The 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 such as clover, and other Herbaceous plant. Grasslands occur naturally on all continents except Antarctica and are found in most of the Earth. Furthermore, grasslands are one of the largest on Earth and dominate the landscape worldwide.
Definitions
Included among the variety of definitions for grasslands are:
-
"...any plant community, including harvested forages, in which grasses and/or legumes make up the dominant vegetation."
-
"...terrestrial ecosystems dominated by herbaceous and shrub vegetation, and maintained by fire, grazing, drought and/or freezing temperatures." (Pilot Assessment of Global Ecosystems, 2000)
-
"A region with sufficient average annual precipitation (25-75 cm) to support grass..." (Stiling, 1999)
Semi-natural grasslands are a very common subcategory of the grasslands biome.
-
Grassland existing as a result of human activity (mowing or livestock grazing), where environmental conditions and the species pool are maintained by natural processes.
They can also be described as the following:
-
"Semi-natural grasslands are one of the world's most biodiverse habitats on a small spatial scales."
-
"Semi-natural grasslands belong to the most species rich ecosystems in the world."
-
"...have been formed over the course of centuries through extensive grazing and mowing."
-
"...without the use of pesticides or fertilisers in modern times."
There are many different types of semi-natural grasslands, e.g. hay .
Evolutionary history
The
are among the most versatile
organism. They became widespread toward the end of the
Cretaceous period, and
of fossilized
dinosaur feces have been found containing
of a variety of grasses that include grasses that are related to modern
rice and
bamboo.
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 favourable to the evolution of grasslands.
Around 5 million years ago during the Late Miocene in the New World and the Pliocene in the Old World, the first true grasslands occurred. Existing forest biomes declined, and grasslands became much more widespread. It is known that grasslands have existed in Europe throughout the Pleistocene (the last 1.8 million years).
The semi-natural grasslands first appeared when humans started farming. So for the use of agriculture, forests got cleared in Europe. Ancient meadows and pastures were the parts that were suitable for cultivation. The semi-natural grasslands were formed from these areas.
Most of the grassland areas have been turned to arable fields and disappeared again. The grasslands permanently became arable cropping fields due to the steady decrease in organic matter.
Ecology
Biodiversity
Grasslands dominated by unsown wild-plant communities ("unimproved grasslands") can be called either natural or "semi-natural" habitat. Although their plant communities are natural, their maintenance depends upon anthropogenic activities such as grazing and cutting regimes. The semi-natural grasslands contain many species of wild plants, including grasses, sedges, rushes, and herbs; 25 plant-species per 100 square centimeters can be found.
A European record that was found on a meadow in Estonia described 76 species of plants in one square meter.
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 little 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 and by the use of fertilizers.
Almost 90% of the European semi-natural grasslands do not exist anymore due to political and economic reasons. This loss took place during the 20th century.
Unfortunately, a large amount of red-listed species are specialists of semi-natural grasslands and are affected by the landscape change due to agriculture of the last century.
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.
Vegetation
Grassland
vegetation can vary considerably depending on the grassland type and on how strong it is affected by human impact. Dominant trees for the semi-natural grassland are
Quercus robur,
Betula pendula,
Corylus avellana,
Crataegus and many kinds of herbs.
In chalk grassland, the plants can vary from very tall to very short. Quite tall grasses can be found in tallgrass prairie, grasslands, and 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 deheza.
As flowering and trees, grasses grow in great concentrations in where annual ranges between .
Fauna
Grasslands support the greatest aggregations of large animals on Earth, including jaguars, African wild dogs,
pronghorn, black-footed ferret,
plains bison,
mountain plover, African elephant, Sunda tiger, black rhino, white rhino, savanna elephant, greater one-horned rhino, Indian elephant and
swift fox. Grazing animals, herd animals, and
Predation in grasslands, like lions and
live in the grasslands of the African savanna.
, insect
larvae,
, and
inhabit deep soil, which can reach 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.
The plants and animals that live in grasslands are connected through an unlimited web of interactions. But the removal of key species—such as buffalo and prairie dogs within the American West—and introduction of invasive species, like cane toads in northern Australia, have disrupted the balance in these ecosystems and damaged a number of other species.
Ecosystem services
Grasslands provide a range of marketed and non-marketed ecosystem services that are fundamental to the livelihoods of an estimated one billion people globally.
Carbon sequestration
Grasslands hold about twenty percent of global
soil carbon stocks.
