Fertilization of hybrid poplar (HP) plantations with papermill by-products is a promising solution to improve soil fertility and nutrient availability, increase plantation productivity, and provide added value to these materials that would otherwise be incinerated or sent to the landfill. We assessed the growth and foliar nutrition of a HP clone (Populus × canadensis × Populus maximowiczii) at six plantation sites aged 3–5 years in southern Quebec, Canada. Sites received a fertilization treatment consisting of a mixture of papermill biosolids (120 to 140 t·ha-1, depending on site) and lime mud (10 to 15 t·ha-1) before being planted, or no fertilization (control). Tree growth was significantly improved by fertilization, with fertilized trees showing a mean annual height increment of 1.3 m (all-site mean; SD = 0.2), compared with 0.5 m (SD = 0.4) for unfertilized trees. Foliar calcium and magnesium increased following fertilization and levels met optimal thresholds at all sites, whereas nitrogen, phosphorous, and potassium concentrations also increased, but nutritional deficiencies remained for these elements at several sites. Our results confirm the benefits of fertilizing hybrid poplars with papermill by-products, but they also indicate that adjustments in application rates or type of by-products could be made to fully satisfy nutritional requirements and thus optimize tree growth. © 2021 Authors Palma Ponce, Miquel, Lafleur, Brais, and Bélanger, and The Crown.

Monitoring of forest response to gradual environmental changes or abrupt disturbances provides insights into how forested ecosystems operate and allows for quantification of forest health. In this chapter, we provide an overview of Smartforests Canada, a national-scale research network consisting of regional investigators who support a wealth of existing and new monitoring sites. The objectives of Smartforests are threefold: (1) establish and coordinate a network of high-precision monitoring plots across a 4400 km gradient of environmental and forest conditions, (2) synthesize the collected multivariate observations to examine the effects of global changes on complex above- and belowground forest dynamics and resilience, and (3) analyze the collected data to guide the development of the next-generation forest growth models and inform policy-makers on best forest management and adaptation strategies. We present the methodological framework implemented in Smartforests to fulfill the aforementioned objectives. We then use an example from a temperate hardwood Smartforests site in Quebec to illustrate our approach for climate-smart forestry. We conclude by discussing how information from the Smartforests network can be integrated with existing data streams, from within Canada and abroad, guiding forest management and the development of climate change adaptation strategies.

{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.}

Global forest area is declining rapidly, along with degradation of the ecological condition of remaining forests. Hence it is necessary to adopt forest management approaches that can achieve a balance between (1) human management designs based on homogenization of forest structure to efficiently deliver economic values and (2) naturally emerging self-organized ecosystem dynamics that foster heterogeneity, biodiversity, resilience and adaptive capacity. Natural disturbance-based management is suggested to provide such an approach. It is grounded on the premise that disturbance is a key process maintaining diversity of ecosystem structures, species and functions, and adaptive and evolutionary potential, which functionally link to sustainability of ecosystem services supporting human well-being. We review the development, ecological and evolutionary foundations and applications of natural disturbance-based forest management. With emphasis on boreal forests, we compare this approach with two mainstream approaches to sustainable forest management, retention and continuous-cover forestry. Compared with these approaches, natural disturbance-based management provides a more comprehensive framework, which is compatible with current understanding of multiple-scale ecological processes and structures, which underlie biodiversity, resilience and adaptive potential of forest ecosystems. We conclude that natural disturbance-based management provides a comprehensive ecosystem-based framework for managing forests for human needs of commodity production and immaterial values, while maintaining forest health in the rapidly changing global environment.

A combination of wildfires and defoliating insect outbreaks play an important role in the natural successional dynamics of North American boreal mixedwood forests, which, in the long term, change the post‐disturbance composition and structure of forest stands. After stand‐replacing disturbances (mainly wildfires), early successional hardwoods typically dominate the affected areas. Provided enough time following disturbances, the increasing recruitment of mid‐ to late‐successional softwoods as well as the mortality of hardwoods gradually change forest composition from hardwoods to admixtures of hardwood‐conifer species and conifer‐dominated stands in mid and late successional stages, respectively. Such mixedwoods are abundant across the southern Canadian boreal forest. In boreal Canada, mixedwoods are the most structurally heterogeneous forest ecosystems, are highly productive, and form an important source of timber supply. Here we present the EASTERN BOREAL MIXEDWOODS CANADA data set, which documents the changes in composition and structure of stands originating from eight different wildfires representing a chronosequence of 249 yr since fire in eastern Canada. This data set has been used in several different projects to study and model the influence of natural (e.g., insect outbreaks) and anthropogenic disturbances (e.g., harvesting) on the dynamics of post‐fire stands. The data set covers a high range of variability in stand composition and structure, explained by species establishment, dominance, and mixture. It thus constitutes a useful source of information to trace the dynamics of the main boreal tree species of eastern North America, from their establishment to their replacement at different spatial scales (e.g., from stand to landscape level). Please cite this data paper when the data are used in publications. We also request that researchers and teachers inform us of how they are using the data. We are open to collaborate in developing or co‐authoring relevant research projects based on this data set.

Over the past two decades, partial harvesting has been increasingly used in boreal forests as an alternative to clearcutting to promote irregular stand structures and maintain a balance between biodiversity preservation and continued timber production. However, relatively little is still known about the silvicultural potential of partial harvesting in Canada’s boreal forest, especially in areas prone to organic matter accumulation (paludification), and most prior research has focused on biodiversity responses. In this study, we assess the effects of partial harvesting on stand development (recruitment, growth, and mortality) ten years after harvesting in previously unmanaged black spruce stands and quantify its effectiveness in reducing the impacts on ecosystem structures. Our analyses revealed that pre-harvest stand structure and site characteristics, especially initial basal area, sapling density, tree diameter, and organic layer thickness (OLT) were major factors involved in stand development ten years following these partial harvesting treatments. Depending on pre-harvest structure and site characteristics, partial harvesting can result in either an increase in post-harvest tree recruitment and growth or a loss of stand volume because of standing tree mortality. To increase the chances of partial harvesting success in ensuring an increase in decennial stand yield after harvest in black spruce forest stands, sites prone to paludification (i.e., where OLT >17 cm) should be left unharvested. This study illustrates the importance of taking into account pre-existing structure and site characteristics in the selection of management strategies to maximize the potential of partial harvesting to achieve sustainable forest management in black spruce stands.

