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

Abstract

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.

Growth Response of Northern White-Cedar (Thuja occidentalis) to Natural Disturbances and Partial Cuts in Mixedwood Stands of Quebec, Canada

“…The growth increase for birch and white-cedar at the OR site suggests that the outbreak at this location caused sufficient tree mortality and openings to favor the subsequent development of non-host species. This outbreak has been reported as a severe one [24], which could explain the persistence of the increased growth. Growth patterns for the PL site were more difficult to interpret for this period and did not show the same pattern.…”

“…According to Bouchard et al [22,25], who studied the region immediately north of our own, this outbreak was of a shorter duration and probably caused little mortality since stands remaining from the previous infestation were young and less vulnerable at that time. However, Blais [24], although providing no quantitative evidence, states that this outbreak may well have been quite severe within the study region. According to Morin [11], the growth decrease was not as marked compared with the other two outbreaks that occurred in the same century.…”

“…A second budworm outbreak was reported around 1939-1949 [22,24,25], with the minimum growth of the host species occurring in 1949. The exact importance of this outbreak for the region is more difficult to define.…”

“…Peak of mortality in mixedwood stands was observed around 1977 [24] with minimum growth of host species occurring in 1978. According to Jardon et al [23], the outbreak was already well developed in this region around 1971.…”

Impacts of insect outbreaks on tree mortality, productivity, and stand development

“…In New Brunswick, indications are that the 1870s spruce budworm outbreak was severe and stand-replacing (Etheridge et al 2005). Blais (1981) suggested that every second budworm outbreak caused higher mortality, because a standreplacing outbreak results in a less vulnerable younger stand for the succeeding outbreak. Long-term data on forest composition changes tend to be limited and localised.…”