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How do trees increase soil fertility in the areas around their trunks?

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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 savanna ecosystems, where overall soil fertility is typically low, trees play a vital role in creating localized nutrient-rich areas around their trunks.

This process occurs primarily through two mechanisms:

Nutrient Cycling: Trees possess deep root systems that draw mineral nutrients up from deeper soil layers that are otherwise inaccessible to surface vegetation.1
Litter Decomposition: As trees shed their leaves and other organic debris, this material accumulates on the soil surface near the trunk. The subsequent decomposition of this litter releases these stored nutrients back into the immediate soil, resulting in higher fertility levels near the tree compared to the open areas between trees.1

This dynamic contributes to the small-scale variations in soil quality often observed across savanna landscapes.1