Growth and mortality of balsam fir- and spruce-tolerant hardwood stands as influenced by stand characteristics and spruce budworm defoliation

Balsam fir (Abies balsamea (L.) Mill.)-tolerant hardwood and spruce (Picea sp.)-tolerant hardwood stands were compared along a gradient of hardwood composition (12–82%). Using permanent sample plot data, influences of stand hardwood content, past spruce budworm (Choristoneura fumiferana Clem.) defoliation, insecticide spraying, and stand and site characteristics on survivor growth and mortality of the fir–spruce (spruce budworm host species) component of these stands were determined. Balsam fir basal area, spruce basal area, and percent hardwood were significantly related to fir–spruce survivor growth. Balsam fir mixedwood plots had four to five times higher fir–spruce cumulative mortality (3.7–13.3 m2 ha−1) than spruce mixedwood plots (1.0–2.9 m2 ha−1) from 1990 to 2004, and sustained higher wind-related mortality after the budworm outbreak ended in the late 1980s to early 1990s. A mortality regression tree model indicated that initial balsam fir basal area, % tolerant hardwood basal area, maximum cumulative defoliation from 1983 to 1987, and spruce budworm outbreak zone (north, central, south New Brunswick) all significantly influenced fir–spruce mortality. Results demonstrated differences in stand dynamics between these stand types, with lower spruce budworm vulnerability and higher longevity and more stable structure of spruce mixedwood than of fir mixedwood.

► We evaluated dynamics of balsam fir versus spruce mixed with 12–82% hardwood.
► Fir–spruce growth was related to fir and spruce basal area and % hardwoods.
► Stand dynamics differed between fir-hardwood and spruce-hardwood stand types.
► Cumulative mortality was 4–5 times higher in fir- than in spruce-hardwoods.
► Mortality was related to fir basal area, % hardwood, insect defoliation and region.