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Changing climates are altering the structural and functional components of forest ecosystems at an unprecedented rate. Simultaneously, we are seeing a diversification of public expectations on the broader sustainable use of forest resources beyond timber production. As a result, the science and art of silviculture needs to adapt to these changing realities. In this piece, we argue that silviculturists are gradually shifting from the application of empirically derived silvicultural scenarios to new sets of approaches, methods and practices, a process that calls for broadening our conception of silviculture as a scientific discipline. We propose a holistic view of silviculture revolving around three key themes: observe, anticipate and adapt. In observe, we present how recent advances in remote sensing now enable silviculturists to observe forest structural, compositional and functional attributes in near-real-time, which in turn facilitates the deployment of efficient, targeted silvicultural measures in practice that are adapted to rapidly changing constraints. In anticipate, we highlight the importance of developing state-of-the-art models designed to take into account the effects of changing environmental conditions on forest growth and dynamics. In adapt, we discuss the need to provide spatially explicit guidance for the implementation of adaptive silvicultural actions that are efficient, cost-effective and socially acceptable. We conclude by presenting key steps towards the development of new tools and practical knowledge that will ensure meeting societal demands in rapidly changing environmental conditions. We classify these actions into three main categories: re-examining existing silvicultural trials to identify key stand attributes associated with the resistance and resilience of forests to multiple stressors, developing technological workflows and infrastructures to allow for continuous forest inventory updating frameworks, and implementing bold, innovative silvicultural trials in consultation with the relevant communities where a range of adaptive silvicultural strategies are tested. In this holistic perspective, silviculture can be defined as the science of observing forest condition and anticipating its development to apply tending and regeneration treatments adapted to a multiplicity of desired outcomes in rapidly changing realities.

As part of the Carrefour Forêts 2019 Conference, the forest research branch of Quebec’s Ministère des Forêts, de la Faune et des Parcs and the Canadian Wood Fibre Centre of Natural Resources Canada organized jointly the symposium “Adaptive silviculture to climate change: from concepts to reality”. The symposium brought together a hundred actors from the forest sector including engineers, biologists, technicians, private woodlot owners, professors, researchers and students. Speakers from different regions of Quebec and Vermont shared current knowledge related to adaptive silviculture in order to nurture reflexion and dialogue about actions that can be taken today to help forests to adapt with the anticipated changes. The symposium covered the theoretical framework of adaptive silviculture, then offered case studies of different ecosystems, innovative tools or research initiatives. The symposium also included presentations by practitioners who described the challenges and opportunities offered by adaptive silviculture in their professional practice. Based on an interactive discussion, participants identified increased drought events, partial cuttings and mixed-species plantations as important topics on which researchers and practitioners should interact in years to come.

Dans le cadre du Carrefour Forêts 2019, le colloque Sylviculture d’adaptation aux changements climatiques : des concepts à la réalité, organisé conjointement par la Direction de la recherche forestière du ministère des Forêts, de la Faune et des Parcs du Québec et le Centre canadien sur la fibre de bois de Ressources naturelles Canada, a rassemblé une centaine d’acteurs du milieu forestier incluant des ingénieurs forestiers, des biologistes, des techniciens forestiers, des propriétaires de boisés, des professeurs, des chercheurs et des étudiants. Des conférenciers de différentes régions du Québec et du Vermont ont partagé les connaissances actuelles en rapport avec la sylviculture d’adaptation afin d’amorcer la réflexion et le dialogue quant aux actions pouvant être entreprises dès maintenant pour favoriser l’adaptation des forêts aux changements anticipés. Les présentations ont abordé les fondements théoriques de la sylviculture d’adaptation, pour ensuite présenter des réflexions ou des études de cas portant sur différents types d’écosystèmes, des outils innovants ou des initiatives de recherche. Le colloque a également laissé une place aux praticiens qui ont pu décrire les défis et les occasions qu’offre la sylviculture d’adaptation dans leur pratique professionnelle. Au terme d’une activité interactive, les participants ont ciblé les sécheresses accrues, les coupes partielles et les plantations mélangées comme des thèmes importants qui devraient animer les échanges entre les chercheurs et les praticiens dans les années à venir.

Ecosystem-based management aims to maintain the natural proportion of native species over a given landscape. White spruce (Picea glauca (Moench) Voss) is a species sensitive to environmental conditions; it is especially demanding in terms of nutrients and its regeneration is negatively affected by clearcut harvesting. Its proportion is now significantly lower than what it was in the preindustrial forests of Québec (Canada). As a native species in boreal Québec, efforts to maintain its proportion in the landscape are undertaken for white spruce, but little is known about the best practices to maximize establishment success of seedlings planted in the balsam fir (Abies balsamea)–white birch (Betula papyrifera) bioclimatic domain. Our general objective was to identify planting practices as related to microsite treatment that favour white spruce sapling survival and size after 11 growing seasons following enrichment planting of sites harvested by mechanized careful logging in an ecosystem-based management context. We also aimed at comparing white spruce performance with that of black spruce (Picea mariana (Mill.) BSP), a native species that is less sensitive to abiotic stress. Finally, we wanted to assess stand composition at this juvenile stage, as a function of microsite treatment and planted species. Localized site preparation did not significantly affect growth or survival for white spruce compared to control conditions. Furthermore, localized site preparation did not increase the proportion of white and black spruce, as evaluated by basal area. Our results suggest that white spruce can be successfully established in enrichment planting in fir-dominated boreal forests, without site preparation.

Sustainable forest management implies successful regeneration after disturbances. Low N availability and competition can, however, limit tree establishment in boreal ecosystems. To develop silviculture strategies that maintain productivity in such context, we established a field trial in northern Québec, Canada. We evaluated if a companion N2-fixing species ( Alnus alnobetula) promotes or hinders Picea mariana and Pinus banksiana establishment over six growing seasons. We tested if Alnus has a facilitation effect through nutritional processes and a competition effect through light interception. Foliar stable nitrogen isotope ratio ( δ15N = 15N/14N, ‰) results confirmed that Alnus obtains a substantial part of its N through biological fixation and represents an N source in this system. Although we did not observe increased foliar N concentrations in either conifer species in the presence of Alnus, Pinus growth was nonetheless higher in presence of Alnus, whereas no difference was observed for Picea. In the plots where Alnus cohabited with the conifers, the former had a negative impact on seedling growth, suggesting a significant competition for light. Overall, the net effect of Alnus was positive for Pinus and neutral for Picea. Our results have significant implications for silviculture in N-limited systems, especially in the context of climate change that imposes increased levels of stress on regeneration.

