Predicting balsam fir mortality in boreal stands affected by spruce budworm

Abstract During spruce budworm (Choristoneura fumiferana (Clem.)) outbreaks, stands dominated by balsam fir (Abies balsamea (L.) Mill.) suffer marked levels of mortality after consecutive years of defoliation. The balsam fir mortality rate is also influenced by such factors as stand age and forest composition. Nonetheless, the relative importance of these factors when predicting balsam fir mortality during an outbreak remains uncertain. Here we present models for predicting balsam fir mortality rates on the basis of defoliation duration and intensity, stand age, and forest composition. We found a significant increase in balsam fir mortality rates with an increased duration and intensity of defoliation. Balsam fir experienced >22% mortality after five years of continuous severe defoliation. The model identified cumulative defoliation—a commonly used measure in Canada of the sum of annual defoliation intensities experienced by a stand over a given defoliation period—as an effective index for predicting mortality. Detailed cumulative defoliation, a factor specifying the frequency of different levels of annual defoliation intensities experienced by a stand over a given defoliation period, provided even more reliable results. Mature stands experienced 5% greater mortality than immature stands, and mortality was 3% and 8% greater in pure balsam fir stands than in mixed balsam fir–spruce and mixed balsam fir–hardwood stands, respectively. The proportion of other tree species also affected the mortality prediction model. We attained 70% accuracy for our best model for predicting balsam fir mortality rate in heterogeneous mixed stands; this model included detailed cumulative defoliation, stand age, and the proportion of other species in the stand. Therefore, these factors must be included in decisions related to recovery cutting in forests affected by spruce budworm outbreaks.