The title is only a slight exaggeration. For any stress, there are always survivors in a large enough population. But they may not be enough to do their job anymore. In the case of trees, the fewer there are, the more humans die.
Mitakuye oyasin. We are all related. That's not just a cultural tidbit; it is a very real, very physical truth. We are part of an ecosystem. If we do not take care of all our relations, they will no longer be able to take care of us, and we will die. We don't even understand all the things they do for us, or why we are affected; but that won't stop people from dying. Dead trees = dead humans.