Forest management aims to better understand effects of natural disturbance regimes on forest dynamics and use this knowledge to formulate guidelines in forest planning, thereby narrowing gaps between managed and unmanaged forest landscapes. Using forest simulators to reconstruct forest dynamics in relation to ecosystem processes, including disturbances, could help forest managers to better understand harvesting effects on forest dynamics. Using SORTIE-ND, a spatially explicit forest simulator, we generated stand dynamics for 100 years following simulated clear-cut and partial harvests (dispersed vs aggregated, with 30% and 60% basal area removal). Based on the hardwood: conifer basal area ratio, we grouped post-fire stands into three stand types corresponding to natural post-fire succession (deciduous, mixed deciduous, and mixed coniferous) and assessed long-term effects of clear-cutting and partial harvesting on each. Our results suggest that spatial configurations of harvested and residual trees had a greater effect on stand dynamics than did tree removal intensity. Following dispersed partial harvesting, both deciduous and mixed deciduous stands had species composition and structure similar to unharvested stands of the next successional stage. In these same stand types, aggregated harvests and clear-cutting favoured increased regeneration and basal area increments of aspen, which set succession back to aspen dominance, as has been observed after wildfire. Dispersed partial harvests (both 30% and 60%) and 30% aggregated cuts, in mixed coniferous stands, maintained recruitment and dominance of conifers to levels comparable with unmanaged stands. Clear-cutting in all stand types greatly modified stand compositional and structural attributes, and, when conducted in stands where aspen was abundant, performed as a stand-replacing disturbance, setting succession back to early developmental stages, i.e., to aspen dominance. We conclude that partial harvesting, which emulates gap dynamics similar to undisturbed stands, can maintain natural stand dynamics.

Over the last three decades, the ecological basis for the generalized use of even-aged silviculture in boreal forests has been increasingly challenged. In boreal mixed-wood landscapes, the diminishing proportion of conifers, to the benefit of intolerant hardwoods, has been a primary concern, coupled with the general rarefication of old-growth conifer-dominated stands. In this context, partial cutting, extended rotations and forest renewal techniques that eliminate or reduce regenerating hardwoods have been proposed as means of regaining greater conifer cover. As a result, experimentation and industrial application of various forms of both variable retention and partial harvesting are occurring across the commercial Canadian boreal forest. In this study, we compared the effects of two harvesting intensities, clearcutting and low-intensity partial cutting (removal of 25–31% of tree basal area), on hardwood and conifer regeneration levels 7–19 years following treatments in aspen (Populus tremuloides)-dominated stands and verified whether regeneration differences existed between micro-sites on and off machinery trails. The abundance of aspen regeneration increased with percent basal area removal and was positively correlated to the abundance of mature aspen prior to harvesting. The abundance of fir (Abies balsamea) regeneration after partial cutting was similar to controls and higher than after clear-cutting and was positively correlated with ground cover of mixed litter (i.e., mixture of needles and leaves) and negatively correlated with ground cover of broadleaf litter. These results suggest that it is possible in boreal mixed-woods to control aspen abundance and promote or maintain conifer regeneration through silvicultural treatments that limit canopy opening and promote mixed forest floor litter.

Natural disturbances, such as fire and insect outbreaks, play important roles in natural forest dynamics, which are characterized over long time scales by changes in stand composition and structure. Individual-based forest simulators could help explain and predict the response of forest ecosystems to different disturbances, silvicultural treatments, or environmental stressors. This study evaluated the ability of the SORTIE-ND simulator to reproduce post-disturbance dynamics of the boreal mixedwoods of eastern Canada. In 1991 and 2009, we sampled all trees (including seedlings and saplings) in 431 (256 m2) plots located in the Lake Duparquet Research and Teaching Forest (western Quebec). These plots were distributed in stands originating from seven wildfires that occurred between 1760 and 1944, and which represented a chronosequence of post-disturbance stand development. We used the 1991 inventory data to parameterize the model, and simulated short- to long-term natural dynamics of post-fire stands in both the absence and presence of a spruce budworm outbreak. We compared short-term simulated stand composition and structure with those observed in 2009 using a chronosequence approach. The model successfully generated the composition and structure of empirical observations. In long-term simulations, species dominance of old-growth forests was not accurately estimated, due to possible differences in stand compositions following wildfires and to differences in stand disturbance histories. Mid- to long-term simulations showed that the secondary disturbance incurred by spruce budworm did not cause substantial changes in early successional stages while setting back the successional dynamics of middle-aged stands and accelerating the dominance of white cedar in late-successional post-fire stands. We conclude that constructing a model with appropriate information regarding stand composition and disturbance history considerably increases the strength and accuracy of the model to reproduce the natural dynamics of post-disturbance boreal mixedwoods.

Changes in the light availability in forests generated by diversified retention patterns (e.g., clear cut, partial harvest) have been shown to strongly filter the plant species present. Modified soil microsite conditions due to post-harvest site preparation (e.g., mechanical site preparation, prescribed fire) might also be an important determinant of plant diversity. The objective here was to detect how retention pattern and post-harvest site preparation act as filters that explain the understory functional diversity in boreal forests. We also assessed whether these effects were dependent on forest attributes (stand type, time since fire, and time since harvest). We retrieved data from seven different studies within 101 sites in boreal forests in Eastern Canada. Our data included forests harvested with two retention patterns: careful logging and clear cut, plus unharvested control forests. Three post-harvest site preparation techniques were applied: plow or disk trenching after careful logging, and prescribed fire after clear cut. We collected trait data (10 traits) representing plant morphology, regeneration strategy, or resource utilization for common species. Our results demonstrated significant variation in functional diversity after harvest. The combined effect of retention pattern and site preparation was the most important factor explaining understory diversity compared to retention pattern only and forest attributes. According to RLQ analysis, harvested forests with site preparation favored traits reflecting resistance or resilience ability after disturbance (clonal guerilla species, geophytes, and species with higher seed weight). Yet harvested forests without site preparation mainly affected understory plant species via their light requirements. Forest attributes did not play significant roles in affecting the relationship between site preparation and functional diversity or traits. Our results indicated the importance of the compounding effects of light variation and soil disturbance in filtering understory diversity and composition in boreal forests. Whether these results are also valid for other ecosystems still needs to be demonstrated.

