Abstract
Hypotheses of forest decline are summarized. Stressed trees may be a more suitable food source for invertebrate herbivores than unstressed trees because stress causes an increase in the tissue content of soluble nitrogenous compounds. There is reasonable correlative evidence that air pollutants influence the outbreak patterns of forest insect species. In some cases, formerly innocuous insects may become pests. The occurrence of specific insect pests with specific air contaminants is reviewed. Emphasis is placed on the decline of Fraser fir (Abies fraseri (Pursh) Poir.) in the southern Appalachians and the occurrence of the balsam woolly adelgid. Variable mortality patterns of Fraser fir may be caused by locally distributed air pollutants acting in combination with other types of stress including that caused by the adelgid.
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