Savanna woody plant dynamics: the role of fire and herbivory, separately and synergistically

Although the demography of woody plants in savannas has long been shown to be due to many factors, there still is no consensus as to the relative importance of the top-down processes of fire and herbivory, nor on how fire and herbivory affect plant demography. We review the recent literature and suggest that further progress depends on the following: (i) a demographic framework with clear terminology and which focuses on recruitment, transitions and mortality, (ii) an understanding of mechanisms of how fire actually damages plants and how plants survive and out-grow this damage, mainly through height, bark thickness or diameter growth, (iii) an understanding of how losses in biomass due to herbivory may affect plant demography and, (iv) a consideration of interactions between fire and herbivory. Our synthesis suggests (i) strong recruitment limitation as well as some evidence of transition limitation by both fire and herbivory, (ii) that in some cases herbivory alone, notably by elephants and impala, can be more significant than fire alone, on woody plant population size, (iii) that fire and herbivory together are a lethal combination for woody plants and, (iv) that differences in strategies and responses of savanna plants to fire and herbivory are poorly explored.