Développement de sapinières éclaircies exposées à une épidémie de tordeuse des bourgeons de l'épinette

Abstract

In 1968, three intensities of low thinning were applied in 33-year-old balsam fir (Abies balsamea [L.] Mill.) stands of eastern Québec in order to remove 0, 20 and 30% of the total basal area. Sample plots were located in two distinct areas, one of which was annually sprayed with insecticides between 1978 and 1982 while the other was never protected against a spruce budworm (Choristoneura fumiferana [Clem.]) outbreak that occurred between 1976 and 1983. A total of 32 0.04-ha sample plots were periodically measured between 1968 and 1994. Low thinnings mostly removed non merchantable trees (DBH < 9.1 cm) and left 5 000 to 7 000 trees/ha while stand density of control plots reached 10 000 stems/ha. From 1968 to 1976, net annual increment in merchantable volume was around 8 m3/ha but no differences were detected between thinning levels. During the period of severe defoliation caused by spruce budworm (1976-1985), net annual increment in merchantable volume of sample plots sprayed with insecticides decreased by 50% but get back to its initial level after 1985. Net annual increment of sample plots that were never protected against spruce budworm was strongly negative during the outbreak period until the extinction of all trees was completed. While thinning level only slightly affected merchantable yield of sample plots during the 26-year period, mean annual increment in DBH was inversely proportional to stand density. This advantage of thinned plots could have been greater if stand density had been reduced further at the beginning of the study. Indeed, the analysis of the evolution of stand density as a function of mean volume of trees shows that mortality rate of all sample plots was similar to that given by the self-thinning rule. This suggests that even trees of most heavily thinned plots suffered from strong intra-specific competition. Key words: balsam fir, low thinning, spruce budworm, growth and yield, diameter increment, self-thinning rule