A savanna or savannah is a mixed
woodland
A woodland () is, in the broad sense, land covered with trees, or in a narrow sense, synonymous with wood (or in the U.S., the ''plurale tantum'' woods), a low-density forest forming open habitats with plenty of sunlight and limited shade (see ...
-
grassland
A grassland is an area 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 natur ...
(i.e. grassy woodland)
ecosystem
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
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
Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials.
Definitions of "herb" and "herbaceous"
The fourth edition of t ...
layer consisting primarily of grasses.
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.
[Smith, Jeremy M.B.. "savanna". ]Encyclopedia Britannica
An encyclopedia (American English) or encyclopædia (British English) is a reference work or compendium providing summaries of knowledge either general or special to a particular field or discipline. Encyclopedias are divided into articles ...
, 5 Sep. 2016, https://www.britannica.com/science/savanna/Environment. Accessed 17 September 2022.
Savannas maintain an open canopy despite a high tree density. 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.
[Manoel Cláudio da Silva Jánior, Christopher William Fagg, Maria Cristina Felfili, Paulo Ernane Nogueira, Alba Valéria Rezende, and Jeanine Maria Felfili 2006 “Chapter 4. Phytogeography of Cerrado Sensu Stricto and Land System Zoning in Central Brazil” in “Neotropical Savannas and Seasonally Dry Forests: Plant Diversity, Biogeography, and Conservation” R. Toby Pennington, James A. Ratter (eds) 2006 CRC Press][Abdullahi Jibrin 2013 “A Study of Variation in Physiognomic Characteristics of Guinea Savanna Vegetation” Environment and Natural Resources Research 3:2][Erika L. Geiger, Sybil G. Gotsch, Gabriel Damasco, M. Haridasan, Augusto C. Franco & William A. Hoffmann 2011 “Distinct roles of savanna and forest tree species in regeneration under fire suppression in a Brazilian savanna” Journal of Vegetation Science 22][Scholz, Fabian G.; Bucci, Sandra J.; Goldstein, Guillermo; Meinzer, Frederick C.; Franco, Augusto C.; Salazar, Ana. 2008 “Plant- and stand-level variation in biophysical and physiological traits along tree density gradients in the Cerrado”, Brazilian Journal of Plant Physiology] 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
The hectare (; SI symbol: ha) is a non-SI metric unit of area equal to a square with 100-metre sides (1 hm2), or 10,000 m2, and is primarily used in the measurement of land. There are 100 hectares in one square kilometre. An acre is a ...
(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.
Savannas are also characterised by seasonal water availability, with the majority of rainfall confined to one season; they are associated with several types of ]biome
A biome () is a biogeographical unit consisting of a biological community that has formed in response to the physical environment in which they are found and a shared regional climate. Biomes may span more than one continent. Biome is a broader ...
s, and are frequently in a transitional zone
Transition or transitional may refer to:
Mathematics, science, and technology Biology
* Transition (genetics), a point mutation that changes a purine nucleotide to another purine (A ↔ G) or a pyrimidine nucleotide to another pyrimidine (C ↔ ...
between forest
A forest is an area of land dominated by trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, and ecological function. The United Nations' ...
and desert
A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...
or grassland
A grassland is an area 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 natur ...
, though they're mostly a transition between desert to forest. Savanna covers approximately 20% of the Earth's land area. Unlike the prairies in North America and steppe
In physical geography, a steppe () is an ecoregion characterized by grassland plains without trees apart from those near rivers and lakes.
Steppe biomes may include:
* the montane grasslands and shrublands biome
* the temperate grasslands, ...
s in Eurasia
Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelago a ...
, 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.
Etymology
The word derives from the Spanish ''sabana'', which is itself a loanword from Taíno
The Taíno were a historic Indigenous peoples of the Caribbean, indigenous people of the Caribbean whose culture has been continued today by Taíno descendant communities and Taíno revivalist communities. At the time of European contact in the ...
, which means "treeless grassland" in the West Indies
The West Indies is a subregion of North America, surrounded by the North Atlantic Ocean and the Caribbean Sea that includes 13 independent island countries and 18 dependencies and other territories in three major archipelagos: the Greater A ...
