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Successive disturbances such as fire can affect post-disturbance regeneration density, with documented adverse effects on subsequent stand productivity. We conducted a simulation study to assess the potential of reactive (reforestation) and proactive (variable retention harvesting) post-fire regeneration failure mitigation strategies in a 1.37 Mha fire-prone boreal landscape dominated by black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.). We quantified their respective capacity to maintain landscape productivity and post-fire resilience, as well as their associated financial returns under current and projected (RCP 8.5) fire regimes. While post-fire reforestation with jack pine revealed to be the most effective strategy to maintain potential production, associated costs quickly became prohibitive when applied over extensive areas. Proactive strategies such as an extensive use of variable retention harvesting, combined with replanting of fire-adapted jack pine only in easily accessible areas, appeared as a more promising approach. Despite this, our results suggest an inevitable erosion of forest productivity due to post-fire regeneration failure events, highlighting the importance of integrating fire a priori in strategic forest management planning as well as its effects on long-term regeneration dynamics.

Clear-cutting has been the dominant harvesting method used in boreal forest silviculture. Reducing the potential negative effects of intensive forestry activities on ecosystems, e.g., the simplification and homogenization of stand structure, requires diversifying silvicultural practices to promote forest resilience in the face of climate change. Priority therefore lies in developing, evaluating, and adapting partial cutting as a potential silvicultural option for ensuring the sustainable management of boreal forests. In this chapter, we summarize the findings of two large-scale experiments conducted in Canadian boreal forests that tested new silvicultural approaches and explore their implications for forest management. We discuss the effects of these treatments on tree growth, tree mortality, regeneration, and biodiversity, and we examine the challenges of existing silvicultural approaches in the context of climate change.

In boreal forests in Canada, broadleaf stands are characterized by generally well-drained soils and a humus-rich layer. In contrast, spruce-moss stands are often characterized by more poorly drained soils and acidic humus layer. However, presence of these two forest types in various degrees of mixture in stands can be beneficial to spruce seedlings productivity. It was hypothesized that leaf litter and humus from pure spruces-moss stands, pure broadleaf stands, and mixed stand may influence Black spruce (Picea mariana (Mill.) BSP) seedling growth and development differently. A greenhouse experiment was carried out to evaluate the effect of different leaf litter and different humus on spruces seedlings. Our results suggest better development for seedlings grown in humus from mixed stands and pure broadleaf stands compared to humus from pure B. spruce or standard forest nursery substrate. Furthermore, leaf litter from broadleaf trees species, such as species Speckle alder (Alnus rugosa (Du Roi) R.T. Clausen), T. aspen (Populus tremuloides Michx), Willows (Salix spp.) and Paper birch (Betula papyrifera Marsh.), has shown distinct results in the growth and development of B. spruce seedlings in greenhouse. Furthermore, promotion of mixed stand can increase B. spruce productivity by improving the physicochemical composition of the forest floor.

In managed boreal forests, partial harvesting has been proposed to promote forest structural complexity and to therefore maintain associated biodiversity. However, there have been few studies identifying forest structures that should be maintained within the forest matrix, and fewer still on changes in these structures during succession. Consequently, there is no tool to identify these different structures in the field or their sequence along natural succession. This study proposes a key that can be used in the field and allows for the identification of different forest diameter structures along a successional sequence in the black spruce boreal forest. The specific objectives of this study were (1) to classify the types of forest structures encountered in natural black spruce boreal forest based on their diameter distribution, and (2) to link this classification to time since last fire and its spatial homogeneity at the stand level. This study shows that the forest stand structure, in black spruce forests, is varied and that this structural variety is mainly controlled by time since last fire. It also shows that the timing of stand structural maturation varies with severity of the last fire and surficial deposit. The identification key suggests that based on tree diameter distribution it is possible to discriminate among young, mature and old forest structures, which could help forest managers select stands to be harvested according to different objectives and hence maintain the variety of black spruce forest structures at the landscape scale.

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&rsquo, 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 &gt, 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.

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.

