Prairies are maintained by periodic fires.
http://www.museum.state.il.us/muslink/prairie/htmls/pr_fire.html
Fire has always been a partner with healthy prairies. In dry conditions early in the spring or late in summer and early fall, lightning could strike and set a prairie ablaze. Later, Native Americans and Euro-Americans set prairie fires to help hunt animals and clear the land. Today, prairies are managed and maintained by prescribed burns.
During each burn, non-native plants are removed, allowing prairie plants more nutrients and room to grow. Prairie plants can survive fires since they have deep roots and grow from a point underground. A prescribed burn is a crucial component in prairie restoration.
Burns are conducted early in restoration projects to prepare the land for planting. Prairies are burned at regular intervals to help keep them healthy.
When the English first set foot in Virginia they wrote that a man on horseback could ride across the countryside at a full gallop for miles and miles. There was no underbrush beneath the trees to slow him down.
The Powhattans had a practice of setting fires to burn off the scrub that would grow beneath the trees. Some speculate that this was to remove hiding places for both enemies and game animals. I think it was to eradicate the plant from hell, poison ivy, but maybe that was just a side benefit.