Studies of tree cover dynamics and tree-grass ratios in savannas have progressed along two parallel trajectories: the demographic/trophic perspective emphasizes the role of disturbance and herbivory, while the mechanistic/ecophysiological perspective focuses on tree-grass competition for water. I draw on research in the Serengeti ecosystem of East Africa to demonstrate the importance of the top-down processes highlighted by the first approach while exploring some of its limitations. I show, for example, that hydrology constrains the set of conditions under which fire and herbivores are able to influence tree cover and the distribution of woodland and grassland across the landscape. I then draw on my research in South African savannas to more broadly explore differences between trees and grasses in terms of water use and partitioning, with an emphasis on differences in rooting depth. I propose that the importance of such differences may vary systematically across environmental gradients, with important consequences for savanna responses to climate change.