Defoliation level interacts with tree species and soil richness to determine volume increment reduction and recovery from simulated spruce budworm attack

Defoliation and tree species determine growth during spruce budworm outbreaks, with site showing inconsistent effects. We determined effects of artificial defoliation treatments (0%, 50%, 100%, 100%+bud) for 3 years on volume increment of 240 balsam fir, black spruce, and white spruce trees, initially 7–10 years old and 2.3–4.1 m tall, on four soil drainage/richness classes. Current annual increment was significantly affected by species × site and site × defoliation interactions, with DBH as covariate. Specific volume increment (SVI) was significantly affected by a species × site × defoliation treatment interaction. Combining species and sites, volume reductions after 3 years were 21%–28%, 43%–51%, and 55%–66% for 50% defoliation, 100% defoliation, and 100%+bud. SVI was negatively, linearly related to cumulative defoliation. Marginal r 2 and conditional r 2 showed that 64% of variance of SVI was explained by site, species, cumulative defoliation, and DBH, but 90% was explained by fixed and random variables. After 3 years of defoliation, SVI of white spruce was 21%–23% greater than balsam fir and black spruce, and SVI on rich was 55%–79% greater than on poor sites. Cumulative defoliation was a good predictor of growth, and soil richness had a stronger effect on growth after defoliation than in previous studies.