An "open-grown oak" perches on the side of a glacial moraine. You can tell "open-grown" trees from their thick, spreading lower branches. In tighter clusters the canoy will cause enough shade to limit lower branch growth. It takes a prarie environment for this to happen.
"It has been estimated that only about 0.01% of the original oak savanna of Wisconsin still remains. The remaining fragments of oak savanna thus constitute a rare and highly endangered community." from Pleasant Valley Conservancy, Oak Savanna Page