Abstract
A method was developed to estimate yield reduction at rotation age in overstory Douglas-fir trees ( Pseudotsuga menziesii var. glauca (Beissn.) Franco) under defoliation by a single outbreak of the western spruce budworm ( Choristoneura occidentalis (Freeman)). Budworm impact was expressed as correction factors which could be applied to the expected volume at rotation. These factors varied by site quality, stand age and with the duration and severity of defoliation. A trial of the method in the interior of British Columbia indicated that yield loss in relation to number of years of defoliation and defoliation severity was greater in stands attacked at younger ages than in stands attacked when older. Sensitivity analysis of the variables in the method showed that loss estimates were most sensitive to changes in site quality and defoliation duration and severity; variation of the rotation age at which final stand characteristics were evaluated also influenced the estimates.
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