Prairie

Most of the prairie has developed since the most recent Ice Age, as determined from the dating of fossilized pollen grains to about 8,300 years ago. The retreating glaciers left a central strip of flat or slightly depressed topography overlying clay soil, or in the western states, rocky dolomite shelves. Climate, weather, soil, and topography then created the initial conditions for the prairie to develop. The central prairie is subject to the stresses of extreme changes in temperature over the course of a year, drought, occasional accumulation of standing water just below the ground surface, and drying westerly winds from the Rocky Mountains. That situation favored the growth of plants with hardy root systems and underground growing points, but whose aerial (or aboveground) parts could die back each year. Perennial grasses and low, hardy shrubs could survive in such a climate; unprotected trees could not. It is thought that the post-Ice Age climate set the stage for the development of the prairie, with soil types and frequent fires then favoring the growth of grasses and forbs.

Fire does not start a prairie, but it is a crucial factor in maintaining it. The pre-settlement fires were landscape-wide and moved rapidly, driven by westerly winds that traveled unimpeded across the plains. The aerial parts of prairie plants burn, but the roots, which in perennial grasses form a deep, thick tangle underground, do not. The fast-moving fires also consume litter, the dried stalks and plant remains that had died in previous seasons and fallen to the ground. Removal of litter gave the next season's growth greater access to air and sunlight. The burns also killed shrubs and trees, which might otherwise have invaded the prairie and displaced its species. Some prairie fires were started by lightning; others were set by Native Americans, who saw the advantage to their horses and to the bison herds they hunted of having fresh vegetation to eat.

Bison, the primary grazers on the prairie, contributed to upkeep of the ecosystem by consuming young shoots of trees and shrubs along with their main food of grasses and forbs. Although they were massive animals, their wide-ranging habit ensured they would not remain in one spot to churn up and destroy the roots of prairie grasses, as fenced-in cattle would later do.

Climate, bison, and fire, maintained the dynamic boundary between prairie and forest. The prairie was not devoid of trees, however. Cottonwoods, green ash, and box elder grew as a riparian community along riverbanks, and long fingers of forest extended into the prairie, often bounded on their western edges by a watercourse that served as a natural firebreak. During periods without fire, plum trees and crabapple could take hold at the edges of the prairie. Copses of trees and patches of flowers interrupted the "seas of grass" and gave an overall more mosaic appearance to the prairie.