Human impacts and bistability of forest-savanna in Africa

Abstract

Bimodality in the distribution of tropical tree cover at intermediate environmental conditions is cited as evidence for the presence of bistability in the forest-savanna system. However, bimodality can also emerge because of human activities that cause changes in tree cover, such as the deforestation resulting from expansion in settlements, agricultural and pasture lands. Also, the impact of such activities on tree cover can persist beyond the site of disturbance. Here, I study the relation between human influence and tree-cover bimodality in sub-Saharan Africa, in order to draw conclusions regarding the vegetation dynamics prevalent in the region. I divide remote-sensed vegetation data into regions with low and high degree of human influence. In addition, I devise an environmental predictor of tree cover using relevant climatic and edaphic variables. It is hypothesized that a higher extent of bimodality will be observed in regions with high human influence, because of bimodality arising from human activities, as opposed to bistability. The results show that bimodality is found in regions with both high and low human impacts. However, the extent of bimodality is lower in the regions with low human influence. This indicates that while bistability is restricted to smaller spatial extents than previously assumed, it is common across sub-Saharan Africa, regardless of the presence of human influence.