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	=== savannas are being replaced by forest and shrub thickets with little herbaceous layer.		=== savannas are being replaced by forest and shrub thickets with little herbaceous layer.

			The consumption of herbage
	The consumption of herbage by introduced grazers in savanna woodlands has led to a reduction in the amount of fuel available for burning and resulted in fewer and cooler fires.<ref name=r1>Pressland, A. J., J. R. Mills, et al. (1988). Landscape of Australia ISBN 0-9590948-9-X.</ref>

	===Grazing and browsing animals===
	], ]]]

	The closed forest types such as broadleaf forests and rainforests are usually not grazed owing to the closed structure precluding grass growth, and hence offering little opportunity for grazing.<ref>Lodge, G. M. and R. D. B. Whalley (1984). Temperate rangelands. Management of Australia’s Rangelands. G. N. Harrington and A. D. Wilson. Melbourne, ].</ref> In contrast the open structure of savannas allows the growth of a herbaceous layer and are commonly used for grazing domestic livestock.<ref>Mott, J. J., Groves, R.H. (1994). Natural and derived grasslands. Australian Vegetation. R. H. Groves. Cambridge, Cambridge University Press.</ref> As a result much of the world's savannas have undergone change as a result of grazing by sheep, goats and cattle, ranging from changes in pasture composition to woody weed encroachment.<ref>{{cite book \|year=1991 \|title=Savanna Ecology and Management: Australian Perspectives and Intercontinental Comparisons \|editor=Patricia A. Werner \|publisher=] \|location=] \|isbn=978-0-632-03199-3 \|url=http://books.google.com/?id=pDouvnGGgKEC \|first=W. H. \|last=Winter \|chapter=Australia's northern savannas: a time for change in management philosophy \|pages=181–186}}</ref>

	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.<ref>Burrows, W. H., J. C. Scanlan, et al. (1988). Plant ecological relations in open forests, woodlands and shrublands. Native pastures in Queensland their resources and management. W. H. Burrows, J. C. Scanlan and M. T. Rutherford eds. Brisbane, Department of Primary Industries ISBN 0-7242-2443-2.</ref> In addition to this effect, the removal of fuel reduces both the intensity and the frequency of fires which may control woody plant species.<ref>Smith, G., A. Franks, et al. (2000). Impacts of domestic grazing within remnant vegetation. Native Vegetation Management in Queensland. S. L. Boulter, B. A. Wilson, J. Westrupet al. Brisbane, Department of Natural Resources ISBN 0-7345-1701-7.</ref> Grazing animals can have a more direct effect on woody plants by the browsing of palatable woody species. There is evidence that unpalatable woody plants have increased under grazing in savannas.<ref>Florence, R. G. (1996). . Collingwood, CSIRO Publishing ISBN 0-643-10252-3.</ref> Grazing also promotes the spread of weeds in savannas by the removal or reduction of the plants which would normally compete with potential weeds and hinder establishment.<ref name="wilson" /> In addition to this, cattle and horses are implicated in the spread of the seeds of weed species such as Prickly Acacia ('']'') and Stylo ('']'' spp.).<ref name=r1/> Alterations in savanna species composition brought about by grazing can alter ecosystem function, and are exacerbated by overgrazing and poor land management practices.

	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.<ref>Foran, B. D. (1984). Central arid woodlands. Management of Australia’s Rangelands. G. N. Harrington and A. D. Wilson. Melbourne, CSIRO Publishing ISBN 0-643-03615-6.</ref> 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.

	===Tree clearing===

	Large areas of Australian and South American savannas have been cleared of trees, and this clearing is continuing today. For example until recently 480,000 ] of savanna were cleared annually in Australia alone primarily to improve pasture production.<ref name="wilson" /> Substantial savanna areas have been cleared of woody vegetation and much of the area that remains today is vegetation that has been disturbed by either clearing or thinning at some point in the past.

	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. The removal of trees from savanna land removes the competition for water from the grasses present, and can lead to a two to fourfold increase in pasture production, as well as improving the quality of the feed available.<ref>Scanlan, J. and C. Chilcott (2000). Management and production aspects. Native Vegetation Management in Queensland. S. L. Boulter, B. A. Wilson, J. Westrupet al. Brisbane, Department of Natural Resources.</ref> Since stock carrying capacity is strongly correlated with herbage yield, there can be major financial benefits from the removal of trees,<ref>Harrington, G. N., M. H. Friedel, et al. (1984). Vegetation ecology and management. Management of Australia's Rangelands. G. N. Harrington and A. D. Wilson. Melbourne, CSIRO Publishing ISBN 0-643-03615-6.</ref> such as assisting with grazing management: regions of dense tree and shrub cover harbors predators, leading to increased stock losses, for example,<ref>Harrington, G. N., D. M. D. Mills, et al. (1984). Semi-arid woodlands. Management of Australia's Rangelands. G. N. Harrington and A. D. Wilson. Melbourne, CSIRO Publishing ISBN 0-643-03615-6.</ref> while woody plant cover hinders mustering in both sheep and cattle areas.<ref>Harrington, G. N., D. M. D. Mills, et al. (1984). Management of Rangeland Ecosystems. Management of Australia's Rangelands. G. N. Harrington and A. D. Wilson. Melbourne, CSIRO Publishing ISBN 0-643-03615-6.</ref>

	A number of techniques have been employed to clear or kill woody plants in savannas. Early pastoralists used felling and ], the removal of a ring of ] and ], as a means of clearing land.<ref>Partridge, I. (1999). Managing grazing in northern Australia. Brisbane, Department of Primary Industries ISBN 0-7345-0035-1.</ref> 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. The 1980s also saw the release of soil-applied arboricides, notably ], that could be utilized without cutting and injecting each individual tree.

