Determining the mechanism of 'protective effects' of hardwoods on spruce budworm defoliation of balsam fir

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Balsam fir-spruce susceptibility and vulnerability to spruce budworm defoliation are reduced by the presence of hardwoods. Fir stands sustain less budworm defoliation when >30-60% hardwood composition is exceeded. The greatest effect of higher hardwood content is reduction of fir mortality, thought to result from higher parasitism rates of budworm or greater losses of dispersing first- and second-instar spruce budworm larvae in stands with greater hardwood content. Hardwood composition effects on defoliation also influence fir-spruce tree growth, as lower mean radial growth reductions have been observed with higher hardwoods. The evidence for a 'protective effect' of hardwood content reducing defoliation, growth reduction, and mortality of host species during spruce budworm outbreaks is strong. But several questions remain: 1) Is the mechanism due to higher parasitism rates of budworm or greater losses of dispersing first- and second-instar spruce budworm larvae in stands with greater hardwood content, or both? 2) Does the 'protective' effect between differ among hardwood species, in particular between shade-tolerant (sugar maple, yellow birch) versus shade-intolerant species (white birch, trembling aspen, red maple, etc.)? 3) How does the 'protective effect' of hardwoods vary over the course of a budworm outbreak, and how should it be incorporated into stand growth model and decision support system projections of stand and forest development? The timing is right to address these questions because spruce budworm populations are on the rise in eastern Canada. Area of defoliation in Quebec has generally doubled in each of the last 4 years, to over 2.4 million ha in 2012 and pheromone traps have shown increases in spruce budworm populations in northern New Brunswick. I propose to determine the mechanism of 'protective effects' of hardwoods on budworm defoliation of fir-spruce and to address the above three questions, using a mix of budworm population and tree measurements in permanent plots in northern New Brunswick or Quebec during the expected upcoming outbreak.