Fire, resprouting and variability: a recipe for grass-tree coexistence in savanna

1 Savanna ecosystems are characterized by the codominance of two different life forms: grasses and trees. An operational understanding of how these tw o different life forms coexist is essential for understanding savanna funct ion and for predicting its response to future environmental change. 2 The existing model, which proposes that grasses and trees coexist by a se paration of rooting niches, is not supported by recent empirical investigat ions. Our aim was to define an alternative mechanism of grass-tree coexiste nce in savanna ecosystems. The model we have built concentrates on life his tory-disturbance interactions between grasses and trees. 3 The model demonstrates coexistence for a wide range of environmental cond itions, and exhibits long periods of slow decline in adult tree numbers int erspersed with relatively infrequent recruitment events. Recruitment is con trolled by rainfall, which limits seedling establishment, and fire, which p revents recruitment into adult size classes. Decline in adult tree numbers is the result of continuing, but low levels, of adult mortality. Both aspec ts of the dynamics are consistent with an established non-equilibrium mecha nism of coexistence (the storage effect). 4 A sensitivity analysis indicated that data on tree resprouting ability, s tem growth rates and the relationship between seedling establishment and we t season drought are essential for predicting both the range of conditions for which coexistence is possible and the response of savanna ecosystems to environmental change. 5 Our analysis suggests that understanding grass-tree interactions in savan na requires consideration of the long-term effects of life history-disturba nce interactions on demography, rather than the fine-scale effects of resou rce competition on physiological performance.