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Vegetation–fire feedbacks are important for determining the distribution of forest and savanna. To understand how vegetation structure controls these feedbacks, we quantified flammability across gradients of tree density from grassland to forest in the Brazilian Cerrado. We experimentally burned 102 plots, for which we measured vegetation structure, fuels, microclimate, ignition success and fire behavior. Tree density had strong negative effects on ignition success, rate of spread, fire-line intensity and flame height. Declining grass biomass was the principal cause of this decline in flammability as tree density increased, but increasing fuel moisture contributed. Although the response of flammability to tree cover often is portrayed as an abrupt, largely invariant threshold, we found the response to be gradual, with considerable variability driven largely by temporal changes in atmospheric humidity. Even when accounting for humidity, flammability at intermediate tree densities cannot be predicted reliably. Fire spread in savanna–forest mosaics is not as deterministic as often assumed, but may appear so where vegetation boundaries are already sharp. Where transitions are diffuse, fire spread is difficult to predict, but should become increasingly predictable over multiple fire cycles, as boundaries are progressively sharpened until flammability appears to respond in a threshold-like manner.

Department of Plant and Microbial Biology North Carolina State University, Campus Box 7612

Department of Fisheries Wildlife and Conservation Biology North Carolina State University

Departamento de Ciências Ambientais Universidade Federal Rural do Rio de Janeiro

Laboratório de Ecologia e Hidrologia Florestal Floresta Estadual de Assis Instituto Florestal

Departamento de Biologia Universidade Estadual Paulista (UNESP), Campus de Jaboticabal

Departamento de Biologia Universidade Estadual Paulista (UNESP), Campus de Jaboticabal