Abstract
Defoliation by spruce budworm (Choristoneura fumiferana Clem.) on balsam fir (Abies balsamea (L.) Mill.) is more severe in fir than in mixed fir-hardwood stands. Previous studies assumed that defoliation in fir-hardwood stands was reduced in proportion to percent hardwood regardless of outbreak severity. We tested the influence of stand composition on defoliation during the first 5 years of a spruce budworm outbreak near Amqui, Quebec, by sampling 27 fir-hardwood plots selected to represent three percent hardwood basal area classes (0%–25%, 40%–65%, and 75%–95%). Balsam fir defoliation was significantly lower (p < 0.001) as hardwood content increased, but the relationship varied with overall defoliation severity each year. Annual plot defoliation in fir-hardwood plots, estimated using: (1) defoliation in pure fir plots and the assumption that defoliation in fir-hardwood plots was reduced in proportion to percent hardwood; (2) a generalized linear mixed-effects model with defoliation in pure fir plots, percent hardwood, and interaction as fixed-effects; and (3) Random Forests prediction incorporating 11 predictor variables, resulted in r = 0.77, 0.87, and 0.92 versus measured defoliation, respectively. Average defoliation severity in softwood plots and percent hardwood content were the most important variables in Random Forests analysis. Data on average defoliation level in softwood stands, as an indicator of overall outbreak severity, improves prediction of balsam fir defoliation in mixed stands.
Highlights
Effective forest pest management in heterogeneous landscapes and in mixed-species forest stands requires knowledge about how tree diversity affects insect herbivory
We evaluated two predictions: (1) fir-hardwood stands with higher hardwood content will have less severe annual defoliation of balsam fir and (2) the simplified linear model can be used to estimate budworm defoliation in fir-hardwood mixed stands if other predictor variables are not available, but incorporating more variables will improve the accuracy of predictions
Examining the fitted relationships from GLMM, balsam fir defoliation was negatively related to percent hardwood content each year from 2012 to 2016 (Figure 2)
Summary
Effective forest pest management in heterogeneous landscapes and in mixed-species forest stands requires knowledge about how tree diversity affects insect herbivory. A meta-analysis of a worldwide data set of 119 studies by Jactel and Brockerhoff [1] showed a significant reduction in herbivory with increasing forest diversity for oligophagous insects (i.e., species that exploit one or a few closely related genera of hosts). This seems to be the case for spruce budworm Outbreaks usually last 5–15 years and severe defoliation causes growth loss and tree mortality over large areas [7,8], peaking at over 52 million ha of defoliation of forests in eastern Canada [9]. Spruce budworm defoliation can be assessed by conducing aerial survey, ground assessment with binoculars, and branch sampling with pole pruners, of which branch sampling with pole pruners and rating defoliation on individual shoots is considered to be the most accurate technique [10,11,12]
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