SUSCEPTIBILITY OF BALSAM FIR TO SPRUCE BUDWORM DEFOLIATION AS AFFECTED BY THINNING

Spruce budworm, C. fumiferana (Clemens) defoliation of balsam fir, A. balsamea (L.) Mill, was studied in thinned stands of the Lower St. Lawrence region of Quebec [Canada]. The object of the study was to quantify defoliation levels in treated and control plots in order to determine what effect, if any, thinning had on the suceptibility of balsam fir. Three main forest types were studied: coniferous, mixed and hardwood with a fir understory. The coniferous forest type was subdivided into 3 types: pure balsam fir; balsam fir-white spruce, Picea glauca (Moench) Voss: (3) balsam fir-hardwoods. Defoliation levels were estimated using 2 techniques, the Dorais-Hardy (1976) and Fettes (1950) methods. Susceptibility of balsam fir to spruce budworm defoliation was not affected after thinning in coniferous and hardwood forest types; susceptibility was increased in mixed stands when part of the hardwood cover was removed. In any situation, defoliation of fir was more intense with an increase of the basal area of the coniferous species (balsam fir, red spruce, Picea rubens Sarg. and white spruce) while an increase of the basal area of hardwoods resulted in lower defoliation levels; a higher number of stems/ha brought higher defoliation levels of fir. Defoliation varies with stand composition; susceptibility of fir in the 3 main cover types decreased in the following order: coniferous, mixed, hardwood. Stand age was not a factor influencing the susceptibility of balsam fir for the 2 age classes studied (30 and 50 yr).