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Environmental Predisposition to Subcortical Insects and Fungi Associated with Declining Red Pine Stands
Although the above description may partially describe within-stand dynamics, it cannot explain the onset of decline.
To address this question, we have conducted a variety of experiments on the effects of environmental stress on host suitability
to insects and pathogens associated with Red Pine Decline. The results suggest that a relatively broad range of biotic and
abiotic factors can improve host suitability to these organisms (Table 3). For example, defoliation decreases resin flow,
reduces tree ability to confine O. ips, and increases feeding preference to H. pales (Krause and Raffa 1994). Likewise,
nitrification increases feeding, development, and survival by H. pales (Hunt et al. 1993). Reduced light availability also
decreases host ability to respond to L. terebrantis (Klepzig et al. 1994c). Both defoliation and reduced light availability
reduce the overall pool of photosynthate for carbon-based defenses such as terpenes and phenolics (Wright et al. 1979,
Ericsson et al. 1985, Lorio and Summers 1986, Miller and Berryman 1986, Peet and Christensen 1987, Waring 1987, Dunn et
al. 1990, Reich et al. 1992, Krause et al. 1993, Krause and Raffa 1994). These stresses can themselves be initiated by biotic
agents. For example, outbreaks by insect (or microbial) defoliators are often followed by mortality due to bark beetles
(Wright et al. 1984, Paine and Baker 1993), densely crowded stands more commonly experience bark beetle outbreaks
(Nebeker et al. 1993), and suppressed trees are less able to resist attack (Safranyik et al. 1975, Raffa and Berryman 1982b,
Mitchell et al. 1983, Waring and Pitman 1985, Paine and Baker 1993, Preisler and Mitchell 1993).
Our current understanding of the biotic and abiotic interactions contributing to Red Pine Decline is depicted in Figure
4. Each of the major assumptions underlying this model has been supported, although some of the mechanisms are only
poorly understood. In particular, our knowledge of the relative importance, impact, and tolerance ranges of the various
inciting factors lags behind our knowledge of within-stand spread. Our current understanding is now sufficient, however, to
test putative natural and anthropogenic stress agents on specific tree physiological, insect behavioral and developmental, and
microbial developmental parameters, and to project these impacts to the stand level. This model can also serve as a framework
for expanding our basic understanding of interactions among feeding guilds, the interaction of constitutive and inducible
allelochemicals in host plant resistance, and microbial mediation of plant-insect interactions.
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..... g g g ..... ..... .
Figure 4._Sequence of events in the infection, decline, and mortality of plantation red pines.
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