• Su, Qiong
  University of New Brunswick, Faculty of Forestry and Environmental Management
  
• MacLean, David A.
  Canadian Forest Service-Atlantic
  
• Needham, Ted D.
  University of New Brunswick, Faculty of Forestry and Environmental Management
  

Changing stand composition by increasing hardwood content has been suggested as a long-term method for reducing susceptibility and vulnerability of balsam fir (Abies balsamea (L.) Mill.) to spruce budworm (Choristoneura fumiferana (Clem.)). Twenty-five mixed balsam fir - hardwood stands were selected in northern New Brunswick, with five stands in each 20% hardwood class (0-20, 21-40%, etc.). Defoliation each year from 1989 to 1993 was significantly (p < 0.0001) related to hardwood content, with r² ranging from 0.57 to 0.81. As hardwood content increased, defoliation of balsam fir decreased. From 1989 to 1992, the years of moderate to severe defoliation, balsam fir stands with <40% hardwoods sustained 58-71% defoliation, on average, versus 12-15% defoliation in stands with >80% hardwood. A generalized model combining hardwood content and the estimated defoliation in pure softwood stands in a given year explained 77% of the variation in defoliation over stands and years. This study indicated that mixed balsam fir - hardwood stand management, with hardwood content >40%, could substantially reduce losses during spruce budworm outbreaks. Further research is warranted to elucidate the mechanism involved, but our working hypothesis is that greater hardwood content increased the diversity or populations of natural enemies such as birds and parasitoids.