Prairies

Areas that were mostly covered with grasses and subdominant forbs (broadleaved herbs) prior to human intervention are known as grasslands. Extensive tracts of these grassland ecosystems in the drier interiors of continents, particularly in North America, are known as prairies. Grasslands occur where there is not enough precipitation to support a forest but there is enough for herbaceous plants. The grasses and lack of precipitation are just two of the characteristics that set apart the prairies from the other ecoregions of North America.

The beginnings of the North American grasslands occurred during the Miocene-Pliocene transition, perhaps 5 million to 7 million years ago. This time period is associated with widespread drying. The increased aridity resulted from the cooling of the oceans as the Antarctic ice sheet spread and from the uplift of the Rocky Mountains. The mountains provided a partial barrier against the moist Pacific air.

There is limited precipitation in this central region, with very warm ground and air temperatures in the summer and extremely cold temperatures in winter. The prairies extend several hundred kilometers, from north-central Mexico to the southern sections of the Canadian provinces of Alberta, Saskatchewan, and Manitoba. The terrain is most often level, though it may also include rolling topography such as the Flint Hills, which stretch from north-central Oklahoma to the northeastern section of Kansas. Prairies are often categorized into three types: short-grass prairies, tallgrass prairies, and mixed-grass prairies.

The western sections of the prairie consist primarily of short grasses, which often grow in tufts. In this region, temperatures are higher in summer than in the prairies to the east, and the annual precipitation may fall below 25 centimeters. The grasses are generally less than 0.65 m (meters) in [Page 2280]height and can adapt better to occasional drought conditions and extreme temperatures. This type of prairie is often found not only in the semiarid climate of the Great Plains of North America but also in Eurasia, where it is referred to as steppe.

Tallgrass prairies once covered 140 million acres of North America, but now only 4% remain. These prairies are predominantly composed of taller grasses, sometimes reaching a height of 2.5 m. These grasses also have extensive and deep root systems. Nutrients have a stronger tendency to remain in the soil rather than leaching, since there is less rainfall here than in the forests. The intricate root systems are very efficient at using whatever water is available. The roots also stimulate carbonates and other nutrients such as calcium and magnesium to join the humus-rich surface layer.

A mixed-grass prairie is a landform where the grass composition is constantly shifting between tall and short grasses. This species transformation occurs as climate, precipitation, fires, and frequency of grazing vary from season to season.

North American prairies developed under the influence of grazing by a diverse group of native herbivores. Because the growth of grasslands is linked with a rise in the number of grazing and browsing animals, the association of grasses and grazers is most likely an ancient one. Several commonly known habitants were deer, antelope, and vast herds of bison that liberally fed themselves as they roamed. However, their constant nibbling did not destroy the grasses. Unlike woody plants, grass plants adapt well to grazing. The growing tissues in grasses are found at the base of the leaves near the ground surface. Therefore, when animals graze on the leaf blades, the continued growth process of each plant is protected. The adjacent forbs also survive because they may have thorns, tough structures, or simply a bad taste that discourages grazing.

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