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.

Regional analyses assessing the vulnerabilities of forest ecosystems and the forest sector to climate change are key to consider the heterogeneity of climate change impacts but also 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 that 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 habitat (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 economical 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.

 

Balsam fir (Abies balsamea) is the most vulnerable species to the spruce budworm (Choristoneura fumiferana), one of the most devastating defoliators in the world. For decades, pest managers have advocated for reducing its abundance in the landscape to minimize losses to the spruce budworm (SBW). Although reduction of fir occurred during the endemic phase of the SBW cycle, there is little information about the extent to which this general principle (reduction of fir) was applied during an outbreak and whether it occurs at both stand and landscape levels. The objective of this paper is to compare the effect of insect and harvest disturbances on forest structure during the 1970–80s outbreak in Québec. We evaluate whether, (i) forest management activities targeted fir forests and whether patch size of host species influences management or SBW disturbance, (ii) SBW outbreaks and logging have similar or divergent effects on forest composition. Although data are from an earlier outbreak, they are at a scale rarely studied and will be useful in guiding decisions made at larger scales in the current and future outbreaks. Our results show that spruce was targeted preferentially by harvesting (up to 69% of plots) during the outbreak period, while it represented less than one third of plots defoliated by the SBW. On the other hand, fir stands represented up to 75% of plots that were defoliated by the SBW but less than 35% of plots that underwent harvesting. Harvesting targeted large blocks of spruce forest more than large blocks of fir-dominated forest while the opposite was observed for the SBW. In terms of regeneration, SBW tends to reduce fir and favor spruce recruitment, along with non-host species, whereas the opposite tendency was observed following harvesting. In terms of spatial organization of stands, our results support the suggestion that small stands of fir and large stands of spruce undergo the least SBW damage. Thus, in order to attenuate SBW impacts in the future, efforts should be made to ensure that spruce recruitment is favored and that its abundance increases at both the stand and landscape scale.

1. As major alterations are occurring in climate and pest ranges, it is imperative to evaluate their combined contribution to tree mortality in order to propose mitigation measures and limit losses in forest productivity. The objective of this study was to explore the association between declines in tree growth resulting from climatic and biotic (spruce budworm) disturbances, and their interactions on tree mortality of two dominant tree species, Abies balsamea and Picea mariana, of the eastern North?American boreal forest.
2. We disentangle the influences of abiotic and biotic components on growth through a combination of model?data comparison techniques. First, we characterized the variability in tree growth and mortality in the study area using a network of tree?ring width measurements collected from living and dead trees. Subsequently, a bioclimatic simulation model was used to estimate the past annual, nonlinear, responses of stand?level net primary production (NPP) to climate variability (period 1902–2012). From these two data sources, we defined the biotic stress events as the variance in the tree?ring data unexplained by the bioclimatic forest growth simulation.
3. Throughout the 20th century, two periods of adverse climatic conditions preceded spruce budworm outbreaks episodes and induced tree mortality. Climatic stress events were associated with cold springs, warmer than average summers. We found that past stress history in interaction with tree characteristics and species predisposed trees to mortality. In addition, co?occurring events (climatic and biotic) increased the severity of mortality episodes.
4. Synthesis. Our study challenges the belief that spruce budworm outbreak is the primary driver of broad?scale tree mortality in eastern boreal forest. Rather, tree mortality is the result of cumulative events that combine unfavourable conditions for growth, resulting in loss of tree vigour and subsequently, mortality. Co?occurrence of stresses in the future may lead to more severe episodes of mortality, as extreme climatic events become more frequent.

Climate change is predicted to alter relationships between trophic levels by changing the phenology of interacting species. We tested whether synchrony between two critical phenological events, budburst of host species and larval emergence from diapause of eastern spruce budworm, increased at warmer temperatures in the boreal forest in northeastern Canada. Budburst was up to 4.6 ± 0.7 days earlier in balsam fir and up to 2.8 ± 0.8 days earlier in black spruce per degree increase in temperature, in naturally occurring microclimates. Larval emergence from diapause did not exhibit a similar response. Instead, larvae emerged once average ambient temperatures reached 10°C, regardless of differences in microclimate. Phenological synchrony increased with warmer microclimates, tightening the relationship between spruce budworm and its host species. Synchrony increased by up to 4.5 ± 0.7 days for balsam fir and up to 2.8 ± 0.8 days for black spruce per degree increase in temperature. Under a warmer climate, defoliation could potentially begin earlier in the season, in which case, damage on the primary host, balsam fir may increase. Black spruce, which escapes severe herbivory because of a 2?week delay in budburst, would become more suitable as a resource for the spruce budworm. The northern boreal forest could become more vulnerable to outbreaks in the future.

