The rapid permeability and instability of the sandy soils discouraged the development of natural surface drainage. This combined with the impermeability of the underlying glacial till maintained a high water table in the swales throughout the year. During late winter and spring, groundwater levels would often be above the surface, with depths of several feet apparently not uncommon. In late summer and fall, levels would drop and the swales would dry. This extreme oscillation favored herbaceous vegetation and kept the growth of trees and shrubs to a minimum.
Fire was the second primary process involved in maintaining the Oak Openings communities. On the ridges and dunes, the savannas and woodlands experienced periodic fires that generally favored the growth of the herbaceous vegetation. Trees remained as part of the vegetative community in the uplands not only because of the better drainage, but also likely as a result of a variable fire frequency, weather conditions and soils. The wet prairies, although primarily influenced by groundwater, were, during dry periods, undoubtedly effected by fire as well.
Our understanding of the impact animals may have had on the vegetation is minimal. Many of the larger mammals were extirpated from the region very early on and we can only speculate as to how their grazing and browsing may have effected the vegetation.
Source: US EPA website.