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Responses of Northern U.S. Forests to Environmental Change
ISBN 0-387-98900-5

Chapter 4: Forest Declines in Response to Environmental Change

Philip M. Wargo and Allan N.D. Auclair
Decline diseases, linked to stress and environmental change, have increased significantly over the past century and in particular in the last two decades. A well-developed theoretical basis explains decline diseases in terms of the interactions between predisposing factors, inciting factors, and contributing factors. If the theoretical models are correct, then increased levels of various interacting stressors in the Northeast are likely to lead to increased incidence of decline disease. Increasing environmental stress is occurring at the same time as many species reach biological maturity across much of their range, a consequence of past land use impacts. Aging forests are known to be more susceptible to decline disease.

Drought and defoliation are the most common stressors associated with decline disease in the Northeast. Other important stressors include sucking insects, defoliation from late spring frost, and fungal leaf pathogens. Examples of the occurrence of decline disease include: (1) mature and abundant sugar maple in Northwest Pennsylvania, affected by biotic factors (defoliating insects, borers, and canker fungus), a series of droughts, and acid deposition; (2) red spruce in the Northeast, affected by winter injury and acid deposition; and (3) a series of widespread and simultaneous declines associated with climatic extremes over the last century. The extent and severity of declines seems

(a) Century trends in annual and cumulative loss to mortality and reduced growth due to dieback in Appalachian region (Vermont, New York, Pennsylvania, West Virginia, Maryland) of the Northeastern U.S.
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(b) The local stress index based on extreme freezing and drought stresses at Burlington, VT. 1910 to 1995.
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