The Smokies have some natural change from storms, fire, and native pine beetles, but most of the forest composition change is coming from introduced pests. The most spectacular, at the present time, is the damage from the balsam wooly alegid. It bores into Fraser fir trees and eventually kills them. Much of the high country above fifty-five hundred feet is more or less denuded. Almost all the mature Fraser firs are dead; seedlings tend to die off after they reach the thirty year mark or so.
I understand that other insects are migrating into the Park from outside, among them foreign pests that infect hemlocks and oaks: two of the most common trees in the Park. One can imagine what would happen if both species died out on a scale equitable with, say, the American chestnut. It would not be pretty. Of course, the environment will eventually adjust, with other species filling in the gaps. The impact would still be immense, though, as anyone familiar the abundance and role of the hemlock and oak in the mountains could attest.
Back in the late 60's and early 70's I watched what you are describing happen to the forests west of Spokane, Wash.