In the managed portion of the boreal forest of eastern North America, logging has replaced fire as the most important disturbance agent. There, a large proportion of timber is harvested in forests susceptible of accumulating a thick Sphagnum layer that decreases forest productivity, a process called paludification. In such a context, understanding how disturbance type and severity of soil disturbances may affect post-disturbance microhabitat characteristics and understory community composition is critical for forest management. Different management techniques have been used such as careful logging and clearcutting, as well as winter and summer harvests, with various impacts on soils and forest regeneration. In the current study, we used 55 study sites representing a gradient of soil disturbance severity by harvesting (winter and summer) and fire (low and high severity) to compare their impacts on understory plant communities in the Clay Belt area of eastern Canada. At each site, understory community composition (vascular plants, bryophytes, and lichens) was assessed. We found a strong response of communities to overall severity as represented by disturbance type (careful logging, clearcutting, and fire), but little impact of fine scale disturbance severity (winter vs. summer, low vs. high severity disturbance) within each type of disturbance. Differences in community composition were reflected in the abundance of the various plant functional types, with invaders being more common in harvested sites, endurers being common in all disturbances except high severity fires, and avoiders being more common in older sites. Understory communities in harvested sites (<40?years old) were similar to communities typical of old sites originating from natural wildfire disturbances (75–100?years old low severity fires or 200?years old high severity fires) in terms of composition, but also Sphagnum spp. abundance. In order to maintain long-term forest productivity and manage forests in ways that more closely reproduce post-fire conditions, logging operations should aim at increasing soil disturbances, for example by using prescribed burns, in the Clay Belt area of eastern North America.

In temperate and boreal mixedwood forests of eastern North America, partial disturbances such as insect outbreaks and gap dynamics result in the development of irregular forest structures. From a forest ecosystem management perspective, management of these forests should therefore include silvicultural regimes that incorporate medium- to high-retention harvesting. We present 12-year results of a field experiment undertaken to evaluate the effects of variable retention harvesting on stand structure, recruitment, and mortality. Treatments were gap harvesting (GAP), diameter-limit harvesting (DL), careful logging (CL), and careful logging followed by scarification (CL + SCAR), and an unharvested control. Although post-harvest basal area in the GAP treatment was significantly lower than that of controls, it maintained a diameter distribution profile and densities of balsam fir regeneration similar to those of pre-harvest conditions. Lower retention treatments (DL, CL, and CL + SCAR) tended to favor regeneration of pioneer, shade-intolerant species. Except for black spruce (for which mortality was highest in DL), stem mortality was similar among harvesting treatments. From an ecosystem management perspective, this study suggests that gap harvesting can maintain, in the short term, forest stand composition and structure similar to unharvested forests, and could be used where management objectives include the maintenance of late successional forest conditions.

A plant’s genotype, their environment, and the interaction between them influence its growth and development. In this study, we investigated the effect of these factors on the growth and biomass yield of willows in short-rotation coppice (SRC) under different harvesting cycles (i.e., two- vs. three-year rotations) in Quebec (Canada). Five of the commercial willow cultivars most common in Quebec, (i.e., Salix × dasyclados Wimm. ‘SV1’, Salix viminalis L. ‘5027’, Salix miyabeana Seeman ‘SX61’, ‘SX64’ and ‘SX67’) were grown in five sites with different pedoclimatic conditions. Yield not only varied significantly according to site and cultivar, but a significant interaction between rotation and site was also detected. Cultivar ‘5027’ showed significantly lower annual biomass yield in both two-year (average 10.8 t ha?1 year?1) and three-year rotation (average 11.2 t ha?1 year?1) compared to other cultivars (15.2 t ha?1 year?1 and 14.6 t ha?1 year?1 in two- and three-year rotation, respectively). Biomass yield also varied significantly with rotation cycle, but the extent of the response depended upon the site. While in some sites the average productivity of all cultivars remained fairly constant under different rotations (i.e., 17.4 vs. 16 t ha?1 year?1 in two- and three-year rotation, respectively), in other cases, biomass yield was higher in the two- than in the three-year rotation or vice versa. Evidence suggests that soil physico-chemical properties are better predictors of willow SRC plantation performance than climate variables.

Canada’s boreal forest represents an important contributor of the world’s wood supply industry. However, maintaining or increasing productivity of the boreal forest may be challenging in areas dominated by forested peatlands. Moreover, sustainable management of these forests must also consider other important aspects of the forest ecosystem such as biodiversity and carbon sequestration. To address these concerns, ecosystem-based management has been implemented in some Canadian jurisdictions, such as in regions where a large portion of the boreal forest is dominated by forested peatlands. The objectives of this paper are (1) to summarize our current understanding of how natural disturbances influence stand dynamics and biodiversity in forested peatlands, and (2) to review the main differences between natural and managed forest stands with respect to soil properties, stand productivity, understory plant communities. We also discuss how even-age management regime succeeds or fails to preserve old forests and how this loss affects both forest structure and habitat diversity at the landscape level. We conclude by showing how, in boreal forested peatlands, forest management could contribute to carbon sequestration and mitigate projected climate change.

The establishment of short-rotation willow coppice plantations for bioenergy production is proposed as a land management practice to reduce the use of fossil fuels. However, there is little information on the performance of different willow (Salix spp.) cultivars on various types of soils and over climate gradients. This study aimed to determine which set of soil, climatic conditions, and cultivars are conducive to greater growth in eastern Canada. The performance of five willow cultivars was followed over three growing seasons on eight sites representing a large hydroclimatic gradient. Both geographic location and cultivar had a significant effect on annual yields. Annual yields were on average greater in the southern part of the climatic gradient. Across all cultivars, annual yields were positively correlated to average annual temperature (r = 0.23), total annual precipitation (r = 0.77), average growing season temperature (r = 0.21), average growing season precipitation (r = 0.47), and degree days (r = 0.18), as well as soil pH (r = 0.37) and soil extractible P (r = 0.38), and negatively correlated to soil clay content (r = 0.33). Cultivars of the SX group (i.e., Salix miyabeana SX61, SX64, and SX67) showed greater yield than did cultivars of the SV group (i.e., Salix × dasyclados SV1 and Salix viminalis SV5027). These results indicate that at the landscape level, climate variables, especially climate-related variables, largely explain the yield of the selected willow cultivars. Nonetheless, soil pH, extractable P, and clay content likely play an important role in plantation yield.

