Natural disturbances are fundamental to forest ecosystem dynamics and have been used for two decades to improve forest management, notably in the boreal forest. Initially based on fire regimes, there is now a need to extend the concept to include other types of disturbances as they can greatly contribute to forest dynamics in some regions of the boreal zone. Here we review the main descriptors—that is, the severity, specificity, spatial and temporal descriptors and legacies, of windthrow and spruce bud worm outbreak disturbance regimes in boreal forests—in order to facilitate incorporating them into a natural disturbance-based forest management framework. We also describe the biological legacies that are generated by these disturbances. Temporal and spatial descriptors characterising both disturbance types are generally variable in time and space. This makes them difficult to reproduce in an ecosystem management framework. However, severity and specificity descriptors may provide a template upon which policies for maintaining post harvesting and salvage logging biological legacies can be based. In a context in which management mainly targets mature and old-growth stages, integrating insect and wind disturbances in a management framework is an important goal, as these disturbances contribute to creating heterogeneity in mature and old-growth forest characteristics.

Boreal mixedwood understory communities are highly dynamic because they are simultaneously influenced by canopy succession, small-scale environmental variation and disturbances. In such a system, fire initiates succession but canopy gaps, resulting from spruce budworm outbreaks or other disturbances, also affect forest dynamics during succession. In this study, we assessed the short-term response of understory communities along a canopy gap severity gradient in mature and old-growth forest stands. The objective was to determine if resource heterogeneity is driving understory community composition and diversity. We created artificial gaps of varying severities in stands of two late successional forest stages. Four treatments, i.e., total cut, conifer cut, girdling of conifers and a control treatment, were each applied over an area of 100 m2 and replicated three times in both types of forest stands. The cover of each understory species was measured before treatment application and 1, 2 and 11 years following it, in seven 1 m2 plots per replicate. Percent transmission of photosynthetic photon flux density and soil temperature were also measured on different occasions after the beginning of the experiment. Species richness, alpha diversity and total cover all increased following gap creation, and this increase was sustained up to 11 years. Increases were more pronounced in total cut than in other treatments. The increase in species richness and diversity was also more pronounced in old-growth stands, while total cover increase was greater in mature stands. Gap creation resulted in significant changes in abundance patterns that were associated with the resident species assemblages. The magnitude of changes in community composition did not significantly differ between mature and old-growth forests and differences in species composition among stands were maintained following gap creation. This study supports the evidence that resource heterogeneity resulting from small-scale disturbances will contribute to maintaining diversity among understory communities by allowing species more typical of early-successional communities to coexist with species from late-successional ones. Such a process may be essential for maintaining the species pool likely to re-invade the stands following major disturbances

Regional analyses assessing the vulnerabilities of forest ecosystems and the forest sector to climate change are key to considering the heterogeneity of climate change impacts as well as the fact that risks, opportunities, and adaptation capacities might differ regionally. Here we provide the Regional Integrated Assessment of climate change on Quebec's forests, a work that involved several research teams and focused on climate change impacts on Quebec's commercial forests and on potential adaptation solutions. Our work showed that climate change will alter several ecological processes within Quebec's forests. These changes will result in important modifications in forest landscapes. Harvest will cumulate with climate change effects to further alter future forest landscapes, which will also have consequences on wildlife habitats (including woodland caribou habitat), avian biodiversity, carbon budget, and a variety of forest landscape values for Indigenous peoples. The adaptation of the forest sector will be crucial to mitigate the impacts of climate change on forest ecosystem goods and services and improve their resilience. Moving forward, a broad range of adaptation measures, notably through reducing harvest levels, should be explored to help strike a balance among social, ecological, and economic values. We conclude that without climate adaptation, strong negative economic and ecological impacts will likely affect Quebec's forests.

Top-down effects, like predation, are drivers of insect outbreaks, but bottom-up effects, like host nutritional quality, also influence outbreaks and could in turn be altered by insect-caused defoliation. We evaluated the prediction that herbivory leads to a positive feedback on outbreak severity as nutrient concentration in plant tissues increases through improved soil nutrient availability from frass and litter deposition. Over seven years of a spruce budworm outbreak, we quantified litter nutrient fluxes, soil nitrogen availability, and host tree foliar nutrient status along a forest susceptibility gradient. As the outbreak progressed, both soil nutrient fluxes and availability increased which, in turn, improved foliage quality in surviving host trees. This is consistent with boosted insect fitness and increased population density and defoliation as outbreaks grow. Our results suggest that a positive bottom-up feedback to forest ecosystems from defoliation may result in conditions favorable to self-amplifying population dynamics in insect herbivores that can contribute to driving broad-scale outbreaks.