Most of Ohio was covered by mature oak-hickory forests in antiquity, and up to the time to settlement. But we also know that vast prairie openings were also here: either naturally or maintained by humans. They were probably an essential feature of earthwork sites. Dr. Brad Lepper explains:

There was a well-developed prairie soil beneath what most people think was the earliest element in the Newark Earthworks, the Great Circle. I believe the prairie had been there for hundreds of years, perhaps thousands of years prior to the Hopewell building it. Hunting and gathering peoples all over the world burn off sections of forest and maintain them artificially as prairies. In a fully mature oak-hickory forest there aren’t very many deer, so they would burn off sections of forest and keep them burned off to maintain it as a prairie, to cultivate the ground for the purposes of keeping high densities of game.