Grass-dominated ecosystems that contain a significant number of widely spaced trees are termed savannas. Trees may make up as little as 5 percent or as much as 30 percent of the cover of all plants in savannas, but grasses and grasslike plants form a continuous ground cover. Originally, savanna was a term used to describe primarily tropical and subtropical grasslands, which usually have more woody plants than temperate grasslands. These tropical and subtropical savannas occupy large land areas. Almost 65 percent of Africa is covered by savanna, much of the northern region of Australia has savanna vegetation and South America has extensive savannas. Worldwide, estimated land cover by savanna is nearly 20 percent.

Savannas may be a product of climatic factors, they may result from unique soil types, or they may be narrow to broad transitional zones between forests and grasslands. Fire and periods of water limitation are present in all savannas...