A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or 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 C4 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 prairie. Savanna covers approximately 20% of the Earth's land area. The largest area of savanna is in Africa.
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.
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 over-simplified 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.
"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 from 25-80% of an area.
Two factors common to all savanna environments are rainfall variations from year to year, and dry season wildfires. Savannas around the world are also dominated by tropical grasses which use the C4 type of photosynthesis. In the Americas, e.g. in Belize, Central America, savanna vegetation is similar from Mexico to South America and to the Caribbean. In North America nearby trees are of subtropical types, ranging from southwestern Pinyon pine to southeastern Longleaf Pine and northern chestnut oak.
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 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 authors 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. 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.
The closed forests 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. In contrast the open structure of savannas allows the growth of a herbaceous layer and are commonly used for grazing domestic livestock. 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.
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. There is evidence that unpalatable woody plants have increased under grazing in savannas. 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. 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.). 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. 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.
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. Since stock carrying capacity is strongly correlated with herbage yield there can be major financial benefits from the removal of trees. The removal of trees also assists grazing management. For example in sheep grazing regions of dense tree and shrub cover harbours predators, leading to increased stock losses while woody plant cover hinders mustering in both sheep and cattle areas.
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. 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. 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.
Savanna ecoregions are of several different types:
Category:Grasslands Category:Ecosystems Category:Temperate grasslands, savannas, and shrublands Category:Tropical and subtropical grasslands, savannas, and shrublands
ar:سافانا az:Savannalar be:Саванна be-x-old:Савана bs:Savana bg:Савана ca:Sabana cv:Саванна cs:Savana da:Savanne de:Savanne et:Savann es:Sabana eo:Savano eu:Sabana fa:ساوانا fr:Savane fy:Savanne gl:Sabana ko:사바나 기후 hi:सवाना hr:Savana io:Savano id:Sabana it:Savana he:סוואנה ka:სავანა kk:Саванна sw:Savana la:Savanna lv:Savanna lt:Savana ln:Esóbé hu:Szavanna ml:സവേന my:ဆာဗားနားမြက်ခင်းပြင် nl:Savanne (landschap) ja:サバナ (地理) no:Savanne nn:Savanne oc:Savana pnb:سوانا pl:Sawanna pt:Savana ro:Savană ru:Саванна sq:Savana si:සවානා simple:Savanna sk:Savana sl:Savana sr:Савана sh:Savana fi:Savanni sv:Savann ta:புல்நிலம் tr:Savan uk:Савана vi:Xavan bat-smg:Savana zh:熱帶乾濕季氣候This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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