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Subtropical and Tropical
The tropical and subtropical savannas typically occur either because of the climate or soil conditions. Most occur in areas with warm or hot climates and annual rainfalls of between 30 and 40 inches. The rainfall is concentrated in a wet season lasting six to eight months a year and is followed by a long dry season prone to fires. If the heavy rainfall was spread out across the year, many areas that support savannas would become tropical forests, the San Diego Zoo reports. Some savannas are the result of hard crust separating the shallow soil that the grasses grow in from deeper, moisture-rich soil. In these areas, the trees grow from cracks in the crust, their roots tapping the deep, moist soil.
Temperate Savannas
Some savannas are found in the temperate climates of North America. For example, in the United States, oak savannas were once common from Minnesota to Texas, encyclopedia.com states. These savannas grow in areas that don't receive enough rainfall for forests and have high risk of fire. If fire danger is controlled, they can be converted to forests. About 99 percent of the oak savannas in the United States have been turned into farmland.
Fire
Fire plays an important role in the savanna, according to the University of California Museum of Paleontology. These fires typically occur at the height of the dry season and are often caused by people. For example, poachers often burn away dead grass so they can see their prey. The majority of the animals killed by the fires are insects. Most of the larger animals quickly escape. Predators such as birds eat up the insects, mice and lizards that are killed or flee the fire. The dead leaves and stems of the grasses and shrubs are destroyed by the fire but their roots survive underground and send up new growth when the rain returns. Trees survive the fire with a variety of adaptations, such as water-retaining trunks and fire-resistant bark.
Biodiversity
Biologists consider savannas areas as having a high biodiversity because of the variety of plants and animals that live within them, the Global Environment website states. They contain forest and grassland species plus some species found only in savannas. For example, oak savannas contain oak trees, prairie grasses and several plants adapted to partial shade and fire. In subtropical and tropical savannas, the grasses share the landscape with acacia, baobab trees and eucalyptus. The rich abundance of plant life found in savannas support herbivores. For example, the African savannas support wildebeests, zebras, giraffes and elephants. Elephants play an especially important role. When there aren't enough elephants, trees and shrubs become too dense and the grasses die off. When there are too many elephants, trees die off and species that depend on shade and wood disappear.