I think the original point was that the large stands of aspen in Colorado and Utah weren't really there in the first place before people chopped down the existing forests, and if they are replaced in turn by other members of the ecosystem it isn't necessarily a disaster.

There are natural cycles to these things. When I moved out here, every summer the live oaks were filled with caterpillars hanging on threads. Annoying as hell. In the past several years, there haven't been any. Not in the towns, not in the wild areas. Plants and animals come and go.

This kind of thing has been going on for thousands of years, and looking at it through the narrow slit of human lifetimes can also be misleading.