(non-wooded) vegetation dominates grasslands and carbon is stored in the roots and soil underground. Above-ground biomass carbon is relatively short-lived due to grazing, fire, and
senescence. Grassland species have an extensive fibrous root system, with grasses often accounting for 60-80% of the biomass carbon in this ecosystem. This underground biomass can extend several meters below the surface and store abundant carbon into the soil, resulting in deep, fertile soils with high organic matter content. For this reason, soil carbon accounts for about 81% of the total ecosystem carbon in grasslands. The close link between soil carbon and underground biomass leads to similar responses of these carbon pools to fluctuations in annual precipitation and temperature on a broad spatial scale. Because plant productivity is limited by grassland precipitation, carbon stocks are highest in regions where precipitation is heaviest, such as the high grass prairie in the humid temperate region of the United States. Similarly, as annual temperatures rise, grassland carbon stocks decrease due to increased evapotranspiration.
Grasslands have suffered large losses of organic carbon due to soil disturbances, vegetation degradation, fires, erosion, nutrient deficiencies, and water shortages. The type, frequency and intensity of the disturbance can play a key role in the soil organic carbon (Soil carbon) balance of grasslands. Bedrock, irrigation practices, soil acidification, liming, and pasture management can all have potential impacts on grassland organic carbon stocks.
There is a lack of agreement on the amount of carbon that can be stored in grassland ecosystem. This is partly caused by different methodologies applied to measure soil organic carbon and limited respective datasets. Further, carbon accumulation in soils changes significantly over time and point in time measurements produce an insufficient evidence base.
Other ecosystem services
-
promotion of genetic diversity
-
weather amelioration
-
provision of wildlife habitat
Degradation
Grasslands are among the most threatened ecosystems.
Global losses from grassland degradation are estimated to be over $7 billion per year.
According to the International Union for the Conservation of Nature (IUCN), the most significant threat to grasslands is human land use, especially agriculture and mining.
The vulnerability of grasslands stems from a range of factors, such as misclassification, poor protection and cultivation.
Causes
Land use intensification
Grasslands have an extensive history of
human activity and disturbance.
To feed a growing human population, most of the world's grasslands are converted from natural landscapes to fields of corn, wheat or other crops. Grasslands that have remained largely intact thus far, such as the East African
, are in danger of being lost to agriculture.
Grasslands are very sensitive to disturbances, such as people hunting and killing key species, or plowing the land to make more space for farms.
Grassland vegetation is often a plagioclimax; it 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 .
Climate change
Grasslands often occur in areas with annual precipitation is between and and average mean annual temperatures ranges from −5 and 20 °C.
However, some grasslands occur in colder (−20 °C) and hotter (30 °C) climatic conditions. 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. Another common predicament often experienced by the ill-fated grassland creatures is the constant burning of plants, fueled by oxygen and many expired photosynthesizing organisms, with the lack of rain pushing this problem to further heights.
When not limited by other factors, increasing CO
2 concentration in the air increases plant growth, similarly as water use efficiency, which is very important in drier regions. However, the advantages of elevated CO
2 are limited by factors including water availability and available
, particularly nitrogen. Thus effects of elevated CO
2 on plant growth will vary with local climate patterns, species adaptations to water limitations, and nitrogen availability. Studies indicate that nutrient depletion may happen faster in drier regions, and with factors like plant community composition and grazing. Nitrogen deposition from air pollutants and increased mineralization from higher temperatures can increase plant productivity, but increases are often among a discount in
biodiversity as faster-growing plants outcompete others. A study of a California grassland found that global change may speed reductions in diversity and forb species are most prone to this process.
Afforestation or introduction of invasive species
Misguided afforestation efforts, for example as part of the global effort to increase carbon sequestration, can harm grasslands and their core ecosystem services.
Forest centric restoration efforts can create the risk of misreading and misclassifying of landscapes.
A map created by the World Resources Institute in collaboration with the IUCN identifies 2 billion hectares for potential forest restoration. It is criticised for including 900 million hectares of grasslands.
It is expected that non-native grasses will continue to outperform native species under warmer and drier conditions that occur in many grasslands due to climate change.
Management
The type of land management used in grasslands can also lead to grassland loss or degradation. Many grasslands and other open ecosystems depend on disturbances such as
,
or
grazing to persist, although this subject is still controversial.
A study in Brazilian Subtropical Highland Grasslands found that grasslands without traditional land management—which uses fire every two years and extensive cattle grazing—can disappear within 30 years.
This study showed that grasslands inside
protected areas, in which fire is not allowed and cattle grazing is banned, were quickly replaced by shrubs (shrub encroachment). Moreover, the absence of fire in grasslands can lead to an increase in the dominance of a few species and litter accumulation, resulting in a reduction in the number of herbaceous species and changes in herbaceous species composition.
