Naturally occurring grasslands are found toward the interiors of continental masses (Fig. 26.5). They tend to intergrade with forests, woodlands, or deserts at their margins, depending on precipitation patterns and amounts. A grassland may receive as little as 25 centimeters (10 inches) of rainfall or as much as 100 centimeters (39 inches) annually. Temperatures can range from 45°C (113°F) in midsummer to -45°C (-50°F) in midwinter. North American grasslands known as prairies grew on fertile soils and provided grazing for huge herds of buffalo. The buffalo disappeared, however, as the settlers cultivated more and more of the land and hunters increasingly slaughtered the large animals. By 1889, there were only 551 left, but breeding and conservation subsequently increased their numbers. Large areas are now used for growing cereal crops (particularly corn and wheat) and for grazing cattle.
Before they were destroyed, American prairies were a remarkable sight. In Illinois and Iowa, the grasses grew over 2 meters (6 feet 6 inches) tall during an average season and another meter taller during a wet one. A dazzling display of wildflowers began before the young perennial grasses emerged in the spring and continued throughout the growing season. I have counted over 50 species of flowering plants in bloom at the same time on 1 hectare (2.47 acres) of protected grassland in the middle of spring.
Areas with a Mediterranean climate (e.g., the Great Central Valley of California), where most of the precipitation takes place in the winters, generally support grasses that are shorter than those of the midwestern prairies. Such grasslands usually include vernal pools, some of which may be more than 50,000 years old. The vernal pools are temporary accumulations of rain water that evaporate after the rains come to an end. Their unique floras include an orderly sequence of flowering plants, some appearing initially at the pool margins, with each species forming a distinct zone or band until the water is gone (Fig. 26.6). Some species flower only in the damp soil and drying mud that remains. The seeds of many species germinate under water.
Grassland animals include cottontails, jackrabbits, gophers, mice, and pronghorns. As indicated earlier, buffalo were once abundant. More than 20,000 of these large animals now are protected in national parks, game preserves,
Stern-Jansky-Bidlack: Introductory Plant Biology, Ninth Edition
© The McGraw-H Companies, 2003
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and private ranches. Various sparrows (e.g., vesper and savannah sparrows) and other birds still find homes in uncultivated tracts of this biome.
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