Partial cutting has lower canopy removal intensities than clearcutting and has been proposed as an alternative harvesting approach to enhance ecosystem services, including carbon sequestration and storage. However, the ideal partial cutting/clearcutting proportion that should be applied to managed areas of the eastern Canadian boreal forest to enhance long-term carbon sequestration and storage at the landscape scale remains uncertain. Our study projected carbon dynamics over 100 years (2010–2110) under a portfolio of management strategies and future climate scenarios within three boreal forest management units in Quebec, Canada, distributed along an east–west gradient. To model future carbon dynamics, we used LANDIS-II, its Forest Carbon Succession extension, and several extensions that account for natural disturbances in the boreal forest (wind, fire, spruce budworm). We simulated the effects of several management strategies on carbon dynamics, including a business-as-usual strategy (clearcutting applied to more than 95 % of the annually managed area), and compared these projections against a no-harvest natural dynamics scenario. We projected an overall increase in total ecosystem carbon storage, mostly because of increased productivity and broadleaf presence under limited climate change. The drier Western region under climate scenario RCP8.5 was an exception, as stocks decreased after 2090 because of the direct negative effects of extreme climate change on coniferous species’ productivity. Under the natural dynamic scenario, our simulations suggest that the Quebec Forest in the Central and Western regions may act as a carbon sink, despite high fire-related carbon emissions, particularly under RCP4.5 and RCP8.5 Conversely, the eastern region periodically switched from carbon sink to source following SBW outbreaks, thus being a weak sink over the simulation period. Applying partial cutting to over 50 % of the managed forest area effectively mitigated the negative impacts of climate change on carbon balance, reducing differences in stand composition and carbon storage between naturally dynamic forests and those managed for timber. In contrast, clearcutting-based scenarios, including the business-as-usual approach, substantially reduced total ecosystem carbon storage— by approximately double (10 tC ha−1 yr−1) compared to partial cutting scenarios (<5 tC ha−1 yr−1). Clearcutting led to higher heterotrophic respiration due to the proliferation of fast-decomposing broadleaves, resulting in lower carbon accumulation compared to partial cuts. Our findings underscore the importance of balancing canopy removal intensities to increase carbon sequestration and storage while preserving other ecosystem qualities under climate change.

Climate change poses a serious risk to sustainable forest management, particularly in boreal forests where natural disturbances have been projected to become more severe. In three Quebec boreal forest management units, biomass carbon storage under various climate change and management scenarios was projected over 300 years (2010–2310) with a process-based dynamic landscape model (PnET-succession for Landis-II). Several strategies varying in their use of partial cuts and clear cuts, including business as usual (BAU) (clear-cut applied on more than 95% of the managed area), were tested and compared to conservation scenarios (no-harvest). Based on simulation results at the landscape scale, the clearcut-based scenarios such as BAU could result in a decrease of biomass carbon stock by 10 tC ha−1 yr−1 compared to the natural scenario. However, this reduction in carbon stock could be offset in the long term through changes in composition, as clearcut systems promote the expansion of trembling aspen and white birch. In contrast, the use of strategies based on partial cuts on more than 75% or 50% of the managed area was closer to or better than the natural scenario and resulted in greater coniferous cover retention. These strategies seemed to be the best to maximize and stabilize biomass carbon storage and ensure wood supply under different climate change scenarios, yet they would require further access and appropriate infrastructure. Furthermore, these strategies could maintain species compositions and age structures similar to natural scenarios, and thus may consequently help achieve forest ecosystem-based management targets. This study presents promising strategies to guide sustainable forest management in Eastern Canada in the context of climate change.

