Prairies are one of the most recently developed ecosystems in North America, formed after the period of Pleistocene glaciation. About 18,000 years ago, much of Illinois was covered by glaciers. As the glaciers melted, the land was covered at first with tundra type vegetation, then by spruce forests. As the climate became warmer and drier, between 14,000 and 10,000 years ago, a cool mesic hardwood forest with ash, oak, elm, maple, birch, and hickory trees replaced the spruce forest. About 8,300 years ago, the climate became substantially warmer and drier, and within the relatively short time of 500 to 800 years, most of the forests in Illinois died out, except along stream banks, and prairies spread over the landscape. For more information about glaciation in North America, see the excellent book by E. C. Pielou, After the Ice Age (1991).
During the last 1,000 years the climate has become slightly cooler and wetter, making conditions more favorable to trees. Savannas, characterized by a grassy prairie-type ground cover underneath an open tree canopy, were common in northeastern Illinois. Scattered out on the prairie were patches of rich forests completely surrounded by prairie; these forests are called prairie groves.
Prairies developed and were maintained under the influence of three major non-biological stresses: climate, grazing, and fire. Occurring in the central part of North America, prairies are subject to extreme ranges of temperatures, with hot summers and cold winters. There are also great fluctuations of temperatures within growing seasons.
Rainfall varies from year to year and within growing seasons as well. The prairie region is also subject to droughts. Usually there is a prolonged dry period during the summer months, and in addition there are major droughts lasting for several years that occur every 30 years or so. People are often surprised to hear that the annual rainfall in Champaign, Illinois is seven inches MORE per year than London, England (35" vs. 28"), but London rarely has the kind of severe droughts found in central Illinois or the prairie region in general.
Before European settlement, the eastern boundary of the prairie was in a state of flux. During periods of drought, trees died and prairie plants took over previously forested regions. When rainfall was abundant, the trees and forest were able to reestablish themselves.
Prairie fires, started either by lightning or by Native Americans, were commonplace before European settlement. Any given parcel of land probably burned once every one to five years. These prairie fires moved rapidly across the prairie, and damaging heat from the fire did not penetrate the soil to any great extent. Fire kills most saplings of woody species, removes thatch that aids nutrient cycling, and promotes early flowering spring species. Today fire also is beneficial to control non-native herbaceous species that can invade prairie remnants.
A considerable portion of the above ground biomass of a prairie was consumed each year by the grazing of a wide range of browsing animals, such as bison, elk, deer, rabbits, and grasshoppers. This grazing was an integral part of the prairie ecosystem, and therefore grasslands and ungulate mammals coevolved together. Grazing increased growth in prairies, recycles nitrogen through urine and feces, and the trampling opens up habitat for plant species that prefer some disturbance of the soil.
Prairie plants have adapted to these stresses by largely being herbaceous perennials with underground storage/perennating structures, growing points slightly below ground level, and extensive, deep root systems. The tender growing points of prairie plants occur an inch or so below ground and are usually not injured by prairie fires, which move rapidly across the prairie. These underground growing points are also left unharmed by browsing animals. During droughts, the deep roots of prairie plants are able to take up moisture from deep in the soil.
Return to Ken Robertson's homepage