The story with the trees.
Above 4,500 feet or so a Canadian Forest type is/was located in the Smokys. During the last ice age these trees and attendant shrubs and ground cover were "pushed" south as the continental ice sheet moved south. The advance was slow enough that each generation spread its seeds in front of the ice and the forest was able to "move" south as the world cooled. The ice sheets did not reach the Southern Appalachians but the forest in front of it did and it did well for 10,000 years or so.
Acid rain began to bathe the mountains in an acidic bath in the 60s and with the introduction of the balsam wooly adelgid in the 70s the fir tress began to die. Other trees have been weakened by air pollution and the warming trend we are now seeing.
I remember this forest when I hiked the high trails in the early 70s. They were giants and their needled branches were so tightly woven that only filtered light struck the ground. What you are looking at now isn't a forest: it’s a graveyard.
Old fir roots put out new trees each year and as they reach 7-10 feet they are again attacked by the adelgids. As for the spruce, mountain ash, witch hobble and other plants of this community, it is environmental factors that are doing them in.