“Grassland”的意思、由来-开放百科全书

Grasslands are areas where the vegetation is dominated by grasses (Poaceae); however, sedge (Cyperaceae) and rush (Juncaceae) families can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur naturally on all continents except Antarctica. Grasslands are found in most ecoregions of the Earth. For example, there are five terrestrial ecoregion classifications (subdivisions) of the temperate grasslands, savannas, and shrublands biome (ecosystem), which is one of eight terrestrial ecozones of the Earth's surface.

Vegetation

Grassland vegetation can vary in height from very short, as in chalk grassland, to quite tall, as in the case of North American tallgrass prairie, South American grasslands and African savanna.

Woody plants, shrubs or trees may occur on some grasslands – forming savannas, scrubby grassland or semi-wooded grassland, such as the African savannas or the Iberian dehesa.^[1]

As flowering plants and trees, grasses grow in great concentrations in climates where annual rainfall ranges between {{convert|500|and|900|mm|abbr=on}}.^[2] The root systems of perennial grasses and forbs form complex mats that hold the soil in place.

Evolution

The grass-like graminoids are among the most versatile life forms. They became widespread toward the end of the Cretaceous period, and coprolites of fossilized dinosaur feces have been found containing phytoliths of a variety of grasses that include grasses that are related to modern rice and bamboo.^[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.^[1]

Climates

Grasslands often occur in areas with annual precipitation is between {{convert|600|mm|abbr=on}} and {{convert|1500|mm|abbr=on}} and average mean annual temperatures ranges from −5 and 20 °C (Woodward et al. 2004). However, some grasslands occur in colder (−20 °C) and hotter (30 °C) climatic conditions.^[4] Grassland can exist in habitats that are frequently disturbed by grazing or fire, as such disturbance prevents the encroachment of woody species. Species richness is particularly high in grasslands of low soil fertility such as serpentine barrens and calcareous grasslands, where woody encroachment is prevented as low nutrient levels in the soil may inhibit the growth of forest and shrub species.

Biodiversity and conservation

Grasslands dominated by unsown wild-plant communities ("unimproved grasslands") can be called either natural or "semi-natural" habitat. 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, which maintains these grasslands through grazing and cutting regimes. These grasslands contain many species of wild plants, including grasses, sedges, rushes, and herbs; 25 or more species per square meter is not unusual.{{Fact|date=September 2007}} Chalk downlands in England can support over 40 species per square meter. In many parts of the world, few examples have escaped agricultural improvement (fertilizing, weed killing, plowing 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; there are also many species of birds that are grassland "specialists", such as the snipe and 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, 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. In many parts of the world, "unimproved" grasslands are one of the most threatened types of habitat, and a target for acquisition by wildlife conservation groups or for special grants to landowners who are encouraged to manage them appropriately.

Human impact and economic importance

Grassland vegetation often remains dominant in a particular area usually due to grazing, cutting, or natural or man-made fires, all discouraging colonization by and survival of tree and shrub seedlings. Some of the world's largest expanses of grassland are found in the African savanna, and these are maintained by wild herbivores as well as by nomadic pastoralists and their cattle, sheep or goats.

Grasslands may occur naturally or as the result of human activity. Grasslands created and maintained by human activity are called anthropogenic grasslands. Hunting cultures 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 U.S. Midwest may have been extended eastward into Illinois, Indiana, and Ohio by human agency. Much grassland in northwest Europe developed after the Neolithic Period when people gradually cleared the forest to create areas for raising their livestock.

The professional study of grasslands falls under the category of rangeland management, which focuses on ecosystem services associated with the grass-dominated arid and semi-arid rangelands of the world. Rangelands account for an estimated 70% of the earth's landmass; thus, many cultures including those of the United States are indebted to the economics that the world's grasslands have to offer, from producing grazing animals, tourism, ecosystems services such as clean water and air, and energy extraction.

Types of grassland

Schimper (1898)

Ellenberg and Mueller-Dombois (1967)

Laycock (1979)

Other

Tropical and subtropical

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 South America.

