• Crook, Gregory W.
  Université Laval, Québec
  
• Vézina, Paul E.
  Université Laval, Québec
  
• Hardy, Yvan
  Université Laval, Québec
  

Spruce budworm, Choristoneura fumiferana (Clemens), defoliation of balsam fir, Abies balsamea (L.) Mill., was studied in thinned stands of the Lower St. Lawrence region of Quebec. 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 susceptibility of balsam fir.

Three main forest types were studied: (a) coniferous, (b) mixed, and (c) hardwood with a fir understory. The coniferous forest type was subdivided into three types: (1) pure balsam fir; (2) balsam fir - white spruce, Picea galuca (Moench) Voss; (3) balsam fir - hardwoods. Defoliation levels were estimated using two current techniques: the Dorais-Hardy (1976) and the Fettes (1950) methods.

Susceptibility of balsam fir to spruce budworm defoliation was not affected after thinning in coniferous and hardwood forest types; however, susceptibility was increased in mixed stands when part of the hardwood cover was removed. In any situation, defoliation of fir was found to be 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; likewise, a higher number of stems per hectare brought higher defoliation levels of fir. Defoliation was also found to  vary with stand composition; susceptibility of fir in the three main cover types decreased in the following order: (a) coniferous, (b) mixed, (c) hardwood. Stand age was not a factor influencing the susceptibility of balsam fir for the two age classes studied (30 and 50 years).