Spruce budworm (Choristoneura fumiferana) severely defoliates balsam fir (Abies balsamea) and spruce (Picea spp.) in large periodic outbreaks that cover millions of hectares. Epidemics typically cause high levels of defoliation for 5-15 years and result in extensive growth reduction and mortality of host species. In this paper, I will review factors that cause variability in susceptibility and vulnerability to spruce budworm damage, and present results of three recent studies in New Brunswick that: 1) evaluated differential defoliation levels among host species (balsam fir, white, red, and black spruce (P. glauca, P. rubens, and P. mariana)); 2) determined post-outbreak, legacy effects of budworm on stand development; and 3) evaluated how radial growth reductions varied with hardwood content of surrounding forest landscapes, insecticide spray history, and elevation.

Results/Conclusions

An extensive permanent sample plot (PSP) dataset, with over 27,000 tree and 1,117 stand measurements from 1984-1992, revealed a clear and consistent hierarchy of host species defoliation susceptibility. Regardless of defoliation severity and other stand variables tested, white, red, and black spruce had approximately 72%, 41%, and 28% as much defoliation as balsam fir, respectively. Balsam fir defoliation was a highly significant predictor of spruce defoliation. 34% of variation in defoliation of red-black spruce hybrids was explained by fir defoliation and an interaction with spruce hybrid index.

The trend and rate of volume development in PSPs was related to past budworm outbreak severity. PSPs with decreasing volume from 1985-2005, well after the outbreak ended, had sustained 1.5 years and 104% more moderate-severe defoliation than PSPs with increasing volume. Decreasing PSPs that experienced 5-8 years of defoliation had depressed periodic annual increment, which did not start to recover until 15-20 years later, as a result of increased post-outbreak wind-related mortality. Budworm-caused mortality of canopy trees increased vulnerability of more open post-outbreak stands to wind disturbance events.

Stepwise regression was used to build models describing relationships between growth reduction and explanatory variables. Hardwood content of surrounding forest landscapes entered into all significant regression models (P < 0.05), explaining 12-41% of the variance. Differences in growth reductions were greatest during the initial years of the outbreak, and relationships between growth reduction and elevation or insecticide spray history were weak and less consistent than with landscape hardwood content.

Variability in system response to budworm is largely determined by host species, stand composition, and within-stand contagious mortality patterns.