This is really a way to see vegetational (ecological) succession in action.  Not so long ago (less than a century) almost all arable land was farmed and tilled.  With advances in transportation and technology the subsistence farms vanished and much of the land went unused.  First small herbaceous plants and weeds grew in these abandoned areas, then more brush and finally woody plants and trees populate the land.  Given enough time the land will revert to forests, in fact, more of the state is forested now than was 100 years ago.