A Continental-Scale Analysis of Tree Cover in African Savannas

We present a continental scale analysis that explores the processes controlling woody community structure in tropical savannas of Africa. We analyze how biotic and abiotic factors interact to promote and modify tree cover, examine alternative ecological hypotheses with semi-empirical modeling and Bayesian statistics and quantify disturbance effects using satellite estimates of tree cover. Woody community structure across African savannas is best represented by a sigmoidal response of tree cover to mean annual rainfall (MAP), with a dependency on soil texture, which is modified by separate effects of fire, domestic livestock, human population density and cultivation intensity. This model explains ~66% of the variance in tree cover, and appears to be consistent across all the savanna regions. The analysis provides new understanding of the importance and interaction of environmental and disturbance factors that create the broad spatial patterns of tree cover observed in African savannas. Woody cover increases with rainfall, but is modified by disturbances. These "perturbation" effects depend on MAP regimes: in arid savannas (MAP<400mm), they are generally small (less than 1% decrease in cover), while in semi-arid and mesic savannas (400-1600mm MAP), perturbations result in an average 2 to 23% decrease in cover; fire frequency and human population have more influence than cattle, and cultivation appears, on average, to lead to small increases in woody cover. Wet savannas (1600-2200mm MAP) are controlled by perturbations that prevent canopy closure and reduce tree cover by 24-34%. Full understanding of the processes determining savanna structure requires consideration of resource limitation and disturbance dynamics.