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. 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. 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. 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.