Dynamics of balsam fir forests in relation to spruce budworm outbreaks in the Boreal Zone of Quebec

Age structures and growth curves were used to determine the origin and to follow the development of 17 balsam fir (Abiesbalsamea (L.) Mill.) stands in the middle of the Boreal Zone in the north of Lake Saint-Jean, Quebec. Every growth curve from the studied sites presented a more or less long period of suppression in the early growth of the balsam fir, followed by a marked release synchronous between the trees of a given site. Generally, a balsam fir cohort established just before the growth release. Characteristic growth releases generally occurred after the formation of exceptionally narrow rings, which took place around 1952, 1914, and between 1860 and 1890 in the sampled trees. Unimodal even-aged stands regenerated from balsam fir advanced growth following spruce budworm (Choristoneurafumiferana (Clem.)) outbreaks that rapidly defoliated the trees and caused openings, sometimes associated with blowdown; these outbreaks occurred between 1944 and 1953 (one stand), between 1909 and 1923 (six stands), and possibly between ca. 1860 and 1900 (seven stands). Bimodal and uneven-aged stands were associated with two spruce budworm outbreaks. In one stand, some of the trees regenerated from the spruce budworm outbreak that occurred between 1909 and 1923, but the 1944–1953 outbreak was intense enough to cause important defoliation and mortality, which released a second cohort. Finally, two stands regenerated from the possible spruce budworm outbreak that occurred around 1860–1900 and from the 1909–1923 outbreak. The presence of a fir cohort and associated growth release between 1860 and 1890 strongly suggests that an outbreak occurred at that time in the studied region. Every sampled stand from the Boreal Zone north of Lake Saint-Jean regenerated from a balsam fir seedling bank following a spruce budworm outbreak that affected the mature balsam fir canopy. These results are in good agreement with the hypothesis that spruce budworm outbreaks and balsam fir forests form an interdependent self-regulating system. Our study shows the drastic effect of spruce budworm outbreaks in the boreal forest, where black spruce (Piceamariana (Mill.) B.S.P.) dominates