Back in the 80s I did several feature stories for American Forests Magazine on Waldsterben. That German word literally translates to "forest death." Back then the world was just learning about acid rain and how it was killing vast tracts of forest. Much of the pioneer work on the subject was being done in Germany and at that time I was traveling a great deal there, particularly in the Schwarzwald, or Black Forest, of southwestern Germany where Waldsterben was taking place.
Nowadays it seems out of fashion to speak of "forest death," though research continues and in a few places some attention is being given it. You can read a very thoughtful analysis of the situation in Maine at and the "Air Pollution & Forest Death" page.
Back in Germany I photographed and reported on lots of dying and unhealthy forests. Often in those forests seemingly healthy trees toppled over during modest storms revealing very underdeveloped root systems. Bark-beetle infestations blighted large areas. Many trees died from no apparent cause.
Nowadays it's accepted by most serious researchers that acid rain and cycles of clearcutting stress forest ecosystems, causing the immune systems of forest trees to weaken to the point that they can no longer ward off insects and pathogens, especially fungi, which during earlier times would have given few problems.
Often on my hikes here I find sections of the forest looking suspiciously like Waldsterben. Especially pines and madrones are dying, with occasional oaks. Bark beetles are killing many pines, several cankers affect the madrones, and I fear that Sudden Oak Death may be appearing here. All this certainly looks like what I saw in the Black Forest.
Often I stand with a slope occupied by several dying trees on one side and on the other side there's a view of the Central Valley down below. Of course usually it's a hazy view with air pollution, or smog, completely hiding the towns, lakes and highways I know to be there.
To my mind, Waldsterben on these slopes is one very tangible expense not properly factored in when the costs of indulging in a car-based society are reckoned. When people pay for their gas at the pump nowadays they think they pay far too much, yet no one has even considered paying for the long-term environmental damage the car-using lifestyle is causing.