.[ 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
In linguistics, a grapheme is the smallest functional unit of a writing system.
The word ''grapheme'' is derived and the suffix ''-eme'' by analogy with ''phoneme'' and other names of emic units. The study of graphemes is called ''graphemics' ...
when transcribed into English.
The word originally entered English as the ''Zauana'' in a description of the ''ilands of the kinges of Spayne'' from 1555.['']Oxford English Dictionary
The ''Oxford English Dictionary'' (''OED'') is the first and foundational historical dictionary of the English language, published by Oxford University Press (OUP). It traces the historical development of the English language, providing a com ...
'', "savannah", ''n.'' Oxford University Press (Oxford), 2012. This was equivalent in the orthography
An orthography is a set of conventions for writing a language, including norms of spelling, hyphenation, capitalization, word breaks, emphasis, and punctuation.
Most transnational languages in the modern period have a writing system, and mos ...
of the times to ''zavana'' (see history of V
History (derived ) is the systematic study and the documentation of the human activity. The time period of event before the invention of writing systems is considered prehistory. "History" is an umbrella term comprising past events as well ...
). Peter Martyr reported it as the local name for the plain around Comagre, the court of the cacique
A ''cacique'' (Latin American ; ; feminine form: ''cacica'') was a tribal chieftain of the Taíno people, the indigenous inhabitants at European contact of the Bahamas, the Greater Antilles, and the northern Lesser Antilles. The term is a Spa ...
Carlos in present-day Panama
Panama ( , ; es, link=no, Panamá ), officially the Republic of Panama ( es, República de Panamá), is a transcontinental country spanning the southern part of North America and the northern part of South America. It is bordered by Cos ...
. The accounts are inexact, but this is usually placed in present-day Madugandí or at points on the nearby Guna Yala
Guna Yala, formerly known as San Blas, is a ''comarca indígena'' (indigenous province) in northeast Panama. Guna Yala is home to the indigenous people known as the Gunas. Its capital is Gaigirgordub. It is bounded on the north by the Cari ...
coast opposite Ustupo
Ustupo is a town in the Kuna Yala province of Panama. The town comprises several (at least six) small, bridge-linked islands around off the coast.
Ustupo Airport runway 15/33 bridges between two of the islands. Another runway is onshore.
Sou ...
or on Point Mosquitos
Point Mosquitos ( es, Punta Mosquitos, links=no or ''Mosquito'') is a point on the Caribbean coast of Panama in Central America.Hydrographic Office of the United States NavyH.O. No. 130: ''Central America and Mexico Pilot (East Coast): From Gallin ...
. These areas are now either given over to modern
Modern may refer to:
History
* Modern history
** Early Modern period
** Late Modern period
*** 18th century
*** 19th century
*** 20th century
** Contemporary history
* Moderns, a faction of Freemasonry that existed in the 18th century
Phil ...
cropland or jungle
A jungle is land covered with dense forest and tangled vegetation, usually in tropical climates. Application of the term has varied greatly during the past recent century.
Etymology
The word ''jungle'' originates from the Sanskrit word ''jaṅ ...
.
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
The Köppen climate classification is one of the most widely used climate classification systems. It was first published by German-Russian climatologist Wladimir Köppen (1846–1940) in 1884, with several later modifications by Köppen, notabl ...
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
Congo or The Congo may refer to either of two countries that border the Congo River in central Africa:
* Democratic Republic of the Congo, the larger country to the southeast, capital Kinshasa, formerly known as Zaire, sometimes referred to a ...
and Amazon River
The Amazon River (, ; es, Río Amazonas, pt, Rio Amazonas) in South America is the largest river by discharge volume of water in the world, and the disputed longest river system in the world in comparison to the Nile.
The headwaters of t ...
s 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
The dry season is a yearly period of low rainfall, especially in the tropics. The weather in the tropics is dominated by the tropical rain belt, which moves from the northern to the southern tropics and back over the course of the year. The te ...
wildfire
A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire ...
s. In the Americas
The Americas, which are sometimes collectively called America, are a landmass comprising the totality of North and South America. The Americas make up most of the land in Earth's Western Hemisphere and comprise the New World.
