Savanna

A savanna (also spelled savannah) is a mix of woodland and grassland, sometimes called grassy woodland. It is a type of biome and ecosystem where the trees are spaced far enough apart that their canopy does not close. Sunlight reaches the ground, so a layer of plants made mostly of grasses can grow across it.

There are four main forms of savanna. In savanna woodland, trees and shrubs form a light canopy. Tree savanna has scattered trees and shrubs. Shrub savanna has shrubs spread across the land. In grass savanna, trees and shrubs are mostly missing.

Many people think savannas have only a few trees scattered far apart, but this is not always true. In many savannas, there are more trees than in forests, and they are often spaced more evenly. Some South American savannas, such as the cerrado, can have as many trees as nearby forests. Savannas also keep their open canopy even with many trees.

Savannas get most of their rain during one season. They are often found between forests and deserts or grasslands, and they are mostly a bridge between deserts and forests. Savannas cover about 20% of the Earth's land. Unlike the prairies of North America and the steppes of Eurasia, which have cold winters, savannas are usually found in warm to hot places such as Africa, Australia, South America, and India. You can also read about Savannah, Georgia, a city in the U.S. state of Georgia, which shares a similar name.

Etymology

The word comes from the Spanish sabana, which was borrowed from Taíno. In the West Indies, it meant "treeless grassland." The word first entered English as Zauana in a 1555 description of the ilands of the kinges of Spayne. In the orthography of the time, this was the same as 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. Accounts of this place are not exact, but it is usually placed in present-day Madugandí or along the nearby Guna Yala coast, near Ustupo or on Point Mosquitos. Those areas are now used for modern cropland or are covered by jungle.

English explorer William Dampier used the spelling "savannah," with an 'h' at the end, throughout his 1697–1705 volumes of A New Voyage Round the World. This spelling became the British English form of the word. He used it to describe woodlands on the island of "Trist" in the "Bay of Campeachy" (Bay of Campeche) in the Gulf of Mexico, and also for similar places in Sumatra and the Philippines, and in New Holland (northern Australia).

Before the middle of the 1800s, many grassy lands and mixed areas of trees, shrubs, and grasses were called savanna, until the idea of a tropical savanna climate became established. The Köppen climate classification system was strongly shaped by how temperature and rainfall affect tree growth. Simple assumptions led to a view of tropical savanna as a "climatic climax" formation. Everyday use of the word now conflicts with this simple climate idea, so some large savannas north and south of the Congo and Amazon Rivers have sometimes been left off savanna maps. In parts of North America, "savanna" has been used in place of "barrens", "prairie", "glade", "grassland" and "oak opening". Authors have set tree cover limits from 5–10% at the lowest to 25–80% at the highest. Yearly rainfall changes and dry season wildfires are two things all savannas share. Savanna plants are similar across the Americas, including in Belize, Central America, Mexico, South America and the Caribbean. Because the line between woodland and savanna is unclear, the two can be grouped as one biome, since both have open-canopy trees that usually cover 25–60% of the ground.

Over many large tropical areas, climate alone cannot predict whether forest, savanna, or grassland will dominate, because historical events such as fire also play a key role. Some areas can have more than one stable biome. Savanna rainfall ranges from 500 mm (19.69 in) to 1,270 mm (50.00 in) a year, usually falling over six or eight months, followed by a dry period. In climatic geomorphology, many savannas are noted to occur in areas of pediplains and inselbergs. It has been suggested that river incision is not prominent, and that rivers in savanna landscapes erode more by lateral migration. Flooding and the related sheet wash have been proposed as the main erosion processes in savanna plains.

Ecology

Savannas in different parts of the world have different plants. In tropical America, many savannas have broadleaved trees such as Curatella, Byrsonima, and Bowdichia, along with grasses such as Leersia and Paspalum. In East Africa, acacias, Combretum, baobabs, Borassus, and Euphorbia are common. Drier savannas there have spiny shrubs and grasses, while wetter ones have Brachystegia trees and tall elephant grass (Pennisetum purpureum). Australian savannas are full of evergreen plants, including the eucalyptus, as well as Acacia and Pandanus, with grasses such as Heteropogon and kangaroo grass (Themeda).

Animals in African savannas include giraffes, elephants, buffalo, zebras, gnus, hippopotamuses, rhinoceroses, and antelopes, which eat grass, tree leaves, or both. In Australian savannas, many of the mammals belong to the Macropodidae family, such as kangaroos and wallabies. Humans have also brought in animals such as cattle, horses, camels, donkeys, and the Asian water buffalo.

Threats

Less than three percent of savanna ecosystems can be classified as highly intact. Savannas are harmed in many different ways.