The boreal forest holds the world's largest soil carbon (C) reservoir. A large portion of it is contained in a thick organic layer originating from the slow decay of bryophytes. Because a thick organic layer slows down tree growth, reduces forest productivity, and thereby reduces the potential wood supply, silvicultural treatments that aim to maintain or restore forest productivity after harvesting often involve mechanical site preparation. However, while these treatments can increase growth and C storage in trees, they can also lead to accelerated decomposition of the soil organic matter, reducing C storage. In this study, we assessed the nine-years effect of two silvicultural treatments on soil C dynamics in forested peatlands of northwestern Quebec, compared to unharvested controls: (1) cut with protection of regeneration and soils (CPRS; low soil disturbance, also called careful logging around advanced growth (CLAAG)), (2) CPRS followed by mechanical site preparation (CPRS + MSP, plowing; severe soil disturbance). The mass loss rate of three bryophytes (Pleurozium schreberi, Sphagnum capillifolium, and Sphagnum fuscum) was measured over two growing seasons together with soil organic carbon (SOC) stocks. We also studied the different effects of temperature, water table level, depth, and type of soil layer on mosses decomposition.We observed a significant influence of silvicultural treatments, bryophyte species, and soil layer type (fibric, mesic, humic and mineral) on bryophyte mass loss, which was higher in the CPRS + MSP treatment (21.6 ± 0.13 % standard error) than in control sites (9.5 ± 0.21 %); CPRS alone resulted in an intermediate mass loss of 11.6 ± 0.23 %, for Sphagnum mosses. Bryophyte mass loss was significantly higher in fibric than humic layer. SOC stocks in the uppermost organic soil layer (fibric) were lower in the CPRS + MSP group than in the control group, while the CPRS group was intermediate; however, differences were not statistically significant for the other soil layer and for total SOC. We conclude that while CPRS + MSP accelerates Sphagnum moss decomposition in the topsoil layer, it has limited impact on total soil C stocks that are detectable with stock change methods.

Several recent studies point out that climate change is expected to influence boreal forest succession, disturbances, productivity, and mortality. However, the effect of climate change on those processes and their interactions is poorly understood. We used an ecophysiological-based mechanistic landscape model to study those processes and their interactions and predict the future productivity and composition under climate change scenarios (RCP) for 300 years (2010–2310). The effects of climate change and wildfires on forest composition, biomass carbon sequestration and storage, and mortality were assessed in three management units of Quebec boreal forest, distributed along a longitudinal gradient from west to east: North-of-Quebec (MU1), Saguenay–Lac-Saint-Jean (MU2), and Côte-Nord region (MU3). Coniferous mortality variation was explained by competitive exclusion and wildfires, which are related to climate change. In the studied MU, we found a decrease in coniferous pure occupancy at the landscape scale and an increase in mixed deciduous forests in MU1 and MU2, and an increase in mixed coniferous, mainly black spruce and balsam fir in MU3. On the other hand, for extreme scenarios (RCP4.5 and RCP8.5), in the absence of broadleaves dispersal, the open woodland occupancy could increase to more than 8, 22, and 10% in MU1, MU2, and MU3 respectively. Also, climate change might increase overall biomass carbon stock two times for RCP2.6 and RCP4.5 scenarios compared to the baseline this may be explained by the extension of the growing season and the reduction of potential cold-temperature injuries. Generally, western regions were more sensitive to climate changes than the eastern regions (MU3), in fact under RCP8.5 biomass carbon stock will be decreasing in the long-term for MU1 compared to the current climate. This study provides a good starting point to support future research on the multiple factors affecting forest C budget under global change.

Carbon sequestration and storage in forest ecosystems is often promoted as a solution for reducing CO2 concentrations in the atmosphere. Yet, our understanding is lacking regarding how forest management strategies affect the net removal of greenhouse gases and contribute to climate change mitigation. Here, we present a review of carbon sequestration and stock dynamics, following three strategies that are widely used in boreal, temperate and tropical forests: extensive forest management, intensive forest management and old-growth forest conservation.