Along with th ...
, e.g. in Belize
Belize (; bzj, Bileez) is a Caribbean and Central American country on the northeastern coast of Central America. It is bordered by Mexico to the north, the Caribbean Sea to the east, and Guatemala to the west and south. It also shares a wate ...
, Central America
Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. ...
, savanna vegetation is similar from Mexico
Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...
to South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...
and to the Caribbean
The Caribbean (, ) ( es, El Caribe; french: la Caraïbe; ht, Karayib; nl, De Caraïben) is a region of the Americas that consists of the Caribbean Sea, its islands (some surrounded by the Caribbean Sea and some bordering both the Caribbean Se ...
. 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
The Arizona Board of Regents (ABOR) is the governing body of Arizona's public university system. It provides policy guidance to Arizona State University, Northern Arizona University, the University of Arizona, and their branch campuses.
History
...
. 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 plays also a key role, for example, fire activity. 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
Climatic geomorphology is the study of the role of climate in shaping landforms and the earth-surface processes. An approach used in climatic geomorphology is to study relict landforms to infer ancient climates. Being often concerned about past cl ...
it has been noted that many savannas occur in areas of pediplain
In geology and geomorphology a pediplain (from the Latin ''pes'', genitive case ''pedis'', meaning "foot") is an extensive plain formed by the coalescence of pediments. The processes through which pediplains forms is known as pediplanation. The ...
s 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 Sheet erosion or sheet wash is the even erosion of substrate along a wide area. It occurs in a wide range of settings such as coastal plains, hillslopes, floodplains, beaches, savanna plains and semi-arid plains. Water moving fairly uniformly with ...
have been prosposed as dominant erosion processes in savanna plains.[
]
Ecology
The savannas of tropical America comprises broadleaved trees such as Curatella
''Curatella americana'', commonly known as the wild cashew tree, sambaı́ba, and the sandpaper tree, is a species of tree in the family Dilleniaceae. It is the sole accepted species in genus ''Curatella''.
Description
''Curatella americana'' is ...
, Byrsonima
''Byrsonima'' is one of about 75 genera in the Malpighiaceae, a family of flowering plants in the order Malpighiales. In particular in American English, they are known as locustberries (which also may refer specifically to '' Byrsonima coccolob ...
, and Bowdichia
''Bowdichia'' is a genus of flowering plants in the legume family, Fabaceae. It belongs to the subfamily Faboideae
The Faboideae are a subfamily of the flowering plant family Fabaceae or Leguminosae. An acceptable alternative name for the s ...
, with grasses such as Leersia
''Leersia'' is a genus of plants in the grass family which includes species known generally as cutgrasses.
The genus is widespread across many countries on all the inhabited continents.
It was named for the German botanist Johann Daniel Leers ...
and Paspalum. Bean relative Prosopis is common in the Argentinian savannahs. In the East African savannas, Acacia
''Acacia'', commonly known as the wattles or acacias, is a large genus of shrubs and trees in the subfamily Mimosoideae of the pea family Fabaceae. Initially, it comprised a group of plant species native to Africa and Australasia. The genus na ...
, Combretum, baobab
''Adansonia'' is a genus made up of eight species of medium-to-large deciduous trees known as baobabs ( or ). They are placed in the Malvaceae family, subfamily Bombacoideae. They are native to Madagascar, mainland Africa, and Australia.Tropic ...
s, Borassus
''Borassus'' (palmyra palm) is a genus of five species of fan palms, native to tropical regions of Africa, Asia and Papua New Guinea.
Description
These massive palms can grow up to high and have robust trunks with distinct leaf scars; in so ...
, and Euphorbia are a common vegetation genera. Drier savannas there feature spiny shrubs and grasses, such as Andropogon
''Andropogon'' ( common names: beard grass, bluestem grass, broomsedge) is a widespread genus of plants in the grass family, native to much of Asia, Africa, and the Americas, as well as southern Europe and various oceanic islands.
Over 100 spec ...
, Hyparrhenia
''Hyparrhenia'' is a genus of grasses. Many species are known commonly as thatching grass.
They are mostly native to tropical Africa; some can be found in warmer areas in temperate Eurasia, Australia, and Latin America. These are annual and pere ...
