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.
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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).
If flooding occurs regularly the constitution of vegetation will vary according to the duration of the flood (Lind and Morrison 1974). The structure of the grassland will differ and under extreme conditions the growth of trees and shrubs can be stunted.
Biotic Factors
A large amount of savanna has been subjected to sustained periods of burning, grazing and agriculture
Fire
Fire is an important ecological factor in determining the vegetation of savanna. Fire in the savanna can be caused deliberately by humans (such as farmers) or naturally (by electrical storms in the summer). In semi arid areas of the world, natural fires started by lightning are endemic to the area. Anthropogenic fires have occurred in the tropics for centuries. According to Collinson (1988) some experts argue the development of savanna in West Africa is due to human induced fire. The practice is carried out to flush game or as part of cultivation. Woody plants have become pyrophytic, resistant to fire, adopting characteristics such as adventitious buds, ligneous sprouting roots and thick bark. It is thought the evolution of these plants indicates the long establishment of fire in the savanna (Tivy 1993).
The impact of fire depends on a number of factors. The most important factor being, the season in which the fire occurs. For example following a particularly wet season, there is an overabundance of grass; fire can be exceptionally intense, seriously reducing growth. Fire occurs less frequently in drier seasons due to the reduction of xerophytes (plants able to grow with little water). The time of day the fire is started and the wind speed and direction will also determine the impact of fire (Collinson 1988). In areas used for agriculture the frequency of fires is also reduced due to the removal of fuel in the area. According to Huntley and Walker (1982) the role which fire plays in determining the balance between grass and bush dominated savanna is contradictory. In some drier areas fire plays a part in ensuring the height of trees and shrubs is most suitable for browsing animals to locate grasses for consumption. Alternatively fire can remove sapling-benefiting grasses.
Grazing and Cultivation
Savanna flora has been present in Africa since the onset of the Tertiary era (Van Der Hammen 1992). The evolution of the vegetation has been accompanied by large herbivores of which Africa possesses forty-four species including African elephant (Loxodonta fricana), white rhinoceros (Ceratotherium simum) and Grant’s gazelle (G. granti). The majority of these herbivores possess flocking and migratory habits and are ungulates (species with hoofs). Eating habits vary throughout the species. Therefore a high proportion of above-ground vegetation is cropped (40 per cent of the net primary production).
A high number of herbivores are bulk feeders and browsers that can cause long term vegetation transformation or disturb vegetation cycles (Tivy 1993). Cumming (1982) suggests elephants may have played a detrimental role in the reduction of savanna woodland to grassland in the Serengeti National Park in Tanzania. The size and bulk of the elephant means it is able to uproot trees enabling them to browse. Felling trees this way causes forest clearings that become more susceptible to fire and grazers. The ongoing protection of the elephant in the park and other game reserves has led to serious overgrazing.
The areas of savanna bordering deserts are most vulnerable to desertification. There is often pressure to remove woodland for fuel and overgrazing will reduce the productivity of grasslands. This can lead to heavy rains forming gulleys and wind erodes the surface soil. In agricultural free areas more trees usually exist, therefore grass may not be the original climatic climax vegetation.
Shifting cultivating has been associated with savanna for long periods. The practice involves cultivating an area of cleared land, exploiting the temporarily high level of nutrients (phosphorus and calcium). When the soil is exhausted of nutrients the land is then neglected to the natural succession of the original ecosystem. The practice is still present today. Simmons (1990) states following the abandonment there is an initial rise in nutrient levels, however studies have shown nutrient concentrations are still below their original values six years on. Waugh (2001) suggests there is evidence in countries such as Kenya that the original climax vegetation was forest but this has been transformed through overgrazing, fire and by climate change.
Communities and Specialisations
According to Waugh (2000) tropical grasslands have an average net primary production (NPP) that is significantly less than tropical rainforest. The estimated mean NPP during the 1970’s was 1408 g/m2 per annum, which is similar to the production of a coniferous forest (Simmons 1990). Savanna ranges from ‘open’ at one extreme to ‘closed’ at the other (Waugh 2000).
‘Open’ savanna is dominated by herbaceous stratum in which usually xerophytic perennial grasses and sedges with a tussock habit are the foremost component (Tivy 1993). The grasses grow in tufts, and following the start of the wet season grow quickly often-towering over 3 metres. These herbaceous plants are dominated by a small number of genera (Andropogon, Loudetia, Hyparrhenia and Stipagrostis). Grass such as African elephant grass (pennistum purpureum) is often abundant and can grow over five metres in height (Waugh 2001), however a long dry season will bring low sparse grasses. Pennistum purpureum can reproduce sexually, depositing seeds onto the lands surface during the dry winter until the beginning of the wet season. However the seeds are small and thus poor germinators. Therefore the grass has more success reproducing through its rhizomes (www.bluebiomes.org).