We investigated whether stand species mixture can attenuate the vulnerability of eastern Canada’s boreal forests to climate change and insect epidemics. For this, we focused on two dominant boreal species, black spruce [Picea mariana (Mill.) BSP] and trembling aspen (Populus tremuloides Michx.), in stands dominated by black spruce or trembling aspen (“pure stands”), and mixed stands (M) composed of both species within a 36 km2 study area in the Nord-du-Québec region. For each species in each stand composition type, we tested climate-growth relations and assessed the impacts on growth by recorded insect epidemics of a black spruce defoliator, the spruce budworm (SBW) [Choristoneura fumiferana (Clem.)], and a trembling aspen defoliator, the forest tent caterpillar (FTC; Malacosoma disstria Hübn.). We implemented linear models in a Bayesian framework to explain baseline and long-term trends in tree growth for each species according to stand composition type and to differentiate the influences of climate and insect epidemics on tree growth. Overall, we found climate vulnerability was lower for black spruce in mixed stands than in pure stands, while trembling aspen was less sensitive to climate than spruce, and aspen did not present differences in responses based on stand mixture. We did not find any reduction of vulnerability for mixed stands to insect epidemics in the host species, but the non-host species in mixed stands could respond positively to epidemics affecting the host species, thus contributing to stabilize ecosystem-scale growth over time. Our findings partially support boreal forest management strategies including stand species mixture to foster forests that are resilient to climate change and insect epidemics.

Wildfire is the primary abiotic disturbance in the boreal forest, and its long-term absence can lead to large changes in ecosystem properties, including the availability and cycling of nutrients. These effects are, however, often confounded with the effects of successional changes in vegetation toward nutrient-conservative species. We studied a system of boreal forested lake islands in eastern Canada, where time since last fire ranged from 50 to 1500 years, and where the relative abundance of the most nutrient-conservative tree species, black spruce, was largely independent of time since last fire. This allowed us to disentangle the effects of time since fire and the dominant vegetation on ecosystem properties, including nutrient stocks and concentrations. Effects of time since fire independent of vegetation composition mostly involved an increase in the thickness of the organic layer and in nitrogen concentration in both soil and leaves. Domination by black spruce had strong negative effects on nutrient concentrations and was associated with a shift toward more fungi and Gram-positive bacteria in the soil microbial community. Path modeling showed that phosphorus concentration was inversely related to organic layer thickness, which was in turn related to both time since fire and black spruce abundance, while nitrogen was more directly related to time since fire and the composition of the overstory. We conclude that discriminating between the effects of vegetation and time since fire is necessary for better understanding and predicting the long-term changes that occur in forest nutrient availability and ecosystem properties.

Plant species mixtures are often seen as being able to achieve higher productivity and carbon (C) sequestration than their single-species counterparts, but it is unclear whether this is true in natural forests. Here, we investigated whether naturally-regenerated mixtures of common North American boreal tree species were more productive and stored more C than single-species stands. We also examined how closely the different C pools and fluxes were interrelated and whether these relationships varied with species composition. Single- and mixed-species stands of trembling aspen, black spruce and jack pine on mesic sites were selected in two regions of the Canadian boreal forest to assess aboveground and belowground productivity and C storage. Although previous studies conducted in these stands found synergistic effects of tree species mixtures on specific C pools and fluxes, such as higher organic layer C stocks and higher fine root productivity in some mixtures, no effects were detected on combined C pools or fluxes at the ecosystem level in the current study. Aspen abundance was linked with higher aboveground tree productivity, higher aboveground living biomass and higher soil heterotrophic respiration, indicating that aspen acts as a key driver of ecosystem C storage and fluxes in these natural forest ecosystems, more so than species richness. However, our results do not rule out the possibility of increased productivity and C storage in mixed stands under environmental conditions or stand developmental stages that are different from the ones studied here. Furthermore, when the entire forest ecosystem is considered (not only tree parts), synergistic effects of tree species mixtures may be more difficult to observe because the beneficial effect of species mixing on one specific C pool may be counterbalanced by a negative effect on another pool.

It is crucial to better understand and predict how burnt areas in the boreal forest will evolve under a changing climate and landscape. The objective of the present study was to predict burnt areas at several spatial and temporal scales in the Quebec continuous boreal forest and to compare the influence of weather, vegetation and topographic variables by including them and their interactions in logistic regressions. At the largest spatial scale (350 km2), the best model explained 66% of the data variability and was able to predict burnt areas with reasonable accuracy for 11 years (r = 0.48). Weather and vegetation or topographic variables had an equivalent importance, though no single vegetation or topographic variable was mandatory to the model performance. Interactions between weather and non-weather variables largely improved the model, particularly when several weather indices were used, as the sign of the interaction with a non-weather variable could differ between weather indices. Vegetation and topography are therefore important predictors of fire susceptibility, but risk factors may vary between wind- and drought-driven fire weather. Including at least some vegetation and topographic variables in statistical models linking burnt areas to weather data can greatly improve their predictive power.