, and Themeda. Wetter savannas include Brachystegia trees and Pennisetum purpureum
''Cenchrus purpureus'', synonym ''Pennisetum purpureum'', also known as Napier grass, elephant grass or Uganda grass, is a species of perennial tropical grass native to the African grasslands. arrell, G., Simons, S. A., & Hillocks, R. J. (2002). ...
, and elephant grass type. West African savanna trees consist of , Combretum, and Strychnos. Indian savannas are mostly cleared, but the reserved ones feature Acacia, Mimosa, and Zizyphus over a grass cover comprising Sehima
''Sehima'' is a genus of mostly Asian and African plants in the grass family.[Dichanthium
''Dichanthium'', known commonly as bluestem or bluegrass, is a genus of African, Asian, and Australian plants in the grass family.
Some species have become naturalized in the Americas
The Americas, which are sometimes collectively cal ...]
. The Australian savannah is abundant with sclerophyllous evergreen
In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, which ...
vegetation, which include the eucalyptus
''Eucalyptus'' () is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with several other genera in the tribe Eucalypteae, including '' Corymbia'', they are commonly known as euca ...
, and as well as Acacia, Bauhinia, Pandanus
''Pandanus'' is a genus of monocots with some 750 accepted species. They are palm-like, dioecious trees and shrubs native to the Old World tropics and subtropics. The greatest number of species are found in Madagascar and Malaysia. Common names ...
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 savannah, mammals in the family Macropodidae predominate, such as kangaroos and wallabies, though cattle, horses, camels, donkeys and the ]Asian water buffalo
The water buffalo (''Bubalus bubalis''), also called the domestic water buffalo or Asian water buffalo, is a large bovid originating in the Indian subcontinent and Southeast Asia. Today, it is also found in Europe, Australia, North America, So ...
, 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 wildfire
A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire ...
s 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. Aboriginal burning appears to have been responsible for the widespread occurrence of savanna in tropical Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
and New Guinea
New Guinea (; Hiri Motu
Hiri Motu, also known as Police Motu, Pidgin Motu, or just Hiri, is a language of Papua New Guinea, which is spoken in surrounding areas of Port Moresby (Capital of Papua New Guinea).
It is a simplified version of ...
, and savannas in India
India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
are a result of human fire use. The maquis
Maquis may refer to:
Resistance groups
* Maquis (World War II), predominantly rural guerrilla bands of the French Resistance
* Spanish Maquis, guerrillas who fought against Francoist Spain in the aftermath of the Spanish Civil War
* The network ...
shrub savannas of the Mediterranean
The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...
region were likewise created and maintained by anthropogenic fire.
These fires are usually confined to the herbaceous layer
Stratification in the field of ecology refers to the vertical layering of a habitat; the arrangement of vegetation in layers. It classifies the layers (sing. ''stratum'', pl. ''strata'') of vegetation largely according to the different heights to w ...
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[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 .] 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.[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 legume
A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock f ...
s 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.
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. In contrast the open structure of savannas allows the growth of a herbaceous layer and is 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
''Stylosanthes'' is a genus of flowering plants in the legume family, Fabaceae and contains numerous highly important pasture and forage species. It was recently assigned to the informal monophyletic ''Pterocarpus'' clade of the Dalbergieae. The ...
'' species).[ 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.
]
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 ha of savanna were cleared annually in Australia alone primarily to improve pasture production. 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. Since stock carrying capacity
The carrying capacity of an environment is the maximum population size of a biological species that can be sustained by that specific environment, given the food, habitat, water, and other resources available. The carrying capacity is defined as t ...
is strongly correlated with herbage yield, there can be major financial benefits from the removal of trees, such as assisting with grazing management: regions of dense tree and shrub cover harbors predators, leading to increased stock losses, for example, 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
Bark may refer to:
* Bark (botany), an outer layer of a woody plant such as a tree or stick
* Bark (sound), a vocalization of some animals (which is commonly the dog)
Places
* Bark, Germany
* Bark, Warmian-Masurian Voivodeship, Poland
Arts, ...