Temperate

Mid-latitude grasslands, including the prairie and Pacific grasslands of North America, the Pampas of Argentina, Brazil and Uruguay, 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 and leopards 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 (both Old and New World varieties), grasshoppers, meadowlarks, sparrows, quails, hawks and hyenas.

Flooded

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

Montane

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 tundra.

Tundra grasslands

Similar to montane grasslands, polar Arctic tundra can have grasses, but high soil moisture means that few tundras are grass-dominated today. However, during the Pleistocene glacial periods (commonly referred to as ice ages), a freezing grassland known as steppe-tundra or mammoth steppe occupied large areas of the Northern Hemisphere. These areas were very cold and arid and featured sub-surface permafrost (hence tundra) but were nevertheless productive grassland ecosystems supporting a wide variety of fauna. As the temperature warmed and the climate became wetter at the beginning of the Holocene much of the mammoth steppe transitioned forest, while the drier parts in central Eurasia remained grassland, becoming the modern Eurasian steppe.

Desert and xeric

Also called desert grasslands, this is composed of sparse grassland ecoregions located in the deserts and xeric shrublands biome.

Animals

Mites, insect larvae, nematodes and earthworms inhabit deep soil, which can reach {{convert|6|m}} 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. Some types of fungi make the plants more resistant to insect and microbial attacks.

Grassland in all its form supports a vast variety of mammals, reptiles, birds, and insects. Typical large mammals include the blue wildebeest, American bison, giant anteater and Przewalski's horse.

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.^[8]

There is evidence for grassland being much the product of animal behaviour and movement;^[9] some examples include migratory herds of antelope trampling vegetation and African bush elephants eating acacia saplings before the plant has a chance to grow into a mature tree.

Temperate grasslands, savannas, and shrublands ecoregions

Tropical and subtropical grasslands, savannas, and shrublands ecoregions

References

1. ^¹{{cite web|url=http://www.ucmp.berkeley.edu/exhibits/biomes/grasslands.php|title=University of California Museum of Paleontology Grasslands website|website=Ucmp.berkeley.edu|accessdate=2011-12-01}}
2. ^{{cite web|url=http://earthobservatory.nasa.gov/Laboratory/Biome/biograssland.html|title=NASA Earth Observatory webpage|website=Earthobservatory.nasa.gov|accessdate=2011-12-01}}
3. ^{{cite journal|doi=10.1126/science.1121020|title=Dinosaurs Dined on Grass|year=2005|last1=Piperno|first1=D. R.|journal=Science|volume=310|issue=5751|pages=1126–8|pmid=16293745|last2=Sues|first2=HD }}
4. ^{{cite web|url=http://earthobservatory.nasa.gov/Laboratory/Biome/biograssland.html|title=EO Experiments: Grassland Biome|website=Earthobservatory.nasa.gov|accessdate=2011-12-01}}
5. ^Schimper, A. F. W. 1898. {{lang|de|Pflanzen-Geographie auf physiologischer Grundlage}}. Fisher, Jena. 876 pp. [https://www.biodiversitylibrary.org/bibliography/8099#/summary English translation, 1903].
6. ^Ellenberg, H. & D. Mueller-Dombois. 1967. Tentative physiognomic-ecological classification of plant formations of the Earth [based on a discussion draft of the UNESCO working group on vegetation classification and mapping.] Berichte des Geobotanischen Institutes der Eidg. Techn. Hochschule, Stiftung Rübel, Zürich 37 (1965-1966): 21—55, .
7. ^Laycock, W.A. 1979. Introduction, pp. 1-2, in: French. N R. (ed.). Perspectives in Grassland Ecology. Springer, New York, 204 pp., [https://books.google.com/books?id=v7_kBwAAQBAJ].
8. ^{{cite web|url=http://kids.nceas.ucsb.edu/biomes/grassland.html|title=University of California – Santa Barbara Temperate Grasslands website|website=Kids.nceas.ucsb.edu|accessdate=2011-12-01}}
9. ^{{cite web|url=http://www.managingwholes.com/grazing-heals-land.htm|title=How can grazing heal land?|website=ManagingWholes.com|accessdate=2011-12-01}}