RELATIONSHIPS BETWEEN GROWTH-RATE INDICAT ORS OF THE BALSAM FIR DURING AN INITIAL OUTBREAK AND ITS VULNERABILITY TO THE SPRUCE BUDWORM

Integrated pest management strategies for the spruce budworm (Choristo neura fumiferana (Clem.)) give high priority to preventive silvicultur al treatments that increase the vigour of balsam fir stands to decreas e their vulnerability. The aim of this study was to determine to which extent the survival probability of individual balsam firs (Abies bals amea L.), during the last budworm outbreak (1974-1986), was related to some growth rate indicators evaluated at the beginning of the outbrea k. The study was carried out in two permanent study sites, established initially to follow the impact of precommercial thinnings on growth a nd yield in young boreal fir stands in eastern Quebec: the area of Lak e Matapedia, thinned in 1968 and protected by chemical insecticides, a nd the area of Lake Huit-Milles, thinned in 1960 and not protected. Di scriminant logistic models, based on tree diameters in 1976 and diamet er growth rates between 1968 and 1976, were used to determine if it wa s possible to discriminate the state of the trees (living or dead) at the end of the outbreak. Using Dagnelie's transformation of the logist ic model, we produced linear probability models that were called survi val thresholds. In the protected study area (Lake Matapedia), the mode ls correctly predicted the state of 81% of the individual trees in 198 9 for the validation sample. In the unprotected stands (Lake Huit-Mill es), we observed a higher position of the survival thresholds followin g higher defoliation levels. The relationship between tree growth indi cators and tree vulnerability is stronger when defoliation levels are low to moderate, but is clearly weaker when defoliation is severe. At high levels of defoliation and without insecticide spraying programs, even vigourous trees become vulnerable.