Grassland

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The Konza tallgrass prairie in the Flint Hills of northeastern Kansas.
The Konza tallgrass prairie in the Flint Hills of northeastern Kansas.
An Inner Mongolian grassland in the People's Republic of China.
An Inner Mongolian grassland in the People's Republic of China.

Grasslands are areas where the vegetation is dominated by grasses (Poaceae) and other herbaceous (non-woody) plants (forbs). Plants of the sedge (Cyperacae) and rush (Juncaceae) families can also be frequent in grasslands. Grasslands occur naturally on all continents except Antarctica, and in many other areas they have replaced the natural vegetation due to human influence. In temperate latitudes, such as north-west Europe, grasslands are dominated by perennial species, whereas in warmer climates annual species form a greater component of the vegetation.[1]

Grasslands can be found in most terrestrial climates. Grassland vegetation can vary in height from very short, as in chalk downland where the vegetation may be less than 30 cm high, to quite tall, as in the case of North American tallgrass prairie, South American grasslands and African savannah. Woody plants, shrubs or trees, may occur on some grasslands - forming wooded, scrubby or semi-wooded grassland, such as the African savannahs or the Iberian dehesa. Such grasslands are sometimes referred to as wood-pasture or Savannah woodland. Grasslands cover nearly fifty percent of the land surface of the continent of Africa.[2] While grasslands in general support diverse wildlife, given the lack of hiding places for predators, the African Savanna regions support a much greater diversity in wildlife than do temperate grasslands.[3]

The appearance of mountains in the western United States during the Miocene and Pliocene epochs, a period of some 25 million years, created a continental climate favorable to the evolution of grasslands. Existing forest biomes declined, and grasslands became much more widespread. Following the Pleistocene Ice Ages, grasslands expanded in range in the hotter, drier climates, and began to become the dominant land feature worldwide.[4]

A restored grassland ecosystem at Morton Arboretum in Illinois.
A restored grassland ecosystem at Morton Arboretum in Illinois.

As flowering plants, grasses grow in great concentrations in climates where annual rainfall ranges between 500 mm and 900 mm (approximately 20 to 35 inches).[5] The root systems of perennial grasses and forbs form complex mats that hold the soil in place. Mites, insect larvae, nematodes and earthworms inhabit deep soil, which can reach 6 meters (about 20 feet) underground in undisturbed grasslands on the richest soils of the world. These invertebrates, along with symbiotic fungi, extend the root systems, break apart hard soil, enrich it with urea and other natural fertilizers, trap minerals and water and promote growth (Chadwick 1995). Some types of fungi make the plants more resistant to insect and microbial attacks.

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[edit] Climate and grasslands

Grasslands receive about 380 to 900 mm (15 to 30 in) of rain per year, as compared with deserts, which receive less than 300 mm (12 in) and tropical rainforests, which receive more than 2,000 mm (80 in).[6] While extreme temperatures may occur in some grasslands, normally they range between -20 to 30 degrees Celsius.[citation needed] Tropical grasslands have dry and wet seasons, but remain warm all the time.[citation needed] Temperate grasslands have cold winters and warm summers with rain or some snow. Since some grasses die back above ground annually,[citation needed] the soil and the sod protect the roots and the new buds from the cold of winter or dry conditions.

[edit] Grassland biodiversity and conservation

Grasslands dominated by unsown wild-plant communities ("unimproved grasslands") can be called either natural or 'semi-natural' habitats. The majority of grasslands in temperate climates are 'semi-natural', although their plant communities are natural, their maintenance depends upon anthropogenic activities such as low-intensity farming. Farming maintains these grasslands through grazing and cutting regimes. These grasslands contain many species of wild plants - grasses, sedges, rushes and herbs - 25 or more species per square metre is not unusual.[citation needed] Chalk downlands in England can support over 40 species per square metre. In many parts of the world, few examples have escaped agricultural improvement (fertilising, weed killing, ploughing or re-seeding). For example, original North American prairie grasslands or lowland wildflower meadows in the UK are now rare and their associated wild flora equally threatened. Associated with the wild-plant diversity of the "unimproved" grasslands is usually a rich invertebrate fauna; also there are many species of birds that are grassland "specialists" - for example snipe, or the Great Bustard. Agriculturally improved grasslands, which dominate modern intensive agricultural landscapes, are usually poor in wild plant species due to the original diversity of plants having been destroyed by cultivation,[citation needed] the original wild-plant communities having been replaced by sown monocultures of cultivated varieties of grasses and clovers, such as Perennial ryegrass and White Clover.[citation needed] In many parts of the world "unimproved" grasslands are one of the most threatened habitats, and a target for acquisition by wildlife conservation groups or for special grants to landowners who are encouraged to manage them appropriately.