Short rotation coppice willow (Salix sp.) is well established as an effective phytoremediation crop and is also emerging as an attractive lignocellulosic bioenergy option. The prospect of integrating value-added renewable chemicals as a supplementary component of the crop’s value is explored here in terms of absolute phenolic yields, extractable from field cultivated mature biomass. Five willow cultivars, selected as leading biomass yielding cultivars, were cultivated at four field sites in Quebec, Canada: Salix x dasyclados ‘SV1’, Salix viminalis ‘SV5027’, Salix miyabeana ‘SX61’, ‘SX64’, ‘SX67’. Substantial and significant cultivar variation was observed in groups of compounds including hydroxycinnamic acids and derivatives, benzoic acids derivatives, flavonols and condensed tannins in concentration per ton of biomass and/or yield per hectare per year. The highest phenolic yields were produced by Salix miyabeana ‘SX67’ cultivated at St-Roch at 5.43 (±0.60) kg ha?1 yr?1, which also produced 35.54 (±4.10) kg ha?1 yr?1 condensed tannins. These phenolic yields suggest further exploration of renewable chemicals production as a supplement to other biomass end-uses could be worthwhile. Such flexibility would help provide important advantages for weathering renewables policy and market uncertainty as well as improve the feasibility and competitiveness of sustainable biomass production as part of an integrated green technology platform.

Strategies for increasing the mobilization of forest biomass supply chains for bioenergy production require continuous assessments of the spatial and temporal availability of biomass feedstock. Using remote sensing products at a 250-m pixel resolution, estimates of theoretical biomass availability from harvest residues and fire-killed trees were computed by combining Canada-wide maps of forest attributes (2001) and of yearly (2002–2011) fires and harvests. At the national scale, biomass availability was estimated at 47 ± 18 M ODT year?1 from fire-killed trees and at 14 ± 2 M ODT year?1 from harvest residues. Mean biomass densities in burned and harvested pixels were estimated at 34 ± 3.0 ODT ha?1 and at 24 ± 1.2 ODT ha?1, respectively. Mean biomass densities also varied dramatically among ecozones, from 14 ODT ha?1 to 206 ODT ha?1 and from 6 ODT ha?1 to 63 ODT ha?1 for burned and harvested pixels, respectively. Spatial averaging with a 100-km radius window shows distinct hotspots of biomass availability across Canada. The largest hotspots from fire-killed trees reached 3.6 M ODT year?1 in the Boreal Shield and the Boreal Plains ecozones of northern Alberta and Saskatchewan, where fires are large and frequent. The largest hotspots from harvest residues reached 1.2 M ODT year?1 in the Montane Cordillera ecozone of British Columbia. The use of spatially explicit remote sensing products yields estimates of theoretical biomass availability that are methodologically consistent across Canada. Future development should include validations with on-the-ground forest inventories as well as the factoring in of environmental, technical and economic considerations to implement operational biomass supply chains.

In the Lower Athabasca region of Alberta (Canada), surface mining for bitumen from oil sands creates highly disturbed environments, which need to be restored, after mine closing, to equivalent land capability in terms of biodiversity and ecosystem services. We demonstrate a method to characterize ecosystem diversity and conditions using biophysical indicators of the Lower Athabasca meant for informing land reclamation planning and monitoring by identifying and creating a typology of the main assemblages of topography, soil and forest vegetation at the watershed, landform and ecosite scales, and analysing the relationships among land units of various scales. Our results showed that watersheds could be classified into distinct groups with specific features, even for a region with a generally flat or gently rolling topography, with slope, surficial deposits and aspect as key drivers of differences. Despite the subtle topography, the moisture regime, which is linked to large-scale cycles that are dependent on the surrounding matrix, was of primary importance for driving vegetation assemblages. There was no unique and homogeneous association between topography and vegetation; the specific landforms each displayed a range of ecosites, and the same ecosites were found in different landforms. This suggests that landscapes cannot be defined in a qualitative manner but rather with quantitative indicators that express the proportion occupied by each class of ecological units within the coarser units, therefore requiring during land reclamation that sufficient care is given to create heterogeneity within a given landform in terms of soil texture and drainage so that a mosaic of ecosite conditions is created.

The occurrence of pesticides in groundwater represents an important health issue, notably for population whose drinking water supply source is located in agricultural areas. However, few solutions have been considered with regard to this issue. We tested the efficacy of a vegetal filtering system made of shrub willows planted at a high density (16,000 plants ha(-1)) to filter or degrade pesticides found in the groundwater flowing out of an apple orchard. Ethylene urea (EU), ethylene thiourea (ETU), tetrahydrophthalimide (THPI), atrazine, and desethylatrazine were monitored in the soil solution in willow and control plots over one growing season. ETU and atrazine concentrations were lower in the willow plots relative to the control plots, whereas desethylatrazine concentration was higher in the willow plots. No significant difference was detected for EU and THPI. Furthermore, pesticide concentrations displayed complex temporal patterns. These results suggest that willow filter systems can filter or degrade pesticides, notably ETU and atrazine, and could be used for phytoremediation purposes. Yet, this potential remains to be quantified with further studies using experimental settings allowing more estimation in time and space.

Aboveground biomass yields of short rotation cultures (SRC) of willow can vary substantially depending on site quality. Among others, aboveground biomass yields depend on climatic conditions, soil properties, age of the SRC, and number of harvesting cycles. In this study, we investigated the effects of coppicing on growth variables (i.e., largest basal stem, height, and aboveground biomass) at ten SRC of Salix miyabeana SX67 established on various soils in southern Quebec. More than 1100 shrubs with stool ages varying between 1 and 15 years were measured. Strain analysis was carried out to calculate past annual aboveground productivities, and maximum annual yield potential was quantified at each site. Annual growth rates were highly variable and depended on site and coppicing history. To achieve optimal stool development and aboveground yields, two to three growing seasons following coppicing were necessary for sandy and clayey sites, respectively. The delays for reaching maximum yields were shortened when soil cation exchange capacity was dramatically low and were prolonged when soil was physically restricting stool development. This lag influenced the total yield of the first rotation and also modulated the magnitude of the increase of aboveground biomass that is generally observed in the second rotation. To increase yields in southern Quebec, our results suggest that it is preferable to extend the length of the first rotation instead of coppicing at the end of the first growing season after establishment.

