The average length of the frost-free growing season is greater in the southern portion of the state, with the first frost usually occurring late in October. The first frost occurs early in October in the northern part of the state. In northern Illinois, winters are cold and summers are cool enough to support eastern white pine, a northern conifer . The climate in the south, with its mild winters, hot summers, and abundant moisture, supports southern species such as bald cypress, tulip tree, and sweet gum. This wedge of grassland projects eastward from the Great Plains into the Midwest, stretching from western Iowa to Indiana, with outlying islands as far east as Ohio.
Deciduous forest occurs both north, south, and east of the Prairie Peninsula.
Low moisture levels are stressful and directly harmful to tree growth. Dry conditions favor wildfires. Young trees in particular are sensitive to consecutive years of severe drought. Drought and Fire Drought conditions also increase the probability of fires that kill young trees and damage mature, fire-sensitive species. Drought and fire prevent the forest from invading the prairie.