Savannas, also known as African grasslands, are a unique vegetation type that thrive under hot, seasonally dry climatic conditions. Characterized by an open tree canopy with scattered trees above a continuous grass understory, savannas are home to a diverse array of plant and animal life. The largest areas of savanna are found in Africa, South America, Australia, India, the Myanmar-Thailand region in Asia, and Madagascar.
Savannas arose as rainfall progressively lessened in the edges of the tropics during the Cenozoic Era (66 million years ago to the present)-in particular, during the past 25 million years. Grasses, the dominant plants of savannas, appeared only about 50 million years ago, although it is possible that some savanna-like vegetation lacking grasses occurred earlier.
The South American fossil record provides evidence of a well-developed vegetation, rich in grass and thought to be equivalent to modern savanna, being established by the early Miocene Epoch, about 20 million years ago. Climates across the world became steadily cooler during that period.
Lower ocean surface temperatures reduced water evaporation, which slowed the whole hydrologic cycle, with less cloud formation and precipitation. The vegetation of midlatitude regions, lying between the wet equatorial areas and the moist cool temperate zones, was affected substantially.
The main regions in which savannas emerged in response to that long-term climatic change-tropical America, Africa, South Asia, and Australia-were already separated from each other by ocean barriers by that time. Plant migration across those barriers was inhibited, and the details of the emergence of savannas on each continent varied. In each region different plant and animal species evolved to occupy the new seasonally dry habitats.
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In temperate regions, savannas became much more widespread, at the expense of forests, during the long, cool, dry intervals-contemporaneous with the ice ages, or glacial intervals, of the Pleistocene Epoch (2.6 million to 11,700 years ago). When human beings (Homo sapiens) first appeared, in Africa, they initially occupied the savanna. Later, as they became more adept at modifying the environment to suit their needs, they spread to Asia, Australia, and the Americas.
There their impact on the nature and development of savanna vegetation was superimposed on the natural pattern, adding to the variation seen among savanna types. The savannas of the world currently are undergoing another phase of change as modern expansion of the human population impinges on the vegetation and fauna.
The Southeast African bioregion, part of the Southern Afrotropics subrealm located in the Afrotropics realm, is centered around the inland Highveld, a plateau formed by the Great Escarpment ridge in South Africa dominated by mountain grasslands, which transition to subtropical grasslands in the east and mid-altitude bushlands in the north. The bioregion contains five ecoregions-Central Bushveld (38), Drakensberg Escarpment Savanna and Thicket (40), Drakensberg Grasslands (41), Limpopo Lowveld (48), Highveld Grasslands (81)-with a total area of more than 63 million hectares. Although fragmented, it contains the greatest expanse of remaining grassland in southern Africa and provides habitat for numerous birds and mammal species, including pangolins, honey badgers, and mountain zebras.
The Southeast African Subtropical Grasslands bioregion is part of the Southern Afrotropics subrealm and is made up of five ecoregions: Central Bushveld, Limpopo Lowveld, Highveld Grasslands, Drakensberg Grasslands, Drakensberg Escarpment Savanna and Thicket.
Savannas are also characterised by seasonal water availability, with the majority of rainfall confined to one season. The annual rainfall ranges from 500 mm (19.69 in) to 1,270 mm (50.00 in) per year, with the precipitation being more common in six or eight months of the year, followed by a period of drought.
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Vegetation in African Grasslands
The savannas of tropical America comprise broadleaved trees such as Curatella, Byrsonima, and Bowdichia, with grasses such as Leersia and Paspalum. Bean relative Prosopis is common in the Argentinian savannas.
In the East African savannas, Acacia, Combretum, baobabs, Borassus, and Euphorbia are a common vegetation genera. Drier savannas there feature spiny shrubs and grasses, such as Andropogon, Hyparrhenia, and Themeda. Wetter savannas include Brachystegia trees and Pennisetum purpureum, and elephant grass type.
West African savanna trees include Anogeissus, Combretum, and Strychnos. Indian savannas are mostly cleared, but the reserved ones feature Acacia, Mimosa, and Zizyphus over a grass cover comprising Sehima and Dichanthium.
A number of exotic plants species have been introduced to savannas around the world. Amongst the woody plant species are serious environmental weeds such as prickly acacia (Acacia nilotica), Rubbervine (Cryptostegia grandiflora), Mesquite (Prosopis spp.), Lantana (Lantana camara and L. montevidensis) and Prickly Pear (Opuntia spp.).
A range of herbaceous species have also been introduced to these woodlands, either deliberately or accidentally including Rhodes grass and other Chloris species, Buffel grass (Cenchrus ciliaris), Giant rat's tail grass (Sporobolus pyramidalis) parthenium (Parthenium hysterophorus) and stylos (Stylosanthes spp.) and other legumes.
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Acacia tortilis in Serengeti National Park
Wildlife in African Grasslands
Animals in the African savanna generally include the giraffe, elephant, buffalo, zebra, gnu, hippopotamus, rhinoceros, and antelope, where they rely on grass and/or tree foliage to survive.
Iconic images of Africa typically portray the vast herds of wildebeest, zebra and gazelles which migrate across the plains of East Africa, and the Maasai pastoralists, in their bright attire, who share this vast landscape. This is quintessential Africa - the location for two of Africa's best known world heritage sites - the Serengeti National Park, and Ngorongoro Conservation Area.
