Determining the mechanism of impact of hardwood content on spruce budworm defoliation of balsam fir
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Date
2020
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University of New Brunswick
Abstract
I investigated effects of hardwood content on eastern spruce budworm (Choristoneura fumiferana Clem.) defoliation in mixedwood forests during the first 5 years of an outbreak. I sampled 27 balsam fir (Abies balsamea (L.) Mill.) - hardwood plots representing three percent hardwood basal area classes (0%‒25% (termed softwood), 40%‒65% (mixedwood), and 75%‒95% (hardwood)) near Amqui, Québec, Canada. Balsam fir defoliation was significantly lower (p < 0.001) as hardwood content increased, but the relationship varied with overall defoliation severity each year. We formulated three predictive models to estimate defoliation in fir-hardwood plots, among which a Random Forests procedure incorporating 11 predictor variables generated the best prediction (r = 0.92). Accurate estimation of defoliation in fir-hardwood stands requires data about average defoliation level in that year in addition to percent hardwood content. The observed reduction of defoliation associated with hardwood content could be explained by two hypotheses: habitat fragmentation and/or natural enemy hypotheses. The first one posits that higher early-instar larval dispersal loss occurs in fir-hardwood stands with higher hardwood content while the second suggests that higher parasitism occurs in mixed stands. First- and second-instar larval dispersal loss, stage-specific budworm density, and parasitism rates were assessed in the three stand types through field sampling. Second-instar dispersal losses were significantly influenced by stand type; higher dispersal losses occurred in plots with higher hardwood content. These results suggested that the habitat fragmentation hypothesis is a plausible explanation for the lower spruce budworm density and balsam fir defoliation associated with increasing hardwood content. There was no significant relationship between parasitism and stand type, not supporting the natural enemy hypothesis. Diversity and structure of the primary parasitoid assemblage of spruce budworm was assessed in the three stand types. Species diversity (α-diversity) did not show significant differences between the three stand types. The community structures of parasitoid assemblages differed significantly among the three stand types (β-diversity). Elevation (p = 0.007), population density of spruce budworm (p = 0.065), and percent hardwood content (p = 0.132) were the top three variables correlated with the parasitoid community structure among seven selected explanatory variables.