With that prologue, here's an article about how the Northeast of the U.S. wasn't originally covered in forest, since Native Americans regularly used fire to clear land. The forests appeared as the Native Americans disappeared, which is why they're only a few hundred years old.
Also, an article titled Does a Tiger Lurk in the Middle of a Fearful Symmetry? points out that tigers and other wildlife have returned to the Korean DMZ. Landmines and barbed wire are far less of a threat than highways and subdivisions. Likewise, animals are doing much better with post-Chernobyl radiation than anywhere where there are people.
I feel weird ending a blog post without some kind of bold and ill-thought-out political statement, so perhaps I should say here that the U.S. should depopulate the middle of the country (other than farming, mining, parks, a few roads, trains and Wall Drug) and move everyone into dense cities with populations no smaller than half a million each. This is sort of happening already, but people are doing things to stop it that are shitty economically and worse environmentally.