Climate change is projected to increase fire severity and frequency in the boreal forest, but it could also directly affect post-fire recruitment processes by impacting seed production, germination, and seedling growth and survival. We reviewed current knowledge regarding the effects of high temperatures and water deficits on post-fire recruitment processes of four major tree species (Picea mariana, Pinus banksiana, Populus tremuloides and Betula papyrifera) in order to anticipate the effects of climate change on forest recovery following fire in the boreal biome. We also produced maps of future vulnerability of post-fire recruitment by combining tree distributions in Canada with projections of temperature, moisture index and fire regime for the 2041–2070 and 2071–2100 periods. Although our review reveals that information is lacking for some regeneration stages, it highlights the response variability to climate conditions between species. The recruitment process of black spruce is likely to be the most affected by rising temperatures and water deficits, but more tolerant species are also at risk of being impacted by projected climate conditions. Our maps suggest that in eastern Canada, tree species will be vulnerable mainly to projected increases in temperature, while forests will be affected mostly by droughts in western Canada. Conifer-dominated forests are at risk of becoming less productive than they currently are, and eventually, timber supplies from deciduous species-dominated forests could also decrease. Our vulnerability maps are useful for prioritizing areas where regeneration monitoring efforts and adaptive measures could be developed.

Planting shock can last for several years during which seedling growth may significantly slow down. This duration depends upon the time that is required for the root system to acclimatize to its new environment. We hypothesized that characteristics of the root system are important factors for black spruce (Picea mariana [Mill.] BSP) seedling establishment. To test this hypothesis, we compared growth and physiology of three stock types: containerized seedlings, with a dense initial root system restricted to the volume of a root plug; deeply planted containerized seedlings, with the stem buried during production to develop dense adventitious and initial root systems; and bareroot seedlings, with a less developed but unrestricted initial root system. The three stock types were planted on three microsites that are typical of reforestation sites found in eastern Canadian boreal forest: (1) bare mineral soil; (2) in presence of ericaceous shrubs; or (3) in presence of Sphagnum spp. and ericaceous species. Labelled nitrogen fertilizer was added to half of the seedlings to compare nitrogen uptake. We measured seedling growth (height, diameter, above- and belowground biomass), nutrition (including 15N uptake) and 13C discrimination (as an indicator of water stress) at the end of the second growing season. Except at the root level, there were few interactions between stock types and microsites. For fertilized microsites, we speculate that ericaceous species captured most added nutrients since seedlings that were planted directly on mineral soil had greater height and diameter increments than seedlings with competing vegetation. In unfertilized microsites, Sphagnum spp. and ericaceous species had significant negative effects on seedling growth. For stock type, the presence of a root plug allowed containerized and deeply planted seedlings to have a lower δ13C, suggesting less water stress compared to bareroot seedlings. Strong initial root system development of containerized seedlings resulted in greater total biomass. We hypothesized that the presence of adventitious roots in deeply planted seedlings positively influenced nutrient uptake, particularly N. As nutrient availability is one of the most limiting factors in the boreal forest, we suggest using deeply planted containerized seedlings to reduce planting stress.

Changes in atmospheric N deposition and the global warming associated with human activities may impact soil organic matter mineralization rate, with possible consequences on ecosystem functioning and productivity. In this study, soil temperature was increased by +2–4 °C from April to July with heating cables and artificial rain was applied above the canopy of mature stands from June to September to simulate a 50% increase in N concentration of atmospheric deposition for a period of nine years (2008–2016) in two eastern Canada boreal forests (BER and SIM). We assessed the effects of these treatments on the rate of organic N mineralization and on the growth, phenology and N sources of Abies balsamea (L.) Mill. (balsam fir; BF) and Picea mariana (Mill.) BSP (black spruce; BS) seedlings growing below the mature tree’s canopies. The soil warming (SW) treatment had no significant effect on both net and gross N mineralization rates, whereas canopy N addition (CNA) decreased gross N mineralization rate by 23% and forest floor’s alkyl/O-alkyl C ratio by ~15% as compared to unfertilized plots. This decrease likely resulted from a reduced mining activity by soil microorganisms due to increased inorganic N availability. Foliar δ15N in the control plots was markedly lower in BS than in BF and at BER than at SIM (–4.8‰ and –2.9‰ in BS at BER and SIM, respectively; 1.6‰ and 3.8‰ in BF at BER and SIM, respectively). This was interpreted as a higher contribution of 15N-depleted N derived from mycorrhizal fungi in BS and at BER, the colder and the more N-depleted site. The SW and CNA treatments had nonsignificant effects on seedling growth and foliar chemistry. In contrast, SW caused a premature bud outbreak and faster bud development for both species at both sites. Overall, our results show that increased soil temperature and N deposition in boreal forests may not impact soil fertility and vegetation growth as much as previously thought, but climate warming, by initiating earlier and faster bud development, may however expose seedlings to late spring frosts in the future.

A tradeoff between forage acquisition and predation avoidance contributes to shape space use by herbivores. The manipulation of structural components of the habitat, such as forage and forest cover may alter this tradeoff. The idea of influencing space use of herbivores is appealing for wildlife managers that aim to locally modify herbivore densities and increase their vulnerability to hunting. We attempted to manipulate the tradeoff between forage acquisition and risk avoidance of white-tailed deer Odocoileus virginianus on Anticosti Island (Quebec, Canada) using experimental hunting fields varying in forage production (fertilized or unfertilized fields) and residual forest cover (30 or 60-m-wide forested strips between fields). In this system with high deer density, no natural predator and limited forage, fecal group surveys and camera traps demonstrated greater use of fertilized fields. Residual forest cover did not impact habitat use, suggesting that use of the experimental fields was mainly driven by the benefits of foraging compared to the costs of avoiding hunters. Deer vulnerability to hunting, however, differed with residual forest cover: hunters saw more deer per hour in fields separated by 30-m-wide forested strips compared to fields separated by 60-m-wide forested strips. That hunters did not detect the difference in deer use between fertilized and unfertilized fields suggests that deer vulnerability to hunting and deer use could be modified by different structural components of the habitat. Our results provide useful insights for wildlife managers that have to deal with conflicting goals such as maintaining hunter satisfaction with high observation rates of deer while reducing the negative impacts of high deer densities on their body condition and, on the composition and structure of forests.

Although trees are key to ecosystem functioning, many forests and tree species across the globe face strong threats. Preserving areas of high biodiversity is a core priority for conservation; however, different dimensions of biodiversity and varied conservation targets make it difficult to respond effectively to this challenge. Here, we (i) identify priority areas for global tree conservation using comprehensive coverage of tree diversity based on taxonomy, phylogeny, and functional traits; and (ii) compare these findings to existing protected areas and global biodiversity conservation frameworks. We find that ca. 51% of the top-priority areas for tree biodiversity are located in current protected areas. The remaining half top-priority areas are subject to moderate to high human pressures, indicating conservation actions are needed to mitigate these human impacts. Our findings emphasize the effectiveness of using tree conservation priority areas for future global conservation planning.

