Acid Rain, Bark Borers or what?
Another view from Clingman's Dome to the NW. The evergreens in the foreground were alive just a few years before. Walking through them, I'd guestimate 40-45% were dead and maybe a third of the remaining clearly stressed.
The cause is an insect called a balsam woolly adelgid. It may or may not be assisted by the substantial amount of acid rain and smog from the coal power plants throughout the South. Soil acidification certainly stresses biota in the mountains, but it is this non-native pest that's migrated down from its introduction in New England in the 19th century from Europe.
While the progressing loss of these beautiful spruce fir takes much away from the visual beauty, note that most of the Appalachian chain from this point north into upper New England has been in flux for over a century. Nearly all of the trees visible that I'll see for the next 1200 miles grew up on mountainsides that were clearcut and barren 70 years before.