A savanna or savannah is a mixed woodland-grassland biome and ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of grasses. Four savanna forms exist; savanna woodland, where trees and shrubs form a light canopy; tree savanna, with scattered trees and shrubs; shrub savanna, with distributed shrubs; and grass savanna, where trees and shrubs are mostly nonexistent.