Glossy buckthorn (Frangula alnus Mill.) is an alien species in Canada that is invading many forested areas. Glossy buckthorn has impacts on the biodiversity and productivity of invaded forests. Currently, we do not know much about the species’ ecology and no thorough study of its distribution in temperate forests has been performed yet. As is often the case with invasive plant species, the phenology of glossy buckthorn differs from that of other indigenous plant species found in invaded communities. In the forests of eastern Canada, the main phenological difference is a delay in the shedding of glossy buckthorn leaves, which occurs later in the fall than for other indigenous tree species found in that region. Therefore, our objective was to use that phenological characteristic to map the spatial distribution of glossy buckthorn over a portion of southern Québec, Canada, using remote sensing-based approaches. We achieved this by applying a linear temporal unmixing model to a time series of the normalized difference vegetation index (NDVI) derived from Landsat 8 Operational Land Imager (OLI) images to create a map of the probability of the occurrence of glossy buckthorn for the study area. The map resulting from the temporal unmixing model shows an agreement of 69% with field estimates of glossy buckthorn occurrence measured in 121 plots distributed over the study area. Glossy buckthorn mapping accuracy was limited by evergreen species and by the spectral and spatial resolution of the Landsat 8 OLI.

In eastern Canada, spruces (Picea spp.) and pines (Pinus spp.) are among the main commercial species being logged for their lumber or wood fiber. Annually, about 175 million seedlings are planted in areas totaling ~100,000 ha. Appropriate microsite selection is essential during reforestation operations, given that it can improve the chances of survival and initial growth of the seedlings. In fir (Abies spp.) and spruce forests of eastern Canada, the optimal characteristics of establishment microsites have yet to be identified, these would be determined by different physical and climatic variables operating at several scales. Our study determined the influence of climatic (regional-scale), edaphic (stand-scale), local (microsite-scale) and planting conditions on the establishment substrate and initial growth of black spruce (Picea mariana Britton, Sterns and Poggenb.) and jack pine (Pinus banksiana Lamb.). Substrate characterization and growth monitoring (three growing seasons) for the two species were conducted on 29 planted cutblocks that were distributed over an east&ndash, west climatic gradient (precipitation and temperature) in the balsam fir and black spruce&ndash, feather moss forests of Quebec (Canada). Linear mixed models and multivariate analyses (PCAs) determined the effects of climatic, edaphic and micro-environmental variables and their interactions on the establishment substrate and seedling initial growth. The predictive models explained, respectively, 61% and 75% of the growth variability of black spruce and jack pine. Successful establishment of black spruce and jack pine depended upon regional conditions of precipitations and temperature, as well as on their interactions with stand-scale edaphic variables (surface deposit, drainage and slope) and local variables (micro-environmental) at the microsite-scale (establishment substrate types and substrate temperature). Mineral, organo-mineral and organic establishment substrates exerted mixed effects on seedling growth according to regional precipitation and temperature conditions, as well as their interactions with edaphic and local variables at the stand and microsite-scales, respectively.

High abundance of large herbivore has major impacts on the composition, structure, and functioning of forest ecosystems, which can result in regeneration failures. Reduction of large herbivore density, however, does not warrant the successful establishment of tree species sensitive to browsing. In such contexts, planting in combination with silvicultural treatments can be used to restore forest cover over large areas. Using an experimental plantation on Anticosti Island (Quebec, Canada), we investigated how white-tailed deer ( Odocoileus virginianus ) browsing affects the performance of planted balsam fir ( Abies balsamea L. Mill.) seedlings following site preparation and mechanical release. After seven growing seasons, seedling height, leader length and ground-level diameter were 30% bigger in fenced plots compared to unfenced ones as a result of cumulative browsing. Seedling survival was low (62%) and unaffected by either silvicultural treatments or deer after the onset of the mechanical release treatment. Site preparation and mechanical release did not improve morphological parameters and did not increase browsing occurrence on fir, likely because they had little influence on surrounding plants that compete for resources and may hide seedlings from deer 7 years after planting. Selective browsing on neighbouring plants also acted as a release treatment for balsam fir seedlings, allowing them to reach full sunlight at a lower height (125–146 cm), compared to fir seedlings in deer exclosures (161–184 cm). We propose that managers aim at target deer densities that promote browsing positive effects. In such context, prescription for site preparation and mechanical release should be based on other considerations than promoting seedling growth, such as facilitating practical reforestation work.

Understory vegetation plays an important role in maintaining biodiversity, soil nutrient cycling and carbon stocks, yet the ability of understory functional traits to predict forest productivity after harvesting disturbance is unknown. Our objective was to investigate the utility of individual traits (the community-weighted trait mean) and combined information from multiple traits (functional diversity indices) of understory plants in predicting tree diameter growth of black spruce (Picea mariana (Mill.) B.S.P.) after two harvesting methods (clearcutting and careful logging). We used vegetation and tree growth data from a network of field plots (171 plots on 43 sites) established in black spruce dominated forests across Ontario and Quebec, in northeastern Canada. Functional traits (11 traits) of dominant understory species that reflect plant morphology, regeneration strategy, and resource utilization were collected. We then used generalized linear mixed models (GLMMs) to model the response of tree diameter increment to functional diversity indices, community-weighted trait means and their interactions with harvesting method. Our results showed that: 1) at the functional diversity level, functional richness had a positive relationship with tree growth in clear-cut forests, while functional evenness showed a negative and functional divergence a positive relationship with tree growth in careful logging forests; 2) at the individual trait level, understory species with relatively large morphological trait values often promoted tree growth in both forest types, with stronger relationships in careful logging than in clear-cut forests. For regeneration traits, heavier seed weight or permanent seed bank persistence of understory plants had negative relationships with tree growth, and those relationships were either only found in careful logging forests or did not depend on harvesting type. A positive relationship was found between vegetative propagation species and tree growth, which was stronger in careful logging than in clear-cut forests. Species preferring either humid or xeric habitat had positive relationships with tree growth; both shade-tolerant and shade-intolerant species also positively correlated to tree growth. In conclusion, our study shows the important roles of understory traits in predicting tree growth based on both niche complementarity and mass-ratio hypotheses. Moreover, disturbance severity influenced the balance or degree of the positive and negative interactions that occurred between understory plants and black spruce tree growth. We suggest that identifying the threshold of harvesting disturbance severity that can activate facilitation interactions between understory plants and crop trees would be an important topic for future study.

