I just finished reading this superb essay by Gail Zawacki.
Ground level ozone, and its impact on trees, may be the most important ecological problem no one has heard of, myself included. It is particularly worrisome because planting trees, in addition to shrinking our lifestyles, may be the only effective means of making climate change less bad.
http://witsendnj.blogspot.ca/2014/07/a-fine-frenzy-universal-dance-of.html
Gail has also written a book on the demise of trees that you can download for free here:
http://witsendnj.blogspot.ca/p/pillage-plunder-pollute-llc.html