THE ROLE OF STRESS IN THE MORTALITY OF MIDWESTERN OAKS AS INDICATED BY GROWTH PRIOR TO DEATH

Tree mortality is a critical attribute of forest ecosystems. But the f actors responsible for tree mortality are ,poorly understood, particul arly for trees in relatively healthy forests. I evaluated a conceptual model of the tree-mortality process that attributes tree death to a s equence of environmental stresses: long-term stresses that predispose trees to injury by short-term, inciting stresses. Tree-ring growth dat a from 63 dead overstory oaks (Quercus spp. L.) from seven Midwestern (USA) oak-hickory forests were employed as long-term records of tree v igor. Tree-ring growth data from surviving oaks from the same sites we re utilized for comparison and as an indicator of year-to-year variati ons in environmental stress at each site. Utilizing time-series regres sion analysis, evidence of the action of inciting stresses was sought in the form of sudden and permanent declines, or interventions, in ind ividual tree growth rates coincident with environmental stresses. Util izing hierarchical regression analysis, evidence of the action of the predisposing stresses was sought in the form of reduced growth rates p rior to the interventions. Three-fourths of the dead trees had growth patterns prior to mortality that included growth declines indicative o f inciting stress. The median intervention resulted in a 38% decline i n basal-area growth rate. The interventions were more likely to occur during environmentally stressful years, with five drought years accoun ting for 40% of the interventions experienced by the dead trees. Prior to experiencing interventions, the now-dead trees were growing an ave rage of 18% slower than comparable surviving trees, indicating the act ion of predisposing stresses. After interventions occurred, two decade s typically passed before the trees died. The results indicate that, f or oaks in the forests studied, tree mortality is usually a decades-lo ng process involving a combination of environmental stresses. The obse rved mortality process provides a framework for identifying the causes of tree mortality but suggests limits on the utility of tree mortalit y as an indicator of forest response to specific environmental stresse s.