Tropical Savannas

Savanna

• Referred to as grasslands or plains;• But over time, it was applied to an array of

vegetation types representing a continuum of increasing cover of woody vegetation, from open grassland to widely spaced shrubs or trees to closed woodland.

• Some savannas are natural• Others are seminatural, brought about and

maintained by centuries of human interference.

• may be subdivided into three categories—wet, dry, and thornbush—depending on the length of the dry season.– In wet savannas the dry season typically lasts 3 to

5 months,– in dry savannas 5 to 7 months, and– in thornbush savannas it is even longer.

• An alternative subdivision recognizes:– savanna woodland, with trees and shrubs forming

a light canopy;– tree savanna, with scattered trees and shrubs;

shrub savanna, with scattered shrubs; and– grass savanna, from which trees and shrubs are

generally absent.

• Occur on land surfaces of little relief, often old alluvial plains.

• Moisture appears to control the density of woody vegetation, a function of both rainfall and soil – its texture, structure, and water-holding capacity.

• The soils are low in nutrients, due in part to infertile parent material and a long period of weathering.

• Are associated with a warm continental climate with precipitation ranging between 500mm and 2000mm. Precipitation exhibits extreme season fluctuations.

• Are subject to recurrent fires, and the dominant vegetation is fire-adapted.

Vegetation

• Grass, mostly bunch or tussock, the major and most essential stratum of the savanna ecosystem.

• A woody component adds one or two vertical layers, ranging from about 50-80cm when small woody shrubs are present to about 8m in the tree savanna.

• Competition may exist between grass and woody vegetation for soil moisture, but more intense competition takes place among trees, accounting for the spacing patterns of woody vegetation.

• The tussock grasses form an array of clumps set in matrix of open ground, creating patches of low vegetation with frequent changes in microclimatic conditions.

• The addition of woody growth, the widely spaces shrubs and trees, increases horizontal structure extending to the soil.

• Trees add more organic matter and nutrients to the soil, reduce evapotranspiration, resulting in increased herbaceous and woody shrub growth, and provide patches of shade.

• There are also numerous marshy depressions that support wetland wildlife.

• large ribbons of riverine or gallery forests weave through the savannas.

Animal Life

• Savannas are capable of supporting a large and varied assemblage of herbivores, invertebrate, and vertebrate, grazing and browsing.

• Dominant herbivores are the invertebrates, including acrid mites, acridid grasshoppers, seed-eating ants, and detrital-feeding dung beetles and termites.

• Savannas support an incredible number of insects: flies, grasshoppers, locusts, crickets, carabid beetles, and especially termites and ants, which dominate insect life.

• A strong element of grazing ungulates represented by pampas deer and the capybara (largest rodent).

• Granivorous(grain eater), insectivorous, frugivorous(fruit eater) birds become an important component of the consumer community.

• Wildebeest and zebra, are migratory during the dry season. Others, such as the impala, partially disperse during the dry season.

• Still others, such as the giraffe and Grant’s gazelle, have little or no seasonal dispersal.

• Grazers: zebra and wildebeest• Browsers: giraffe, Thompson’s gazelle, kudu,

black rhino

• Herbivores have short-term and long-term impacts on the savanna

• Over the short-term, the grazing ungulates affect vegetation structure. Elephants can convert woodland to grassland, and large concentrations of grazers, can turn grassland to eroded, bare ground.

• Heavy grazing that reduces grass cover can cause competitive release of woody growth.

• Over evolutionary time the selective pressures of grazing have resulted in the development of structural and chemical defenses against grazing, such as concentration of silica on leaves, and in the alteration of growth process to respond favorably to grazing.

• Highly palatable plants retain a high carbohudrate concentration in their crowns and roots and respond quickly to defoliation.

• Some acacia trees respond to browsing by increasing growth.

• Others have their growth form altered and size reduced by browsing, especially by giraffes.

• Living on the ungulate fauna is an array of carnivores, including the lion, leopard, cheetah, hyena, and wild dog. Subsisting on leftover prey are scavengers, including vultures and jackals.

Function

• a wide range of production exists between grass savanna on one end of the gradient and tree savanna and woodlands on the other.

• On the wet season, moisture releases nutrients from materials accumulated in the dry season and stimulates nutrient translocation from the roots. This would then be followed by a quick flush of growth into grass and woody plants.

• Nutrient movement bet. soil and vegetation is generally higher under the trees than in the open.

• Savanna trees exhibit tight internal cycling. • Nitrogen concentration in the leaves,

decreases as the dry season approaches, with maximum withdrawal before leaf fall.

• Trees transfer some of the nitrogen into new woody growth, but much of it goes to the root reserve, where it is available to stimulate the flush of new season growth.

• ants and termites consume and break down plant litter and modify the soil.

• Mound-building termites excavate and move tons of soil, mixing mineral soil with organic matter.

• Some species galleries, and others accumulate organic matter.

• Comprising over 50 percent of soil biomass, termites have a considerable impact on the physical and chemical properties of savanna soil.

Human Impact

• Humans have had such a close association with savanna vegetation, especially in Africa, that it is difficult to separate human influences from natural influences on the shaping on savanna ecosystem.

• Introduction of crops, grazing animals, and settlements has accentuated the dry season and increased the desiccation of the drier savannas.

• *Desiccation has allowed the desserts to encroach

• cutting and burning of trees for fuel wood,• Destruction by domestic grazing animals,• and the loss of grass cover – Help to expose the soil to wind and water erosion.