Response of tree cover to interannual rainfall variability: the balance of direct and indirect effects

Abstract:

Climate change studies predict an increase both in seasonal and interannual rainfall variability. The impact of such variability on vegetation composition and ecosystem processes is not well understood. Using satellite data or model simulations, previous studies have reported mixed responses of tree cover to interannual rainfall variability in the tropic (i.e., neutral, positive, or negative). The underlying mechanisms behind such patterns, however, are still unclear. This study uses satellite data and develops a new mechanistic model to investigate the response of tree cover to increasing interannual rainfall variability along Kalahari Transect in Southern Africa. This model accounts for the competition between trees and grasses in access to soil water, fire-induced disturbance, and a demographic bottleneck in tree recruitment. Both satellite data and model results show an increase in tree cover with increasing interannual rainfall fluctuations in dry environments (i.e., mean annual rainfall, MAP<700 mm) but a decrease in tree cover in wet environments (i.e., MAP>700 mm). In dry environments, an increase in interannual rainfall variability disfavors grasses with shallow roots, thereby reducing fire-induced mortality in tree seedlings and opening windows of opportunity for tree recruitment (i.e., indirect effects). In wet environments, an increase in interannual rainfall variability leads to more instances of mass flow below the rooting zone of tree seedlings and thus reduces tree recruitment rate (i.e., direct effects). This study highlights the importance of accounting for the direct effects of rainfall variability on trees and the indirect effects mediated by tree-grass interactions to better understand how tree cover respond to increase in rainfall variability along rainfall gradients.