Patterns of balsam fir foliar production and growth in relation to defoliation by spruce budworm

Changes in foliar production caused by spruce budworm (Choristoneura fumiferana (Clem.)) defoliation and patterns of volume increment of surviving trees during and after the outbreak were determined in 20 mature balsam fir (Abies balsamea (L.) Mill.) stands on Cape Breton Island, Nova Scotia. Following the cessation of defoliation, the number of shoots increased 4-fold and mean shoot length doubled, resulting in a 12-fold increase in needle biomass. Average specific volume increment declined from 0.17-0.25 to 0.02-0.04 cm3.cm-2.year-1 after 4 years of severe defoliation; maximum increment reduction was 74-92%. Periodic radial increment and volume increment reduction (percent of mean increment in the previous decade) were both significantly related to cumulative (summed current annual) defoliation; a logistic regression equation explained 77% of the variation in volume loss. Differences in growth recovery among trees were determined by the temporal patterns of defoliation, with a 1-year lag before the beginning of volume increment recovery. Following 5 to 8 years of recovery, surviving trees had regained 48-82% of their predefoliation increment; however, absolute volume losses during the outbreak averaged 12-33 dm3/tree, or 32-48%.