How did prairies form?
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 grew in the midwest. 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 southwestern Michigan died out or got burned down, except along stream banks, and prairies spread over the landscape. 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, are common in areas that border the prairies. 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 p