Savanna Ecology, Types, and Threats

Savanna definition and open-canopy structure

A savanna (or savannah) is a mixed woodland–grassland ecosystem (grassy woodland) defined primarily by an open canopy: trees are spaced widely enough that the canopy does not close. This openness lets sufficient sunlight reach the ground, supporting an unbroken herbaceous layer dominated mainly by grasses. Savannas can occur even where tree density is relatively high, and the common idea of “widely scattered” trees is not always accurate—many savannas have higher and more regular tree spacing than forests. They are also typically shaped by seasonal water availability (most rainfall concentrated in one season) and are often transitional between forest and desert or grassland. Four structural forms are recognized: savanna woodland (light canopy of trees and shrubs), tree savanna (scattered trees and shrubs), shrub savanna (distributed shrubs), and grass savanna (trees and shrubs mostly absent).

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Four savanna forms and vegetation composition

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