Underlying the development and function of aspen forest communities are interactions between aspen and a broad suite of plant species. These plant–plant interactions can be facilitative or antagonistic in nature and their influence varies depending on multiple environmental factors that are changing with human activity. The purpose of this synthesis paper is to identify the patterns, mechanisms and consequences of facilitation and competition in aspen communities and how they vary based on environmental conditions and different aspen forest types.

Across its expansive range, aspen commonly associate with conifers to form mixed forests. There is increasing evidence that facilitation in early stand development alters competitive interactions between aspen and conifers in later stages of development. However, the influences of facilitation and competition vary depending on conifer species and aspen forest type. In drier, montane aspen forests of the western US, shade and higher moisture content at the base of aspen trees facilitate the germination and survival of young fir seedlings. This facilitation effect increases the proximity of aspen and fir which over time creates competitive interactions that favor conifer dominance. In the more mesic conditions of eastern Canada, aspen also promotes fir establishment but the facilitation effect occurs at the stand level and is most likely driven by increased light penetration and more optimal edaphic conditions rather than by mitigating moisture limitations. In the western and central boreal forest, successional transitions are primarily driven by competitive effects in which short fire cycles and competitive inhibition of spruce favors aspen dominance.

Positive and antagonistic interactions between aspen and associated plant species are influenced by environmental conditions that fluctuate according to nature processes and human perturbations. In this review we discuss the impact that plant invasions, global change factors, fire regimes and herbivory have on plant–plant interactions in aspen forest and how they modify successional outcomes. The literature suggests that aspen’s competitive ability is strongly influenced by rising CO2, temperatures, drought and ozone. Conditions resulting in longer fire cycles will tend to promote losses in aspen cover through competitive exclusion through conifer expansion. Finally, competition alters aspen susceptibility to herbivory which is a major threat to aspen resilience in some parts of its range. Identifying the environmental conditions that create the proper balance between facilitative and competitive interactions is paramount in formulating management approaches that promote resilient aspen forests.

Changes in forest composition as a result of forest management, natural disturbances, and climate change may affect the accumulation of soil organic carbon (SOC). We examined the influence of common boreal tree species (trembling aspen, black spruce, and jack pine), either in pure stands or in conifer-broadleaf mixtures, on the amount, distribution, and quality of SOC in two regions of the Canadian boreal biome. Long-term laboratory incubations were used to assess SOC quality by quantifying proportions of fast carbon (C) (that is, proportion of total C released during the first 100 days of incubation) and active C (that is, modeled proportion of total C that can be potentially released). Total amounts of SOC did not differ between stand types, but the effects of stand type on SOC stocks and quality differed with soil depth. Among stand types, aspen stands had the greatest relative proportion of total SOC in deeper mineral layers and the lowest amount of active C in the organic layer. For these reasons, the SOC stock that developed under aspen was more stable than in the other stand types. Although black spruce stands allowed a greater accumulation of SOC in surface layers, these stocks, however, might become more vulnerable to extra losses if environmental conditions are to become more favorable to decomposition in the future. Our work highlights that boreal forest composition influences the stability of SOC stocks and how climate change could alter this large C pool.

• Although fine roots (< 2 mm in diameter) account for a major share of the production of terrestrial ecosystems, diversity effects on fine root productivity and their mechanisms remain unclear.
• We hypothesized that: (i) fine root productivity increases with tree species diversity, (ii) higher fine root productivity is a result of greater soil volume filling due to species-specific patterns of root placement and proliferation, and (iii) differences in fine root productivity and soil volume filling associated with tree species diversity are more pronounced in summer when plants are physiologically active and demand for water and nutrients is at its greatest.
• We investigated the effects of tree species diversity on fine root productivity and soil volume filling of boreal forest stands that have grown naturally for 85 years on similar sites.
• Annual fine root production was 19–83% higher in evenly mixed- than single-species-dominated stands, and increased with tree species evenness, but not tree species richness. Fine root biomass was higher in evenly mixed- than single-species-dominated stands in summer months, but not in spring or fall. Higher fine root productivity in evenly mixed- than single-species-dominated stands was realized by filling more soil volume horizontally and vertically in the forest floor in the mixtures of deep- and shallow-rooted species vs. the deeper mineral soil in the mixtures of deep-rooted species.
• Synthesis. Our results provide some of the first direct evidence for below-ground species complementarity in heterogeneous natural forests, by demonstrating that tree species evenness increases fine root productivity by filling/exploiting the soil environment more completely in space and time, driven by differences in the inherent rooting traits of the component species and variations of root growth within species.