These two adjacent sites form part of the wider Serengeti ecosystem, where the world's greatest migration of large mammals takes place. Local pastoralists are excluded from the national park, but they are allowed to live and graze their livestock in the Conservation Area - a bold experiment in multiple land use.
Elsewhere, in the tall grass savannas north of the equator, are two further world heritage sites - Garamba National Park in northeastern Congo, where the last few northern white rhinos may still find sanctuary, hidden in the elephant grass; and Manovo-Gounda St Floris National Park in the Central African Republic, with its dwindling herds of elephants and other large mammals, threatened by war, the collapse of management and unprecedented levels of commercial poaching.
Iconic Species: The great migrating herds of grazing animals - wildebeest, zebra and gazelles - which roam the grassy plains, are pursued relentlessly by predators - lion, cheetah, and leopard, while scavenging hyaenas, jackals and vultures take advantage of the leftovers. Meanwhile, millions of tons of processed grass are recycled by armies of dung beetles, thus ensuring that nothing is wasted in the complex web of life.
Today there are very few places where this complexity is maintained, and key species are under threat as never before.
Map of African Grasslands
The Role of Fire in Savanna Ecosystems
Savannas are subject to regular wildfires and the ecosystem appears to be the result of human use of fire. Intentional controlled burns typically create fires confined to the herbaceous layer that do little long term damage to mature trees. This prevents more catastrophic wildfires that could do much more damage.
However, these fires either kill or suppress tree seedlings, thus preventing the establishment of a continuous tree canopy which would prevent further grass growth. Prior to European settlement aboriginal land use practices, including fire, influenced vegetation and may have maintained and modified savanna flora. It has been suggested by many authors that aboriginal burning created a structurally more open savanna landscape.
The removal of grass by grazing affects the woody plant component of woodland systems in two major ways. Grasses compete with woody plants for water in the topsoil and removal by grazing reduces this competitive effect, potentially boosting tree growth.
In addition to this effect, the removal of fuel reduces both the intensity and the frequency of fires which may control woody plant species. Grazing animals can have a more direct effect on woody plants by the browsing of palatable woody species.
Introduced grazing animals can also affect soil condition through physical compaction and break-up of the soil caused by the hooves of animals and through the erosion effects caused by the removal of protective plant cover. Such effects are most likely to occur on land subjected to repeated and heavy grazing.
The effects of overstocking are often worst on soils of low fertility and in low rainfall areas below 500 mm, as most soil nutrients in these areas tend to be concentrated in the surface so any movement of soils can lead to severe degradation. Alteration in soil structure and nutrient levels affects the establishment, growth and survival of plant species and in turn can lead to a change in woodland structure and composition.
Large areas of Australian and South American savannas have been cleared of trees, and this clearing continues today. Clearing is carried out by the grazing industry in an attempt to increase the quality and quantity of feed available for stock and to improve the management of livestock.
A number of techniques have been employed to clear or kill woody plants in savannas. Early pastoralists used felling and girdling, the removal of a ring of bark and sapwood, as a means of clearing land. In the 1950s arboricides suitable for stem injection were developed.
War-surplus heavy machinery was made available, and these were used for either pushing timber, or for pulling using a chain and ball strung between two machines. These two new methods of timber control, along with the introduction and widespread adoption of several new pasture grasses and legumes promoted a resurgence in tree clearing.
In many ways "artificial" clearing, particularly pulling, mimics the effects of fire and, in savannas adapted to regeneration after fire as most Queensland savannas are, there is a similar response to that after fire. Tree clearing in many savanna communities, although causing a dramatic reduction in basal area and canopy cover, often leaves a high percentage of woody plants alive either as seedlings too small to be affected or as plants capable of re-sprouting from lignotubers and broken stumps.
Human induced climate change resulting from the greenhouse effect may result in an alteration of the structure and function of savannas. Some authors have suggested that savannas and grasslands may become even more susceptible to woody plant encroachment as a result of greenhouse induced climate change.
Types of Savannas
A savanna can simply be distinguished by the open savanna, where grass prevails and trees are rare; and the wooded savanna, where the trees are densest, bordering an open woodland or forest.
- Tropical and subtropical savannas are classified with tropical and subtropical grasslands and shrublands as the tropical and subtropical grasslands, savannas, and shrublands biome. The savannas of Africa, including the Serengeti, famous for its wildlife, are typical of this type. The Brazilian savanna (Cerrado) is also included in this category, known for its exotic and varied flora.
- Temperate savannas are mid-latitude savannas with wetter summers and drier winters. They are classified with temperate savannas and shrublands as the temperate grasslands, savannas, and shrublands biome, that for example cover much of the plains of southeastern Australia, northern India, Southern Africa, southeastern Argentina and Uruguay.
- 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, including the Temperate Grassland of South Australia, which features eucalyptuses.
- 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.
- Montane savannas are mid- to high-altitude savannas, located in a few spots around the world's high mountain regions, part of the montane grasslands and shrublands biome.
| Savanna Type | Climate | Location Examples |
|---|---|---|
| Tropical and Subtropical | Hot, seasonally dry | Serengeti (Africa), Cerrado (Brazil) |
| Temperate | Mid-latitude, wet summers, dry winters | Southeastern Australia, Northern India |
| Mediterranean | Mild, rainy winters, hot, dry summers | California, South Australia |
| Flooded | Seasonally or year-round flooding | Tropics and subtropics |
| Montane | Mid- to high-altitude | High mountain regions worldwide |
Savanna Ecosystem
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