Ecological thresholds have been recognized to generate alternate stable states in a diversity of ecological systems. Our research in central Brazil has permitted us to quantify the factors that give rise to two such thresholds that govern the distribution of savanna and forest. The first, referred to here as the fire-resistance threshold is defined by the size that a growing tree must reach to avoid topkill (i.e. mortality of the aboveground stem following fire). The fire-suppression threshold is defined by the tree canopy density at which ecosystem flammability is greatly reduced.
The fire-resistance threshold is governed primarily by the bark thickness required to insulate the stem against lethal temperatures, while the fire-suppression threshold is governed by the canopy density at which savanna grasses are suppressed. These thresholds provide a mechanistic explanation for how soil resources and species traits interact with fire regimes to determine the distribution of the savanna and forest biomes. Specifically, high resource availability reduces the time required for both thresholds to be reached during a fire-free interval. Savanna tree species tend to accumulate bark thickness at a faster rate than forest species, so savanna species reach the fire-resistant threshold more quickly. On the other hand, forest species have more regular architecture and tend to be more shade tolerant than savanna species, so stands containing forest species attain the fire-suppression threshold more quickly. These differences can explain common patterns observed in the Brazilian savannas, including abrupt savanna-forest boundaries, largely non-overlapping distributions of savanna and forest species, and the predominance of forest on high-nutrient soils.