Air pollution, including both acid precipitation and nitrogen deposition, has been implicated in extensive dieback of red spruce forests in the Appalachian Mountains. Experts believe it is not immediately threatened in it's large northern range and on a preferred site.
This dieback, called "red spruce decline", has negatively effected tree growth rates and mortality has increased. The decline is apparently more severe at higher elevations, in older stands, and on more exposed sites. Studies on the causes of red spruce decline have failed to make a definitive case for any single cause.
Red Spruce on Cranberry Bog in West Virginia - Photo by Steve Nix, Licensed to About.com