Land survey records show that pre-European forests included oak groves on the prairie where there was water or firebreaks in the form of broken topography. Although climate became wetter after about 3500 years ago and more favorable for trees, prairie continued to persist because of the annual burning of the prairies, which become highly inflammable in late summer when the vegetation dries. Native Americans ignited most of these fires, and therefore played in important role in maintaining the prairie. Native Americans burned the prairies for a number of reasons, especially for driving and flushing game during their fall hunts. Without frequent burning, shrubs and trees rapidly invade present-day Illinois prairies, and controlled burning is an important tool for the managers of these prairies.