Effects of stand and site characteristics on susceptibility and vulnerability of balsam fir and spruce to spruce budworm in New Brunswick

The relationships of spruce budworm (Choristoneura fumiferana (Clem.)) defoliation, and host tree growth and mortality, to soil drainage, tree species, budworm outbreak status (location), and productivity class were examined in 356 spruce (Picea sp.)-balsam fir (Abies balsamea (L.) Mill.) permanent sample plots in New Brunswick, Canada. Tree species, budworm outbreak status, and drainage class were significantly related to susceptibility to spruce budworm, which was quantified using defoliation levels from 1985 to 1988. On average, balsam fir sustained 6-9% more current and calculated cumulative defoliation than white spruce (Picea glauca (Moench) Voss) and 15-19% more than red (Picea rubens Sarg.) or black spruce (Picea mariana (Mill.) BSP). Moderate-to well-drained sites sustained 7-17% more current and 5-7% more cumulative defoliation than imperfectly or poorly drained sites. When pooled for species, rich sites sustained significantly higher cumulative defoliation than poor or medium sites. Soil drainage showed significant main and defoliation x drainage interaction effects on both mortality and net basal area change. However, within stand types, drainage effects were significant only for white spruce under light defoliation and spruce-fir under moderate defoliation, with no significant differences occurring for balsam fir, red spruce, black spruce, or fn-spruce stand types. Therefore, we support the recommendation of Y. Bergeron, A. Leduc, H. Morin, and C. Joyal (1995. Balsam fir mortality following the last spruce budworm outbreak in northwestern Quebec. Can. J. For. Res. 25: 1375-1384) that the emphasis put on soil drainage class and surficial deposit as predictors of stand vulnerability to budworm should be reevaluated.