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

Understanding and measuring forest resistance and resilience have emerged as key priorities in ecology and management, particularly to maintain forest functioning. The analysis of the factors involved in a forest’s ability to cope with disturbances is key in identifying forest vulnerability to environmental change. In this study, we apply a procedure based on combining pathway analyses of forest composition and structure with quantitative indices of resistance and resilience to disturbances. We applied our approach to boreal forests affected by a major spruce budworm outbreak in the province of Quebec (Canada). We aimed to identify the main patterns of forest dynamics and the environmental factors affecting these responses. To achieve this goal, we developed quantitative metrics of resistance and resilience. We then compared forests with different pre-disturbance conditions and explored the factors influencing their recovery following disturbance. We found that post-outbreak forest dynamics are determined by distinct resistance and resilience patterns according to dominant species and stand composition and structure. Black spruce forests are highly resistant to spruce budworm outbreaks, but this resistance is conditioned by the length of the defoliation period, with long outbreaks having the potential to lead the system to collapse. In contrast, balsam fir forests easily change to a different composition after outbreaks but are highly resilient when mixed with hardwood species. Overall, the severity of the disturbance and the tree species affected are the main drivers contributing to boreal forest resistance and resilience. Our procedure is valuable to understand post-disturbance dynamics of a broad range of communities and to guide management strategies focused on enhancing the resistance and resilience of the system. This study was funded by MINECO via EST_RES Project (AGL2015-70425-R) and BIOCLIM (CGL2015-6741R) and the EC through the Marie Curie IRSES Project NEWFORESTS (PIRSES-GA- 2013-612645) and the ERA-NET Foresterra Project INFORMED (Grant Number: 29183). MINECO provided MSP with support through a predoctoral contract (BES-2013-063019) and AA through a Juan de la Cierva fellowship (FJCI-2014-20739). We thank Me´lanie Desrochers and Aure´lie Terrier for technical support and three anonymous reviewers for constructive comments on the manuscript.

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.

In managed boreal forests, variable retention harvest is considered by forest managers as a means of mitigating harvest impacts on biodiversity. Variable retention harvest consists of maintaining within a cutblock structural attributes of the original forest stand in intact forest patches that could provide quality habitat (i.e., with large trees and deadwood) for many forest species during forest regeneration. However, retention patch modalities (size, shape, age of the forest) allowing both persistence and sustainable recruitment of deadwood over time remains unknown. The objective of this study is to evaluate the abundance of recent deadwood in post-harvest and post-fire residual patches and to compare their temporal dynamics in black spruce dominated stands located in northwestern Quebec. Abundance of the recent deadwood, estimated as the sum of recent standing deadwood volume and recently fallen deadwood volume was analyzed in 41 post-fire residual patches, and in 45 post-harvest retention patches of varying ages (i.e. exposure time to the disturbed matrix) and in 37 continuous black spruce forest stands (controls). This study shows that post-fire residual patches appear in general more durable than post-harvest retention patches after disturbance. In a management context, our results indicate that: (1) large island patches and large linear separators oriented to escape windthrow usually have deadwood recruitment dynamics similar to that of post-fire patches; (2) retention patches with an initial stand volume greater than 60 m 3 /ha will generate more deadwood volume over time. This suggests that the selection of large retention patches in the shape of an island or a separator, with high volume (between 60 and 300 m 3 /ha) should help increase the persistence of post-harvest retention patches in black spruce forest, and simultaneously ensure quality habitat for several forest species while the adjacent managed forest regenerates.

Aim In managed forest landscapes, the tolerance of species to contemporary alteration of forest cover is often assumed to reflect their resilience to natural disturbances. We tested this central tenet of ecosystem-based management by comparing the structure of forest bird assemblages among four regions with contrasting historical natural disturbance regimes. Location Canada's boreal and northern hardwood forests. Methods Using point count data from four study regions across Canada, we first determined the relative sensitivity of individual bird species to the contemporary reduction of old forest cover at stand and ‘landscape-context’ (1-km radius) scales with log-linear models. The richness of species most sensitive to loss of old forest (hereafter ‘sensitive species’) was then modelled as a function of landscape-scale changes in old forest cover. Differences in the rate of decline in the richness of sensitive species with contemporary cover of old forest were compared among regions using ANCOVA. We then compared broken-stick regression models with linear models to detect thresholds, if present, in this relationship in each region. Results Bird assemblages from regions with relatively infrequent natural disturbances hosted more species sensitive to contemporary reduction in old forest cover. Those species were also more abundant than in regions with frequent natural disturbances, and the rate of decline in their richness with the loss of old forest was steeper in regions with infrequent natural disturbances than in those where they were frequent. However, we did not detect thresholds in this rate of decline in any study region. Main conclusions Our findings are consistent with the contention that historical natural disturbance regimes shape the response of biota to contemporary landscape alterations through evolutionary adaptation. We argue that forest management conducted within the natural range of variability in stand and landscape structure specific to a region is likely to be ecologically sustainable.

