Limited Differences in Insect Herbivory on Young White Spruce Growing in Small Open Plantations and under Natural Canopies in Boreal Mixed Forests

Simple Summary We conducted this study in the boreal forest of Quebec, Canada, to compare insect damage on white spruce (Picea glauca) trees within open-canopy, multispecies plantations, and trees naturally regenerated under mature mixed-wood canopies. Our plantation sites were established post-clearcutting and compared with naturally regenerated post-fire sites dominated by trembling aspen. Over two years, observations were made on young trees from ten sites in each environment, focusing on overall rates of herbivory, galls, spruce budworm damage, and sawfly defoliation. Despite a minor increase in spruce budworm damage in under-canopy forests from 2020 to 2021, overall, damage levels remained low, suggesting minimal impact on tree growth or mortality at this early outbreak stage. Contrary to initial assumptions, we found that insect damage was comparably low in both plantation and naturally regenerated environments, casting doubt on the idea that the enhanced sunlight and accelerated growth observed in plantations necessarily increase their susceptibility to pest attacks. In the context of global forest restoration efforts and Canada's pledge to plant 2 billion trees by 2050, the resilience of white spruce plantations to insect damage is of paramount importance. Amidst concerns over increasing pest damage due to climate change, the observed resilience of multispecies plantations underpins the effectiveness of ecosystem-based management in maintaining low insect damage levels without compromising growth. This balance highlights the potential of mixed-species plantations to mimic natural forest conditions, contributing to biodiversity conservation and sustainable forestry practices. This research illuminates the intricate dynamics between tree growth, environmental conditions, and pest vulnerability, offering valuable insights for future forest management and conservation strategies.Abstract In managed boreal forests, both plantations and natural regeneration are used to re-establish a cohort of conifer trees following harvest or disturbance. Young trees in open plantations generally grow more rapidly than under forest canopies, but more rapid growth could be compromised by greater insect damage. We compared insect damage on white spruce (Picea glauca (Moench) Voss, Pinaceae) growing in plantations with naturally regenerated trees under mature forest canopies in boreal forests (Quebec, Canada). We selected ten sites in the naturally regenerated forest and in small, multispecies plantations and sampled ten young trees of 2.5-3 m (per site) in late summer 2020 and again in early and late summer 2021. We compared overall rates of herbivory, galls (adelgids), damage by the spruce budworm (Choristoneura fumiferana, Clemens), and defoliation from sawflies. Overall, insect herbivory damage remained at similarly low levels in both habitats; an average of 9.3% of expanding shoots were damaged on forest trees and 7.7% in plantation trees. Spruce budworm damage increased from 2020 to 2021 and remained higher in under-canopy trees, but damage rates were negligible at this early stage of the outbreak (1.5% in forest vs. 0.78% of buds damaged on plantation trees). While damage due to galls was higher in plantations, the overall low level of damage likely does not pose a significant impact on the growth or mortality of young trees.