Clearcutting practices combined with the predicted increase in fire activity may induce post-fire regeneration failure in boreal forest landscapes. This study aims (1) to evaluate if recently managed landscape by clear cut logging is susceptible to be affected by post-fire regeneration failure; and (2) to explore the ecological drivers of black spruce (Picea mariana (Mill.) BSP) post-fire regeneration. In 2014, we surveyed the regeneration of 36 stands in northwestern Quebec that had burned in a major fire in 2005. Fire severity was evaluated for each site with the differenced Normalized Burn Ratio. Using linear models, we explored the relationship between environmental variables (fire severity, pre-fire stand maturity, nature of the seedbed, and physiographic variables) and black spruce post-fire regeneration. Black spruce post-fire seedling density was highly variable (range: 25–16 000 seedlings/ha; mean ± standard deviation: 4549 seedlings/ha ± 4752) within the studied fire, but did not significantly differ between stands that had been logged 50 years prior to fire and those that were mature prior to the 2005 fire. However, post-fire regeneration failure (defined as 50%) in stands affected by past logging activities. Our study illustrates how biological legacies, environmental conditions and fire severity determine post-fire recovery and resilience of black spruce-dominated ecosystems of eastern Canada. By identifying the drivers of post-fire regeneration success, our study will help forest managers allocating resources where restoration of productive forest are truly needed.

Sustainable forest management implies successful regeneration following disturbances. Tree regeneration in subarctic ecosystems can, however, be constrained by limitations to seedling establishment related to cold soils, slow decomposition rates, and competition by ericaceous species. We established a field trial at the northern limit of commercial forests in Québec, Canada, to evaluate to what extent mechanical site preparation (MSP) and planting of a nurse N2-fixing species could promote conifer establishment on a site burned in 2007. The experiment comprised four treatments applied in 2010: standard MSP (disc trenching), standard MSP plus planting of Alnus crispa, intensive MSP, with larger furrows than standard MSP, and a control. Main plots were divided and planted in 2011 with Picea mariana (Mill.) Britton, Stearns & Poggenb. or Pinus banksiana Lamb. We monitored seedling survival, growth, nutrition, and microsite over a 3-year period. Results revealed interactions between treatments and planted species. Mechanical site preparation resulted in higher conifer growth relative to the control conditions, and planting Alnus resulted in growth gains similar to those obtained from intensive MSP. We measured competitive interactions between Alnus and the conifers that might eventually cancel out the initial benefits derived from facilitation by planting the nurse species. Longer term monitoring of interspecific interactions is needed to unravel the mechanisms responsible for the facilitative effect and identify the best management practices.

Research has demonstrated the potential of soil scarification, fertilization, and herbicide application to improve conifer seedling establishment and early growth. However, tree responses to and interactions among silvicultural treatments vary, making it difficult to predict mid- and long-term impacts of silviculture on stand productivity. We thus evaluated the 25-year effects of scarification and herbicide–fertilization combinations on black spruce (Picea mariana), jack pine (Pinus banksiana) and tamarack (Larix laricina) planted on a Kalmia angustifolia-dominated site. Our results show that the effects of scarification and herbicide–fertilization combinations diverged among species. Black spruce was the most responsive species to scarification for height and diameter at breast height. The combination of herbicide and fertilization treatments still had significantly positive effects on the long-term height and diameter growth of all species. Silvicultural treatments resulted in significant reductions in rotation length (based on height) when compared to height in nontreated-plots, depending on the species; reductions in years to attain a given height were greater for black spruce than for the other species. Our results illustrate the need to take species autecology into account when predicting productivity gains associated with early silviculture, and to provide managers with specific guidelines for the reforestation of ericaceous-dominated sites in Canadian boreal ecosystems.

Ecosystem-based management, now a dominant forestry paradigm, implies reducing the gap between variability of natural and managed forests (i.e. ecological distance) to reconcile ecological issues with production of socioeconomic services. Here, we tested whether a trade-off exists between conserving key ecosystem attributes of natural forests and maintaining and/or increasing merchantable wood production at the stand scale in humid boreal stands. Using 20-y data from an experimental design comparing silviculture scenarios of increasing intensity, (i) careful logging around advance growth (CLAAG); (ii) CLAAG followed by pre-commercial thinning; (iii) plantation followed by mechanical release; and (iv) plantation followed by chemical release, we examined plant community composition, stand structure and the quantity and the quality of snags. We also assessed timber productivity by comparing scenarios in terms of conifer and merchantable (diameter at breast height > 9 cm) tree dimensions. We used data from stands originating from a spruce budworm outbreak as a baseline to understand scenario impacts on variability of key attributes and productivity. Our results showed increasing differences in these attributes between natural and managed stands with increasing silviculture intensity: the diameter structure became more homogenized, light demanding species richness and abundance increased and the quantity and the quality of snags decreased. Therefore, our results showed that the ecological distance from naturally disturbed stands was lower after CLAAG than after the other silviculture scenarios. However, CLAAG favored an increase in the density of deciduous trees and a decrease of conifer snag density that have the potential to affect resilience of mature stands. Pre-commercial thinning resulted in crop trees reaching larger diameter than following CLAAG only and in the decrease of birch tree density, with no effect on deciduous regeneration density ⩾ 60 cm in height. We measured higher basal area of merchantable trees in plantations than in stands originating from natural regeneration scenarios, with mechanical and chemical release scenarios resulting in similar crop tree productivity. Globally, our study confirmed a general antagonism between the impacts of silviculture on key ecosystem attributes and forest productivity, posing a challenge for reconciling ecological issues with the production of socioeconomic services. At the stand level, results support that retention forestry could emulate natural disturbances by conserving biological legacies during harvest in humid boreal forests. Further research is needed to determine retention parameters to achieve expected wood production while maintaining variability of key attributes in humid boreal forests.

Considering intraspecific trait variability (ITV) in ecological studies has improved our understanding of species persistence and coexistence. These advances are based on the growing number of leaf ITV studies over local gradients, but logistical constraints have prevented a solid examination of ITV in root traits or at scales reflecting species’ geographic ranges. We compared the magnitude of ITV in above‐ and below‐ground plant organs across three spatial scales (biophysical region, locality and plot). We focused on six understorey species (four herbs and two shrubs) that occur both in disturbed and undisturbed habitats across boreal and temperate Canadian forests. We aimed to document ITV structure over broad ecological and geographical scales by asking: (a) What is the breadth of ITV across species range‐scale? (b) What proportion of ITV is captured at different spatial scales, particularly when local scale disturbances are considered? and (c) Is the variance structure consistent between analogous leaf and root traits, and between morphological and chemical traits? Following standardized methods, we sampled 818 populations across 79 forest plots simultaneously, including disturbed and undisturbed stands, spanning four biophysical regions (~5,200 km). Traits measured included specific leaf area (SLA), specific root length (SRL) and leaf and root nutrient concentrations (N, P, K, Mg, Ca). We used variance decomposition techniques to characterize ITV structure across scales. Our results show that an important proportion of ITV occurred at the local scale when sampling included contrasting environmental conditions resulting from local disturbance. A certain proportion of the variability in both leaf and root traits remained unaccounted for by the three sampling scales included in the design (36% on average), with the largest amount for SRL (54%). Substantial differences in magnitude of ITV were found among the six species, and between analogous traits, suggesting that trait distribution was influenced by species strategy and reflects the extent of understorey environment heterogeneity. Even for species with broad geographical distributions, a large proportion of within‐species trait variability can be captured by sampling locally across ecological gradients. This has practical implications for sampling design and trait selection for both local studies and continental‐scale modelling. A free Plain Language Summary can be found within the Supporting Information of this article.

