It is now recognized that no single causal factor is responsible, although there are a variety of anthropogenic causal factor complexes interacting with natural events and processes that, together, induce stresses in forests that culminate in declines of individual plants and ecosystems. It is the thesis of this article that forest declines involve all biotic and abiotic facets and parameters of forested ecosystems and that the declines are themselves new causal factor complexes that continue to affect the stability of forested ecosystems independently of the initial causal factor complexes. This article attempts to utilize the growing body of information on plant physiological ecology to provide a heuristic framework for evaluating long-term forest decline.