Grassland

thumb|upright=1.35|Typical grassland

thumb|[[Setaria pumila, a species of Poaceae (the dominant plant family in grasslands)]]

A grassland is an area (or ecosystem) where the vegetation is dominated by grasses. Sedges and rushes can also be found along with some of legumes such as clover, and other herbs. Grasslands occur naturally on all continents except Antarctica and are found in most ecoregions of the Earth. Furthermore, grasslands are one of the largest biomes on Earth, covering 31–69% of the Earth's land area. There are different types of grasslands: natural grasslands, semi-natural grasslands, and agricultural grasslands. These can be defined as:

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."

There are many different types of semi-natural grasslands, e.g. hay meadows.

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 removal of the plants by the grazing animals and later the mowing farmers led to co-existence of other plant species around. In the following, the biodiversity of the plants evolve. Also, the species that already lived there adapted to the new conditions. Nowadays, semi-natural grasslands are rather located in areas that are unsuitable for agricultural farming. Owing to semi-natural grasslands being referred to as one of the most-species rich ecosystems in the world and essential habitat for many specialists, also including pollinators,

In chalk grassland, the plants can vary from very tall to very short. Quite tall grasses can be found in 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 dehesa and montado, in Spain and Portugal respectively.

As flowering plants and trees, grasses grow in great concentrations in climates where annual rainfall ranges between . The root systems of perennial grasses and forbs form complex mats that hold the soil in place.

Fauna

thumb|left|Mountain plover

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 predators in grasslands, like lions and cheetahs live in the grasslands of the African savanna. Mites, insect larvae, nematodes, and earthworms 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.

thumb|Cheetah

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.

Carbon sequestration

Grasslands hold about twenty percent of global soil carbon stocks.

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 (SOC) balance of grasslands. Bedrock, irrigation practices, soil acidification, liming, and pasture management can all have potential impacts on grassland organic carbon stocks. Good grassland management can reverse historical soil carbon losses. The relationship of improved biodiversity with carbon storage is subject of research.

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. 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 savannas, are in danger of being lost to agriculture. 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 have an impact on climate change by slower decomposition rates of litter compared to forest environments.

thumb|upright=1.35|Main land-cover trajectories from the 1960s to 2015

Grasslands may occur naturally or as a result of human activity. 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.

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 CO2 concentration in the air increases plant growth, similarly as water use efficiency, which is very important in drier regions. However, the advantages of elevated CO2 are limited by factors including water availability and available nutrients, particularly nitrogen. Thus effects of elevated CO2 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. Forest centric restoration efforts can create the risk of misreading and misclassifying of landscapes. 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 wildfires, controlled burns 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%). It is more likely that intensification will occur in flat semi-natural grasslands, especially if the soil is fertile. On the other hand, grasslands, where the land is drought-prone or less productive, are more likely to persist as semi-natural grasslands than grasslands with fertile soil and low gradient of the terrain. Furthermore, the accessibility of the land is also important, as it is then easier to fertilize, for example. For instance, if it is located near a road. With the development of technology, it is becoming increasingly easy to cultivate land with a steeper gradient, to the detriment of grasslands. The management of grasslands is also changing permanently. There is increased use of mineral fertilizers, furthermore borders and field edges are removed to enlarge fields and leveling the terrain to facilitate the use of agricultural machinery.

Vast areas of grassland are affected by woody encroachment, which is the expansion of woody plants at the expense of the herbaceous layer. Woody encroachment is caused by a combination of human impact (e.g. fire exclusion, overstocking and resulting overgrazing) and environmental factors (i.e. increased CO2 levels in the atmosphere). It can have severe negative consequences on key ecosystem services, like land productivity and groundwater recharge.

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
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.

Types of grasslands

thumb|Meadow by the Desna river in Ukraine

Classifications of grassland

Grassland types by Schimper (1898, 1903):

Meadow (hygrophilous or tropophilous grassland)
Steppe (xerophilous grassland)
Savannah (xerophilous grassland containing isolated trees)thumb|Steppe family: a common grassland animal, the red fox

Grassland types by Ellenberg and Mueller-Dombois (1967):

Formation-class V. Terrestrial herbaceous communities

Savannas and related grasslands (tropical or subtropical grasslands and parklands)
Steppes and related grasslands (e.g. North American "prairies" etc.)
Meadows, pastures or related grasslands
Sedge swamps and flushes
Herbaceous and half-woody salt swamps
Forb vegetationthumb|A hike through the Tallgrass Prairie Heritage Park in Canada

Grassland types by Laycock (1979):

Tallgrass (true) prairie
Shortgrass prairie
Mixed-grass prairie
Shrub steppe
Annual grassland
Desert (arid) grassland
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 mammals, such as wildebeest (Connochaetes taurinus) and zebra (Equus zebra). Notable tropical and subtropical grasslands include the Llanos grasslands of South America.

alt=|left|thumb|[[Cumberland Plain Woodland, a grassy woodland that covers Western Sydney]]

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, cheetahs and leopards are also found in temperate grasslands. Other animals of this region include deer, prairie dogs, mice, jack rabbits, skunks, coyotes, snakes, foxes, owls, badgers, blackbirds, 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. 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 orchids, 300 bird species, and 150 fish species.

Water-meadows are grasslands that are deliberately flooded for short periods.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 ice ages), 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: