Savanna

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A tree savanna in Tanzania, East Africa (Tarangire National Park) Tarangire-Natpark800600.jpg
A tree savanna in Tanzania, East Africa (Tarangire National Park)
A grass savannah in South Africa (Kruger National Park) Savanna Grasslands (199168845).jpeg
A grass savannah in South Africa (Kruger National Park)

A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) 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. [1] [2] [3] According to Britannica , there exists four savanna forms; 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. [4]

Contents

Savannas maintain an open canopy despite a high tree density. [5] 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. [6] [7] [8] [9] 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, [6] [8] [9] 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, [7] while Eastern Australian sclerophyll forests have average tree densities of approximately 100 per hectare, comparable to savannas in the same region. [10]

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 they're mostly a transition between desert to forest. [11] Savanna covers approximately 20% of the Earth's land area. [12] Unlike the prairies in North America and steppes in Eurasia, which would feature cold winters, savannahs are mostly located in areas having warm to hot climates, such as those in Africa, Australia, Thailand, South America and India. [13]

Etymology

The word derives from the Spanish sabana, which is itself a loanword from Taíno, which means "treeless grassland" in the West Indies. [14] [15] The letter b in Spanish, when positioned in the middle of a word, is pronounced almost like an English v; hence the change of grapheme when transcribed into English. [16]

The word originally entered English as the Zauana in a description of the ilands of the kinges of Spayne from 1555. [17] [19] 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, [21] but this is usually placed in present-day Madugandí [22] or at points on the nearby Guna Yala coast opposite Ustupo [23] or on Point Mosquitos. [24] These areas are now either given over to modern cropland or jungle. [25]

Distribution

A savanna woodland in Northern Australia demonstrating the regular tree spacing characteristic of some savannas. Australian savanna.jpg
A savanna woodland in Northern Australia demonstrating the regular tree spacing characteristic of some savannas.

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. [26]

In different parts of North America, the word "savanna" has been used interchangeably with "barrens", "prairie", "glade", "grassland" and "oak opening". [27] 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.[ citation needed ] In the Americas, e.g. in Belize, Central America, savanna vegetation is similar from Mexico to South America and to the Caribbean. [28] 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). [14]

Over many large tropical areas, the dominant biome (forest, savanna or grassland) can not be predicted only by the climate, as historical events plays also a key role, for example, fire activity. [29] In some areas, indeed, it is possible for there to be multiple stable biomes. [30] 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. Savannas may at times be classified as forests. [13]

In climatic geomorphology it has been noted that many savannas occur in areas of pediplains and inselbergs. [31] It has been posited that river incision is not prominent but that rivers in savanna landscapes erode more by lateral migration. [31] Flooding and associated sheet wash have been prosposed as dominant erosion processes in savanna plains. [31]

Ecology

The savannas of tropical America comprises broadleaved trees such as Curatella, Byrsonima, and Bowdichia, with grasses such as Leersia and Paspalum. Bean relative Prosopis is common in the Argentinian savannahs. 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 consist of 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. The Australian savannah is abundant with sclerophyllous evergreen vegetation, which include the eucalyptus, and as well as Acacia, Bauhinia, Pandanus with grasses such as Heteropogon and kangaroo grass (Themeda). [4]

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 savannah, 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. [4]

Threats

Changes in fire management

Bushfire in Kakadu National Park, Australia Bushfire Australia.jpg
Bushfire in Kakadu National Park, Australia

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. [32] Aboriginal burning appears to have been responsible for the widespread occurrence of savanna in tropical Australia and New Guinea, [33] and savannas in India are a result of human fire use. [34] The maquis shrub savannas of the Mediterranean region were likewise created and maintained by anthropogenic fire. [35]

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 [36] and may have maintained and modified savanna flora. [3] [33] It has been suggested by many authors [36] [37] 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. [33] [36] It has been suggested by many authors [37] [38] 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. [39] 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. [40]

Grazing and browsing animals

Grevy's zebras grazing Grevy's Zebra Feeding.jpg
Grevy's zebras grazing

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. [41] In contrast the open structure of savannas allows the growth of a herbaceous layer and is commonly used for grazing domestic livestock. [42] 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. [43]

Iberian pigs feeding on acorns of an holm oak Dehesa Pigs.jpg
Iberian pigs feeding on acorns of an holm oak

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. [44] In addition to this effect, the removal of fuel reduces both the intensity and the frequency of fires which may control woody plant species. [45] 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. [46] 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. [36] 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 species). [39] 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. [47] 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

Savanna in eastern South Africa Upland South Africa Savanna.jpg
Savanna in eastern South Africa
Savanna in Western Sydney Western Sydney (Badgerys Creek) Airport site - Anton Rd.JPG
Savanna in Western Sydney

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. [36] 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. [48] Since stock carrying capacity is strongly correlated with herbage yield, there can be major financial benefits from the removal of trees, [49] such as assisting with grazing management: regions of dense tree and shrub cover harbors predators, leading to increased stock losses, for example, [50] while woody plant cover hinders mustering in both sheep and cattle areas. [51]

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. [52] 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 utilised 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. [53] 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

Acacia savanna, Taita Hills Wildlife Sanctuary, Kenya. Savanna towards the south-east from the south-west of Taita Hills Game Lodge within the Taita Hills Wildlife Sanctuary in Kenya.jpg
Acacia savanna, Taita Hills Wildlife Sanctuary, Kenya.

