Savanna

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Typical tropical savanna in Northern Australia demonstrating the high tree density and regular spacing characteristic of many savannas.

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.[1][2][3] The oak savanna is a common type of savanna in the Northern Hemisphere. Some classification systems[which?] also recognize a grassland savanna from which trees are absent.[4] This article deals only with savanna under the common definition of a grassy woodland with a significant woody plant component.

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[edit]

Cerrado savanna, Brazil.

Although the term savanna is believed to have originally come from an Arawak word describing "land which is without trees but with much grass either tall or short" (Oviedo y Valdes, 1535), by the late 1800s it was used to mean "land with both grass and trees". It now refers to land with grass and either scattered trees or an open canopy of trees.

Spanish explorers familiar with the term "sabana" called the grasslands they found around the Orinoco River "llanos", as well as calling Venezuelan and Colombian grasslands by that specific term. "Cerrado" was used on the higher savannas of the Brazilian Central Plateau.[5]

Distribution[edit]

Many grassy landscapes and mixed communities of trees, shrubs, and grasses were described as savanna before the middle of the 19th century, when the concept of a tropical savanna climate became established. The Köppen climate classification system was strongly influenced by effects of temperature and precipitation upon tree growth, and his oversimplified assumptions resulted in a tropical savanna classification concept which resulted in it being considered as a "climatic climax" formation. The common usage meaning to describe vegetation now conflicts with a simplified yet widespread climatic concept meaning. The divergence has sometimes caused areas such as extensive savannas north and south of the Congo and Amazon Rivers to be excluded from mapped savanna categories.[5]

"Barrens" has been used almost interchangeably with savanna in different parts of North America. Sometimes midwestern savanna were described as "grassland with trees". Different authors have defined the lower limits of savanna tree coverage as 5–10% and upper limits range as 25–80% of an area.[6]

Two factors common to all savanna environments are rainfall variations from year to year, and dry season wildfires. In the Americas, e.g. in Belize, Central America, savanna vegetation is similar from Mexico to South America and to the Caribbean.[7]

Threats[edit]

Changes in fire management[edit]

Savannas are subject to regular wildfires and the ecosystem appears to be the result of human use of fire. For example, Native Americans created the Pre-Columbian savannas of North America by periodically burning where fire-resistant plants were the dominant species.[8] Pine barrens in scattered locations from New Jersey to coastal New England are remnants of these savannas. Aboriginal burning appears to have been responsible for the widespread occurrence of savanna in tropical Australia and New Guinea,[9] and savannas in India are a result of human fire use.[10] The maquis shrub savannas of the Mediterranean region were likewise created and maintained by anthropogenic fire.[11]

Prescribed burn; Wisconsin bur oak savanna

These fires are usually confined to the herbaceous layer and do little long term damage to mature trees. 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[12] and may have maintained and modified savanna flora.[3][9] It has been suggested by many authors[12][13] that aboriginal burning created a structurally more open savanna landscape. Aboriginal burning certainly created a habitat mosaic that probably increased biodiversity and changed the structure of woodlands and geographic range of numerous woodland species.[9][12] It has been suggested by many authors[13][14] that with the removal or alteration of traditional burning regimes many savannas are being replaced by forest and shrub thickets with little herbaceous layer.

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.[15] The introduction of exotic pasture legumes has also led to a reduction in the need to burn to produce a flush of green growth because legumes retain high nutrient levels throughout the year, and because fires can have a negative impact on legume populations which causes a reluctance to burn.[16]

Grazing and browsing animals[edit]

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.[17] In contrast the open structure of savannas allows the growth of a herbaceous layer and are commonly used for grazing domestic livestock.[18] 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.[19]

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.[20] In addition to this effect, the removal of fuel reduces both the intensity and the frequency of fires which may control woody plant species.[21] 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.[22] 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.[12] In addition to this, cattle and horses are implicated in the spread of the seeds of weed species such as Prickly Acacia (Acacia nilotica) and Stylo (Stylosanthes spp.).[15] 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.[23] 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[edit]

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 ha of savanna were cleared annually in Australia alone primarily to improve pasture production.[12] 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.[24] Since stock carrying capacity is strongly correlated with herbage yield, there can be major financial benefits from the removal of trees,[25] such as assisting with grazing management: regions of dense tree and shrub cover harbors predators, leading to increased stock losses, for example,[26] while woody plant cover hinders mustering in both sheep and cattle areas.[27]

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.[28] 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 tebuthiuron, 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.[29] 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[edit]

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 (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. 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.[29][30] Other plant species include: white sage, spotted cactus, cotton seed, rosemary.

Climate change[edit]

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[31] 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.[32]

Savanna ecoregions[edit]

Mediterranean savanna in Alentejo region, Portugal.