Les préoccupations environnementales croissantes en foresterie ont provoqué un intérêt pour l’aménagement mixte comme une stratégie possible dans un contexte d’aménagement forestier durable. Cette revue de littérature se concentre sur les effets des peuplements mixtes sur la biodiversité, incluant les plantes de sous-bois, la faune aviaire, la faune du sol, et les ectomycorrhizes (ECM). Elle examine la diversité et la composition spécifiques à l’échelle des peuplements mais focalise particulièrement sur l’échelle du paysage (diversité gamma) en recherchant la présence d’espèces indicatrices des peuplements mixtes. Les principales conclusions sont les suivantes : (i) L’existence de différentes espèces d’arbres dans la canopée est associée avec une plus grande diversité de microhabitats, permettant l’addition des espèces de plantes de sous-bois associées à chacune des espèces de la canopée. Il n’y a toutefois que peu d’indications quant à l’existence d’espèces de plantes de sous-bois associées exclusivement aux peuplements mixtes. (ii) Certaines espèces d’oiseaux exigent ou préfèrent la présence de différentes espèces d’arbres dans un paysage ou un peuplement. Les peuplements ou paysages forestiers mixtes sont donc critiques pour la conservation de telles espèces. (iii) Quelques études ont montré un effet positif des peuplements ou litières mélangés sur certains groupes d’organismes du sol, mais la forte variabilité de ces résultats rend toute conclusion hasardeuse. (iv) Certains taxa d’ECM sont associés à des hôtes multiples, et pourraient donc bénéficier de la disponibilité de plusieurs hôtes dans les peuplements mixtes. Plusieurs études ont confirmé la plus grande abondance de ces taxa à hôtes multiples dans les peuplements mixtes.

Question: The effect of overstorey composition on above?ground dynamics of understorey vegetation is poorly understood. This study examines the understorey biomass, production and turnover rates of vascular and non?vascular plants along a conifer–broadleaf gradient of resource availability and heterogeneity.

Location: Canadian boreal forests of northwest Quebec and Ontario.

Methods: We sampled mature stands containing various proportions of black spruce (Picea mariana (Mill.) BSP), trembling aspen (Populus tremuloides Michx.) and jack pine (Pinus banksiana Lamb.). Above?ground biomass of the understorey vegetation was assessed through harvesting; annual growth rates were calculated as the differences between biomass in 2007 and 2008, as estimated by allometric relationships, and turnover rates were estimated as net primary production divided by the biomass in 2007.

Results: Higher aspen presence, linked to greater nutrient availability in the forest floor, was generally associated with higher vascular biomass and production in the understorey. This effect was less pronounced in sites of high intrinsic fertility. In contrast, bryophyte biomass was positively associated with conifer abundance, particularly in wet sites of the Quebec study area. Non?linear responses resulted in total understorey biomass being lower under mixed canopies than under pure aspen or pure conifer canopies. Turnover rates did not differ with overstorey composition.

Conclusions: While resource availability is a main driver of understorey productivity, resources as drivers appear to differ with differences in understorey strata components, i.e. vascular versus non?vascular plants. Resource heterogeneity induced by a mixed canopy had overall negative effects on understorey above?ground productivity, as this productivity seemed to rely on species adapted to the specific conditions induced by a pure canopy.