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. [53] [54] Other plant species include: white sage, spotted cactus, cotton seed, rosemary.

Climate change

Human induced climate change resulting from the greenhouse effect may result in an alteration of the structure and function of savannas. Some authors [55] 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. [56]

Savanna ecoregions

Mediterranean savanna in the Alentejo region, Portugal AlentejoPortugal1994.jpg
Mediterranean savanna in the Alentejo region, Portugal
A montane savanna in the Colombian Andes Cordillera oriental Colombia.jpg
A montane savanna in the Colombian Andes

A savanna can simply be distinguished by the open savannah, where grass prevails and trees are rare; and the wooded savannah, where the trees are densest, bordering an open woodland or forest. Specific savanna ecoregions of several different types include:

See also

Related Research Articles

<span class="mw-page-title-main">Temperate grasslands, savannas, and shrublands</span> Terrestrial biome

Temperate grasslands, savannas, and shrublands is a terrestrial biome defined by the World Wide Fund for Nature. The predominant vegetation in this biome consists of grass and/or shrubs. The climate is temperate and ranges from semi-arid to semi-humid. The habitat type differs from tropical grasslands in the annual temperature regime as well as the types of species found here.

<span class="mw-page-title-main">Sclerophyll</span> Type of plant

Sclerophyll is a type of vegetation that is adapted to long periods of dryness and heat. The plants feature hard leaves, short internodes and leaf orientation which is parallel or oblique to direct sunlight. The word comes from the Greek sklēros (hard) and phyllon (leaf). The term was coined by A.F.W. Schimper in 1898, originally as a synonym of xeromorph, but the two words were later differentiated.

<span class="mw-page-title-main">Grassland</span> Area with vegetation dominated by grasses

Grasslands are areas where the vegetation is dominated by grasses (Poaceae). However, sedge (Cyperaceae) and rush (Juncaceae) can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur naturally on all continents except Antarctica and are found in most ecoregions of the Earth. Furthermore, grasslands are one of the largest biomes on earth and dominate the landscape worldwide. There are different types of grasslands: natural grasslands, semi-natural grasslands, and agricultural grasslands. They cover 31–69% of the Earth's land area.

<span class="mw-page-title-main">Tropical and subtropical grasslands, savannas, and shrublands</span> Terrestrial habitat type defined by the World Wide Fund for Nature

Tropical and subtropical grasslands, savannas, and shrublands is a terrestrial biome defined by the World Wide Fund for Nature. The biome is dominated by grass and/or shrubs located in semi-arid to semi-humid climate regions of subtropical and tropical latitudes.

<span class="mw-page-title-main">Veld</span> Type of rural landscape in South Africa

Veld, also spelled veldt, is a type of wide open rural landscape in Southern Africa. Particularly, it is a flat area covered in grass or low scrub, especially in the countries of South Africa, Lesotho, Eswatini, Zimbabwe and Botswana. A certain sub-tropical woodland ecoregion of Southern Africa has been officially defined as the Bushveld by the World Wide Fund for Nature. Trees are not abundant—frost, fire and grazing animals allow grass to grow but prevent the build-up of dense foliage.

<span class="mw-page-title-main">Tropical climate</span> One of the five major climate groups in the Köppen climate classification

Tropical climate is the first of the five major climate groups in the Köppen climate classification identified with the letter A. Tropical climates are defined by a monthly average temperature of 18 °C (64.4 °F) or higher in the coolest month, and feature hot temperatures all year-round. Annual precipitation is often abundant in tropical climates, and shows a seasonal rhythm but may have seasonal dryness to varying degrees. There are normally only two seasons in tropical climates, a wet season and a dry season. The annual temperature range in tropical climates is normally very small. Sunlight is intense in these climates.

<i>Acacia aneura</i> Species of shrub or small tree

Acacia aneura, commonly known as mulga or true mulga, is a shrub or small tree native to arid outback areas of Australia. It is the dominant tree in the habitat to which it gives its name (mulga) that occurs across much of inland Australia. Specific regions have been designated the Western Australian mulga shrublands in Western Australia and Mulga Lands in Queensland.

<span class="mw-page-title-main">Rangeland</span> Biomes which can be grazed by animals or livestock (grasslands, woodlands, prairies, etc)

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<i>Acacia harpophylla</i> Species of legume

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<i>Acacia cambagei</i> Species of plant

Acacia cambagei, commonly known as gidgee, stinking wattle, stinking gidgee in English, or gidjiirr, by transliteration from indigenous languages of north-western NSW, is an endemic tree of Australia. It is found primarily in semiarid and arid Queensland, but extends into the Northern Territory, South Australia and north-western New South Wales. It can reach up to 12 m in height and can form extensive open woodland communities. The leaves, bark, and litter of A. cambagei produce a characteristic odour, vaguely reminiscent of boiled cabbage, gas or sewage that accounts for the common name of "stinking gidgee".

