Concern exists over the effects of interacting environmental stresses on the ecological integrity of temperate forests. Coincidence of sensitivity to drought, increasing occurrence of defoliation, and elevated pollutant deposition has produced speculation that northern hardwood forests may be susceptible to the increased climatic stresses projected for the Great Lakes region. The objective of our study was to examine relationships among environmental stress factors, vigor, mortality, and growth in northern hardwood forests located along a pollution-climate gradient in the Great Lakes region. Between 1987 and 1993, we quantified climatic variables, pollutant deposition, insect defoliation, and tree vigor and growth at five sites along this gradient. Drought and defoliation occurred to varying degrees during the study period. Symptoms of chronic environmental stress, such as cankers and epicormic branching, were most pronounced at extreme ends of the gradient. Periodic diameter increments decreased for suppressed trees but increased for dominant trees from north to south, primarily related to climatic factors. Variation in annual diameter increments was strongly associated with moisture availability, with diameter growth being greatly reduced during episodic droughts at the more southerly sites. At one site that experienced both drought and defoliation in 1988, worsening crown condition, elevated mortality, and reduced growth were consistent with the effects of acute levels of environmental stress. While most northern hardwood forests in the Great Lakes region are currently healthy, our results provide additional evidence that these forests are sensitive to increased severity of environmental stress, and may experience alterations in mortality and growth if climate changes as some have predicted
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