I was once told that "trees die for many reasons". See the following (and references therein and citations thereof):
Mueller-Dombois, D. 1987. Natural Dieback in Forests. BioScience
37 (8, Tree Death: Cause and Consequence): 575-58.
Mueller-Dombois, D. 1985. 'Ohi'a dieback in Hawaii: 1984 synthesis and evaluation. Pacific Science 39:150-170.
Mueller-Dombois, D. et al. 1983. Tree-group death in North American and Hawaiian forests: a pathological problem or a new problem for vegetation ecology? Phytocoenologia 11:117-137
Acid rain has caused the death of many forests and continues to do so worldwide and is caused by exhaust from vehicles, smoke from factories, mining operations, and a host of other polluters. Diseases brought on by fragmentation and global warming also threaten the health of many forests.