Vegetation Change in East Africa Under the Influence of Tectonic Uplift

Abstract:

Today, when we look at the tropical latitudes all around the world, we see rainforests more or less everywhere, except the easternmost parts of tropical East Africa. Why do the environmental conditions elsewhere in the tropics support the rainforest biomes but not in East Africa? The suggested explanation for this is the progressive tectonic uplift of the region. The uplift of eastern African topography, together with the changes in Earth's orbital forcing, resulted in the aridification of the region over the past 10 million years. It follows that there must have been rainforest environments in tropical East Africa before the uplift began and there is evidence to support this claim. Then, it raises the questions about how extensive these forests were, when and under which environmental conditions they started to fragment and grasslands took over. To address these questions we ran a dynamic vegetation model under experimental scenarios with varying precipitation, temperature and atmospheric CO₂ concentration values. Our results show that even an increase of rainfall by 100% with respect to present-day conditions is not enough by itself for sustaining forest biomes at the easternmost parts of tropical East Africa. Within the range of atmospheric CO₂ concentrations estimated for the late Miocene (9 Ma) [250-320 ppmv], decrease in temperature must also accompany the increase in rainfall to support the woody vegetation. Although an increase in arboreal cover at the expense of herbaceous vegetation could clearly be simulated all over the region, a continuous forest cover was only established at the south of the equator which fragmented with the decreasing rainfall and increasing temperatures in the region.