Synopsis and Prognosis

Large areas of the temperate forest ecosystem are currently experiencing major perturbation from air pollution. The influence of a variety of air contaminants on biogeochemical cycling, patterns of succession and competition, and individual tree health, designated Class II interactions in this book, are causing significant forest change in the temperate zone. At the ecosystem level the major perturbations include decreased productivity, biomass and diversity; at the community level, reduced growth; and at the population level, altered species composition. Early and midsuccessional forests are concluded to be at particular risk. Temperate forests have historically been subjected to major change resulting from the activities of human beings. For centuries the major influence was gross destruction for agricultural, fuel, or other wood-product purposes. In the present century reduced need for agricultural land and increased forest management have reduced the adverse impact on forests in temperate latitudes. Human activities of primary contemporary importance to forest structure and function have included the introduction of exotic arthropod and microbial tree pests into forest systems lacking evolutionary exposure to these destructive agents, enhancement of native and natural stresses by cultural practices, and the creation of artificial forests of one or a few commercially important species. In the past several decades, however, we have accumulated sufficient evidence to indicate that an additional major anthropogenic modifier of temperate forest ecosystem development is air pollution.