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Journal Article

The Role of Stress in the Mortality of Midwestern Oaks as Indicated by Growth prior to Death

Brian S. Pedersen
Ecology
Vol. 79, No. 1 (Jan., 1998), pp. 79-93
Published by: Wiley on behalf of the Ecological Society of America
DOI: 10.2307/176866
Stable URL: http://www.jstor.org/stable/176866
Page Count: 15
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The Role of Stress in the Mortality of Midwestern Oaks as Indicated by Growth prior to Death
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Abstract

Tree mortality is a critical attribute of forest ecosystems. But the factors responsible for tree mortality are poorly understood, particularly for trees in relatively healthy forests. I evaluated a conceptual model of the tree-mortality process that attributes tree death to a sequence of environmental stresses: long-term stresses that predispose trees to injury by short-term, inciting stresses. Tree-ring growth data from 63 dead overstory oaks (Quercus spp. L.) from seven Midwestern (USA) oak-hickory forests were employed as long-term records of tree vigor. Tree-ring growth data from surviving oaks from the same sites were utilized for comparison and as an indicator of year-to-year variations in environmental stress at each site. Utilizing time-series regression analysis, evidence of the action of inciting stresses was sought in the form of sudden and permanent declines, or interventions, in individual tree growth rates coincident with environmental stresses. Utilizing hierarchical regression analysis, evidence of the action of the predisposing stresses was sought in the form of reduced growth rates prior to the interventions. Three-fourths of the dead trees had growth patterns prior to mortality that included growth declines indicative of inciting stress. The median intervention resulted in a 38% decline in basal-area growth rate. The interventions were more likely to occur during environmentally stressful years, with five drought years accounting for 40% of the interventions experienced by the dead trees. Prior to experiencing interventions, the now-dead trees were growing an average of 18% slower than comparable surviving trees, indicating the action of predisposing stresses. After interventions occurred, two decades typically passed before the trees died. The results indicate that, for oaks in the forests studied, tree mortality is usually a decades-long process involving a combination of environmental stresses. The observed mortality process provides a framework for identifying the causes of tree mortality but suggests limits on the utility of tree mortality as an indicator of forest response to specific environmental stresses.

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