The demand for wood products continues to increase globally. Productivity of forest plantations can be greater than that of naturally regenerated forests. Plantation forestry could thus be employed to meet up to 75% of global wood supply by 2050. The resilience of natural forests could jeopardize plantation productivity. If the plantation scenario is not oriented in the same direction as the natural successional trajectories that are driven by resilience, then naturally regenerating tree species could recover to the detriment of planted species. Unproductive plantations will likely generate sustainability issues, as they must provide ecosystem services (e.g., wood fiber production) while being economically viable for forest management to be sustainable. Our general objective of our study was to assess whether resilience of natural forests can jeopardized plantation productivity. We studied successional trajectories in recent, young and old black spruce plantations that are located within the balsam fir ecological region on balsam fir ecological site types. To compare with a plantation scenario that was more susceptible to aiming in the same direction as resilience-driven successional trajectories, we also studied successional trajectories in recent, young and old black spruce plantations that are not only located within the balsam fir ecological region, but on black spruce ecological site types. Successional trajectories on balsam fir ecological site types pointed towards recovery of balsam fir and white birch, to the detriment of black spruce. Successional trajectories on black spruce ecological site types pointed towards the recovery of black spruce. Thus, we showed that resilience of natural forests could jeopardize plantation productivity when the plantation scenario is not oriented in the same direction as resilience-driven successional trajectories. It also can enhance plantation productivity when the regime is oriented in the same direction as resilience-driven successional trajectories. To ensure that plantations are economically viable and promote sustainability, forest managers should thus favor plantation scenarios that point in the same direction as resilience-driven successional trajectories. Finally, we suggest adopting such scenarios, and stress that it would also be necessary to develop regimes that would promote sustainability in the context of alternative successional trajectories that will undoubtedly arise because of global changes.

Reforestation in the boreal forest is challenging; trees must survive to large daily temperature variations and to the cold environment. Even if local tree species are adapted to withstand these harsh environmental conditions, spruce plantation failure after artificial regeneration occurs frequently, with important impacts on sustainable forest management. We hypothesized that this regeneration problem is caused by recurrent frost events occurring during the growing season. These events would freeze the terminal bud and the newly formed needles of the planted trees, thus limiting photosynthesis capacity and height growth. Our goal was to identify key permanent physical attributes of the landscape (elevation, slope shape and angular slope) and of microsite conditions (hole vs. mound) that best predict tree height and frost damage to foliage. In summer 2016 and 2017, we sampled tree height of 2,943 white spruce (Picea glauca [Moench] Voss) and black spruce (Picea mariana [Mill.] B.S.P.) trees in 66 monoculture plantations aged between 6 and 13 years distributed in the Clay Belt region of Quebec (Canada), and environment prone to frequent growing-season frosts. Using linear and binomial mixed regression models, we analyzed the effects of the physical attributes of the landscape and of microsite conditions on tree height (linear) and on frost damage (binomial). Tree height increased with increasing elevation and when seedlings were planted on mounds compared to planted in holes. The impact of microsite conditions on tree height increased as plantations aged, but the importance of elevation on tree height decreased with age. The probability of frost damage to foliage decreased for trees planted on mounds compared to trees planted in holes and from concave to convex slopes. These relations were most important in young plantations, but trees showing growth problems were still shorter by 2 m, even 13 years after planting. We also observed differences between species: white spruce was significantly more damaged by frost and was smaller compared to black spruce. Therefore, growing-season frosts can cause growth suppression problems in white spruce plantations established in the boreal mixedwood region. Since microsite conditions also play a key role in driving plantation success, mechanical site preparation techniques should not only focus on reducing the competition between the planted trees and the competing vegetation but should also focus on limiting frost damage by planting trees on elevated microsites. Our results will support forestry practices limiting plantation failure in boreal mixedwoods.

Fire is considered the major disturbance in boreal forests. Nonetheless, in several areas logging has become the primary driver of forest dynamics. In many areas of the boreal forest, stands may undergo paludification (i.e. the accumulation of thick, poorly decomposed organic layers over the mineral soil) in the prolonged absence of fire, which reduces forest productivity. Whereas high-severity fires (HSF) may restore forest productivity by burning the soil organic layer (SOL), low-severity fires (LSF) mainly burn the soil surface and do not significantly reduce SOL thickness. In the Clay Belt region of eastern Canada, an area prone to paludification, forest stands have historically been harvested by clearcutting (CC), but concerns about the protection of soils and tree regeneration lead to the replacement of CC by careful logging (CL). Whereas CC disturbs the SOL and is thought to favor tree growth, CL has little impact on the SOL. Furthermore, it has been suggested that prescribed burning after clearcut (CCPB) could also be used to control paludification. Using a retrospective approach, this study sought to understand how CC, CL, and CCPB compare to LSF and HSF with respect to soil properties, SOL thickness, vegetation ground cover, tree nutrition, and stand height in paludified black spruce stands of the Clay Belt region. HSF led to significantly taller trees than CL and LSF, but did not differ from CC and CCPB. Foliar N was significantly higher in HSF and CCPB sites relative to CL and LSF, with an intermediate value in CC sites. Ground cover of Rhododendron groenlandicum was significantly lower in HSF and CC sites relative to LSF, with intermediate values in CL and CCPB sites. Sphagnum spp. ground cover was significantly lower in HSF and CCPB sites relative to CL, with intermediate values in CC and LSF sites. High-severity fire sites had a significantly thinner SOL than the four other disturbances. Finally, regression tree analysis showed that SOL thickness represented the best predictor of tree height, whereas segmented regression showed that tree height was negatively correlated to SOL thickness and revealed a cut-off point circa 23 cm, which suggests that tree growth is impeded beyond this threshold. These results support the idea that management strategies intending to regenerate paludified forests should primarily aim at reducing organic layer thickness, either through mechanical disturbance or combustion.

