Forest overstory composition and seedling height influence defoliation of understory regeneration by spruce budworm

Higher hardwood content in stands results in lower defoliation of overstory balsam fir (Abies balsamea (L.) Mill.) caused by spruce budworm (Choristoneura fumiferana Clem.). To determine whether higher overstory hardwood content also reduced defoliation and mortality of balsam fir regeneration of varying height classes during a spruce budworm outbreak, we sampled 36 plots representing three classes of hardwood content (0–25%, 40–65%, and 75–95%) across a gradient of fir-hardwood stands. Twenty-seven plots were sampled in nine stands near Amqui, Quebec in an early stage spruce budworm outbreak (3 years of defoliation), and nine plots were sampled in three stands in the North Shore of Quebec in a later stage budworm outbreak (7 years of defoliation). Linear mixed-effects models with repeated measures (years) were used to analyze differences in defoliation of fir regeneration as a function of hardwood content, six height classes, and three years (2013, 2014, 2015). In the Amqui plots, defoliation of fir regeneration was significantly related to all factors and interaction terms except for hardwood content, while in the North Shore plots, defoliation was significantly related to all factors and the hardwood content x height class and hardwood content x year interaction terms. Defoliation of balsam fir regeneration was 85% higher in softwood than in hardwood stands in 2013 and 2014 in the North Shore plots, when the budworm outbreak was severe. Defoliation was at least 10% higher on regeneration taller than 30 cm than on smaller regeneration in the Amqui plots in 2015 and over 15% higher in the North Shore plots. In general, balsam fir regeneration in softwood stands had higher levels of defoliation than in hardwood stands when defoliation was severe, and regeneration taller than 30 cm had higher defoliation than smaller regeneration.