Until they do. Eventually bugs and diseases would kill off huge portions of them. Trees would still be around, but it'd definitely be a different mix than we have now. Right now every ash tree in my state is either dead or dying due to beetles, all the chestnuts died when I was a kid, the native oaks mostly only live where they've been planted because anytime one dies a faster growing maple replaces it.
Until they do. Eventually bugs and diseases would kill off huge portions of them. Trees would still be around, but it'd definitely be a different mix than we have now. Right now every ash tree in my state is either dead or dying due to beetles, all the chestnuts died when I was a kid, the native oaks mostly only live where they've been planted because anytime one dies a faster growing maple replaces it.