In the managed boreal forest, harvesting has become a disturbance as important as fire. To assess whether forest recovery following both types of disturbance is similar, we compared post-disturbance revegetation rates of forests in 22 fire events and 14 harvested agglomerations (harvested areas over 5–10 years in the same vicinity) in the western boreal forest of Quebec. Pre-disturbance conditions were first compared in terms of vegetation cover types and surficial deposit types using an ordination technique. Post-disturbance changes over 30 years in land cover types were characterized by vectors of succession in an ordination. Four post-disturbance stages were identified from the 48 land thematic classes in the Landsat images: “S0” stand initiation phase, “S1” early regeneration phase, “S2” stem exclusion phase, and “S3” the coniferous forest. Analyses suggest that fire occurs in both productive and unproductive forests, which is not the case for harvesting. Revegetation rates (i.e., rapidity with which forest cover is re-established) appeared to be more advanced in harvested agglomerations when compared with entire fire events. However, when considering only the productive forest fraction of each fire, the revegetation rates are comparable between the fire events and the harvested agglomerations. The S0 is practically absent from harvested agglomerations, which is not the case in the fire events. The difference in revegetation rates between the two disturbance types could therefore be attributed mostly to the fact that fire also occurs in unproductive forest, a factor that has to be taken into account in such comparisons.

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.

Dead and decaying trees may be a limited resource for woodpeckers in managed forests, especially for species that rely on dead wood for nesting and foraging. Whereas recent nest web studies greatly increased our understanding of nest tree use by woodpeckers, knowledge on woodpeckers foraging requirements is much less developed. We quantified and compared tree selection patterns and foraging behavior of six bark-foraging woodpeckers – downy woodpecker ( Picoides pubescens ), hairy woodpecker ( Picoides villosus ), American three-toed woodpecker ( Picoides dorsalis ), black-backed woodpecker ( Picoides arcticus ), yellow-bellied sapsucker ( Sphyropicus varius ) and pileated woodpecker ( Dryocopus pileatus ) – that co-occur in eastern boreal forests of North America. A total of 271 observation bouts and more than 600 foraging trees were recorded at three study sites characterized as mixedwood, conifer, and burn. Our results show that dead wood represents an important foraging substrate for most bark-foraging woodpeckers in Canadian eastern boreal forests. However, significant differences in individual species were found with regard to substrate use patterns, foraging behavior and associated prey. Woodpeckers were categorized according to their selection for specific stages of tree degradation, with the yellow-bellied sapsucker and the pileated woodpecker representing opposite ends of this gradient. The black-backed woodpecker showed the highest use of dead wood and was very specific in its tree selection by using mostly recently dead trees. We emphasize that providing foraging substrates for most woodpecker species not only requires maintaining dead wood but also paying heed to the underlying dynamics of dead wood (e.g. recruitment and degradation) in managed boreal forest landscapes.

Habitat loss and degradation induced by human development are among the major threats to biodiversity worldwide. In this study, we tested our ability to predict the response of bird communities (128 species) to land-use changes in southern Quebec (~483,100 km2 ) over the last 30 yr (between 1984-1989 and 2010-2014) by using species distribution models (299,302 occurrences in 30,408 locations) from a hindcasting perspective. Results were grouped by functional guilds to infer potential impacts on ecosystem services, and to relate model transferability (i.e., ability of our models to be generalized to other times and scales) to specific functional and life-history traits. Overall, our models were able to accurately predict, both in space and time, habitat suitability for 69% of species, especially for granivorous, nonmigrant, tree-nesting species, and species that are tied to agricultural areas under intensive use. These findings indicate that model transferability depends upon specific functional and life-history traits, providing further evidence that species' ecologies affect the ability of models to accurately predict bird distributions. Declining bird species were mostly short-distance migrants that were associated with open habitats (agricultural and nonproductive forest) with aerial insectivorous or granivorous diets, which may be related to agricultural intensification and land abandonment. Land-use changes were positive for some forest bird species that were mainly associated with mixed and deciduous forests, generalist diets and tree-nesting strategies. Yet cavity-nesting birds have suffered substantial reductions in their distributions, suggesting that cumulative effects of intensive logging and wildfires on mature forests pose a threat for forest-specialist species. Habitat suitability changes predicted by our coarse-scale species distribution models partially agreed with the long-term trends reported by the North American Breeding Bird Survey. Our findings confirm land-use change as a key driving force for shaping bird communities in southern Quebec, together with the need to explicitly incorporate it into global change scenarios that better inform decision-makers on conservation and management.

Ongoing climate change is likely to result in shifts in successional dynamics in boreal mixedwood stands. Using data from provincial forest inventory databases, we examined the occurrence and abundance of the regeneration of various coniferous species (white spruce, black spruce and balsam fir) along an east-west Canadian gradient in aspen-dominated stands. The interpretation of the results was based on environmental conditions, including climate, natural fire regime and human impacts. We found that conifer regeneration was present in aspen stands along the entire gradient, despite differences in climatic conditions and fire regimes between the west (warmer and drier, with large recurrent fires) and east (more humid with relatively long fire cycles). However, abundance and distribution varied from one conifer species to the next. The abundance of white spruce decreased towards the eastern end of the longitudinal gradient, while balsam fir and black spruce abundance decreased towards the west. Although abundance decreased, balsam fir and black spruce regeneration was still present in western Canada. This study shows that it is difficult to interpret the effects of climate change on conifer recruitment without accounting for the superimposed effects of human activities.

