Al may be old, but he ain't that old.
I don't know how much grass was growing in the forest. One desires of forest for lumber, aside from the fact that that is where most of the trees were, is that the grew straighter and with less branches, due to the close proximity of other trees, except for the canopy, thus producing better lumber without knot holes. Sun light didn't make it to the forest floor very often.
http://mdc.mo.gov/discover-nature/outdoor-recreation/nature-viewing/mammals/elk-restoration/elk-restoration-backgroun-