The biology and management of bark beetles in old growth pine forests of Itasca State Park
| Executive summary part VI: Regional patterns in the susceptibility of red pine to bark beetle Itasca |
| Because the forests at Itasca are very near the western distribution limits of red pine, Itasca forests might be chronically stressed and
generally susceptible to insects and pathogens. We tested this hypothesis by comparing resin defenses of red pines in Itasca with those
growing in a region of west-central Wisconsin where growth rates are near the maximum for red pines anywhere. For comparably
aged red pines, resin flow averaged nearly twice as high in Itasca as in Wisconsin (Fig. 2.13). These results are contrary to the
hypothesis that trees growing on marginal sites, such as Itasca, tend to be less defended against bark beetles than trees on high quality
sites. However, results are completely consistent with a physiological model that predicts secondary metabolism will increase in
environments where water deficiencies limit growth more than photosynthesis (Fig. 2.14). It appears that the low growth of red pines
at Itasca is associated with increased resin flow and reduced risks from bark beetles, which should increase the potential longevity of
the forest. A counterpoint to this good news is that the same physiological model predicts that tree defenses at Itasca could be
compromised during droughts. We lack any basis for speculating on the severity of drought that would be required to increase tree
risks from bark beetles.
All available data indicate that (1) red pine in Itasca are not chronically susceptible to bark beetles compared to red pine growing on better sites and (2) old red pine at Itasca are not chronically susceptible to bark beetles because of reduced resin flow, declining growth, and senescence. Mortality of Itasca red pines from bark beetles is apparently the product of long exposure to low risks rather than an indication of a forest that is in rapid decline or chronically vulnerable to insects. |
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