Savanna–Forest Coexistence Across a Fire Gradient

Tropical forests and savannas can co-occur in a range of macro-environmental conditions. In these conditions, disturbances and resource availability are thought to control savanna and forest transitions, although the mechanisms involved are disputed. We hypothesized that, in Neotropical regions where fire activity is high, fire is the main factor controlling functional differences between savanna and forest, as well as their relative resistance to biome shifts. We sampled plant functional traits and soil and determined fire history, for 198 plots distributed across three landscapes with distinct fire frequencies (high, mid, and low). In each landscape, plots covered a woody cover gradient (from wooded grasslands to forests). We tested whether the sharpness and the magnitude of the functional distinction between savanna and forest were affected by fire. We also computed the environmental hyperspace (niche space) to evaluate how biome relative stability changed in relation to fire. Functional thresholds were detected only in the high and mid landscapes, where savanna and forest plots formed a multidimensional bimodal distribution in functional trait space. The stability of savannas in relation to forest increased abruptly with fire, whereas functional differences between forest and savanna increased gradually. Our results suggest that savanna can occur as an alternative vegetation state to forest where a fire burns every 18 years (on average), but higher frequencies are required for savannas to occupy large unique portions of the environmental niche space.