GrasslandsMANY DIFFERENT TYPES of ecosystems throughout the world are described as grasslands because they are dominated by species of grass with a range of other plant types as subordinates within the community. Grasslands occur at most latitudes and altitudes reflecting the wide range of environmental tolerances that characterize this huge taxonomic group of plants known as the Gramineae. There are approximately 9,000 species within this group. They are described as monocotyledons because there is only one leaf protecting the seed, in contrast to most flowering plants, which have two seed leaves and are known as dicotyledons. Grasses reproduce by generating and dispersing seed; many species spread through the production of underground stems called tillers. These two characteristics have been widely exploited in agricultural systems. First, many species producing particularly carbohydrate-rich seeds have been domesticated, a process begun more than 10,000 years ago, and today they are major cereal crops of arable agriculture, notably rice, maize (corn), wheat, barley, oats, rye, and sorghum. Second, the ability of grasses to produce a turf ground cover through tillering and the placement of the growing points close to the ground facilitates grazing by herbivorous animals and thus underpins pastoral agriculture worldwide. Broadly, grasslands may be classified as natural, seminatural, and artificial. Each of these groups can be further subdivided. Natural grasslands are widespread and tend to occur where climatic or soil conditions prohibit the growth of shrubs and trees; annual precipitation regimes are especially influential, as a pronounced dry season or seasons is characteristic of grasslands, along with a low annual precipitation. The occurrence of natural fire may have also played a role in the formation of continental grasslands. Other factors may give rise to grasslands locally, such as persistently cold, high-speed or salt-loaded winds in coastal areas.
The main categories of natural grasslands are alpine or high mountain grasslands; high-latitude TUNDRA; temperate grasslands such as the prairies of North America; the pampas of South America; the steppes of Eurasia; the grassveld of the high plateaus of South Africa; and tropical grasslands such as those of savanna regions. Most are in continental interiors with moist continental or dry subtropical climates. All of these grasslands can be further subdivided. The prairies, for example, comprise the eastern and western prairies that are separated by the Rocky Mountains. Moreover, the eastern prairies comprise three communities: tall-grass, mixed-grass, and short-grass prairies, which reflect decreasing precipitation along an east to west gradient. A similar gradation exists in the steppes, where tall-grass communities are known as meadow and short-grass communities as typical steppe. Savanna, by definition, is a vegetation type in which grasses form a continuous ground cover. However, there may also be trees and shrubs present in various degrees from sparse to abundant. All of these grasslands have been considerably modified by human activity, especially for arable and pastoral agriculture. Indeed, the prairies and steppes are considered to be the “bread basket” of the world because of their production of wheat. Alpine grasslands are exploited for summer feed for animals, for example, yak production in Mongolia and cattle production in Switzerland, in a practice known as transhumance. Seminatural grasslands occur in areas once occupied by trees and shrubs and where some type of human activity has favored the spread of grasses. Grazing and forestry are the most significant causes of this shift. Grazing prohibits the growth of trees and shrubs, as they are eaten by animals, while favoring grasses that can withstand grazing pressure. Forestry, once land has been cleared of trees either in small patches or through clear felling, generates unshaded conditions favored by grasses. Controlled firing may be a significant component of such management strategies. Consequently, seminatural grasslands exist throughout the world in areas that could and once did support a forest cover. They vary in size from small glades to many hectares and the grass species present reflect local climate and soils as well as the plant community composition of the source areas from which the invaders derived. Artificial grasslands are so called because they occur in areas that could support trees and shrubs but these are prevented from growing through management. Their floristic composition is manipulated through seeding and dressing with fertilizers and herbicides that favor species with high nutritional value for grazing animals. Such grasslands are common in countries where high-technology agriculture is practiced, especially in the developed world.