Savanna ecoregions are of several different types:

See also[edit]

References[edit]

  1. ^ 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.
  2. ^ McPherson, G. R. (1997). Ecology and management of North American Savannas. Tucson, AZ: University of Arizona Press.
  3. ^ a b Werner, Patricia A.; B. H. Walker; P. A Stott (1991). "Introduction". In Patricia A. Werner. Savanna Ecology and Management: Australian Perspectives and Intercontinental Comparisons. Oxford: Blackwell Publishing. ISBN 978-0-632-03199-3. 
  4. ^ grass savanna, Encyclopædia Britannica Online
  5. ^ a b David R. Harris, ed. (1980). Human Ecology in Savanna Environments. London: Academic Press. pp. 3,5–9,12,271–278,297–298. ISBN 0-12-326550-9. 
  6. ^ Roger C. Anderson, James S. Fralish, Jerry M. Baskin, ed. (1999). Savannas, Barrens, and Rock Outcrop Plant Communities of North America. Cambridge University Press. pp. 2–3. ISBN 0-521-57322-X. 
  7. ^ David L. Lentz, ed. (2000). Imperfect balance: landscape transformations in the Precolumbian Americas. New York City: Columbia University Press. pp. 73–74. ISBN 0-231-11157-6. 
  8. ^ "Use of Fire by Native Americans". The Southern Forest Resource Assessment Summary Report. Southern Research Station, USDA Forest Service. Retrieved 2008-07-21. 
  9. ^ a b c Flannery, Timothy Fridtjof (1994). The Future Eaters: An Ecological History of the Australasian Lands and People. Frenchs Forest, New South Wales: Reed New Holland. ISBN 978-0-8076-1403-7. 
  10. ^ Saha, S. (2003). "Patterns in woody species diversity, richness and partitioning of diversity in forest communities of tropical deciduous forest biomes". Ecography 26 (1): 80–86. doi:10.1034/j.1600-0587.2003.03411.x. 
  11. ^ Pyne, Stephen J. (1997). Vestal Fire: An Environmental History, Told through Fire, of Europe and Europe's Encounter with the World. Seattle: University of Washington Press. ISBN 0-295-97596-2. 
  12. ^ a b c d e Wilson, B., S. Boulter, et al. (2000). Queensland's resources. Native Vegetation Management in Queensland. S. L. Boulter, B. A. Wilson, J. Westrupet eds. Brisbane, Department of Natural Resources ISBN 0-7345-1701-7.
  13. ^ a b Lunt, I. D.; N. Jones (2006). "Effects of European colonisation on indigenous ecosystems: post-settlement changes in tree stand structures in EucalyptusCallitris woodlands in central New South Wales, Australia". Journal of Biogeography 33 (6): 1102–1115. doi:10.1111/j.1365-2699.2006.01484.x. 
  14. ^ Archer S, (1994.) "Woody plant encroachment into southwestern grasslands and savannas: Rates, patterns and proximate causes." pp. 13–68 in Vavra, Laycock and Pieper (eds.) Ecological Implications of Livestock Herbivory in the West. Society For Range Management, Denver ISBN 1-884930-00-X.
  15. ^ a b Pressland, A. J., J. R. Mills, et al. (1988). Landscape degradation in native pasture. 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.
  16. ^ Dyer, R., A. Craig, et al. (1997). Fire in northern pastoral lands. Fire in the management of northern Australian pastoral lands. T. C. Grice and S. M. Slatter. St. Lucia, Australia, Tropical Grassland Society of Australia ISBN 0-9590948-9-X.
  17. ^ Lodge, G. M. and R. D. B. Whalley (1984). Temperate rangelands. Management of Australia’s Rangelands. G. N. Harrington and A. D. Wilson. Melbourne, CSIRO Publishing.
  18. ^ Mott, J. J., Groves, R.H. (1994). Natural and derived grasslands. Australian Vegetation. R. H. Groves. Cambridge, Cambridge University Press.
  19. ^ Winter, W. H. (1991). "Australia's northern savannas: a time for change in management philosophy". In Patricia A. Werner. Savanna Ecology and Management: Australian Perspectives and Intercontinental Comparisons. Oxford: Blackwell Publishing. pp. 181–186. ISBN 978-0-632-03199-3. 
  20. ^ 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.
  21. ^ 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.
  22. ^ Florence, R. G. (1996). Ecology and silviculture of eucalypt forests. Collingwood, CSIRO Publishing ISBN 0-643-10252-3.
  23. ^ 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.
  24. ^ 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.
  25. ^ 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.
  26. ^ 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.
  27. ^ 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.
  28. ^ Partridge, I. (1999). Managing grazing in northern Australia. Brisbane, Department of Primary Industries ISBN 0-7345-0035-1.
  29. ^ a b 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.
  30. ^ Tothill, J. C. and C. Gillies (1992). The pasture lands of northern Australia. Brisbane, Tropical Grassland Society of Australia ISBN 0-9590948-4-9.
  31. ^ Archer, S. (1991). "Development and stability of grass/woody mosaics in a subtropical savanna parkland, Texas, USA". In Patricia A. Werner. Savanna Ecology and Management: Australian Perspectives and Intercontinental Comparisons. Oxford: Blackwell Publishing. pp. 109–118. ISBN 978-0-632-03199-3. 
  32. ^ Allen, C. D. and D. D. Breshears (1998). "Drought-induced shift of a forest–woodland ecotone: Rapid landscape response to climate variation". Proceedings of the National Academy of Sciences 95 (25): 14839–14842. doi:10.1073/pnas.95.25.14839. PMC 24536. PMID 9843976. 

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