Interactions with Insects and Disease

The dogwood example raises an interesting question: Is an individual tree's susceptibility to drought-related mortality determined, at least in part, by the local conditions under which the individual developed (Waring 1987)? Gram and Sork (2001) have shown that under sufficient selection pressure, even within a localized area, some species can develop distinct genotypes that are associated with fine-scale mi-crotopographic variation or with a specific set of resource availabilities. For example, Tainter et al. (1990) provided evidence suggesting that periods of prolonged moisture stress can result in differential within-species responses. In their study of the effects of drought on radial increments of trees, two populations emerged after a severe drought: a relatively healthy population and a declining population. In some species, gene switching during a fluctuating local climate—to compensate for periods of reduced resource availability (i.e., low moisture availability)—is a common drought-avoidance mechanism. Chang et al. (1996) demonstrated experimentally that genes with a variety of drought-avoidance functions are water-deficit inducible, particularly those that may fulfill a structural role either directly or through participating in the synthesis of cell wall components necessary for maintaining turgor. However, this mechanism may be ineffective where strong within-species genetic selection for specific resource conditions has occurred. That is, under widely fluctuating soil moisture conditions, the capacity for that form of gene expression to aid in the necessary adjustments in water-use efficiency may be exceeded.

Another mechanism responsible for variation in within-species responses to drought occurs when plants undergoing moisture stress incur increased levels of abscisic acid (ABA), which elicits a myriad of physiological responses, such as increased root/shoot ratios and regulation of stomatal function (Nilsen and Orcutt 1996). Long-term exposure to moisture stress, particularly during development, may result in greater sensitivity to ABA, as well as a more "hardened" physiological state, which would allow quicker responses to moisture stress and the maintenance of a higher level of drought resistance (Nilsen and Orcutt 1996). More research is needed to better explain spatial patterns of within- and among-species responses to stress.

A growing body of evidence in the literature supports the notion that the risk of tree death increases with a decreasing growth rate (Pedersen 1998). The rationale behind this assertion is that recovery from periods of stress becomes increasingly difficult and that the effects of repeated periods of stress compound problems of recovery (Pedersen 1999). Wyckoff (1999), through the use of various growth-mortality functions, showed that the probability of mortality increases with a decreasing growth rate. Specifically, dead trees of the two species he examined (Cornus florida and Acer rubrum) tend to have lower growth rates in the 5 years prior to mortality than their living cohorts. Conversely, fitted mortality functions show that the risk of death decreases with increasing growth for both species. Furthermore, he examined the effect of tree size on growth-mortality functions and found that when small trees and large trees are examined independent of one another, their respective mortality functions diverge, implying that their rates of mortality are driven by tree size.

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