We studied three hybrid poplar plantations in Quebec (Canada) established on sites with varying soil and environmental characteristics to investigate the effects of logging residues on the water potential, carbon isotope ratio and foliar nutrients of planted trees. On each site, four treatments representing different residue loads, as well as treatments aimed at manipulating specific factors of the environment (Herbicide, Geotextile) were applied to test their effects on seedling water potential, carbon isotope ratio and foliar nutrients. Along with analyses of variance, we used structural equation modelling to infer causal relationships of logging residues on height, basal diameter and foliar nutrition of trees through their effects on soil temperature, soil water content and competing vegetation cover. Logging residues decreased soil temperature at all sites and woody plants cover at one site out of three. Height, basal diameter and unit leaf mass were strongly related to each other. Foliar δ13C, N concentration and unit leaf mass increased with decreasing cover of woody plants suggesting an important role of competition for resources. Overall, logging residues had no direct influence on hybrid poplar dimensions after two growing seasons: their effects on the microenvironment of this resource demanding tree species were either cancelling out each other, or were not significant enough to have a significant impact on the growth drivers measured. For example, presence of logging residues might reduce soil temperature, impeding overall seedling performance. Our study highlights the fact that any given silvicultural method aimed at manipulating logging residues has a complex influence involving the interaction of multiple environmental drivers and that the net effect on tree productivity will depend on species and site specific conditions.

We present a case study in which we assessed the effects of a severe soil perturbation on the plant community and soil variables in young hybrid poplar (Populus sp.) plantations of southern Quebec (Canada). Our overall goal was to test if soil perturbation and planting fast-growing species could promote the reestablishment of a relatively diverse plant community. A chronosequence that included three plantations (4, 8, and 12-year old) established after soil scarification, paired with three natural stands representative of the local temperate mixedwood forest comprising both pioneer and late-successional tree species, was implemented. Vegetation surveys and soil collection were performed in 2012 and species traits were provided by the Traits Of Plants In Canada and TRY databases. Principal response curve analyses showed species and functional trait divergence between treated and natural stands at ages 4 and 8, but that those divergences were less at age 12. Species and trait convergences were slowe...

Forest vegetation management can improve planted seedling survival and growth and is thus widely used in plantation silviculture. In some jurisdictions, mechanical release using brushsaws has replaced the traditional use of chemical herbicides for forest vegetation management purposes. However, its associated costs and the increasing difficulty of finding qualified labor represent a challenge. The browsing of competition by large herbivores may represent an alternative to mechanical release when planted seedlings are resistant to browsing. Here, we compare the efficacy of moose browsing relative to mechanical release in controlling competing vegetation and in promoting white spruce growth in plantations. In a high moose density region, we used an experimental design consisting of four pairs of moose exclosures and unfenced plots, fifty percent of both the access-restricted and unrestricted study areas received a mechanical release treatment. Moose browsing was more efficient than mechanical release in diminishing the sapling density and basal area of competing species. Mechanical release only reduced the sapling density of taller competitors (height &gt, 201 cm), whereas browsing reduced the sapling densities of competitors across a greater size range (height &gt, 130 cm). These effects of moose browsing on competition translated into a greater positive effect of moose browsing on the basal area of planted spruces. We attribute the higher effectiveness of moose browsing relative to mechanical release to its chronic nature. Moose browsed continuously throughout the year and for multiple years, whereas mechanical release was applied only one time between the second and fourth years after planting. Our results suggest that pairing wildlife management and silviculture decisions could be in the best interest of both the hunting and forestry industries in regions where plantations are frequent and use browse-resistant crop trees. Favouring browsers in controlling the density of competing species could increase the hunting experience and income, while providing an effective, cost-free, and socially acceptable forest vegetation management service.

The Eastern boreal forests of Quebec, Canada, have been extensively harvested over the past decades. Second-growth stands originating from sites harvested between 1920 and 1950 will soon reach the stage allowing for a second harvest. In order to guide the decision-makers responsible for ecosystem-based management of these forests, their specific management issues must be identified, based on the best knowledge available. In this context, we used the Delphi method and asked experts to identify and prioritize the main ecological issues related to the management of second-growth forests. Fourteen experts participated in at least one round of the process, out of an initial population of 30 known experts. After three rounds of questions, experts identified the maintenance of old-growth forests as the most important issue related to second-growth forest management in this region. The protection of woodland caribou and its habitat, and land fragmentation by forest roads were the second and third most important issues identified by the Delphi survey participants. These issues are not unique to second-growth forests, but should be given priority in considering management strategies associated with second-growth stands.

Black spruce (Picea mariana [Mill.] BSP) is a boreal tree species characterized by the formation of an adventitious root system. Unlike initial roots from seed germination, adventitious roots gradually appear above the root collar, until they constitute most of mature black spruce root system. Little is known about the physiological role they play and their influence on tree growth relative to initial roots. We hypothesized that adventitious roots present an advantage over initial roots in acquiring water and nutrients. To test this hypothesis, the absorptive capacities of the two root systems were explored in a controlled environment during one growing season. Black spruce seedlings were placed in a double-pot system allowing irrigation (25 and 100% water container capacity) and fertilization (with or without fertilizer) inputs independent to initial and adventitious roots. After 14 weeks, growth parameters (height, diameter, biomass), physiology (net photosynthetic rate, stomatal conductance, shoot water potential) and nutrient content (N, P, K, Ca and Mg foliar content) were compared. Most measured parameters showed no difference for the same treatment on adventitious or initial roots, except for root biomass. Indeed, fertilized black spruce seedlings invested heavily in adventitious root production, twice as much as initial roots. This was also the case when adventitious roots alone were irrigated, while seedlings with adventitious roots subjected to low irrigation produced initial root biomass equivalent to that of adventitious roots. We conclude that black spruce seedlings perform equally well through adventitious and initial roots, but if resources are abundant, they strongly promote development of adventitious roots.

