Introduction:
Persistence of the Prairie

Prairies in Illinois developed when climate was drier than today. Although increased aridity undoubtedly encouraged the origin of the prairie, the persistence of the prairie is also the result of other factors. Many of the early accounts and many modern ecologists ascribe the existence of tallgrass prairie in the Prairie Peninsula to wildfire. Tallgrass prairie produces large amounts of fuel that become highly flammable in the fall, and in the past, fires annually burned huge areas. These fires inhibited invasion of the prairie by trees, thus favoring prairie vegetation. Prairie plants are highly adapted to fire. Once they dry out in late summer and early fall, fires no longer cause damage, and the underground rootstocks survive. Moreover, fires hasten nutrient cycling and actually fertilize the prairie. To many early observers who witnessed the fires, the burned woodlands bordering the prairie, the confinement of trees to fire-protected refuges, and the rapid invasion of prairie by forest following the cessation of burning, fire was the obvious cause for the prairie. In 1838 Joseph Nicollet (1976) wrote:

In general they give the name l’ile de bois (isle of woods) to a group of trees that one finds in the great prairies and which are protected from fires by streams or water or other irregularities of the terrain.” He later maintained that in the absence of fire, the prairies of the region would return to forests which existed in another time.

Henry Gleason (1913) described the prairie groves in Illinois that existed because of protection by sloughs. In general, woodlands bordering streams and other water bodies were more extensive on the east and north sides, exhibiting the prevailing movement of fires from the south and west.