As mentioned above, ecosystems in which grasses and grass-like plants (including sedges and rushes and collectively known as graminoids) dominate the vegetation are termed grasslands. In its narrow sense, 'grassland' may be defined as ground covered by vegetation dominated by grasses, with little or no tree cover. UNESCO defines grassland as ''land covered with herbaceous plants with less than 10 percent tree and shrub cover'' and wooded grassland as 10-40% tree and shrub cover. Grassland ecosystems are notable for two characteristics: they have properties that readily allow for agricultural exploitation through the management of domesticated plants or herbivores, and a climate that is quite variable both spatially and temporally. They are found in regions where drought is fairly common but where precipitation is sufficient for their growth. In addition, they can also dominate wetlands in both freshwater and coastal regions. They also occur in sites where more predictable rainfall occurs and soils are shallow or poorly drained, or in areas with topography too steep for woody plants. To put it simply, grasslands usually occupy that area between wetter areas dominated by woody plants and arid desert vegetation.
Grassland biomes occur on every continent except Antarctica. It is estimated that grasslands once covered as much as 25-40% of the Earth's land surface although much of the original extent of native grassland has been plowed and converted to other grass production (corn and wheat) or other row crops such as soybeans. Indeed, grasslands are important from both agronomic and ecological perspectives. Grasslands are the basis of an extensive livestock production industry in North America and elsewhere. In addition, grasslands sequester and retain large amounts of soil carbon and thus, they are an important component of the global carbon cycle.
Indeed, because grasslands store a significant amount of carbon in their soils and they contain relatively high biodiversity, then now play a prominent role in the discussion about biofuel production. Biofuels may offer a mechanism to generate energy that releases less carbon into the atmosphere. Some energy producers recommend intensive agricultural production of corn, or other grasses such as switchgrass or elephant grass for biofuel production. However, agricultural practices have significant energy costs that may reduce the value of these fuel sources. A recent study has suggested, however, that diverse prairie communities on marginal lands are potentially 'carbon negative' because they provide significant biomass for fuel and store carbon belowground. Much additional research is needed to assess the sustainability of grasslands for biofuel production, but the prospects are certainly tantalizing to energy producers and conservationists alike.
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