How can heavier rainfall change dry savannas into shrubland or forest?

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Flat savanna plain with scattered acacia trees under a partly cloudy sky in Serengeti National Park, Tanzania. — Savanna A savanna marked with acacia trees in Serengeti National Park, Tanzania. Africa's Serengeti Plain is commonly used as an example of the savanna biome—a hot, seasonally dry ecological region characterized by an open tree canopy (i.e., scattered trees) above an understory of continuous grasses.

In dry savannas, an increase in rainfall can shift the vegetation toward greater shrub and tree coverage, potentially transforming the landscape into scrubland or forest.1

This transition occurs because the tree component of a savanna generally becomes more prominent as precipitation increases. While savannas are defined by a balance between grasses and woody plants, heavier rainfall provides the necessary moisture to support more vigorous tree growth. Furthermore, an increase in tree density can reduce the frequency and intensity of wildfires, which are critical for maintaining the open, grass-dominated structure of a savanna by killing young saplings. When these fires are suppressed or reduced, woody plants can proliferate, leading to a shift in the ecosystem's profile.234