[edit] Human impact and economic importance

Grassland in Cantabria, northern Spain.
Grassland in Cantabria, northern Spain.

Grasslands are of vital importance for raising livestock for human consumption and for milk and other dairy products.

Grassland vegetation remains dominant in a particular area usually due to grazing, cutting (such as for hay), or natural or man-made fires, all discouraging colonisation by and survival of tree and shrub seedlings.[citation needed] Fire was used in North America by the Native Americans as a means of maintaining grassland.[citation needed] Some of the world's largest expanses of grassland are found in African savannah, and these are maintained by wild herbivores as well as by nomadic pastoralists and their cattle, sheep or goats.[citation needed]

Grasslands may occur naturally or as the result of human activity. Grasslands created and maintained by human activity are called anthropogenic grasslands. Hunting peoples around the world often set regular fires to maintain and extend grasslands, and prevent fire-intolerant trees and shrubs from taking hold. The tallgrass prairies in the American Midwest may have been extended eastward into Illinois, Indiana, and Ohio by human agency. Much grassland in north-west Europe developed after the Neolithic Period, when people gradually cleared the forest to create areas for raising their livestock.

[edit] Types of grassland

[edit] Tropical and subtropical grasslands

These grasslands are classified with tropical and subtropical savannas and shrublands as the tropical and subtropical grasslands, savannas, and shrublands biome. Notable tropical and subtropical grasslands include the Llanos grasslands of northern South America.

[edit] Temperate grasslands

Mid-latitude grasslands, including the Prairie of North America, the Pampa of Argentina, calcareous downland, and the steppes of Europe. They are classified with temperate savannas and shrublands as the temperate grasslands, savannas, and shrublands biome. Temperate grasslands are the home to many large herbivores, such as bison, gazelles, zebras, rhinoceroses, and wild horses. Carnivores like lions, wolves and cheetahs are also found in temperate grasslands. Other animals of this region include: deer, prairie dogs, mice, jack rabbits, skunks, coyotes, snakes, fox, owls, badgers, blackbirds, grasshoppers, meadowlarks, sparrows, quails, and hawks.

[edit] Flooded grasslands

Grasslands that are flooded seasonally or year-round, like the Everglades of Florida or the Pantanal of Brazil, Bolivia and Paraguay. They are classified with flooded savannas as the flooded grasslands and savannas biome and occur mostly in the tropics and subtropics.

[edit] Montane grasslands

High-altitude grasslands located on high mountain ranges around the world, like the Páramo of the Andes Mountains. They are part of the montane grasslands and shrublands biome, and also constitute alpine tundra.

[edit] Polar grasslands

Similar to montane grasslands, arctic tundra can have grasses. However high soil moisture means that few tundras are grass-dominated today. However, during the Pleistocene ice ages, a polar grassland known as steppe-tundra occupied large areas of the Northern hemisphere.

[edit] Xeric grasslands

Also called desert grasslands, these are sparse grasslands located in deserts and xeric shrublands ecoregions.

[edit] See also

[edit] References

  1. ^ NASA Earth Observatory webpage
  2. ^ University of California Museum of Paleontology Grasslands website
  3. ^ University of California- Santa Barbara Temperate Grasslands website
  4. ^ University of California Museum of Paleontology Grasslands website
  5. ^ NASA Earth Observatory webpage
  6. ^ University of California Museum of Paleontology Grasslands website