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
Tebuthiuron is a nonselective broad spectrum herbicide of the urea class. It is used in a number of herbicides manufactured by Dow AgroSciences, and is sold under several trade names, depending on the formulation. It is used to control weeds, wo ...
, 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.[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 .] 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 ('' Acacia nilotica''), Rubbervine ('' Cryptostegia grandiflora''), Mesquite ('' Prosopis'' spp.), Lantana (''Lantana camara
''Lantana camara'' (common lantana) is a species of flowering plant within the verbena family (Verbenaceae), native to the American tropics. It is a very adaptable species, which can inhabit a wide variety of ecosystems; once it has been introduc ...
'' and '' L. montevidensis'') and Prickly Pear (''Opuntia
''Opuntia'', commonly called prickly pear or pear cactus, is a genus of flowering plants in the cactus family Cactaceae. Prickly pears are also known as ''tuna'' (fruit), ''sabra'', ''nopal'' (paddle, plural ''nopales'') from the Nahuatl word f ...
'' spp.) A range of herbaceous species have also been introduced to these woodlands, either deliberately or accidentally including Rhodes grass and other ''Chloris
In Greek mythology, the name Chloris (; Greek Χλωρίς ''Chlōrís'', from χλωρός ''chlōrós'', meaning "greenish-yellow", "pale green", "pale", "pallid", or "fresh") appears in a variety of contexts. Some clearly refer to different char ...
'' species, Buffel grass (''Cenchrus ciliaris
''Cenchrus ciliaris'' (buffel-grass or African foxtail grass; syn. ''Pennisetum ciliare'' (L.) Link) is a species of grass native to most of Africa, southern Asia (east to India), southern Iran, and the extreme south of Europe (Sicily). Other na ...
''), Giant rat's tail grass (''Sporobolus pyramidalis
''Sporobolus pyramidalis'', commonly known as giant rat's tail grass, is a species of grass
Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the ...
'') parthenium
''Parthenium'' is a genus of North American shrubs in the tribe Heliantheae within the family Asteraceae and subfamily Asteroideae.
The name ''Parthenium'' is an evolution of the Ancient Greek name παρθένιον (''parthenion''), which ref ...
(''Parthenium hysterophorus
''Parthenium hysterophorus'' is a species of flowering plant in the family Asteraceae. It is native to the American tropics. Common names include Santa-Maria, Santa Maria feverfew, whitetop weed, and famine weed.McConnachie AJ, Strathie LW, et ...
'') and stylos (''Stylosanthes
''Stylosanthes'' is a genus of flowering plants in the legume family, Fabaceae and contains numerous highly important pasture and forage species. It was recently assigned to the informal monophyletic ''Pterocarpus'' clade of the Dalbergieae. The ...
'' spp.) and other legume
A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock f ...
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.
Other plant species include: white sage, spotted cactus, cotton seed, rosemary.
Climate change
Human induced climate change
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
resulting from the greenhouse effect
The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
may result in an alteration of the structure and function of savannas. Some authors have suggested that savannas and grasslands may become even more susceptible to woody plant encroachment
Woody plant encroachment (also called bush encroachment, shrub encroachment, woody encroachment, bush thickening, or woody plant proliferation) is a natural phenomenon characterised by the increase in density of woody plants, bushes and shrubs, ...
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.
Savanna ecoregions
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
An ecoregion (ecological region) or ecozone (ecological zone) is an ecologically and geographically defined area that is smaller than a bioregion, which in turn is smaller than a biogeographic realm. Ecoregions cover relatively large areas of l ...
of several different types include:
* Tropical 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
The Serengeti ( ) ecosystem is a geographical region in Africa, spanning northern Tanzania. The protected area within the region includes approximately of land, including the Serengeti National Park and several game reserves. The Serengeti ...
, famous for its wildlife, are typical of this type. The Brazilian savanna ( Cerrado) is also included in this category, known for its exotic and varied flora. Other examples include the Kimberley tropical savanna
The Kimberley tropical savanna is a tropical and subtropical grasslands, savannas, and shrublands ecoregion in northwestern Australia, covering portions of Western Australia and the Northern Territory south of the Timor Sea.
Geography
The ec ...
