance taxa that still characterize the landscape today. This work has important implications for the understanding the how climate change will alter the distribution of lowland and highland forests, in particular how disturbance processes influence the rate of vegetation change.