Young plantation trees are often highly vulnerable to insect herbivory in ways that are difficult to predict as underlying mechanisms linked to plant traits and natural enemy pressure interact in context-dependent ways. We compared bottom-up and top-down forces acting on spruce budworm (Choristoneura fumiferana) on young white spruce (Picea glauca) trees in plantations vs in natural regeneration under hardwood canopy. The spruce budworm is the most important outbreaking conifer defoliator in Eastern Canada, and we aim to better understand budworm herbivory on young trees, given its importance for post-outbreak forest succession. We conducted a 4-year field survey in Northwestern Québec, Canada, to compare plant phenology, budworm density, defoliation rates, predator populations, and parasitism between the two habitats. We also designed manipulative experiments with sentinel larvae to assess bottom-up and top-down forces in these habitats. The field survey showed earlier budburst phenology in plantation trees, affecting synchronization with budworm emergence from diapause. Field survey results also included slightly higher budworm density and lower larval parasitism in plantations, but no significant difference in current-year growth defoliation. The bottom-up experiment showed slightly better budworm biological performance, indicated by higher pupal mass, in plantations. The top-down experiment suggested slightly higher predation and parasitism in the natural forests. Together, our results show how mechanisms controlling insect defoliator populations are context-dependent. In plantations both bottom-up and top-down pressures on the spruce budworm appear slightly eased, leading to marginally better biological performance and higher population density of this forest pest. However, differences are so minor that no impact on defoliation is observed.

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 (Québec, 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.

Few studies focused on non-outbreaking herbivorous insects to understand the patterns of damage they inflict on plants. We compared damage by herbivorous insects on young white spruce (Picea glauca) between natural regrowth in the understory of mixed wood forest and small extensively-managed plantations. We observed damage to foliage to quantify damage by different groups of herbivores, including leaf chewers, miners and sap-sucking species. Our hypothesis stated that trees in forest understory environments would have higher diversity of damages caused by insects but that plantation trees would have more damaged tree shoots. Our two sampling methods were branch collection, in which we collected a forty-centimeter branch and recorded foliar damage, and field surveys, where one researcher recorded foliar damage on the saplings for three-minute intervals. We also measured tree growth, canopy openness, soil temperature and humidity. We used these environmental variables in general linear models to test their effects on herbivore damage in the two habitats. The results showed that plantation and understory trees did not differ significantly in the overall amount of insect damage. There was no correlation found with any environmental factor. This pattern indicated that the plantation we sampled maintained insect biodiversity similar to that in mixed wood forests. Thus, small, extensively managed multispecies plantations can be less at risk of insect outbreaks.