, Central Zambezian miombo woodlands
The Central Zambezian miombo woodlands ecoregion spans southern central Africa. Miombo woodland is the predominant plant community. It is one of the largest ecoregions on the continent, and home to a great variety of wildlife, including many large ...
, Guinean forest–savanna mosaic
The Guinean forest-savanna mosaic is an ecoregion of West Africa, a band of interlaced forest, savanna, and grassland running east to west and dividing the tropical moist forests near the coast from the West Sudanian savanna of the interior.
Set ...
, Cape York Peninsula tropical savanna
The Cape York Peninsula tropical savanna is a tropical and subtropical grasslands, savannas, and shrublands, tropical grasslands, savannas, and shrublands ecoregion in northern Australia. It occupies the Cape York Peninsula in Queensland, mai ...
, Somali Acacia–Commiphora bushlands and thickets
The Somali ''Acacia''–''Commiphora'' bushlands and thickets is a semi-arid tropical grasslands, savannas, and shrublands ecoregion in the Horn of Africa. It is home to diverse communities of plants and animals, including several endemic specie ...
, Terai–Duar savanna and grasslands
The Terai–Duar savanna and grasslands is a narrow lowland ecoregion at the base of the Himalayas, about wide, and a continuation of the Indo-Gangetic Plain in India, Nepal and Bhutan. It is colloquially called ''Terai'' in the Ganges Basin eas ...
and the Victoria Basin forest–savanna mosaic.
* Subtropical and 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
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 climate ...
biome, that for example cover much of the plains of southeastern Australia, northern India, Southern Africa, southeastern Argentina and Uruguay. Examples of subtropical and temperate savannas include the Southeast Australia temperate savanna
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.
In Australia, the region is known as, or corresponds to, the Grey Box ...
, Argentine Espinal
The Espinal (NT0801) is an ecoregion of dry, thorny forest, savanna and steppe in Argentina. It has been extensively modified by large scale cattle ranching, but remnants of the original flora remain. It is threatened by the advance of the irriga ...
, Pampas, Cumberland Plain Woodland, Southern Cone Mesopotamian savanna
The Southern Cone Mesopotamian savanna, also known as the Argentine Mesopotamian grasslands, is a flooded grassland ecoregion of Argentina and southern Paraguay.
Setting
The ecoregion covers an area of , lying west of the Uruguay River in Argenti ...
, New England Peppermint Grassy Woodland and the Uruguayan savanna
The Uruguayan savanna, also known as the Brazilian-Uruguayan savanna, is a subtropical grassland and savanna ecoregion which includes all of Uruguay and southernmost Brazil, along with portions of Argentina along the Uruguay River, In Brazil, th ...
.
* Mediterranean savannas are mid-latitude savannas in Mediterranean climate
A Mediterranean climate (also called a dry summer temperate climate ''Cs'') is a temperate climate sub-type, generally characterized by warm, dry summers and mild, fairly wet winters; these weather conditions are typically experienced in the ...
regions, with mild, rainy winters and hot, dry summers, part of the Mediterranean forests, woodlands, and scrub
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 ...
biome. The oak tree savannas of California
California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...
, part of the California chaparral and woodlands
The California chaparral and woodlands is a terrestrial ecoregion of southwestern Oregon, northern, central, and southern California (United States) and northwestern Baja California (Mexico), located on the west coast of North America. It is a ...
ecoregion, fall into this category, including the Temperate Grassland of South Australia
The Iron-grass Natural Temperate Grassland of South Australia is a temperate grassland in the southeast of South Australia that stretches from Orroroo in the north, to Strathalbyn in the south, just straddling the eastern fringes of Adelaide's ...
, which features eucalyptuses. Parts of the Middle East steppe
The Middle East steppe ecoregion (WWF ID: PA0812) stretches in an arc from southern Jordan across Syria and Iraq to the western border of Iran. The upper plains of the Tigris and Euphrates Rivers dominate most of the ecoregion. The terrain is mo ...
and the Eastern Mediterranean conifer–sclerophyllous–broadleaf forests
The Eastern Mediterranean conifer-sclerophyllous-broadleaf forests, also known as the Eastern Mediterranean conifer-broadleaf forests, is an ecoregion in the eastern Mediterranean Basin. It covers portions of Turkey, Syria, Iraq, Lebanon, Isra ...
may also feature savannah-like landscapes.
