A savanna or savannah is a grassland 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.
Savannas maintain an open canopy despite a high tree density.
It is often believed that savannas feature widely spaced, scattered
trees. However, in many savannas, tree densities are higher and trees
are more regularly spaced than in forests. The South American savanna types cerrado sensu stricto and cerrado dense typically have densities of trees similar to or higher than that found in South American tropical forests,
with savanna ranging 800–3300 trees/ha and adjacent forests with
800–2000 trees/ha. Similarly Guinean savanna has 129 trees/ha, compared
to 103 for riparian forest,
while Eastern Australian sclerophyll forests have average tree
densities of approximately 100 per hectare, comparable to savannas in
the same region.
Savannas are also characterised by seasonal water availability, with
the majority of rainfall confined to one season; they are associated
with several types of biomes, and are frequently in a transitional zone between forest and desert or grassland. Savanna covers approximately 20% of the Earth's land area.