Think of it, so many forest-dwelling plants, animals and fungi owe their existence in this region to human action. Is that a good thing or a bad thing? Well, it may be bad for mammoths and grasslands, but I can’t call it bad for warblers and forests.
These animals and plants did not just spring into existence when humans came around and altered the environment, there must have already been a place for them in the ecosystem. Warblers and mammoths were both thriving before humans, now it’s just the warblers. Doesn’t seem like a good thing to me. So no, we wouldn’t have had forests as we know them, we would have had forests much more biodiverse and much more interesting.
I don’t think humans can have a positive impact on an ecosystem other than undoing harm we’ve already done to it in the past.