Tree-grass coexistence in savannas revisited - insights from an examination of assumptions and mechanisms invoked in existing models

Abstract

Several explanations for the persistence of tree–grass mixtures in savannas have beenadvanced thus far. In general, these either concentrate on competition-basedmechanisms, where niche separation with respect to limiting resources such as waterlead to tree–grass coexistence, or demographic mechanisms, where factors such as fire,herbivory and rainfall variability promote tree–grass persistence through their dissimilareffects on different life-history stages of trees. Tests of these models have been largelysite-specific, and although different models find support in empirical data from somesavanna sites, enough dissenting evidence exists from others to question their validity asgeneral mechanisms of tree–grass coexistence. This lack of consensus on determinantsof savanna structure and function arises because different models: (i) focus on differentdemographic stages of trees, (ii) focus on different limiting factors of tree establishment,and (iii) emphasize different subsets of the potential interactions between trees andgrasses. Furthermore, models differ in terms of the most basic assumptions as towhether trees or grasses are the better competitors. We believe an integration ofcompetition-based and demographic approaches is required if a comprehensive modelthat explains both coexistence and the relative productivity of the tree and grasscomponents across the diverse savannas of the world is to emerge. As a first steptowards this end, we outline a conceptual framework that integrates existing approachesand applies them explicitly to different life-history stage of tree