Prairie

I

Introduction

Prairie, an ecosystem in temperate regions in which grasses and herbaceous plants are the dominant vegetation. The term “prairie” is sometimes used by scientists to describe specifically humid, temperate, but generally treeless, grasslands where tall grasses dominate (see Grassland). Another term, “steppe” , is used for drier grasslands with short grasses. However, for most people the term “prairie” means the grasslands of North America, which include both tall and short grass ecosystems depending on the climate. A similar variety of habitats is also found in grasslands in other continents, including the steppes of Eurasia and the pampas of Argentina.

The prairies of central North America are sometimes also called the Great Plains. They include the plains of southern Alberta, Saskatchewan, and Manitoba—the three “Prairie provinces” of Canada—together with the interior plains of the United States, extending south from the Canadian border through the Midwest to central Texas. From west to east, the prairies extend from the foothills of the Rocky Mountains to roughly the longitude of Lake Michigan.

II

Climate

Prairie climates are transitional between desert and temperate forest climates. The average annual rainfall, which occurs mainly in spring and early summer, is between 250 and 750 mm (10 and 30 in). Amounts, however, are variable and prolonged droughts sometimes occur. Evaporation rates are high, especially during the height of summer, when temperatures exceed 20° C (68° F). Temperatures in the northern prairies plummet to below -15° C (5° F) in winter, when snow is common and, periodically, strong winds cause blizzards.

Because the North American prairies extend across a vast area, they exhibit significant climatic variations. Rainfall generally decreases from east to west, and the aridity of the western prairies is intensified by an important climatic phenomenon, the chinook wind. This wind blows down the eastern flanks of the Rocky Mountains, becoming warmer as it descends. When it reaches the foothills, it can raise temperatures rapidly. In winter, chinook winds melt the snow, making grazing possible throughout the year. The effects of the chinook are most marked in the zone east of the Rocky Mountains between southern Colorado and northern Alberta. Strong chinooks bring mild winters, but winters are severe when they fail to occur.

Temperatures vary from north to south. For example, average temperatures in Winnipeg, capital of Manitoba, range from -20° C (-4° F) in January to 20° C (68° F) in July. By contrast, average temperatures in Oklahoma City in the south-central United States vary from 3° C (37° F) in January to 27° C (81° F) in July.

III

Soils

Prairie soils are generally dark and fertile. They are coloured by humus—organic material formed by bacteria and other soil organisms from the rotted remains of dead plants and animals. The most fertile soils are called chernozems, or black earths, like those found in Ukraine and other parts of the Eurasian steppe. Chernozems are black because the light rainfall does not leach, or wash out, humus and other nutrients from the top horizon, or layer, of the soil and deposit them at a lower level. However, gentle leaching does cause a process called calcification, which occurs particularly in the eastern part of the prairies. This occurs when rainwater (or melted snow) seeps down, helping to create fertile soil by removing calcium carbonate from the top horizon and depositing it at a lower level, while leaving behind most plant nutrients, including humus, potash, and phosphates.

Chestnut-coloured soils are a variety of chernozem, but they occur in more arid regions. They are rich in humus, but they are loose and friable. Dark-brown soils, called brunizems or prairie soils, form in more humid regions and are transitional between chernozems and the brown forest soils found in deciduous forest regions east of the prairies. Brunizems are rather less fertile than chernozems, but they are important because they occur in areas with higher rainfall levels, giving them great agricultural importance.

IV

Plant Life

Trees are rare on the North American prairies, except in the east, a transitional zone where the prairie merges into the deciduous forest belt. Even in the true prairie, poplars and willows grow along watercourses, while isolated elms and oaks grow in spots where the water table is close to the surface. But the dominant plants are xerophilous (drought resistant) prairie grasses with extensive root systems, and herbaceous annuals, and perennials, bulb plants, and woody shrubs.

The absence of trees is the result of several factors. In the driest areas, the intense summer drought inhibits tree growth, but in wetter areas other factors are important. They include fires started by lightning during late summer storms. The fires sweep quickly across the prairie, destroying tree seedlings, but they do not harm the roots of grasses, or the bulbs and seeds in the soil. Another factor which once worked against tree growth was the intense grazing and trampling of the North American plains by millions of animals, such as the American bison and the pronghorn. Today, these have been replaced by domestic grazers in large areas. Most grasses are perennials and the grazing of their leaves does not harm them. Rather, it encourages the grasses to produce side shoots, which in turn produce new leaves.

The climatic variations mean that the North American prairie contains three distinct zones. In the wetter east is a zone of tall grasses, where species of Andropogon and Stipa spartea reach heights of 2 m (6 ft 6 in) or more. One species, the big bluestem (Andropogon gerardii), reaches heights of 2.4 m (8 ft) in the wettest parts of the Great Lakes region. Other grasses in the wetter parts of the east include slough grass, found in marshy areas, Indian grass, switch grass, and wild rye. The second zone, mixed-grass prairie, contains short, medium, and tall grass species. Characteristic plants include the little bluestem (Andropogon scoparius), needlegrass (or porcupine grass), and western wheat grass, which grows to heights of between 50 and 120 cm (20 and 47 in). The third zone of short grasses, sometimes called steppe, lies in the dry west; blue grama grass and buffalo grass are characteristic species. Here the grasses often grow in clumps, with bare ground between. Some xerophytes, such as sagebrush and prickly pear, are also found in short grass prairie.

Besides grasses, the prairies contain many flowering plants especially of the composite family, such as asters, cornflowers, goldenrods, and sunflowers, and such legumes as clovers.

Much original vegetation has disappeared and the North American prairies have become one of the world's great grain-growing areas. Instead of native grass species, the prairies now contain vast fields of cultivated grasses, such as maize in the wetter, formerly tall-grass areas; wheat in the drier, mixed-grass zone. The driest short-grass zone in the west now contains large cattle ranches.