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.

Fire is the major disturbance agent in boreal forests that initiates forest regeneration and succession, and plays a major role in determining the composition of terricolous lichen communities. In recent decades however, logging has become an important disturbance agent in boreal forests. In this context, growing concerns about logging effects on endangered species such as woodland caribou (Rangifer tarandus caribou) has emerged and drawn attention on how reindeer lichens (i.e. Cladonia arbuscula, Cladonia mitis, Cladonia rangiferina, Cladonia stellaris, and Cladonia stygia) respond to logging and fire given their importance as winter forage for caribou. We compared critical lichen habitat factors (i.e., forest floor thickness, tree height, and canopy closure) between fire and logged sites and evaluated how Cladonia lichen species richness, biomass, and ground cover were related to these habitat factors in the first decades following disturbance. We found no significant differences in habitat factors and no significant differences in lichen species richness or ground cover between logged and fire sites. However, Cladonia lichen biomass was significantly higher following logging. These results support the hypothesis that Cladonia lichens can persist following logging and suggest that forestry practices that include in their toolbox winter harvesting or machinery traffic restricted to specific trails are likely to preserve undisturbed forest floor habitat conditions with source populations of lichens.

Paludification, the accumulation over the mineral soil of poorly decomposed organic matter mainly originating from Sphagnum spp., transforms black spruce (Picea mariana) boreal forests into forested peatlands in the prolonged absence of fire, which diminishes forest productivity. High-severity wildfires reset this process by burning the soil organic layer (SOL) and reinitiating forest succession. In contrast, low severity wildfires impact mainly the soil surface and tree layer and do not significantly reduce SOL depth. In the Clay Belt region of eastern Canada, an area prone to paludification, the current forest harvest practice (careful logging around advanced growth [CLAAG]) removes trees but has little impact on the SOL and the understorey vegetation. This is thought to further promote paludification, which consequently reduces forest productivity. Conversely, clearcut (CC) disturbs the SOL and the understorey vegetation, and is thought to favor tree growth. Furthermore, prescribed burning after clearcut (CCPB) is used as a site preparation technique, but may also be used to control paludification as it can burn part of the organic soil layer. Using a retrospective approach, our study examines three hypotheses: compared to CLAAG, CC and CCPB: (1) have positive effects on soil conditions (e.g. decomposition level and pH), (2) reduce Sphagnum spp. and ericaceous shrub cover and (3) result in enhanced black spruce growth. We sampled 22 sites in which we measured SOL characteristics (e.g. depth, decomposition state), understorey vegetation cover and black spruce growth. Compared to CLAAG, CCPB resulted in increased soil decomposition level and higher pH. CCPB also reduced Sphagnum spp. cover but not ericaceous shrub cover. Black spruce growth rate was higher following CCPB than CC, and mean dominant tree height was marginally higher following CCPB than CLAAG and CC. Our results demonstrate that CCPB is beneficial to black spruce growth, presumably through its effects on forest understorey and SOL chemistry. While not similar to a high severity fire, prescribed burning after clearcut in paludified stands on the Clay Belt emulates some wildfire effects such as increasing soil pH. We suggest that unlike CLAAG, prescribed burning after clearcut can restore black spruce stand productivity and should be considered in the context of forest ecosystem management.

Forest inventory mapping is used worldwide to describe forests at a large spatial scale via the delimitation of portions of the landscape that are structurally homogeneous. Consequently, there is a significant amount of descriptive forest data in forest inventory maps, particularly with the development of ecosystem classification, which represents a significant potential for use in ecosystem based management. With this study we propose to test whether forest inventory maps can be used to describe not only stand characteristics but also dynamic processes. The results indicate that stand types identifiable in forest inventory maps do not in fact represent unique developmental stages, but rather confound stands at multiple developmental stages that may be undergoing different ecological processes. The reasons for this are linked to both the interaction between succession, fire severity and paludification. Finally, some aspects of the process of forest inventory mapping itself contribute to the disjunction between forest types and forest succession. Given the low similarity between spruce mapping types and their actual description following forest inventories, it would be too ambitious to infer the dynamic aspects of spruce forest by map units.

In the North American boreal forest, the presence of aspen (Populus tremuloides Michx.) is thought to be constrained on sites where thick (>25 cm) soil organic layers (SOL) prevail. Aspen can reproduce both by seeds and suckers, but it is still unknown how SOL thickness influences both modes of reproduction. In this study, we sought to determine how SOL thickness and chemistry in black spruce dominated stands influences aspen regeneration and growth. Aspen abundance was negatively related to SOL thickness and logistic regression indicated that the probability to detect an aspen declined from 30% at SOL = 0 cm to 10% at 30 cm. Our results also indicated that aspen diameter at breast height was significantly negatively correlated with SOL thickness and black spruce abundance, and positively correlated with soil Ntot, Ca, CEC and pH. Finally, we failed to detect any significant effect of SOL on aspen mode of regeneration (i.e. seeds or suckers). Our study shows that through changes in physical and chemical soil properties, SOL accumulation equally hinders both aspen seedling germination and growth, and sucker development.

Background Successional paludification, a dynamic process that leads to the formation of peatlands, is influenced by climatic factors and site features such as surficial deposits and soil texture. In boreal regions, projected climate change and corresponding modifications in natural fire regimes are expected to influence the paludification process and forest development. The objective of this study was to forecast the development of boreal paludified forests in northeastern North America in relation to climate change and modifications in the natural fire regime for the period 2011?2100.MethodsA paludification index was built using static (e.g. surficial deposits and soil texture) and dynamic (e.g. moisture regime and soil organic layer thickness) stand scale factors available from forest maps. The index considered the effects of three temperature increase scenarios (i.e. +1?C, +3?C and +6?C) and progressively decreasing fire cycle (from 300?years for 2011?2041, to 200?years for 2071?2100) on peat accumulation rate and soil organic layer (SOL) thickness at the stand level, and paludification at the landscape level.ResultsOur index show that in the context where in the absence of fire the landscape continues to paludify, the negative effect of climate change on peat accumulation resulted in little modification to SOL thickness at the stand level, and no change in the paludification level of the study area between 2011 and 2100. However, including decreasing fire cycle to the index resulted in declines in paludified area. Overall, the index predicts a slight to moderate decrease in the area covered by paludified forests in 2100, with slower rates of paludification.ConclusionsSlower paludification rates imply greater forest productivity and a greater potential for forest harvest, but also a gradual loss of open paludified stands, which could impact the carbon balance in paludified landscapes. Nonetheless, as the thick Sphagnum layer typical of paludified forests may protect soil organic layer from drought and deep burns, a significant proportion of the territory has high potential to remain a carbon sink.

