Demographic variability in the tree component of savannas, arising from both spatial and temporal variability in fire, herbivory, and climate, is thought to be a key driver maintaining tree-grass co-existence in savannas. Modeling work has shown that this variability can be vital to the persistence of savanna as a biome and that variability in climate, herbivory and fire affect tree establishment. However, empirical evaluations of the idea that savanna tree establishment is variable have so far been limited to tests of the effects of major events (such as the East African rinderpest epidemic) on tree establishment. The degree to which tree establishment varies in savannas under less extreme variability in climate, herbivory and fire remains unknown.
We tested whether establishment of adult trees was constant or episodic using tree ring counting of three Acacia species in Hluhluwe iMfolozi Park in South Africa. Patterns of establishment of adult trees were analyzed at three spatial scales for temporal and spatial patchiness. Establishment of adult trees was not constant for any of the three species of Acacia, and all three species showed significant spatial patchiness and larger-scale temporal variability in establishment patterns. Spatial patchiness highlights the importance of landscape processes like fire and herbivory, while larger-scale temporal variability indicates that climatic variability also structures tree demography.
These results suggest that current tree cover in savannas is a product of ecological history and is not necessarily determined by current ecological processes. An historical perspective is essential for interpreting patterns of tree cover in savannas, in models of the effects of climate, fire and herbivory (especially elephants), in experimental work, and in analyses of continental patterns of tree cover alike.