Savannas are often hit by regular wildfires, and the ecosystem seems to be shaped by people's use of fire. For example, Native Americans created the Pre-Columbian woodlands of North America by burning often, which favored fire-resistant plants. Fire-stick farming seems to have created much of the savanna in tropical Australia and New Guinea, and savannas in India come from human fire use. The maquis shrub savannas of the Mediterranean region were also created and kept by human fire. Planned controlled burns usually stay near the herbaceous layer and cause little long-term harm to mature trees. However, these fires can kill or hold back young tree seedlings, which stops a continuous tree canopy from forming. Grazing animals also reduce the fuel available for fires, and introduced legumes lower the need to burn.

Grazing by sheep, goats, and cattle has changed much of the world's savannas, from the types of pasture plants that grow to woody plant encroachment. Grazing removes grass that would otherwise compete with trees for water, and it can lower how often fires burn. Animals also eat tree species they like, and weeds such as prickly acacia (Acacia nilotica) and stylo (Stylosanthes species) can spread through savannas. Trampling and removing plant cover can also damage soil, especially where grazing is heavy.

Large areas of savanna in Australia and South America have been cleared of trees, and clearing continues today. In Queensland, 4,800 km² (1,900 sq mi) of savanna were being cleared each year in the 2000s, mostly to grow more pasture for livestock. Clearing also uses methods such as felling, pushing, and pulling trees, and the use of tebuthiuron. Some exotic plant species have also spread widely, including Rubbervine, Mesquite, Lantana, Prickly Pear, and grasses such as Buffel grass. These plants can change how savannas burn, compete with native plants, and take over empty spaces in nature.

Climate change caused by the greenhouse effect may change the structure and function of savannas. Some authors suggest that savannas and grasslands may become even more open to woody plant encroachment because of greenhouse induced climate change. A recent case also showed a savanna growing into forest areas in response to climate changes.

Savanna ecoregions

Tropical savanna in Kenya.

Temperate savanna in New South Wales

Mediterranean savanna in the Alentejo region, Portugal

Nile Delta flooded savanna.

A montane savanna in the Colombian Andes

A savanna can be recognized in two main ways. An open savanna is mostly grass, with few trees. A wooded savanna has its thickest trees, and it borders an open woodland or forest. Savannas are divided into several ecoregions of different types. Tropical and subtropical savannas belong to the tropical and subtropical grasslands, savannas, and shrublands biome, and the savannas of Africa, including the Serengeti, which is famous for its wildlife, are typical of this type. The Brazilian savanna (Cerrado) is also in this group and is known for its exotic and varied flora. Other examples are the Kimberley tropical savanna, Central Zambezian miombo woodlands, Guinean forest–savanna mosaic, Cape York Peninsula tropical savanna, Somali Acacia–Commiphora bushlands and thickets, Southern Acacia–Commiphora bushlands and thickets, Terai–Duar savanna and grasslands, and the Victoria Basin forest–savanna mosaic. Temperate savannas are found in mid-latitudes, with wetter summers and drier winters. They belong to the temperate grasslands, savannas, and shrublands biome and cover much of the plains of southeastern Australia, northern India, Southern Africa, southeastern Argentina and Uruguay. Examples include the Southeast Australia temperate savanna, Argentine Espinal, Pampas, Cumberland Plain Woodland, Southern Cone Mesopotamian savanna, New England Peppermint Grassy Woodland, and the Uruguayan savanna.

Mediterranean savannas are mid-latitude savannas in regions with a Mediterranean climate, which has mild, rainy winters and hot, dry summers. They are part of the Mediterranean forests, woodlands, and scrub biome. The oak tree savannas of California, part of the California chaparral and woodlands ecoregion, belong to this group, as does the Temperate Grassland of South Australia, which features eucalyptus trees. Parts of the Middle East steppe and the Eastern Mediterranean conifer–sclerophyllous–broadleaf forests may also have savanna-like landscapes. Flooded savannas are flooded either seasonally or all year. They belong to the flooded grasslands and savannas biome, which is found mostly in the tropics and subtropics. Examples include the Everglades, Mesopotamian Marshes, Pantanal, Nile Delta flooded savanna, Lake Chad flooded savanna, Zambezian flooded grasslands, and the Sudd. Montane savannas are found at mid to high altitudes in a few places in the world's high mountain regions, and are part of the montane grasslands and shrublands biome. The Bogotá savanna is an example. It lies at an average altitude of 2,550 metres (8,370 ft) on the Altiplano Cundiboyacense, in the Eastern Ranges of the Andes. The savannas of the Angolan Scarp savanna and woodlands ecoregion are a lower example, reaching up to 1,000 metres (3,300 ft). Others include the Al Hajar montane woodlands and the southern part of the Eastern Anatolian montane steppe.