‘Closed’ savanna will be dominated by a differing ratio of drought resistant woody species varying from low bush to tall trees (Tivy 1993) with areas of grasses. Many trees are xerophytes. Therefore able to survive by maximizing their water consumption and minimizing their water loss (through morphological and physiological resistance). Trees have developed long stretching roots probing for underground moisture. A number of tree types drop their leaves during the dry season to facilitate reduced transpiration. Leaves that do appear are relatively small and waxy (Waugh 2001). The trees are often gnarled, stunted with thick bark, and have developed protective mechanisms to prohibit grazing fauna denuding them. Many of the savannas trees are pyrophytic. The effects of fire have also led to the destruction of seeds. This problem is exacerbated by the ravenous feeding habit of termites. Trees have reacted by producing huge numbers of seeds each year (Briggs et al 1997).
There are over 700 species of Acacia present in Africa. It is possible to view some of the above traits in the Umbrella Thorn Acacia (acacia tortillas.)
The Acacia is the most recognisable tree of the African savanna due to their flat crowns (figure 1.1) stunted by the trade winds. The tree can grow up to 12 metres in height and is available to survive the extremities of the biome (temperatures as high as 122 degrees Fahrenheit during the day and below zero at night) and drops its leaves during droughts (Begon et al 1996). In contrast Acacia faidherbia albidafound that is located in and around Sudan retains its leaves (Collinson 1988). Acacia tortillis has developed two types of sharp thorns (one set long and straight and the other short and hooked) on its branches as a form of protection from grazers. These thorns are hidden amongst flower clusters that grow on the tree. Acacia karoo is able to produce masses of seeds and releases up to 20,000 each year; despite the high fertility of these seeds (90%) very few will grow into trees (Briggs et al 1997).
Figure 1.1 Acacia trees beneath Mount Kilmanjaro, Tanzania.
The Baobab tree (figure 1.2), another pyrophyte, possesses a trunk with a large girth, the diameter measuring up to 10 metres. The branches of the baobab resemble roots and carry little foliage to reduce transpiration. Successful Baobabs can live for long periods with some species living to be several thousand years old (Waugh 2001).
Figure 1.2 Baobob trees (adansonia digitata)
The relationship between the density of perennial herbacceous vegetation and woody plants is a contentious issue. In some areas trees and grasses have reached a balanced equilibrium where the two life forms can co-exist, occasionally benefiting each other (Boulliere and Hadley 1992). Generally it is perceived the climate will determine the success of vegetation. For example the number of trees will reduce in the drier areas of the savanna. Details of this relationship can be found in the Climate section earlier in the essay.
Some experts such as Walter (1984) assert the two are direct competitors. This is especially true towards the climax of the dry season when water is scarce. Grasses are more likely to be successful on fine soils, here their dense roots can capitalise the soil moisture reserves. Therefore trees will become more successful on stony soils where their long, protruding roots can exploit deep-water reserves.
Terms of Reference
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Bourliere,F. Hadley,M. (1992) Present-day savannas: an overview.
In: Bourliere, F. (E) (1992) Ecosystems of the World: Tropical Savannas. Elsevier Science Publishers.
Briggs, D., Smithson, P., Addison, K. & Atkinson, K. (1997) Fundamentals of the Physical Environment. Routledge.
Burrows, C.J. (1990) Processes of Vegetation Change. Unwin Hyman Ltd.
Collinson, A.S. (1988) Introduction to World Vegetation. Unwin Hyman Ltd.
Cumming, D.D.M. (1982) The influence of large herbivores on savanna structure in Africa. In: Huntley, B.J.& Walker, B. Ecology of Tropical Savannahs. Berlin.
Hobley, L.F. (1970) Deserts and Savannahs of the World. MacMillan and Co. Ltd.
Hopkins, B. (1977) Forest and Savanna. Heineman Educational Books Ltd.
Huntley, B.J.& Walker, B. (1982) Ecology of Tropical Savannahs. Berlin.
Lind, E.M & Morrison, M.E.S. (1974) East African Vegetation. Longman Group Ltd.
Montgomery, R.F. & Askew, G.P. (1992) Soils of tropical savannas. In: Bourliere, F. (E) Ecosystems of the World: Tropical Savannas. Elsevier Science Publishers.
Nix, H.A. (1992) Climate of Tropical Savannas. In: Bourliere, F. (E) Ecosystems of the World: Tropical Savannas. Elsevier Science Publishers.
Park, C. (2001) The Environment. Routledge.
Simmons, I.G. (1990) Changing the Face of the Earth: Culture, Environment, History. Basil Blackwell Ltd.
Tivy, J. (1993) Biogeography: A study of Plants in the Ecosphere. Longman Scientific & Technical.
Tricart, J. (1972) Landforms of the humid tropics ,forests and savannas. Longman Group Ltd.
Van der Hammen, T. (1992) The Palaeoecology and Paleogeography of Savannas. In: Bourliere, F. (E) (1992) Ecosystems of the World: Tropical Savannas. Elsevier Science Publishers.
Walter, H (1984) Vegetation of the Earth. Berlin: Springer.
Waugh, D. (2000) Geography: An Integrated Approach. Nelson.
Werger (1984) Tropical grasslands, savannas, woodlands : natural and man made. In Man’s impact on vegetation, W. Holzner, M.J.A. Werger and I. Ikusima (eds). The Hague:Junk.
Whitlow, J. (1984) The Penguin Dictionary of Physical Geography. Penguin Books.
Young, A. (1976) Tropical Soils and Soil Survey. Cambridge University Press.
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