L’objectif de cette étude était de modéliser le développement des tourbières forestières du nord-est de l’Ontario en fonction des changements de températures, de précipitations et de régime de feux pour la période 2011-2100, et explorer les effets de la paludification ou de l’accumulation de tourbe sur des sols minéraux secs. Les projections du modèle indiquent une diminution légère ou modérée de la surface occupée par les tourbières forestière pour l’horizon 2100, ainsi qu’un taux de paludification plus bas. La diminution projetée du taux de paludification implique une productivité forestière accrue et une augmentation potentielle de la récolte. Ces résultats suggèrent aussi une perte graduelle de peuplements paludifiés ouverts, ce qui pourrait affecter le bilan de carbone régional ainsi que la disponibilité d’habitats pour les espèces associées aux habitats ouverts. Finalement, nous faisons des recommandations afin d’améliorer le modèle ainsi que la planification des aménagements forestiers.

In boreal forested peatlands, we disturbed Sphagnum spp. and Pleurozium schreberi carpets to see how disturbance influenced substrate physico-chemistry, and growth and foliar nutrition of planted Picea mariana seedlings. Carpets were hand disturbed using gardening tools to a depth of approximately 25 cm. Carpet disturbance was aimed at disrupting only the organic layer and did not result in the mixing of organic matter with mineral soil. Disturbed carpets, whether P. schreberi or Sphagnum spp., were warmer than undisturbed carpets and had a lower cover of ericaceous shrubs. Pleurozium schreberi carpets had a higher decomposition index than Sphagnum spp. carpets, whereas disturbance had no effect on this variable. Pleurozium schreberi had higher N_tot and dissolved organic N concentrations (DON) than Sphagnum spp., whereas disturbance increased NH ₄ ⁺ availability in both substrates. Moss disruption increased seedling growth rates as well as their foliar N and P concentrations in both substrates and these variables remained higher in P. schreberi than in Sphagnum spp. within a given treatment. Seedling growth was positively correlated to substrate N_tot, NH ₄ ⁺ and DON concentrations, and to foliar N and P concentrations, and negatively to substrate C/N and ericaceous shrub cover. Disruption of the moss carpets without mineral soil mixing improved black spruce seedling growth and nutrition in both moss types but the superiority of P. schreberi compared to Sphagnum spp. as a growing substrate remained present.

Context

Following forest harvest, mechanical site preparation (MSP) is commonly used to regenerate harvested sites. In boreal forested peatlands, however, the effectiveness of MSP to regenerate harvested sites is likely to be hampered by thick organic layers.

Aim

We sought to determine the capability of different MSP techniques to improve growth conditions of planted black spruce seedlings in boreal forested peatlands where closed-crown productive forests could revert to unproductive forested peatlands in the absence of severe soil disturbance.

Methods

The effects of disc scarification, mounding and patch scarification on soil chemistry and seedling growth were contrasted.

Results

Seedlings of site-prepared plots were 15% taller than those of untreated ones, irrespective of the MSP technique used, likely owing to the greater abundance of exposed mineral soil and mesic substrates created. Mounding and patch scarification were able to expose mineral soil over a greater proportion (>25% vs. <10%) of the treated area compared with disc scarification and control, whereas the combined surface area of exposed mineral soil and mesic substrates was higher in every MSP treatments relative to the control (>57% vs. 41%, respectively). Individual seedling growth was influenced by substrate type and drainage. Seedlings planted in moderately and well-drained mesic substrates and mineral soil were 25% taller than those planted in poorly drained fibric substrates.

Conclusion

All three MSP techniques were effective because they succeeded in creating high-quality microsites despite thick organic layers.