Scottsdale Reserve is a 1,328-hectare (3,280-acre) nature reserve on the Murrumbidgee River in south-central New South Wales, Australia. It is 79 kilometres (49 mi) south of Canberra, and 4 kilometres (2.5 mi) north of Bredbo. It is owned and managed by Bush Heritage Australia (BHA), which purchased it in 2006. The purchase was supportive of projects aiming to connect existing fragmented remnant habitat such as K2C. Since the 1870s up until 2006, the land was used for agriculture – primarily sheep grazing with some minor cropping. A significant component of the Reserve has been cleared of native vegetation.

<span class="mw-page-title-main">Flora of Australia</span> Plant species of Australia

The flora of Australia comprises a vast assemblage of plant species estimated to over 30,000 vascular and 14,000 non-vascular plants, 250,000 species of fungi and over 3,000 lichens. The flora has strong affinities with the flora of Gondwana, and below the family level has a highly endemic angiosperm flora whose diversity was shaped by the effects of continental drift and climate change since the Cretaceous. Prominent features of the Australian flora are adaptations to aridity and fire which include scleromorphy and serotiny. These adaptations are common in species from the large and well-known families Proteaceae (Banksia), Myrtaceae, and Fabaceae.

<span class="mw-page-title-main">Victoria Plains tropical savanna</span>

The Victoria Plains tropical savanna is a tropical and subtropical grasslands, savannas, and shrublands ecoregion in northwestern Australia.

<span class="mw-page-title-main">Southeast Australia temperate savanna</span>

The Southeast Australia temperate savanna ecoregion is a large area of grassland dotted with eucalyptus trees running north–south across central New South Wales, Australia.

<i>Melinis minutiflora</i> Species of grass

Melinis minutiflora, commonly known as molasses grass, is a species of grass.

<span class="mw-page-title-main">Mediterranean forests, woodlands, and scrub</span> Habitat defined by the World Wide Fund for Nature

Mediterranean forests, woodlands, and scrub is a biome defined by the World Wide Fund for Nature. The biome is generally characterized by dry summers and rainy winters, although in some areas rainfall may be uniform. Summers are typically hot in low-lying inland locations but can be cool near colder seas. Winters are typically mild to cool in low-lying locations but can be cold in inland and higher locations. All these ecoregions are highly distinctive, collectively harboring 10% of the Earth's plant species.

<span class="mw-page-title-main">Tropical vegetation</span> Vegetation in tropical latitude

Tropical vegetation is any vegetation in tropical latitudes. Plant life that occurs in climates that are warm year-round is in general more biologically diverse that in other latitudes. Some tropical areas may receive abundant rain the whole year round, but others have long dry seasons which last several months and may vary in length and intensity with geographic location. These seasonal droughts have great impact on the vegetation, such as in the Madagascar spiny forests. Rainforest vegetation is categorized by five layers. The top layer being the upper tree layer. Here you will find the largest and widest trees in all the forest. These trees tend to have very large canopy's so they can be fully exposed to sunlight. A layer below that is the middle tree layer. Here you will find more compact trees and vegetation. These trees tend to be more skinny as they are trying to gain any sunlight they can. The third layer is the lower tree area. These trees tend to be around five to ten meters high and tightly compacted. The trees found in the third layer are young trees trying to grow into the larger canopy trees. The fourth layer is the shrub layer beneath the tree canopy. This layer is mainly populated by sapling trees, shrubs, and seedlings. The fifth and final layer is the herb layer which is the forest floor. The forest floor is mainly bare except for various plants, mosses, and ferns. The forest floor is much more dense than above because of little sunlight and air movement.

<span class="mw-page-title-main">Central Great Plains (ecoregion)</span> Temperate grasslands, savannas, and shrublands ecoregion of the United States

The Central Great Plains are a semiarid prairie ecoregion of the central United States, part of North American Great Plains. The region runs from west-central Texas through west-central Oklahoma, central Kansas, and south-central Nebraska.

<span class="mw-page-title-main">Woody plant encroachment</span> Vegetation cover change

Woody plant encroachment is a natural phenomenon characterised by the increase in density of woody plants, bushes and shrubs, at the expense of the herbaceous layer, grasses and forbs. It predominantly occurs in grasslands, savannas and woodlands and can cause biome shifts from open grasslands and savannas to closed woodlands. The term bush encroachment refers to the expansion of native plants and not the spread of alien invasive species. It is thus defined by plant density, not species. Bush encroachment is often considered an ecological regime shift and can be a symptom of land degradation. The phenomenon is observed across different ecosystems and with different characteristics and intensities globally.

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