Forest regeneration is a key element in achieving sustainable forest management. Partial harvest methods have been used extensively in temperate broadleaf and mixedwood ecosystems to promote regeneration on poorly stocked sites and to maintain forest composition and productivity. However, their effectiveness in promoting conifer establishment has yet to be demonstrated in unmanaged boreal forests, especially those dominated by black spruce (Picea mariana (Mill.) BSP) where constraints for regeneration differ from those found in more meridional regions. We aimed to evaluate conifer seedling density and dimensions, 10 years after the onset of a gradient of silvicultural treatments varying in harvesting intensities, and to identify the critical factors driving the regeneration process. Study blocks of even-aged black spruce stands in the eastern Canadian boreal forest were submitted to three variants of shelterwood harvesting: a seed-tree harvest, a clear-cut and an untreated control. Shelterwood and seed-tree harvesting were combined with spot scarification to promote regeneration. Shelterwood and seed-tree harvesting produced a density of conifer regeneration sufficient to maintain forest productivity, but they did not promote seedling growth. Black spruce was the predominant species in terms of regeneration density, with proportions 3–5× higher than that for balsam fir (Abies balsamea (L.) Mill.). Ten years after treatment, seed-origin black spruce seedlings were abundant in skidding trails, while layers dominated the residual strips. Balsam fir density was not influenced by treatment nor by tree position relative to skidding trails. Balsam fir and black spruce had different responses to treatment in terms of height and diameter, the former exhibiting a better growth performance and larger diameter in the residual strips. Spot scarification created micro-sites that had a significant impact on the regeneration process. Overall, our results support that shelterwood and seed-tree harvesting combined with scarification enable adequate regeneration in black spruce stands, confirming these treatments as viable silvicultural alternatives to clear-cutting when required by sustainable forest management objectives.

Monitoring and mapping species diversity using indicators can allow the detection of changes in communities. Conclusions regarding these changes greatly depend on the choice of indicator. Here, we propose a new metric, the distance biochange index (DBCI), that enables the characterization and quantification of both the level and direction of change in biological communities relative to a given reference state. The proposed metric uses conditional probabilities to assess the probability of observing a complete change in a given community and can be decomposed into four conditional probabilities of change: no change, complete change in species composition only, complete change in species richness only, and complete change in both species composition and richness. In this study, we compared the properties of DBCI and BCI, a similarity version of DBCI, with those of other widely used indices. We also proposed a new approach, based on the use of partial derivatives, to assess the sensitivity of six similarity indices over a wide range of contrasting scenarios of change. Finally, we extended the application of DBCI to a simulated case study of the predicted evolution of suitable habitats for 20 species under climate change. Results from this simulation demonstrated that DBCI provides an accurate assessment of the level and direction of change. Results also show that DBCI can be used to reflect the effect of ecological gradients on species composition and species richness in biological communities.

Use of fast-growing tree plantations on dedicated areas is proposed as a means of reconciling fibre production with conservation objectives. Success of this approach however requires fine-tuning silvicultural scenarios so that survival and growth are optimized while management and environmental costs are minimized. This is particularly challenging for hybrid larch ( Ã Coaz), a shade-intolerant species planted on fertile sites in Quebec (Canada) where legislation prevents the use of chemical herbicides. In this context, multiple motor-manual release treatments are often required, with high impacts on costs and social issues related to the scarcity of a qualified workforce. We established a split-split-plot design on a recently harvested site to assess the main and interaction effects of mechanical site preparation (MSP) intensity (five modalities of trenching or mounding), motor-manual release scenario (one or two treatments) and planting depth (0â3 cm or 3â10 cm) on hybrid larch seedling growth and survival six years after planting. Mechanical site preparation intensity and planting depth did not influence seedling growth after 6 years. The lack of significant interaction between MSP and release scenarios indicates that these operations should be planned independently. A more intensive MSP treatment cannot replace a second motor-manual release on fertile sites, as proposed to reduce costs. Our results also show the significant advantage of performing two motor-manual release treatments two years apart (the first one early in the scenario), over performing a single treatment. Our study provides silvicultural guidelines for the establishment of high-yield exotic larch plantations.Larixmarschlinsii

Many deer populations have recently increased worldwide leading to strong direct and indirect ecological and socioeconomical impacts on the composition, dynamic, and functions of forest ecosystems. Deer directly modify the composition and structure of vegetation communities, but they also indirectly affect other species of the ecosystem by modifying the structure of the vegetation. Here we review the results of a research program on overabundant white-tailed deer (Odocoileus virginianus) in the boreal forest of Anticosti Island (Québec, Canada) aimed at identifying deer densities compatible with forest regeneration. Various silvicultural systems and treatments failed to regenerate deer habitat at high deer densities, but planting size-adapted seedlings could be effective at moderate densities. Using a controlled deer density experiment, we found vegetation recovery at deer densities ≤ 15 deer/km2. The same experiment revealed that other groups of organisms such as insects and birds responded favorably to a reduction of deer density. We also found that alternative successional trajectories may occur after a certain period of heavy browsing during early succession. We conclude that one of the most important remaining research gaps is the need to identify habitat-specific threshold densities at which deer impacts occur and then to design effective wildlife and forest management strategies to limit deer impacts and sustain ecosystem integrity.

Roots directly affect planted seedling adaptation to new growing conditions at reforestation sites. To test the influence of root characteristics on the short-term response of seedlings to limiting resources (water, nutrient, or oxygen), we conducted two experiments. We compared (1) the growth and physiology of three types of four-year-old black spruce (Picea mariana (Mill.) BSP) seedlings (Containerized, highly developed initial roots restricted to a plug, bareroot, less developed but unrestricted initial roots, deeply-planted containerized, restricted initial and adventitious roots) to different combinations of irrigation and fertilization. We also investigated (2) the cellular plasticity of adventitious and initial roots to three irrigation regimes including flooding. Bareroot seedlings had better relative growth rates in height than containerized seedlings, probably due to their larger initial size. On the other hand, containerized seedlings took better advantage of fertilization, as shown by a higher relative growth rate in diameter compared to bareroot seedlings and were less affected by water limitation, possibly due to the root plug acting as an additional water reserve capacity. For containerized seedlings, the presence of adventitious roots was beneficial to height growth and physiological performances compared to seedlings with initial roots only. Adventitious roots showed great cell plasticity, particularly under flooding conditions.

Managing competing vegetation is crucial in stand establishment strategies, forecasting the abundance, composition, and impact of competing vegetation after harvesting is needed to optimize silviculture scenarios and maintain long-term site productivity. Our main objective was to identify factors influencing the short-term abundance and composition of competing vegetation over a large area of the Canadian boreal forest. Our second objective was to better understand the mid-term evolution of the regeneration/competing vegetation complex in cases of marginal regeneration conditions. We used operational regeneration surveys of 4471 transects sampled &asymp, 5 years after harvesting that contained data on regeneration, competing vegetation, elevation, ecological classification, soil attributes, and pre-harvest forest stands. We performed a redundancy analysis to identify the relationships between competing vegetation, harvesting and biophysical variables. We then estimated the probability of observing a given competing species cover based on these variables. In 2015, we re-sampled a portion of the sites, where conifer regeneration was marginal early after harvesting, to assess the temporal impact of different competing levels and species groups on the free-to-grow stocking, vigour and basal area of softwood regeneration. Results from the first inventory showed that, after careful logging around advance growth, ericaceous shrubs and hardwoods were not associated with the same sets of site attributes. Ericaceous shrubs were mainly found on low fertility sites associated with black spruce (Picea mariana (Mill.) BSP) or jack pine (Pinus banksiana Lamb.). The distinction between suitable environments for commercial shade-intolerant hardwoods and non-commercial hardwoods was less clear, as they responded similarly to many variables. Analysis of data from the second inventory showed a significant improvement in conifer free-to-grow stocking when commercial shade-intolerant hardwood competing levels were low (stocking 0%&ndash, 40%) and when ericaceous shrubs competing levels were moderate (percent cover 26%&ndash, 75%). In these conditions of marginal regeneration, the different types and intensities of competition did not affect the vigour or basal area of softwood regeneration, 9&ndash, 14 years after harvesting.

