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Savanna

   Superficially savanna is regarded as a 'grassland'. However this general description fails to identify the diversity of the biome. A number of academics have categorised savanna into different sub zones. Werger (1983) defines four formation types.
  • Grassland – tree coverage is less than 1% of the surface.
  • Savanna – trees coverage spans from 1 – 10 %. In some areas scrub such as thickets 1 –10%.
  • Dense savanna – trees or shrub coverage spans 10 – 50%.
  • Savanna woodland – dominant tree layer. Canopy has a coverage that spans 50–90 %.

   Some shrubs will appear in undergrowth, there will be some areas of developed grass.
There are five factors that together determine what form of savanna is successful at any particular location. These are climate, edaphic, hydrological and geomorphologic factors, fire and grazing (Bourliere and Hadley 1992).

   Savanna is a dynamic system found in every tropical climatic area of the globe (Collinson 1988), located mainly between the latitudes of 5 degrees and 15 degrees north and south of the equator (Park 2001). Nix (1992) argues savanna can be found as far as 30 degrees north and south of the equator. The vegetation zone can be found in the interior of continents, sandwiched midway between the open desert steppes and the humid equatorial zones, it covers approximately 11.6 % of the earths land surface (Briggs et al 1997). The physiognomic composition of savanna (the external features) will alter with increased aridity. Generally the vegetation of the savanna becomes less dense as it moves further away from the equator (Hobley 1970). The vast majority of savanna is secondary; this is mainly attributable to anthropogenic or human activities such as cultivation (Hopkins 1977). It is nearly impossible to categorise contemporary savannas as “natural” or “anthropogenic” (Bourliere and Hadley 1992). Examples of global savanna include Llanos (Venezuela), the Campos (Brazilian Highlands), parts of Mexico and Northern Australia (Waugh 2001)

   The majority of the world’s savanna is located in Africa, the remainder of the essay details the determining factors of African Savannah, the communities that exist and the specialisation of members of these communities.

Abiotic Factors

Climate

   Savanna is formed in regions with a tropical continental climate where high temperatures occur. These regions have a marked wet and dry season with high evaporation rates. The wet season occurs when the sun travels overhead in correspondence with the Inter Tropical Convergence Zone (ITCZ) and the equatorial low pressure belt causing heavy convectional storms (80 % of rainfall can occur in four or five months). Temperatures can decrease slightly in the wet season. As the ITCZ moves away the area is left with ‘tradewinds’ bringing warm, moisture less air (Waugh 2001). This reduced humidity is a characteristic of the dry season. The biome is drier than the rainforest (Park 2001).

   In parts of Africa rainfall ranges can vary between 100mm and 400mm per year. Precipitation levels have a direct influence on the type of vegetation that can survive.
  • If the region experiences very low rainfall approximately between 100 – 200mm only grasses will survive the dry season.
  • If the region experiences rainfall approximately 250-300 mm the soil will be able to retain enough moisture to sustain shrubs and grasses through the dry season.
  • If the region experiences rainfall in excess of 300mm there will be ample water to support solitary trees.
  • If the region experiences rainfall in excess of 400mm the soil can retain enough water through the dry season, which can facilitate tree growth. The trees should be successful enough to form a canopy that will shade out grasses. (www.pupilvision.com)

   Some regions are exceptions for the above guidelines. For example Werger (1983) details areas in West Africa that support only ephemeral grasses when the average annual rainfall is as high as 300mm. This region will only support woody vegetation when the average annual rainfall is over 500mm.

   Frost can also influence the vegetation types. Frost damaged trees have been reported in some tropical lands (southern Brazil, southern Transvaal, highlands of Angola etc.) (Collinson 1988). Where frost is an ordinary characteristic of the environment broad-leaved or fine-leaved thorny woodlands can be successful (Werger 1983)
Savanna organisms have developed and adopted a range of behavioural, morphological and physiological methods to deal with the seasonality and unpredictable climate of the biome (Bourliere and Hadley 1992).

Soil

   Soil can act as a primary of indirect factor in determining savanna vegetation. A number of soil types can be found supporting vegetation in the savanna, therefore it is difficult to define a typical savanna soil (Montgomery and Askew 1992). There are a variety of soil types found supporting savanna. According to Young’s (1976) classification scheme soils include ferruginous (dominated by hydrous and kaolinite oxides of aluminium and iron) and both weathered and weakly ferralitic (similar to ferruginous with a lower cation exchange capacity and base saturation level). The distribution of these soils is dependent on the climate, geology and geomorphology (Montgomery and Askew 1992). Savanna only has a thin layer of humus, produced mainly by the decomposition of plant and less frequently animal matter (Hopkins 1977).

   Soil of the savanna is porous, during the wet season rapid leaching occurs, removing silica from the upper layers and depositing red-coloured oxides of aluminium and iron (ferruginization) (Waugh 2000). The soil suffers from a lack of nutrients; this problem is exacerbated when slope processes are active. Ferruginous soils tend to be acidic and soft. However, during the dry season if the soil is exposed at the surface a laterite crust may form. This outer layer can impede drainage and plant root penetration and also leave the upper layer more vulnerable to erosion from wind and water (Waugh 2000).

   Vegetation will differ depending on the texture and depth of the soil. Soils composed of a fine clay or silt are able to retain adequate water in the upper layers, therefore will only support grasses and forbs (Tivy 1993). In regions with an annual rainfall exceeding 900mm Werger states “broad-leaved woodlands are likely with fibrous tuft grasses adapted to leached soils”. Woody vegetation is also supported by heavy clay soils, with grassland or shrub communities found in dryer regions and vegetation with thinly divided compound foliage in wetter areas. Extreme halomorphic soils (intrazonal soils which have developed in regions where salt have collected at the surface) will only support low, open vegetation such as grassland, with a variable amount of trees or shrubs.

   The nutrient cycle is fast, due to the rapid breakdown of organic matter by soil organisms. The high temperatures of the tropics facilitate chemical action. Silica can often make up a large proportion of the nutrient budget and is distinctly soluble in the Tropics. This can often lead to the formation of amorphous silica in some leaves. Recycled nutrients that are returned to the soil are more resistant to leaching. This can be attributed to lower rainfall leading to milder leaching and weathering. Clay soils also have a higher cation exchange capacity to construct more efficient bonds with nutrients. The majority of nutrient loss in these regions can be credited to soil erosion. Agricultural activities such as overgrazing and tillage leave the soil vulnerable during heavy rainstorms (Briggs et al 1997).

Hydrological and Geomorphological

  Vegetation type is also heavily related to the altitude and precipitation levels of the plateau surface. Elevated areas with high humidity (found in Zimbabwe) are severely dissected, “capped by weathered ferralitic soils and associated latheritic horizons which form steep sharp cliffs” (Tivy 1993). As altitude decreases the lower plateaux often covered with sand extend into drier areas (Cole 1986). Consequently topography and drainage will have an effect on vegetation growth; this is especially true in dry areas (Tricart 1972). Here the vegetation is accustomed to a short growing period and plants in the herb-layer grow more rapidly during the wet season. These plants will immediately transpire at their maximum rate, absorbing water through pores, rather than waiting for soils to replenish their moisture (Briggs et al 1997).

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