Wildfires were frequent in parts of our area prior to European settlement. They would burn for days, weeks & over entire modern-day townships in early spring & in late fall. Fire frequency varied from as little as every 1 to 3 years on the prairies to 6-12 years in barrens & savannas & perhaps 10-15 years in oak-hickory woods. In the deep coves of American Beech, Sugar Maple & Tuliptree, like those around the steep slopes of Wildcat Creek or in the ravines of Carroll, parts of Tippecanoe & Miami counties, you may not have a fire reach such a location in 80-100 years.
Obviously, flatter terrain with lack of fire breaks perpetuated prairie from Iowa & Illinois into Indiana. Large firebreaks & more eastward & southeastward extent saw larger areas of forests perpetuate.
It is the very deep, coal black soils that were in prairie for a very, very long time.............thousands of years.
Other soils that were more dark brown & gray ("gray prairies") were in a state of flux between barrens, savannas & prairies. You see this a lot in west-central & southwestern Indiana where the Hypsithermal put an area in prairie, then the barrens & savanna encroached back in on it. It may have gone back to forest off & on before being seen as prairie in the early land surveys. This flux from the warm, dry periods of the Hypsithermal, Medieval & Roman prairie periods & waxing & waning Native American & bison populations & movements led to more dark brown & gray soils under the prairies there. This due to locations not being in consistent prairie for such a long period of time. These were not in prairie as long as the the heart of Benton County or in northwest Clinton or northern Warren counties. There, thousands & thousands of years of prairie root & off & on leaf matter composition led to the deep, deep black soils we see today.
The Texas Parks and Wildlife Department summed up prairie, barrens & savanna driving forces well. This applies to our area, too:
The Blackland Prairie and Post Oak Savannah landscapes were formed and maintained by two major forces:
frequent fire and grazing of bison. Recurrent fires ignited either by lightning or humans (American Indian) were the major force that molded the prairie and savannah landscapes. These fires were typically very large in scale and would traverse the countryside until they reached landforms or conditions that would contain them (rivers, creek bottoms, soil change, topographical change, climatic change, or fuel change). Fire maintained these plant communities by suppressing invading woody species and stimulating growth of prairie grasses and forbs. Large herds of bison, sometimes as large as 1,000 animals, ranged the prairies and savannahs, where they would consume large quantities of grasses, trample organic matter, and then distribute seed into the disturbed soil. The grazing pressure was not continuous, however, and the large herds would move on allowing the range time to recover.