Mortality of balsam fir and white spruce following a spruce budworm outbreak in the Ottawa River watershed in Quebec

Spruce–fir stands in the Ottawa River watershed in Quebec were subjected to defoliation by spruce budworm, Choristoneurafumiferana (Clem.), between 1967 and 1975. Eighteen study plots were established in mixed and coniferous mature stands to determine impact of the infestation on balsam fir, Abiesbalsamea Mill., and white spruce, Piceaglauca (Moench) Voss, and the protracted mortality of these two species following the end of the infestation. The plots were established in 1975, the last year of defoliation, and revisted each year until 1979. Between 1975 and 1979, fir mortality increased from 44 to 91%, and spruce mortality from 17 to 52%. Thus, more than 50% of the trees of both species that died did so during the 4 years after cessation of defoliation. Mortality for both spruce and fir was as high for mixed (hardwoods and budworm hosts) as for coniferous (predominantly spruce and fir) stands. A survey along 710 km of forest roads conducted in the study area in 1980 indicated the degree of mortality observed in the plots was representative of that for the whole region, and that mortality in young stands (20–50 years) was almost as high as in mature fir stands. Since the beginning of the twentieth century, intervals between budworm outbreaks lasted about 30 years in the Ottawa River watershed. However, the high incidence of mortality of white spruce and of immature fir that occurred during the recent infestation was not observed during earlier infestations. Regeneration of fir is plentiful throughout the area, but it will take some time before these young trees attain maturity and become susceptible to budworm attack. The interval between this recent and the next budworm infestation should therefore be longer than preceding ones.