Savanna definition and open-canopy structure

Summary

A savanna (or savannah) is a mixed woodland–grassland ecosystem in which trees are spaced widely enough that the canopy does not close. This open-canopy structure lets substantial sunlight reach the ground, supporting an unbroken herbaceous layer dominated mainly by grasses. Savannas can persist even where tree density is relatively high, and they are often transitional between forest and grassland or desert. Savannas are commonly described by their vegetation structure: 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). Although savannas are frequently pictured as having widely scattered trees, many savannas actually have higher and more regularly spaced tree densities than forests, while still maintaining an open canopy. They also tend to have seasonal water availability, with most rainfall concentrated in one season, and they are shaped by factors such as fire and grazing that help maintain the open structure.

Key Points

• 1

  Savannas are defined by an open canopy that allows enough light for a continuous grass-dominated herb layer.

• 2

  Savanna structure varies into four forms: savanna woodland, tree savanna, shrub savanna, and grass savanna.

• 3

  Tree spacing in savannas prevents canopy closure even when tree density can be relatively high.

• 4

  Seasonal rainfall and recurring fire/grazing help maintain the open-canopy savanna state.