1.As major alterations are occurring in climate and pest ranges, it is imperative to evaluate their combined contribution to tree mortality in order to propose mitigation measures and limit losses in forest productivity. The objective of this study was to explore the association between declines in tree growth resulting from climatic and biotic (spruce budworm) disturbances, and their interactions on tree mortality of two dominant tree species, Abies balsamea and Picea mariana, of the eastern North?American boreal forest.

2.We disentangle the influences of abiotic and biotic components on growth through a combination of model?data comparison techniques. First, we characterized the variability in tree growth and mortality in the study area using a network of tree?ring width measurements collected from living and dead trees. Subsequently, a bioclimatic simulation model was used to estimate the past annual, non?linear, responses of stand?level net primary production (NPP) to climate variability (period 1902?2012). From these two data sources, we defined the biotic stress events as the variance in the tree?ring data unexplained by the bioclimatic forest growth simulation.

3.Throughout the 20th century, two periods of adverse climatic conditions preceded spruce budworm outbreaks episodes and induced tree mortality. Climatic stress events were associated with cold springs, warmer than average summers. We found that past stress history in interaction with tree characteristics and species predisposed trees to mortality. In addition, co?occurring events (climatic and biotic) increased the severity of mortality episodes.

4.Synthesis: Our study challenges the belief that spruce budworm outbreak is the primary driver of broad?scale tree mortality in eastern boreal forest. Rather, tree mortality is the result of cumulative events that combine unfavourable conditions for growth, resulting in loss of tree vigour and subsequently, mortality. Co?occurrence of stresses in the future may lead to more severe episodes of mortality, as extreme climatic events become more frequent.

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Au Québec, le jardinage est le système sylvicole le plus pratiqué dans les forêts à structure inéquienne. Ce type d'aménagement vise à améliorer la qualité des peuplements en prélevant périodiquement le tiers de la surface terrière. La priorité lors de la récolte est accordée aux tiges de faible vigueur, qui sont choisies individuellement ou par petits groupes. Ceci permet de créer des conditions de croissance favorables pour les arbres résiduels, en plus de réduire la mortalité naturelle après la coupe. Cependant, les opérations forestières réalisées périodiquement dans un même peuplement sont une source de préoccupation, puisque les arbres résiduels sont soumis de façon répétitive à des blessures d'exploitation. De plus, le passage de la machinerie dans les sentiers de débardage perturbe le sol et brise des racines, causant possiblement un stress hydrique chez les arbres situés près des sentiers. La première partie de cette étude avait pour but d'évaluer l'incidence de ces perturbations sur la mortalité des arbres ainsi que de repérer des facteurs prédisposants possibles pour des érablières jardinées. Des inventaires ont été réalisés dans des peuplements soumis à des coupes de jardinage à différentes années. Les analyses ont révélé que les blessures d'exploitation ainsi que des causes de mortalité inconnues étaient associées aux arbres en bordure des sentiers, représentant respectivement 21 % et 26 % de la mortalité récente observée. La taille des arbres et leur espèce ont été cernées comme facteurs prédisposants : la mortalité était supérieure dans les classes de diamètre inférieures et chez le bouleau jaune, qui a aussi été trouvé plus sensible aux blessures d'exploitation. La deuxième partie de l'étude visait à évaluer l'effet des perturbations liées à la machinerie lourde sur le bilan hydrique d'arbres résiduels. Dans une érablière soumise à une coupe de jardinage en 2006, 35 érables à sucre vivants ont été choisis de façon à former un gradient de distance au sentier de débardage. Le flux de sève de ces arbres a été mesuré à l'aide de capteurs. Une zone d'influence circulaire a été estimée autour de chacun des arbres et le pourcentage de cette zone sous un sentier a été calculé (MD). Une régression linéaire segmentée a montré une diminution du flux de sève après un seuil MD de 18,17 %. Des modèles mixtes linéaires ont révélés que bien que les perturbations n'avaient pas d'effet sur le flux de sève sous cette valeur seuil, une augmentation de 1 % de MD après le seuil entraînait une diminution de 1.8 % du flux de sève. Pour un arbre avec 40.26 % de MD, ceci représente une diminution de 39,76 %, valeur comparable à ce qui a été observé durant des périodes de sécheresse dans d'autres études. Aucune différence n'a été observée entre les arbres près de sentiers primaires et ceux près de sentiers secondaires. Ceci suggère que la majorité des dommages sont causés par le premier passage de machinerie. Ces résultats indiquent que même sous des conditions climatiques favorables, telles que celles observées durant la période d'échantillonnage, les perturbations liées à la machinerie peuvent causer un stress hydrique important chez les arbres en bordure de sentiers. Ce stress hydrique pourrait expliquer une partie des causes de mortalité inconnues observées dans la première partie de l'étude. Les perturbations liées à la récolte pourraient donc être responsables de près de 47 % de la mortalité après coupe de jardinage. De plus, la disproportion entre les classes de taille et les espèces pourraient avoir des effets négatifs à long terme sur la structure et la composition du peuplement.