EPA contract 68-03-2442 provided support for three years of the studies to determine the chronic effects of photochemical oxidant air pollutants on a western mixed conifer forest ecosystem. This report deals with the year 1976-77 and is the final publication on EPA contract 68-03-2442. Computer simulation programs have been written for some of the subsections. Subsystems which received greatest attention during this study were: major tree species response to oxidant dose, tree population dynamics, tree growth, moisture dynamics, soil chemical and physical properties, tree mortality relative to disease, insects and other factors, epidemiology of forest tree pathogens with emphasis on Fomes annosus, cone and seed production, tree seedling establishment, litter production and litter decomposition relative to microfloral decomposer populations.

This volume presents a body of research conducted over more than thirty years, including an intensive interdisciplinary five-year study begun in 1991. Chapters include studies of the relationships of biogeography and climate to the region's air pollution, the chemical and physiological mechanisms of ozone injury, as well as the impacts of nitrogen-containing pollutants and natural stresses on polluted forests.

Photochemical oxidants are secondary air pollutants formed under the influence of sunlight by complex photochemical reactions in air which contains nitrogen oxides and reactive hydrocarbons as precursors. The most adverse components formed by photochemical reactions in polluted air are ozone (0 ) 3 and peroxyacetyl nitrate (PAN), among many other products such as aldehydes, ketones, organic and inorganic acids, nitrates, sulfates etc. An analysis and evaluation of the available knowledge has been used to characterize the relationships among emissions, ambient air concentrations, and effects, and to identify the important controlling influences on the formation and effects of photochemical oxidants. The biological activity of photochemical oxidants was first clearly manifested during the early 1940's, when vegetation injury was observed in the Los Angeles Basin in the United States. Since that time, as a consequence of the increasing emissions of photochemical oxidant precursors, the photochemical oxidants have become the most important air pollutants in North America. In other parts of the world, for example South and Central America, Asia, and Australia, photo chemical oxidants threaten vegetation, particularly the economic and ecological performance of plant life. According to my knowledge, the first observations of ozone and PAN injury to vegetation in Europe were made by Dr. Ellis F. Darley (Statewide Air Pollution Research Center, University of California, Riverside, California) during a study visit (1963/64) to the Federal Republic of Germany.

EPA contract 68-03-2442 provided support for three years of the studies to determine the chronic effects of photochemical oxidant air pollutants on a western mixed conifer forest ecosystem. This report deals with the year 1976-77 and is the final publication on EPA contract 68-03-2442. Computer simulation programs have been written for some of the subsections. Subsystems which received greatest attention during this study were: major tree species response to oxidant dose, tree population dynamics, tree growth, moisture dynamics, soil chemical and physical properties, tree mortality relative to disease, insects and other factors, epidemiology of forest tree pathogens with emphasis on Fomes annosus, cone and seed production, tree seedling establishment, litter production and litter decomposition relative to microfloral decomposer populations.