* 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. Examples include the Everglades
The Everglades is a natural region of tropical climate, tropical wetlands in the southern portion of the U.S. state of Florida, comprising the southern half of a large drainage basin within the Neotropical realm. The system begins near Orland ...
, Mesopotamian Marshes
The Mesopotamian Marshes, also known as the Iraqi Marshes, are a wetland area located in Southern Iraq and in southwestern Iran. The marshes are primarily located on the floodplains of the Euphrates and Tigris rivers bound by the cities of Basra, ...
, Pantanal, Nile Delta flooded savanna
The Nile Delta flooded savanna, ecoregion (WWF ID: PA0904) covers both the Nile Delta proper, where the Nile River enters the Mediterranean Sea, as well as the river floodplains of the Nile up-river to the Aswan Dam. Since the Aswan Dam was co ...
, Lake Chad flooded savanna
The Lake Chad flooded savanna is a flooded grasslands and savannas ecoregion in Africa. It includes the seasonally- and permanently-flooded grasslands and savannas in the basin of Lake Chad in Central Africa, and covers portions of Cameroon, Ch ...
, Zambezian flooded grasslands
The Zambezian flooded grasslands is an ecoregion of southern and eastern Africa that is rich in wildlife.
Setting
The Zambezian flooded grasslands can be found on seasonally- or permanently-flooded lowlands in the basin of the Zambezi and neighbo ...
, and the Sudd
The Sudd (' or ', Dinka language, Dinka: Toc) is a vast swamp in South Sudan, formed by the White Nile's ''Mountain Nile, Baḥr al-Jabal'' section. The Arabic language, Arabic word ' is derived from ' (), meaning "barrier" or "obstruction". The ...
.
* Montane savannas are mid- to high-altitude savannas, located in a few spots around the world's high mountain regions, part of the montane grasslands and shrublands biome. The Bogotá savanna, located at an average altitude of on the Altiplano Cundiboyacense, Eastern Ranges of the Andes
The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
, is an example of a montane savanna. The savannas of the Angolan Scarp savanna and woodlands
The Angolan Scarp savanna and woodlands is an ecoregion located on the coast of Angola, an area with a variety of habitats and rich in wildlife including many endemic birds and animals.
Geography
This ecoregion consists of the strip of land that ...
ecoregion are a lower altitude example, up to .Angolan Scarp savanna and woodlands
/ref> Other examples include the Al Hajar montane woodlands
The Al Hajar montane woodlands is a temperate grasslands, savannas and shrublands ecoregion in the Al Hajar Mountains on the eastern Arabian Peninsula, which extends across portions of Oman and the United Arab Emirates.
Geography
The ecoregion ...
and the southern part of the Eastern Anatolian montane steppe
The Eastern Anatolian montane steppe is a temperate grasslands, savannas, and shrublands ecoregion. It is located in the Armenian Highlands, covering parts of eastern Turkey, Armenia, Azerbaijan, southern Georgia, and northwestern Iran.
Geograp ...
.
See also
* Pampas
*Pasture
Pasture (from the Latin ''pastus'', past participle of ''pascere'', "to feed") is land used for grazing. Pasture lands in the narrow sense are enclosed tracts of farmland, grazed by domesticated livestock, such as horses, cattle, sheep, or swine ...
* Prairie
*Rangeland
Rangelands are grasslands, shrublands, woodlands, wetlands, and deserts that are grazed by domestic livestock or wild animals. Types of rangelands include tallgrass and shortgrass prairies, desert grasslands and shrublands, woodlands, savannas ...
*Steppe
In physical geography, a steppe () is an ecoregion characterized by grassland plains without trees apart from those near rivers and lakes.
Steppe biomes may include:
* the montane grasslands and shrublands biome
* the temperate grasslands, ...
*Veld
Veld ( or ), 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 Bots ...
References
External links
The Savanna
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{{Authority control
Ecosystems
Grasslands
Temperate grasslands, savannas, and shrublands
Tropical and subtropical grasslands, savannas, and shrublands