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Current and past defoliation by western spruce budworm (Choristoneura occidentalis Freeman) in an 80-year-old predominantly Douglas-fir (Pseudotsuga menziesii var. glauca [Beissn.] Franco) stand thinned to 14 by 14 feet was significantly lower than in a nearby untreated stand of similar age, species, and site conditions. Analysis of periodic radial growth ratios indicated that prior to thinning, Douglas-fir had been heavily defoliated by budworm and growth was seriously depressed. Following thinning, the host leave trees developed dense crowns and 10-year radial growth increased an average of 57 percent: meanwhile, radial growth of nonhost ponderosa pine (Pinus ponderosa Dougl.) increased 38 percent. Several hypotheses individually or collectively may explain this effect of thinning on budworm defoliation. The thinning may have caused increased mortality of budworm larval stages, the trees simply released and outgrew the insect, or the defensive chemistry of leave trees was enhanced. Radial growth of ponderosa pine accelerated in both the thinned and unthinned stands prior to thinning. This prethinning release of pine probably was in response to the defoliation and reduced competition of Douglas-fir.


SuDocs call # A 13.78:INT-349