I can't speak for that area specifically but I used to work at a county forest preserve during my summers in college. Typically when you are trying to restore a natural environment it takes a bit of work to make sure invasive species don't turn your forest into a bramble patch. One particularly common invasive species is Buckthorn that will choke out more desirable trees like Oaks which grow slowly. Mowing is one way that keeps the undesirable invasive species from taking over

http://www.dnr.state.mn.us/invasives/terrestrialplants/woody...