Types of degradation
Land cover change
Land cover has always changed during the years. The following relates to the changes between 1960 and 2015. There has been a decrease in semi-natural grasslands and an increase in areas with
arable land, forest and land used for
infrastructure and buildings. The line style and relative thickness of the lines indicates the percentage of the total area that changed. Changes less than 1% and land-cover classes with all changes less than 1% (i.e. semi-natural wetlands and water) are not included.
In 1960 most of the land, 49.7%, was covered with forest and there was also more semi-natural grassland (18.8%) than arable land (15.8%). In 2015 this has changed drastically. The forest cover has increased (50.8%) and arable land has also increased (20.4%), but the semi-natural grassland cover has decreased. Although it still covers a large area of the earth (10.6%).
A quarter of semi-natural grassland was lost through intensification, i.e. it was converted into arable or pasture land and forests.
The professional study of dry 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.
Conservation and restoration
Despite growing recognition of the importance of grasslands, understanding of restoration options remains limited.
Cost of grassland restoration is highly variable and respective data is scarce.
Successful grassland restoration has several dimensions, including recognition in policy, standardisation of indicators of degradation, scientific innovation, knowledge transfer and data sharing.
Restoration methods and measures include the following:
-
prescribed fires
-
appropriate management of livestock and wild herbivores: in light of land use intensification caused by global food demand, grassland land use practices may need to be adjusted to better support key ecosystem services.
-
tree cutting
-
shrub removal
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invasive species control
-
reintroduction of native grasses and forbs via seeding or transplant: a main challenge for grassland restoration is how to overcome seed limitation.
For the period 2021–2030 the United Nations General Assembly has proclaimed the UN Decade on Restoration, involving a joint resolution by over 70 countries. It is led by the United Nations Environment Programme and the Food and Agriculture Organization.
Types of grasslands
Classifications of grassland
Grassland types by Schimper (1898, 1903):
-
Meadow (hygrophilous or tropophilous grassland)
-
Steppe (xerophilous grassland)
-
(xerophilous grassland containing isolated trees)
Grassland types by Ellenberg and Mueller-Dombois (1967):[Ellenberg, H. & D. Mueller-Dombois. 1967. Tentative physiognomic-ecological classification of plant formations of the Earth based Berichte des Geobotanischen Institutes der Eidg. Techn. Hochschule, Stiftung Rübel, Zürich 37 (1965-1966): 21—55, [1] .]
Formation-class V. Terrestrial herbaceous communities
-
Savannas and related grasslands (tropical or subtropical grasslands and parklands)
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Steppes and related grasslands (e.g. North American "prairies" etc.)
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Meadows, or related grasslands
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Sedge and flushes
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Herbaceous and half-woody salt swamps
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Forb vegetation
Grassland types by Laycock (1979):
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Tallgrass (true) prairie
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Shortgrass prairie
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Mixed-grass prairie
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Shrub-steppe
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Annual grassland
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Desert (arid) grassland
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High mountain grassland
General grasslands types
Tropical and subtropical
These grasslands can be classified as the tropical and subtropical grasslands, savannas and shrublands biome. The rainfall level for that grassland type is between 90 and 150 centimeters per year. Grasses and scattered trees are common for that ecoregion, as well as large
, such as
wildebeest (
Connochaetes taurinus) and
zebra (
Equus zebra). 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
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,
,
,
, and
.
Carnivores like
,
wolf,
and
are also found in temperate grasslands. Other animals of this region include
deer,
,
mouse,
,
,
,
,
,
,
, blackbirds,
,
, sparrows,
,
and
.
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. The species that live in these grasslands are well adapted to the hydrologic regimes and soil conditions. The Everglades—the world's largest rain-fed flooded grassland—is rich in 11,000 species of seed-bearing plants, 25 species of
Orchidaceae, 300
bird species, and 150
fish species.
Antelope Valley, California">AntelopeValleyCAgrassland.JPG|thumb|Grassland in the Antelope Valley, California
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 can be tropical, subtropical, and temperate. The plants and animals, that can be found in the tropical montane, are able to adapt to cool, wet conditions as well as intense sunlight.
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
), a 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 increased and the climate became wetter at the beginning of the
Holocene much of the mammoth steppe transitioned to forest, while the drier parts in central Eurasia remained as a grassland, becoming the modern
Eurasian steppe.
Desert and xeric
Also called desert grasslands, they are composed of sparse grassland ecoregions located in the deserts and xeric shrublands biome. Temperature extremes and low amounts of rainfall characterise these kinds of grasslands. Therefore, plants and animals are well adapted to minimize water loss.
Temperate grasslands, savannas, and shrublands ecoregions
The grassland
ecoregions of the temperate grasslands, savannas, and shrublands
biome are:
Tropical and subtropical grasslands, savannas, and shrublands ecoregions
See also
Further reading
External links
