A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) biome and ecosystem characterised 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.McPherson, G. R. (1997). Ecology and management of North American Savannas. Tucson, AZ: University of Arizona Press. Four savanna forms exist; savanna woodland, where trees and shrubs form a light canopy; tree savanna, with scattered trees and shrubs; shrub savanna, with distributed shrubs; and grass savanna, where trees and shrubs are mostly nonexistent.Smith, Jeremy M.B.. "savanna". Encyclopedia Britannica, 5 Sep. 2016, https://www.britannica.com/science/savanna/Environment. Accessed 17 September 2022.
Savannas maintain an open canopy despite a high tree density.Alexandro Solórzano, Jeanine Maria Felfili 2008 "Comparative analysis of the international terminaoolgy for cerrado" IX Symposio Nacional Cerrado 13 a 17 de outubro de 2008 Parlamundi Barsilia, DF 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 forests. The South American savanna types cerrado sensu stricto and cerrado dense typically have densities of trees similar to or higher than that found in South American tropical forests, with savanna ranging from 800 to 3300 trees per hectare (trees/ha) and adjacent forests with 800–2000 trees/ha. Similarly Guinean savanna has 129 trees/ha, compared to 103 for riparian forest, while Eastern Australian sclerophyll forests have average tree densities of approximately 100 per hectare, comparable to savannas in the same region.Tait, L 2010, Structure and dynamics of grazed woodlands in North-eastern Australia, Master of Applied Science Thesis, Central Queensland University, Faculty of Science, Engineering and Health, Rockhampton.
Savannas are also characterised by seasonal water availability, with the majority of rainfall confined to one season. They are associated with several types of biomes, and are frequently in a transitional zone between forest and desert or grassland, though mostly a transition between desert to forest. Savanna covers approximately 20% of the Earth's land area. Unlike the prairies in North America and steppes in Eurasia, which feature cold winters, savannas are mostly located in areas having warm to hot climates, such as in Africa, Australia, South America, and India.
Etymology The word derives from the Spanish , which is itself a loanword from Taíno, which means "treeless grassland" in the West Indies.
The word originally entered English as the Zauana in a description of the ilands of the kinges of Spayne from 1555.Oxford English Dictionary, "savannah", n. Oxford University Press (Oxford), 2012. This was equivalent in the orthography of the times to zavana (see history of V). Peter Martyr reported it as the local name for the plain around Comagre, the court of the cacique Carlos in present-day Panama. The accounts are inexact, but this is usually placed in present-day Madugandí or at points on the nearby Guna Yala coast opposite Ustupo or on Point Mosquitos. These areas are now either given over to modern cropland or jungle.NASA. "[earthobservatory.nasa.gov/Experiments/ICE/panama/Images/igbp_panama2000289_lg.gif Land Cover Classification]" from Earth Observatory. The Image Composite Explorer. Exercise 4: Vegetation Vital Signs. Accessed 1 August 2014.
English explorer William Dampier, in his 1697–1705 volumes of A New Voyage Round the World, used the spelling "savannah" (with the terminal 'h') throughout, which became the British English form of the word. Dampier describes woodlands he saw on the island of "Trist" (Isla del Triste or Trist Island) in the 'Bay of Campeachy' (Bay of Campeche) in the Gulf of Mexico: "The West part is dry and sandy, bearing a sort of long Grass, growing in Tufts very thin. This is a sort of Savannah, with some large Palmeto Trees growing in it". He also used that spelling for descriptions of similar environments in Sumatra and the Philippines in accounts of a later voyage to New Holland (northern Australia).
Distribution
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 oversimplified assumptions resulted in a tropical savanna classification concept which considered it as a "climatic climax" formation. The common usage to describe vegetation now conflicts with a simplified yet widespread climatic concept. 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.
In different parts of North America, the word "savanna" has been used interchangeably with "barrens", "prairie", "glade", "grassland" and "oak opening". Different authors have defined the lower limits of savanna tree coverage as 5–10% and upper limits range as 25–80% of an area. 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. The distinction between woodland and savanna is vague and therefore the two can be combined into a single biome as both woodlands and savannas feature open-canopied trees with crowns not usually interlinking (mostly forming 25–60% cover).WOODLAND SAVANNA Wrangle – WORLD RANGELAND LEARNING EXPERIENCE. 2022 Arizona Board of Regents. Retrieved 17 September 2022.
Over many large tropical areas, the dominant biome (forest, savanna or grassland) can not be predicted only by the climate, as historical events also play a key role—fire activity, for example. In some areas, indeed, it is possible for there to be multiple stable biomes. The annual rainfall ranges from to per year, with the precipitation being more common in six or eight months of the year, followed by a period of drought. Savannas may at times be classified as forests.
In climatic geomorphology it has been noted that many savannas occur in areas of pediplains and inselbergs. It has been posited that river incision is not prominent but that rivers in savanna landscapes erode more by lateral migration. Flooding and associated sheet wash have been proposed as dominant erosion processes in savanna plains.
Ecology The savannas of tropical America comprise broadleaved trees such as Curatella, Byrsonima, and Bowdichia, with grasses such as Leersia and Paspalum. The bean relative Prosopis is common in the Argentinian savannas. In the East African savannas, acacias, Combretum, baobabs, Borassus, and Euphorbia are 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 acacias, Mimosa, and Zizyphus over a grass cover comprising Sehima and Dichanthium. The Australian savanna is abundant with sclerophyllous evergreen vegetation, which include the eucalyptus, as well as Acacia, Bauhinia, and Pandanus with grasses such as Heteropogon and kangaroo grass (Themeda).
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. In the Australian savanna, mammals in the family Macropodidae predominate, such as kangaroos and wallabies, though cattle, horses, camels, donkeys and the Asian water buffalo, among others, have been introduced by humans.
Threats It is estimated that less than three percent of savanna ecosystems can be classified as highly intact. Reasons for savanna degradation are manifold, as outlined below.
Changes in fire management
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 woodlands of North America by periodically burning where fire-resistant plants were the dominant species. Fire-stick farming appears to have been responsible for the widespread occurrence of savanna in tropical Australia and New Guinea, and savannas in India are a result of human fire use. The maquis shrub savannas of the Mediterranean region were likewise created and maintained by anthropogenic 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 vegetationWilson, 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 . 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. Aboriginal burning certainly created a habitat mosaic that probably increased biodiversity and changed the structure of woodlands and geographic range of numerous woodland species. It has been suggested by many authorsArcher 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 . 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.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 . 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.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 .
Grazing and browsing animals