Responses of American Forests to Photochemical Oxidants

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Photochemical oxidants produced over urban areas in North America contribute to the development of regional levels of air pollution which have been injurious to forests. Ozone data recorded since 1974 suggest that the Pacific Southwest and the Atlantic North-east have the greatest potential for tree damage. Ozone persists longer in nonurban areas because of the absence of chemical scavengers. Factors influencing oxidant accumulation include terrain features which result in the formation of inversion layers, elevation, and seasonal patterns of air movement. Ozone concentrations typical of several mountain sites are presented. Information on the effects of ozone on North American forest species is limited. Caution is recommended in the use of sensitivity rankings from the limited available data because of variability in methods used to assess ozone injury. Tree and forest decline in response to ozone has been demonstrated in a limited number of studies in North America. Chronic injury to forests has been reported in the transverse mountain ranges of southern California, the Blue Ridge and southern Appalachian Mountains, and the Cumberland Plateau of Tennessee. Recent observations indicate that similar levels of chronic injury are occurring in the mountains southwest of Mexico City. Increased ability to characterize oxidant-caused foliar damage Symptoms may also implicate ozone and associated oxidants as important elements of tree and forest decline observed in other regions of the United States and Europe.