Edge fires drive the shape and stability of tropical forests

The shape of the areas we protect may be more important than the size, too thin exposes them to death by fire, too round inhibits growth

In tropical regions, fires that start in grassland typically burn only the edges of forested regions, but do not spread within the forest. This behaviour starkly contrasts with wildfires in temperate forests, where the fire spreads readily through the trees via local ignition of neighbouring trees.

Using our spatial model, we showed that commonly observed ‘edge effects’ drive the stability and fate of individual forest patches. Importantly, the two competing processes of exposure to fire and propagative growth do not balance because of their very different time scales. For a forest of a given size, having a large perimeter means that it is more likely to be exposed to fire before it receives any benefit from increased tree cover due to propagative growth. Contrarily, if too dense, a forest will not grow sufficiently quickly to offset its exposure to fire, which will lead to an increase in perimeter by encroachment of grassland at the forest edges. The forests that do survive are those that end up with intermediate shapes to ensure enough perimeter growth to offset their exposure to fire.

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