Wednesday, May 11, 2011

Savanna ecosystem

Savanna ecosystem

                A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses. Some classification systems also recognize a grassland savanna from which trees are absent.This article deals only with savanna under the common definition of a grassy woodland with a significant woody plant component.

          It is often believed that savannas feature widely spaced, scattered trees. However, in many savannas, tree densities are higher and trees are more regularly spaced than in forest. Savannas are also characterized by seasonal water availability, with the majority of rainfall confined to one season. Savannas are associated with several types of biomes. Savannas are frequently in a transitional zone between forest and desert or prairie. Savanna covers approximately 20% of the earth's land area.

         Many grassy landscapes and mixed communities of trees, shrubs, and grasses were described as savanna before the middle of the 19th century, when the concept of a tropical savanna climate became established. The K√∂ppen climate classification system was strongly influenced by effects of temperature and precipitation upon tree growth, and his over-simplified assumptions resulted in a tropical savanna classification concept which resulted in it being considered as a "climatic climax" formation. The common usage meaning to describe vegetation now conflicts with a simplified yet widespread climatic concept meaning. The divergence has sometimes caused areas such as extensive savannas north and south of the Congo and Amazon Rivers to be excluded from mapped savanna categories.

         "Barrens" has been used almost interchangeably with savanna in different parts of North America. Sometimes Midwestern savanna were described as "grassland with trees". Different authors have defined the lower limits of savanna tree coverage as 5-10% and upper limits range from 25-80% of an area.
   
       Two factors common to all savanna environments are rainfall variations from year to year, and dry season wildfires. Savannas around the world are also dominated by tropical grasses which use the C4 type of photosynthesis.In the Americas, e.g. in Belize, Central America, savanna vegetation is similar from Mexico to South America and to the Caribbean. In North America nearby trees are of subtropical types, ranging from southwestern Pinyon pine to southeastern Long leaf Pine and northern chestnut oak.

Savanna ecoregions

Savanna ecoregions are of several different types:-
  • Tropical and subtropical savannas are classified with tropical and subtropical grasslands and shrub lands as the tropical and subtropical grasslands, savannas, and shrub lands biome. The savannas of Africa, including the Serengeti, famous for its wildlife, are typical of this type.
  • Temperate savannas are mid-latitude savannas with wetter summers and drier winters. They are classified with temperate savannas and shrub lands as the temperate grasslands, savannas, and shrub lands biome, that for example cover much of the Great Plains of the United States. (See areas such as the Central forest-grasslands transition).
  • Mediterranean savannas are mid-latitude savannas in Mediterranean climate regions, with mild, rainy winters and hot, dry summers, part of the Mediterranean forests, woodlands, and scrub biome. The oak tree savannas of California, part of the California chaparral and woodlands ecoregion, fall into this category.
  • Flooded savannas are savannas that are flooded seasonally or year-round. They are classified with flooded savannas as the flooded grasslands and savannas biome, which occurs mostly in the tropics and subtropics.
  • Mountain savannas are high-altitude savannas, located in a few spots around the world's high mountain regions, part of the mountain grasslands and shrub lands biome. The highland savannas of the Angolan Scarp savanna and woodlands ecoregion are an example.
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