Processes governing tree interspecific interactions, such as facilitation and competition, may vary in strength over time. This study tried to unveil them by performing dendrometrical analyses on black spruce Picea mariana, trembling aspen Populus tremuloides and jack pine Pinus banksiana trees from pure and mixed mature boreal forest stands in the Clay Belt of northwestern Quebec and on the tills of northwestern Ontario. We cored 1430 trees and cut 120 for stem analysis across all stand composition types, tree species and study regions. Aspen annual growth rate was initially higher when mixed with conifers, but then progressively decreased over time compared to pure aspen stands, while jack pine growth rate did not differ with black spruce presence throughout all stages of stand development. When mixed with aspen, black spruce showed a contrary response to aspen, i.e. an initial loss in growth but a positive gain later. On the richer clay soil of the Quebec Clay Belt region, however, both aspen and spruce responses in mixed stands reversed between 37 and 54 years. Overall, our results demonstrate that interspecific interactions were present and tended to change with stand development and among species. Our results also suggest that the nature of interspecific interactions may differ with soil nutrient availability. © 2011 The Authors.

Bien qu'il soit reconnu que la forêt boréale canadienne contribue de façon non négligeable au stockage du carbone, le fonctionnement des peuplements mixtes demeure mal compris. Ces derniers sont pourtant fréquents dans les forêts naturelles et il existe plusieurs hypothèses qui y prévoient un accroissement du stockage de carbone par rapport aux peuplements monospécifiques : en plus de possibles phénomènes de facilitation, le recoupement des niches écologiques peut se révéler moindre entre individus d'espèces différentes, induisant ainsi une baisse de la compétition et une utilisation plus vaste des ressources du milieu, et donc une augmentation de la productivité. La végétation du sous-bois n'est pas en reste, pouvant être largement affectée – positivement ou non – par la diversité de microhabitats créée par une canopée mixte. Les résultats expérimentaux sur ces sujets sont toutefois peu nombreux et contradictoires, et aucune étude ne semble s'être intéressée spécifiquement au rôle des peuplements mixtes dans le stockage du carbone. C'est pourquoi cette thèse vise à déterminer l'effet du mélange des essences sur la productivité aérienne, mais n'est qu'une partie d'un projet plus vaste qui englobe des études concernant la productivité racinaire et la respiration hétérotrophe du sol, l'objectif ultime étant de modéliser le bilan des flux d'entrée et de sortie du carbone en fonction de la composition spécifique des peuplements forestiers étudiés.

Des peuplements purs et mixtes d'environ 90 ans, issus de feu et peu perturbés, ont été sélectionnés dans deux zones géographiques distinctes : sur la ceinture d'argile abitibienne, au nord-ouest du Québec, et sur des tills au nord-ouest de l'Ontario. Les mélanges étudiés au Québec réunissaient le peuplier faux-tremble et l'épinette noire, et ceux en Ontario comportaient du tremble, de l'épinette noire et du pin gris.

La mesure des stocks de carbone dans les tiges vivantes à 90 ans ont révélé des relations plutôt neutres entre les espèces à l'échelle de la tige individuelle. À l'échelle du peuplement, les stocks de carbone dans les peuplements mixtes ne sont généralement pas différents de ce qui peut être prédit à partir des peuplements purs associés, et les quelques différences observées peuvent être expliquée par l'effet de la densité. Les analyses dendrochronologiques ont toutefois révélé que les relations entre les espèces ne sont pas neutres mais que les effets positifs et négatifs évoluent et s'annulent au fil du temps. La végétation de sous-bois quant à elle n'est guère favorisée par les canopées mixtes. Les plantes vasculaires bénéficient des surplus de ressources apportés par le tremble, mais leur productivité chute drastiquement dès lors que la proportion de conifère augmente. Inversement, les bryophytes qui envahissent les sous-bois des pessières ne tolèrent pas que la proportion de tremble dans la canopée augmente un tant soit peu. Ainsi, la biomasse et la croissance du sous-bois des peuplements mixtes est inférieure à celle de chacun des peuplements purs correspondants.