	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.<ref name="scanlantree">Scanlan, J. C. (1988). Managing tree and shrub populations. Native pastures in Queensland their resources and management. W. H. Burrows, J. C. Scanlan and M. T. Rutherford. Queensland, Queensland Government Press ISBN 0-7242-2443-2.</ref> 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. A population of woody plants equal to half or more of the original number often remains following pulling of eucalypt communities, even if all the trees over 5 metres are uprooted completely.

	===Exotic plant species===

	], ].]]

	A number of exotic plants species have been introduced to the savannas around the world. Amongst the woody plant species are serious environmental weeds such as Prickly Acacia ('']''), Rubbervine ('']''), Mesquite ('']'' spp.), Lantana ('']'' and '']'') and Prickly Pear ('']'' spp.) A range of herbaceous species have also been introduced to these woodlands, either deliberately or accidentally including Rhodes grass and other '']'' species, Buffel grass ('']''), Giant rat's tail grass ('']'') ] ('']'') and stylos ('']'' spp.) and other ]s. These introductions have the potential to significantly alter the structure and composition of savannas worldwide, and have already done so in many areas through a number of processes including altering the fire regime, increasing grazing pressure, competing with native vegetation and occupying previously vacant ecological niches.<ref name="scanlantree" /><ref>Tothill, J. C. and C. Gillies (1992). The pasture lands of northern Australia. Brisbane, Tropical Grassland Society of Australia ISBN 0-9590948-4-9.</ref>
	Other plant species include: white sage, spotted cactus, cotton seed, rosemary.

	===Climate change===
	There exists the possibility that human induced climate change in the form of the greenhouse effect may result in an alteration of the structure and function of savannas. Some authors<ref>{{cite book \|year=1991 \|title=Savanna Ecology and Management: Australian Perspectives and Intercontinental Comparisons \|editor=Patricia A. Werner \|publisher=] \|location=] \|isbn=978-0-632-03199-3 \|url=http://books.google.com/?id=pDouvnGGgKEC \|first=S. \|last=Archer \|chapter=Development and stability of grass/woody mosaics in a subtropical savanna parkland, Texas, USA \|pages=109–118}}</ref> have suggested that savannas and grasslands may become even more susceptible to woody plant encroachment as a result of greenhouse induced climate change. However, a recent case described a savanna increasing its range at the expense of forest in response to climate variation, and potential exists for similar rapid, dramatic shifts in vegetation distribution as a result of global climate change, particularly at ecotones such as savannas so often represent.<ref>{{cite journal\|author=Allen, C. D. and D. D. Breshears \|year=1998\|title=Drought-induced shift of a forest–woodland ecotone: Rapid landscape response to climate variation \|volume=95\|pages=14839–14842\|pmid=9843976\|journal=Proceedings of the National Academy of Sciences\|doi=10.1073/pnas.95.25.14839\|issue=25\|pmc=24536}}</ref>

	==Savanna ecoregions==		==Savanna ecoregions==

Revision as of 14:41, 18 June 2013

"Savannah" redirects here. For other uses, see Savannah (disambiguation).

A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently 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 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 grassland. Savanna covers approximately 20% of the Earth's land area.

Etymology

Although the term savanna is believed to have originally come from an Arawak word

Distribution

Many grassy landscapesare rainfall variations from year to year, and dry season [[

Threats

=== savannas are being replaced by forest and shrub thickets with little herbaceous layer.

The consumption of herbage

Savanna ecoregions

Savanna ecoregions are of several different types:

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.
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 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.
Montane savannas are high-altitude savannas, located in a few spots around the world's high mountain regions, part of the montane grasslands and shrublands biome. The lowland savannas of the Angolan Scarp savanna and woodlands ecoregion are an example.

References

Anderson, Roger A., Fralish, James S. and Baskin, Jerry M. editors.1999. Savannas, Barrens, and Rock Outcrop Plant Communities of North America. Cambridge University Press.
McPherson, G. R. (1997). Ecology and management of North American Savannas. Tucson, AZ: University of Arizona Press.
Werner, Patricia A. (1991). "Introduction". In Patricia A. Werner (ed.). Savanna Ecology and Management: Australian Perspectives and Intercontinental Comparisons. Oxford: Blackwell Publishing. ISBN 978-0-632-03199-3. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
grass savanna, Encyclopædia Britannica Online

External links

The Savanna

Biogeographic regionalisations

Biomes

Terrestrial
biomes

Polar/montane	Tundra Taiga Montane grasslands and shrublands Alpine tundra
Temperate	Coniferous forests Broadleaf and mixed forests Deciduous forests Grasslands, savannas, and shrublands
Tropical and subtropical	Tropical rainforest Coniferous forests Moist broadleaf forests Dry broadleaf forests Grasslands, savannas, and shrublands
Dry	Mediterranean forests, woodlands, and scrub Deserts and xeric shrublands Steppe
Wet	Flooded grasslands and savannas Riparian Wetland Mangrove

Aquatic
biomes

Other biomes

Endolithic zone

Biogeographic
realms

Terrestrial	Afrotropical Antarctic Australasian Holarctic Indomalayan Nearctic Neotropical Oceanian Palearctic
Marine	Antarctic/Southern Ocean Arctic Central Indo-Pacific Eastern Indo-Pacific Temperate Australasia Temperate Northern Atlantic Temperate Northern Pacific Temperate South America Temperate Southern Africa Tropical Atlantic Tropical Eastern Pacific Western Indo-Pacific
Subdivisions	Biogeographic provinces Bioregions Ecoregions Lists of ecoregions Global 200 ecoregions

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