The frequency and intensity of rainfall determine the temporal dynamics of soil moisture, with potentially major consequences for plant communities. Predictions that climate change will cause shifts to less frequent, more intense rainfall, even where mean annual rainfall does not change, highlight the importance of understanding relationships between rainfall climatology and vegetation. In tropical savannas – a widespread biome notable for coexistence of trees and grasses – it is uncertain whether less frequent, more intense rainfall promotes or suppresses tree cover. Small-scale experiments suggest that greater rainfall intensity favors deeper-rooted trees over shallow-rooted grasses by promoting deeper infiltration, such that we would expect tree cover to increase with rainfall intensity, but continental-scale analyses of tree cover and rainfall climatology have found the opposite pattern. However, past analyses have not considered how the response of tree cover to rainfall intensity may depend on environmental context, particularly soil texture (which changes infiltration dynamics) and fire (which feeds back on biotic responses to climate). Here, we analyzed remote sensing data on tree cover, rainfall, fire frequency, and soil texture across savannas in sub-Saharan Africa. Our statistical analyses tested how these factors and interactions between them determine savanna tree cover.
Results/Conclusions
Although savanna tree cover generally declined with increasing rainfall intensity for a given mean wet-season rainfall, this relationship depended on both soil texture and fire. Tree cover declined most dramatically with increasing rainfall intensity on soils with low sand content. On sandier soils, by contrast, tree cover responded negligibly or even increased with less frequent, more intense rainfall. Accounting for fire was also important, as tree cover declined with increasing fire frequency, but it was less clear whether the effect of fire interacted meaningfully with rainfall. We suggest that the influence of soil texture is a consequence of differences in infiltration and the feasibility of trees taking advantage of deeper water availability in the event of larger storms, or could also be an outcome of differences in runoff or waterlogging. Our findings imply that the response of savanna vegetation to potential changes in rainfall regimes will not look the same everywhere – a shift to less frequent, more intense rainfall could be met with simultaneous decreases in tree cover on clay soils and increases on sandy soils. Projections of future vegetation change must account for interactions between climate, soils, and fire.