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A savanna or savannah is a grassland with widely spaced trees, and occurs in several types of biomes. In savannas, grasses and trees are co-dominant vegetation types, with trees and grasses often alternating in dominance over time. The herbaceous layer is usually a mixture of grasses and herbs with trees and shrubs scattered individually or in small clumps. Savannas are frequently seen as a transitional zone, occurring between forest or woodland regions and grassland or desert regions. Savannas are targets of regular fires. Most savannas experience fire at least twice a decade and annual fires are common in many savanna types. These fires are usually confined to the herbaceous layer and do little long term damage to mature trees. However these fires do serve to either kill or suppress tree seedlings, thus preventing the establishment of a continuous tree canopy which would prevent further grass growth. Browsing animals such as elephants, antelope and deer also play an important role in supressing tree growth in savannas. Savannas appear to be the result of human use of fire. For example Native Americans created subtropical savannas by periodic burning in some areas of the US southeastern coast where fire-resistant Longleaf Pine was the dominant species. Aboriginal burning appears to have been responsible for the widespread occurance of savanna in tropical Australia and New Guinea and savannas in India are a creation of human fire use . With the removal or lateration of traditional burning regimes many savannas are being replaced by forest and shrub thickets with little herbaceous layer. Although the term savanna is believed to have originally come from an Amerindian word describing "land which is without trees but with much grass either tall or short" (Oviedo y Valdes, 1535), by the late 1800s it was used to mean "land with both grass and trees". It now refers to land with grass and either scattered trees, or an open canopy of trees. Savannah, Georgia is named after such an area. Savanna ecoregions are of several different types:
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