Une fois tout cela mis ensemble, il apparaît que comparé aux arbres, le sous-bois contribue très peu aux stocks de carbone 90 ans après feu. Au niveau des intrants de carbone en revanche, le sous-bois contribue largement plus, particulièrement les bryophytes dans les pessières en Abitibi et les arbustes dans les tremblaies en Ontario. Par conséquent, l'effet négatif des peuplements mixtes sur la productivité du sous-bois fait en sorte que la productivité aérienne dans les mixtes est plus proche de celle du peuplement pur le moins productif que de celle du peuplement pur le plus productif. La quantité de carbone accumulée après 90 ans dans la biomasse aérienne des peuplements mixtes étudiés est, en revanche, plus proche de la moyenne entre celles des peuplements purs associés.

Abstract: This study investigates the potential of mixed forest stands as better aboveground carbon sinks than pure stands. According to the facilitation and niche complementarity hypotheses, we predict higher carbon sequestration in mature boreal mixedwoods. Aboveground carbon contents of black spruce (Picea mariana (Mill.) Britton, Sterns, Poggenb.) and trembling aspen (Populus tremuloides Michx.) mixtures were investigated in the eastern boreal forest, whereas jack pine (Pinus banksiana Lamb.) and trembling aspen were used in the central boreal forest. No carbon gain was found in species mixtures; nearly pure trembling aspen stands contained the greatest amount of aboveground carbon, black spruce stands had the least, and mixtures were intermediate with amounts that could generally be predicted by linear interpolation with stem proportions. These results suggest that for aspen, the potentially detrimental effect of spruce on soils observed in other studies may be offset by greater light availability in mixtures. On the other hand, for black spruce, the potentially beneficial effects of aspen on soils could be offset by greater competition by aspen for nutrients and light. The mixture of jack pine and trembling aspen did not benefit any of these species while inducing a loss in trembling aspen carbon at the stand level.

Résumé : Cette étude vise à déterminer si les peuplements forestiers mixtes peuvent être des puits de carbone aériens plus efficaces que les peuplements purs. En accord avec les hypothèses de facilitation et de séparation des niches écologiques, nous prédisons une plus grande séquestration du carbone dans les peuplements mixtes. Nous avons donc déterminé les stocks de carbone aériens dans des mélanges d’épinettes noires (Picea mariana (Mill.) Britton, Sterns, Poggenb.) et de peupliers faux-tremble (Populus tremuloides Michx.) situés dans la forêt boréale de l’est, tandis que des mélanges de pin gris (Pinus banksiana Lamb.) et de peupliers faux-tremble furent plutôt utilisés dans la forêt boréale centrale. Les mélanges d’espèces ne présentaient aucun gain en carbone. Les peuplements dominés par le tremble contenaient la plus grande quantité de carbone aérien, les pessières la plus petite quantité, et les mélanges en contenaient des quantités intermédiaires qui pouvaient généralement être prédites par interpolation linéaire en fonction de la proportion de tiges de chaque espèce. Ces résultats suggèrent que dans le cas du tremble, l’effet potentiellement néfaste de l’épinette sur les sols peut être compensé par une meilleure disponibilité de la lumière dans les peuplements mixtes. Parallèlement, dans le cas de l’épinette noire, l’effet potentiellement bénéfique du peuplier faux-tremble sur les sols pourrait être contrebalancé par une compétition accrue pour la lumière et les nutriments en sa présence. Le mélange de pin gris et de peuplier faux-tremble n’a apporté aucun bénéfice pour aucune des deux espèces tout en induisant une perte en carbone chez le tremble à l’échelle du peuplement.

European gorse (Ulex europaeus L.) N2 fixation rate (%Ndfa) was studied in a maritime pine (Pinus pinaster Aït.) oligotrophic forest. Fertilization field trials were carried out on 5 sites with various inputs of phosphorus (0–240 kg P2O5·ha?1). Seven to ten years after pine planting, gorse were sampled to evaluate the effect of P fertilization on gorse %Ndfa, determined using the 15N natural abundance method. One of the prerequisites of this method is the existence of a significant difference between the 15N/14N ratios in the atmospheric N reference and in the stand soil N references. This prerequisite was satisfied for 80 of 120 cases. The average %Ndfa was high (70 ± 3%) but with high local variability. No significant difference in %Ndfa was detected among P treatments. Nitrogen concentration of gorse was significantly higher in the highest dose treatments compared to the control.