Savanna
Savanna, also savannah, tropical grassland with a scattering of shrubs and small and large trees. Savannas may result from soil conditions, from periodic fires caused by lightning or set by humans, or from climatic influences. Climatically determined savannas, as found in western and southwestern Africa, develop in regions with marked wet and dry seasons, where rainfall ranges between 100 and 400 mm (4 and 16 in) a year. These savannas vary from open-canopied forests with a grassy understory to true savannas in which grasses are dominant. When the rainfall is 100 to 200 mm (4 to 8 in), generally only grasses can survive the dry season. When rainfall reaches 300 mm (12 in), the soil holds enough water to sustain shrubs through the dry season as well. When rainfall exceeds 300 mm, enough water is left to support solitary trees; and when rainfall exceeds 400 mm, enough moisture remains during the dry season to allow trees to grow more densely and to form a canopy, shading out the grasses