En écologie, la notion de résilience se définit comme la capacité d'un système naturel à se réorganiser suite à une perturbation. Dans le cas d'un écosystème forestier, par exemple, la résilience pourra être évoquée pour décrire sa capacité à se reconstituer à la suite d'un feu sans subir de transformations importantes de composition ou de structure. La différence entre la persistance d'un système (i.e. sa résilience) ou sa transformation (i.e. un changement d'état) dépend de l'équilibre entre des rétroactions négatives qui tendent à maintenir le système dans son état actuel et des rétroactions positives qui tendent à le faire basculer dans un nouvel état. Cette définition de la résilience suppose donc, pour un écosystème donné, la possibilité que celui-ci se trouve dans des états stables alternatifs. Du point de vue des sociétés humaines, tous les états stables alternatifs d'un écosystème ne sont pas désirables. La valeur qu'on accordera à un état stable dépendra, entre autres, de son utilité sociale ou économique, et du contexte dans lequel se fait la gestion des ressources tirées de cet écosystème. Une vaste région du nord-ouest québécois est constituée de peuplements d'épinette noire susceptibles à la paludification, i.e. à la transformation graduelle de forêt productive en tourbière forestière. Dans cette région, les feux constituent un des plus importants agents de perturbation et dynamisation des forêts et exercent un contrôle important sur le processus de paludification. Selon la sévérité de la perturbation des sols lors d'un incendie de forêt, les pessières à mousses de la ceinture d'argile sont susceptibles d'osciller entre deux états stables alternatifs, soit entre un peuplement productif et une tourbière forestière. Des études récentes menées dans la région ont par ailleurs suggéré que les pratiques sylvicoles visant à protéger les sols pourraient contribuer au déclin à long terme de la productivité forestière en favorisant ou en accélérant le processus de paludification. Dans le contexte de l'aménagement forestier, il importe donc de bien comprendre comment les interventions en forêts peuvent contribuer à maintenir les écosystèmes forestiers dans leur état actuel ou de les faire basculer dans un autre état. Dans ce contexte, il s'avère donc nécessaire d'améliorer nos connaissances des effets des pratiques sylvicoles sur la paludification et la productivité des peuplements postrécolte et d'identifier non seulement celles qui sont les plus susceptibles de maintenir ou de restaurer la productivité forestière, mais aussi de préserver la diversité biologique, structurelle et fonctionnelle du paysage forestier régional et d'assurer sa résilience. L'objectif général de cette thèse est de raffiner nos connaissances des effets de diverses pratiques sylvicoles sur les propriétés des sols et sur la croissance de l'épinette noire dans des peuplements susceptibles à la paludification. Plus spécifiquement, nous chercherons à déterminer (i) les effets à l'échelle du paysage de deux méthodes de récolte (coupe avec protection de la régénération et des sols [CPRS] et coupe totale) et de cinq types de dépôts-drainage sur la régénération et la croissance de peuplements d'épinette noire, (ii) comment, 10 à 30 ans après la récolte, la perturbation des sols créée par diverses méthodes et saisons de récolte influence la productivité forestière, (iii) comment la perturbation de l'humus forestier influence la température, l'humidité, la décomposition et la disponibilité des éléments nutritifs dans des tapis de Pleurozium schreberi et de Sphagnum spp. et comment ces modifications influencent la nutrition et la croissance de semis plantés d'épinette noire et (iv) si l'épaisseur de la couche organique peut influencer la capacité de la préparation de terrain à créer des microsites favorables à la croissance de semis d'épinette noire plantés. Nos résultats montrent qu'en forêt paludifiée, la perturbation des sols permet d'augmenter la croissance et la productivité forestière, du moins à court terme, et que cet accroissement est fortement influencé par les propriétés physico-chimiques des sols. Ensuite, nos résultats soutiennent l'hypothèse qu'en forêt boréale paludifiée la régénération et la croissance des arbres suivant la récolte sont influencées par la sévérité de la perturbation de la couche organique à l'échelle du peuplement, ces résultats s'appliquant à une vaste gamme de dépôts de surface. De plus, nos résultats montrent qu'une perturbation mécanique suffisamment sévère (p. ex., une coupe totale effectuée l'été) de la couche organique est capable de régénérer des peuplements dont la croissance et la productivité est similaire à celle produite par des feux qui brûlent sévèrement la couche organique. Enfin, nos résultats montrent que l'épaisseur de la couche organique avant la récolte a peu d'influence sur la capacité de la préparation mécanique de terrain à créer des microsites favorables à la croissance de semis d'épinette noire. Depuis peu, le concept d'aménagement durable des forêts se trouve au coeur de la politique forestière québécoise. Le maintien de la biodiversité et des processus écologiques fait donc désormais partie des éléments qui doivent être pris en compte lors de la préparation des plans d'aménagement forestier. Parallèlement, il s'est développé au cours des dernières années un intérêt grandissant pour le développement d'approches d'aménagement forestier basées sur la dynamique des perturbations naturelles. Afin de parvenir à créer des paysages aménagés ayant les mêmes caractéristiques que les paysages naturels, les aménagistes forestiers devront non seulement raffiner leurs connaissances de la dynamique des perturbations naturelles et de leurs effets sur les écosystèmes forestiers, mais également diversifier et adapter les traitements sylvicoles en fonctions des caractéristiques, parfois particulières, des territoires aménagés. Les résultats de cette thèse suggèrent donc que l'utilisation adéquate et simultanée de méthodes de récolte visant à protéger les sols et de la coupe totale, ainsi que celle d'autres méthodes de récolte et de traitements sylvicoles (par ex. la coupe partielle, coupe à rétention variable et la préparation de terrain), pourrait permettre d'élargir le gradient des effets des opérations forestières à l'échelle du paysage et ainsi aider à maintenir la diversité biologique, structurelle et fonctionnelle des paysages forestiers.

It has been suggested that without sufficient soil disturbance, harvest in boreal forested peatlands may accelerate paludification and reduce forest productivity. The objectives of this study were to compare the effects of harvest methods (clearcutting vs. careful logging) and season (summer vs. winter harvest) on black spruce regeneration and growth in boreal forested peatlands of eastern Canada, and to identify the soil variables that favour tree growth following harvest. Moreover, we sought to determine how stand growth following harvest compared with that observed following fire. The average tree height of summer clearcuts was greater than that of summer carefully logged stands and that of all winter harvested sites. Summer clearcutting also resulted in a higher density of trees > 3 m and > 4 m tall and in a 50% reduction in Rhododendron groenlandicum cover, a species associated with reduced black spruce growth. Height growth of sample trees was related to foliar N and P concentrations, and to soil total N, pH and available Ca and Mg but not to harvest method or season. Our results also indicate that summer clearcutting could produce stand productivity levels comparable to those observed after high-severity soil burns. These results suggest that summer clearcutting could be used to restore forest productivity following harvest in forested peatlands, and offer further support to the idea that sufficient levels of soil disturbance may be required to restore productivity in ecosystems undergoing paludification.

Abstract: Machinery traffic restrictions during forest harvest have been adopted to minimize soil damage and protect tree regeneration. However, this practice is questioned for paludifying black spruce (Picea mariana (Mill.) BSP) stands in which severe soil disturbance by wildfire restores forest productivity. The objective of this study was to determine, 8 years after harvest, how soil disturbance created by clearcutting and careful logging affected black spruce natural regeneration and growth and how this effect varied by soil type. While regeneration density was higher following careful logging, stocking was not influenced by harvest method. Regenerating stands were taller following clearcutting despite potentially greater damages to preestablished regeneration. Compared with careful logging, clearcutting also resulted in reduced cover of Sphagnum spp. and ericaceous shrubs. Spruce stem density and stocking were both higher on organic and subhydric soils and lower on mesic soils. No significant interactions were observed between harvest method and soil type, indicating that the observation of taller black spruce stands and adequate stocking with clearcutting may be applicable to all soil types considered in this study. These results suggest that an adequate level of soil disturbance is an important part of forest regeneration, particularly in ecosystems where an autogenic reduction in productivity occurs.

