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Savanna

Savanna

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 whether they occur in the tropics or in temperate zones.

Tropical and Subtropical Savannas

The extensive savannas of Africa, South America (locally known as cerrado) and Australia are primarily a result of either the climate or unique soil characteristics. Climatically derived savannas are warm all year but have distinct wet and dry (winter) seasons with annual rainfall varying widely from 30 to more than 100 centimeters of rainfall. What is critical about these climates is that during the dry season, rainfall amounts are very low. It is during these dry periods that the grasses are dormant and the trees experience water stress. Fires are also common at this time because the fine, dry fuel (dead foliage) that the grasses produce is very flammable. This allows fires, started by lightning or humans, to start easily and spread quickly. This combination of water stress and fire keeps the tree density low and distinguishes savanna from the adjacent forest.

Other savannas occur in areas where there are unique soil conditions. Although most tropical and subtropical savanna soils are poor in nutrients, some also have a hard crust or barrier at some depth in the soil. This crust separates the shallow soil layer that the grasses rely on for water, and which dries during periods of low rainfall, from the deeper soil layers that may retain moisture all year. Trees in these savannas are located where cracks in the crust occur. In these places the roots of trees can access this deep soil water. Such savannas are referred to as edaphic (related to the soil) savannas.

Savannas as Transitional Zones

Savannas in both tropical and temperate zones may occur along the edge of forests where the dominant vegetation shifts from trees to grasses. This tension zone (also called an ecotone) between forest and grassland may be relatively narrow or 50 to more than 100 kilometers wide. These savannas usually occur where annual rainfall is not quite high enough to support a closed forest and where fire is common. In North America, the aspen parkland of Canada is an example of a temperate savanna and in the United States there are the oak savannas that extend from Minnesota to Texas. Historically, the region occupied by oak savanna moved eastward in periods of aridity and with frequent fire. In contrast, with fire suppression these savannas may be converted to closed canopy forest. Much of the original extent of North American savanna was found on deep fertile soils, but greater than 99 percent has been lost because the land was so valuable for row crop agriculture.

Savannas and Biodiversity

Savannas contain a mixture of forest and grassland species, as well as some species unique to this ecosystem type. Because of this they are important zones of high biodiversity for both plants and animals. In North America, oak trees embedded in tallgrass prairie vegetation are joined by species specifically adapted to partial shade and frequent fire. In Africa and Australia, thorny acacia, eucalyptus, and baobab trees are scattered among the grasses. Savanna grasses are well adapted to fire because their buds are protected below ground. Like the grasses, some mature savanna tree species are resistant to fire, particularly compared to other forest species. Many savanna trees (such as the oaks) have thick insulating bark that protects the inner growing layers of the tree from fire. Others, such as the baobab tree in Africa, can store tremendous amounts of water in their bark and trunk, protecting them from both fire and drought. Other savanna trees are capable of resprouting vigorously after fire. Despite these adaptations, frequent fire decreases the density of trees in most savannas, with tree seedlings especially susceptible to fire.

Savannas support a diverse array of herbivores , especially so in the African savannas. Grassland grazers such as zebras and wildebeest are found with herbivores that feed on trees, such as giraffes and elephants. Elephants have been termed a keystone species of African savannas for the role they play in determining the density of trees. When elephant populations are low, acacia trees and shrubs may become so dense that the grasses are shaded out and grassland species disappear. Conversely, if elephant populations are too high, the trees may disappear along with those species that depend on woody plants for food and shelter. Another group of organisms that is particularly notable in tropical savannas for their diversity and numbers, if not their individual size, are the termites. Conspicuous above-ground termite mounds are present in Australian and African savannas, but most termites live underground without building mounds. Termites fill a very important role as one of the major decomposers in savannas. As much as 90 percent of the grass biomass that is decomposed in some savannas can be attributed to termites. Thus, these organisms are valuable for making nutrients available to plants.

Savanna Management and Conservation

Most of the larger savannas in tropical and subtropical regions are grazed by livestock. Fire is used as a management tool to keep the density of trees low and stimulate the productivity of the grasses. Savannas can produce abundant plant biomass for grazers in regions with high rainfall, but savannas in the driest regions and with the most nutrient-poor soils can support only a modest number of livestock. In Africa, human-induced shifts in the populations of large native herbivores (elephants) have altered the density of trees in some savannas, and conservation programs for these species must also take into account their effect on other species as well as the savanna vegetation.

Interest in conserving and restoring savanna ecosystems is also great in North America, where the greatest proportion of the original savanna ecosystems has been lost. Controlled fire and even mechanical removal of woody species is typically used in areas where dense shrubs and tree seedlings have displaced the grasses.

see also Biome; Fabaceae; Grasses; Grasslands.

Alan K. Knapp

Bibliography

Archibold, O. W. Ecology of World Vegetation. New York: Chapman & Hall, 1995.

Barbour, Michael G., Jack H. Burk, Wanna D. Pitts, Frank S. Gilliam, and Mark W. Schwartz. Terrestrial Plant Ecology. Menlo Park, CA: Addison Wesley Longman, Inc., 1998.

Cole, M. M. The Savannas: Biogeography and Geobotany. New York: Academic Press, 1986.

Daubenmire, Rexford. Plant Geography. New York: Academic Press, 1978.

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savanna

savanna or savannah (both: səvăn´ə), tropical or subtropical grassland lying on the margin of the trade wind belts. The climate of a savanna is characterized by a rainy period during the summer when the area is covered by grasses, and by a dry winter when the grasses wither. Savannas near the equatorial belt, e.g., in Nigeria, support clumps of trees. The most extensive savannas—all important pasture lands—are in Africa; others include the llanos and the campos of South America.

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savannah

savannah An extensive tropical vegetation dominated by grasses with varying admixtures of tall bushes and/or trees in open formation. Savannah occurs in diverse tropical environments, although most experience a dry season. Much savannah is no doubt climatic climax, although extensive tracts are anthropogenic fire climaxes and others are edaphically controlled; it is generally difficult to distinguish one type from the other.

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"savannah." A Dictionary of Ecology. . Encyclopedia.com. 22 Aug. 2017 <http://www.encyclopedia.com>.

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"savannah." A Dictionary of Ecology. . Retrieved August 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/savannah

savannah

savannah An extensive tropical vegetation dominated by grasses with varying admixtures of tall bushes and/or trees in open formation. Savannah occurs in diverse tropical environments, although most experience a dry season. Much savannah is no doubt climatic climax, although extensive tracts are anthropogenic fire climaxes, and it is generally difficult to distinguish one type from the other.

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savanna

savanna Plain with coarse grass and scattered tree growth, particularly the wide plains of tropical and subtropical regions. An extensive example is the savanna of the East African tableland.

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savanna

sa·van·na / səˈvanə/ (also sa·van·nah) • n. a grassy plain in tropical and subtropical regions, with few trees.

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savanna

savanna See grassland.

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