Heavy browsing pressure from large ungulates is a multicontinent phenomenon that causes regeneration failure of many palatable tree species and induces important socioeconomic and ecological impacts in forest ecosystems. The development of forest management practices that address adequately this issue, however, remains scarce and challenging because (1) large herbivores are both a resource and a source of disturbance; (2) the management of forests and ungulate populations remains largely disconnected in practice; and (3) we still lack a good understanding of the role of critical factors, especially deer densities, vegetation attributes, and their interactions, on the magnitude of browsing damages on forest regeneration. We bring new insights into these challenging issues by critically reviewing the current methods used by managers and conservationists to mitigate deer impacts on forest regeneration, emphasizing the spatial scale at which these methods are undertaken. Specifically, we review management actions at multiple scales on both deer populations (e.g., hunting) and vegetation (e.g., silvicultural treatments) that are common to most deer–forest systems and, for that reason, deserve priority investigation. We identify strengths and limitations of current management actions and highlight the main research gaps. Based on this review, we propose a new integrated management scheme that explicitly addresses: (1) the integration and prioritization of management actions, (2) the development of adaptive management plans, and (3) the participation of stakeholders. Conflicting demands by different stakeholders have challenged the effectiveness of management strategies in deer–forest systems. To reverse this situation, we advocate for a shift of paradigm and the development of integrated strategies that (1) bridge the gap between management actions and the design of in situ experiments and (2) coordinate actions at multiple spatial scales on both deer populations and forests. We propose a new framework informed by key objectives and grounded in the adaptive management paradigm to support this transition, and suggest a research agenda for the next decade(s).

International audience; The impact of mechanical site preparation (MSP) on soil biogeochemical structure in young larch plantations was investigated. Soil samples were collected in replicated plots comprising simple trenching, double trenching, mounding and inverting site preparation. Unlogged natural mixed forest areas were used as a reference. Analysis of soil nutrients, abundance of bacteria and gas exchanges unveiled no significant difference among the plots. However, inverting site preparation resulted in higher variations of gas exchanges when compared with trenching, mounding and unlogged natural forest. A combination of the biological and physicochemical variables was used to define a multifunctional classification of the soil samples into four distinct groups categorized as a function of their deviation from baseline ecological conditions. According to this classification model, simple trenching was the approach that represented the lowest ecological risk potential at the microsite level. No relationship was observed between MSP method and soil bacterial community structure as assessed by high-throughput sequencing of bacterial 16S rRNA gene; however, indicator genotypes were identified for each multifunctional soil class. This is the first identification of multifunctional molecular indicators for baseline and disturbed ecological conditions in soil, demonstrating the potential of applied microbial ecology to guide silvicultural practices and ecological risk assessment.

Covers with capillary barrier effects (CCBE) are used to prevent acid mine drainage from mine wastes in the short term. However, the long-term efficiency of CCBE can be affected by trees because their roots may reduce the ability of covers to limit oxygen migration and also physically damage the CCBE. Two plant species that are native to boreal Canada, bluejoint () and sheep laurel (, were selected as bio-barrier species (BBS) to test if they reduce the growth and root system architecture of trees established on mine covers (balsam poplar [], willow [ spp], and black spruce []). The experiment was established in 2008 on a mine tailings impoundment located in northwestern Quebec, Canada. Trees were measured for height, diameter, and biomass. Coarse roots were excavated from the plots and digitized in three dimensions. Compared with the control (no BBS), bluejoint strongly decreased tree height and diameter increment, biomass, maximum root depth and radial extension, total root length and volume, and number of second- and third-order tree roots. Height and diameter increment, biomass, maximum root depth and volume, and number of second-order roots of balsam poplar increased with sheep laurel compared with control conditions, whereas willow showed no response to this treatment. Most characteristics of black spruce (except root-to-shoot ratio and number of second-order roots) improved in the presence of sheep laurel compared with the control. Thus, bluejoint was a more efficient BBS than sheep laurel. Bio-barriers comprised of bluejoint can be used as a countermeasure for controlling tree invasion of CCBE.

International audience; - In Québec (Canada), the use of large planting stock is being applied in combination with scarification, since herbicide use is forbidden in public forest. Large containerized and bare-root stock of black spruce were planted on two sites located within the sub-boreal mixedwood region of eastern Québec. We analyzed data to detect main effects and interactions among scarification, competing vegetation and stock type on seedling growth and physiology during the first three growing seasons. Scarification did not improve seedling water relations, third-year height and ground-level diameter, and foliar nutrient concentration. After three years, the two stock types showed similar water relations and nutritional status but the large containerized seedlings performed slightly better than the large bare-root stock in terms of diameter and height growth. Competing vegetation greatly reduced seedling diameter, foliar-N concentration, compared to competition-free seedlings. We discuss results in relation with treatment effects on above- and belowground resource availability to newly planted conifers.; - Performance et physiologie de plants de fortes dimensions d'épinette produits en récipients et à racines nues en relation avec le scarifiage et la végétation compétitive au Québec (Canada). Au Québec (Canada), l'utilisation de phytocides chimiques est proscrite en forêt publique. Le reboisement de plants de fortes dimensions (PFD) est pratiqué, en combinaison avec le scarifiage, sur les stations à haut risque de compétition. Des PFD d'épinette noire en récipients (RC) et à racines nues (RN) ont été plantés sur deux sites afin d'évaluer les effets du scarifiage, de la compétition, du type de plant et de leurs interactions sur l'établissement des plants pendant les trois premières saisons de croissance. Le scarifiage n'a pas d'effet marqué sur les dimensions et la physiologie des semis après trois ans. Les deux types de PFD présentent, après trois saisons, des relations hydriques et des statuts nutritifs équivalents. Les RC atteignent des dimensions légèrement supérieures à celles des RN. La végétation a peu d'impacts sur la croissance en hauteur, mais affecte négativement le diamètre et le statut nutritif. Nous discutons des résultats à l'égard de leurs effets sur la disponibilité des ressources environnementales pour les plants.