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Biologic Markers of Air-PoDution Stress and Damage in Forests INTRODUCTION The effects of air pollution on forests have long been the object of study and public concern. For example, smelters in Copper Hill, Tennessee, in Sudbury, Ontario, and in Palmerton, Pennsylvania (reviewed by Kozlowski, 1985) have devastated forests over large areas. In contrast, the effects of New York City's air pollution on the growth of corticolous lichen populations on Long Island (Brodo, 1966) have been subtle: the changes documented were so obscure that even an experienced botanist observing a segment of the gradient might overlook the changes and dismiss the differences as being well within the normal range of variation. Similarly, patchiness in the distribution, vigor, reproductive success, and other attributes of trees in forests often is accepted as normal. But the extent of normal variation is not well known, so understanding normal variability has taken on new importance with the accumulation of evidence of declines over the last 25 years in the vitality of many forests in the United States and in Europe (e.g., Johnson and Siccama, 1983; Andersson, 1984; Schutt and Cowling, 1985; McLaughlin et al., 1987; Sheffield and Cost, 1987; Woodman and Cowling, 1987; Pitelka and Raynal, 1989). Even if forests are distant from sources of pollutants, ambient concentrations of airborne chemicals can be sufficient to produce visible injury, alter biochemical and physiologic processes that control metabolism and carbon allocation, reduce resistance to disease, reduce resistance to abiotic stress, and lead to the death of individual trees (Berry and Riperton, 1963; Linzon, 1966; Dochinger, 1968; Cobb and Stark, 1970; Miller and McBride, 1975; Cowling, 1985; Kozlowski, 1985; McBride et al., 1985; McLaughlin, 1985~. However, the cause-and-effect relationships in such examples often are difficult to discern. The pollutants most often suspected in cases of forest stress and damage are combinations of sulfuric acid and sulfur oxides, nitric acid and nitrogen oxides, and ozone. As research progresses, the list might grow to include an array of other organic and inorganic substances. The effects of a wide variety of stresses on trees and forests often are similar, and it remains to be seen whether a particular set of symptoms can be s