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How do sandy soils favor grasses in savanna ecosystems?

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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.

Sandy soils favor the grassy component of savanna ecosystems because of their specific water-retention properties. These soils are capable of supplying abundant moisture during rainy periods, which allows grasses to grow rapidly. However, sandy soils also dry out almost completely in the absence of rain.1

Because grasses in these environments are adapted to survive long dry periods as dormant buds near the soil surface or as annual seeds, they are well-suited to this cycle. In contrast, trees generally require access to water in at least small amounts throughout all seasons, necessitating deeper soil layers that can provide a consistent supply of moisture. Consequently, sandy soils support grass-rich savannas with few trees, whereas deeper, coarser soils are more favorable to tree-rich environments.1