Résumé : Afin de préserver les sols et la régénération durant la récolte, la circulation de la machinerie forestière est fréquemment restreinte à des sentiers définis. Toutefois, cette pratique est remise en question dans les peuplements d’épinette noire (Picea mariana (Mill.) BSP) susceptibles à la paludification où de sévères perturbations des sols causées par le feu sont aptes à améliorer la productivité des peuplements. L’objectif de cette étude était de déterminer comment la perturbation des sols causée lors de la coupe totale et la coupe de protection affectent la régénération de l’épinette noire 8 ans après la récolte, et comment cet effet est conditionné par le type de sol. Alors que la densité de régénération de l’épinette noire était supérieure après la coupe de protection, le coefficient de distribution n’était pas influencé par la méthode de récolte. Les peuplements d’épinette régénérés après coupe totale étaient plus hauts en dépit d’un potentiel de dommage plus grand à la régénération préétablie. Comparée à la coupe de protection, la coupe totale a aussi résulté en une réduction de la couverture au sol de la sphaigne et des éricacées. La densité et le coefficient de distribution de l’épinette noire étaient supérieurs sur les sols organiques et subhydriques et inférieurs sur les sols mésiques. Aucune interaction significative n’a été observée entre la méthode de récolte et le type de sol, indiquant qu’une plus grande hauteur des peuplements et un coefficient de distribution d’épinette adéquat après coupe totale pourrait être applicable à toute la gamme de types de sol considérée dans cette étude. Ces résultats suggèrent qu’un niveau de perturbation des sols suffisant lors des opérations forestières est nécessaire, particulièrement dans les écosystèmes où des processus autogéniques réduisant la productivité s’opèrent.

Abstract: Plant species distribution and plant community composition vary along environmental gradients. At the continental scale, climate plays a major role in determining plant distribution, while at the local and regional scales vegetation patterns are more strongly related to edaphic and topographic factors. The projected global warming and alteration of the precipitation regime will influence tree physiology and phenology, and is likely to promote northward migration of tree species. However the influence of soil characteristics on tree species migration is not as well understood. Considering the broad tolerance of most tree species to variations in soil factors, soils should not represent a major constraint for the northward shift of tree species. However, locally or regionally, soil properties may constrain species migration. Thus, while climate change has the potential to induce a northward migration of tree species, local or regional soil properties may hinder their migratory response. These antagonistic forces are likely to slow down potential tree migration in response to climate change. Because tree species respond individualistically to climate variables and soil properties, new tree communities are likely to emerge from climate change.

Résumé : La distribution des espèces et la composition des communautés végétales varient le long de gradients environnementaux. À l’échelle continentale, le climat joue un rôle primordial dans la distribution des espèces végétales, alors qu’aux échelles locale et régionale la distribution des espèces est surtout influencée par les variables édaphiques et la topographie. Le réchauffement climatique projeté et l’altération du régime des précipitations sont susceptibles d’influencer la physiologie et la phénologie des arbres, et d’entraîner leur migration vers le nord. Toutefois, l’influence des caractéristiques des sols sur le potentiel migratoire des arbres est peu connue. Puisque la plupart des espèces d’arbres possèdent une grande tolérance aux variations édaphiques, les sols ne devraient pas constituer un obstacle majeur à leur migration vers le nord. Toutefois, localement ou régionalement, les sols pourraient posséder des propriétés qui pourraient restreindre la migration des espèces. Alors que les changements climatiques ont le potentiel de provoquer la migration des espèces vers le nord, localement ou régionalement les propriétés des sols pourraient limiter leur capacité migratoire. Ces forces en opposition pourraient restreindre le potentiel migratoire des arbres en réponse aux changements climatiques. Parce que les espèces d’arbres répondent de manière individuelle aux variables climatiques et aux propriétés des sols, de nouvelles communautés d’arbres pourraient résulter des changements climatiques.

Plant species distribution and plant community composition vary along environmental gradients. At the continental scale, climate plays a major role in determining plant distribution, while at local and regional scales vegetation patterns rely more on edaphic and topographic factors. The projected global warming and alteration of the water regime will influence plant physiology and phenology, and is likely to promote northward migration of forest tree species. However the influence of soil factors on tree species migration remains unknown. The objective of this work is to investigate the effects of soil factors and processes, as mediate by climate change, on the migration of tree species of the boreal and temperate forests of northeastern North America. In addition, because of their importance in shaping tree communities and their possible influence on tree species migration, other biotic and abiotic are considered.

The projected climate change is expected to increase soil temperature and alter water availability, which control many terrestrial biogeochemical processes such as soil respiration, litter decomposition, nutrient mineralization, fine root dynamics, and plant nutrient uptake. Because tree species respond individually to environmental factors, it can then be speculated that altered soil factors will influence different species in different ways. Ultimately, these differences could be translated in different colonization abilities and migration rates, which will likely alter current tree communities. Furthermore, other biotic and abiotic factors such as genetic adaptation, disturbance regime, habitat fragmentation, and competition are known to influence tree species establishment and tree community composition.

Considering the broad tolerance of most tree species to soil factors, soils should not represent major constraint for the northward shift of populations located at the southern limit or at the center of their distribution range. For populations located at the northern limit of their distribution range, short-distance (e.g. <10 km) migration should be possible as soil factors do not vary much over these distances, while long-distance (e.g. >100 km) migration, could be constrained by soil factors that prevent seed germination and seedling establishment. Because the presence of soil is a prerequisite for the establishment of trees, the absence of soil or its low development may be the most important factor constraining the establishment of trees in the tundra, along with climate variables and genetic adaptation. The relative importance of soil factors should thus be high in the tundra. In contrast, in the boreal and temperate forests because soils are already developed, soil relative importance for tree establishment should be lower. In these last biomes, other factors, such as competition and disturbance regime should be as important for tree species migration. The migration of tree species is thus highly probable within the area currently occupied by the boreal and temperate forest, but will be restricted to the more favorable sites in the tundra because of the absence of extended soils. However, during pedogenesis, soils may develop properties that can be favorable or detrimental for some plant species. For instance, podzols of boreal forests are generally low in nutrient content compared to hardwood forest brunisols. Thus, nutrient-demanding species typical of hardwood forests may not be able to thrive in podzols following their establishment. Moreover, some parent materials may lead to the development of soils, such as serpentine, which support unique type of vegetation. Locally or regionally, such irreversible soil development processes may hinder tree migration in response to climate change. Thus, while climate change has the potential to improve soil conditions for plant growth, this forcing will have cope with soil properties inherited from slow pedogenic processes that started at the end of the last ice age and that are still in progress today. These antagonistic forces are likely to slow down potential tree migration in response to climate change.