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.

Spatial information about forest soil base cations is important because of its implications for forest health and resilience to disturbance and management. The relevance of using geochemical data from lake and stream sediments collected for mining prospection purposes was evaluated to assess on a relative basis the forest soil base cation status and availability across the Quebec (Canada) forest landbase, an area for which information on soil properties is scarce. The relevance of the acid extraction of the fine fraction ( Values of element concentrations of sediments were then extrapolated spatially from their sampling points to the whole of the Quebec forest landbase, based on their geographical location and on the topographical features of their position using the k-nearest neighbour imputation method. The acid extraction used to determine the geochemical composition of sediments yielded concentrations of Ca, Mg and K that were well correlated with the observed exchangeable base cation concentrations, effective cationic exchange capacity and presence of clay-sized particles of soil samples. Also, spatial imputation of the geochemical signature of sediments to the forest landbase produced values that successfully represented variations in acid-extractable K and Mg in forest soils, as validated with two independent forest soil datasets, but it failed to capture variations in acid-extractable Ca concentrations. Nevertheless, the spatialized values broadly illustrated the gradient of forest soil exchangeable base cation reserves, soil clay content and site richness at a coarse scale across landscapes.

Over the last 25 years, greater understanding of natural dynamics in the boreal forest has led to the integration of forest ecosystem management principles into forest policy of several Canadian provinces and, in turn, to greater interest in developing silvicultural treatments that are grounded in natural stand-level dynamics – often referred to as natural disturbance-based silviculture. As a result, alternative silvicultural practices including variants of partial cutting are increasingly being applied in the boreal forest as an approach to balancing economic and ecological management objectives. While the numerous benefits of partial cutting reported in the literature are acknowledged, the objective of this paper is to provide an overview of factors or constraints that potentially limit the application of these practices in boreal Canada in the context of forest ecosystem management and natural disturbance-based silviculture. Among constraining factors, numerous studies have reported elevated mortality rates of residual stems following partial cutting, initial growth stagnation of residual trees, problems related to recruitment of desirable species and, on certain flat or lowland sites, risks of long-term decline in site and stand productivity. A number of operational challenges to partial cutting in the boreal forest are also presented and several avenues of research are proposed.

Variable retention harvest promotes biodiversity conservation in managed boreal forests by ensuring forest continuity and structural complexity. However, do post-harvest and post-fire patches maintain the same structural complexity? This study compares post-harvest and post-fire residual patches and proposes retention modalities that can maintain the same structural complexity as in natural forests, here considering both continuous forest stands and post-fire residual patches. In boreal black spruce forests, 41 post-fire residual patches, and 45 post-harvest retention patches of varying size and ages (exposure time to disturbed matrix) and 37 continuous forest stands were classified into six diameter structure types. Types 1 (inverted-J) and 2 (trunked-unimodal) characterized stands dominated by small trees. The abundance of small trees decreased and the abundance of large trees increased from Type 1 to Type 6. Type 6 had the most irregular structure with a wide range of diameters. This study indicates that: (1) old post-harvest residual retentions maintained the range of structural complexity found in natural stands; (2) Types 1 and 2 were generally associated with young post-fire patches and post-harvest retention clumps; (3) the structure of residual patches containing only small trees was usually younger (in terms of the age of the original forest from which residual patches were formed) than those with larger trees. To avoid the risk of simplifying the structure, retention patches should be intentionally oriented towards Types 3–6, dominated by intermediate and large trees.

Severe crown fires are determining disturbances for the composition and structure of boreal forests in North America. Fire cycle (FC) associations with continental climate gradients are well known, but smaller scale controls remain poorly documented. Using a time since fire map (time scale of 300 years), the study aims to assess the relative contributions of local and regional controls on FC and to describe the relationship between FC heterogeneity and vegetation patterns. The study area, located in boreal eastern North America, was partitioned into watersheds according to five scales going from local (3 km2) to landscape (2800 km2) scales. Using survival analysis, we observed that dry surficial deposits and hydrography density better predict FC when measured at the local scale, while terrain complexity and slope position perform better when measured at the middle and landscape scales. The most parsimonious model was selected according to the Akaike information criterion to predict FC throughout the study area. We detected two FC zones, one short (159 years) and one long (303 years), with specific age structures and tree compositions. We argue that the local heterogeneity of the fire regime contributes to ecosystem diversity and must be considered in ecosystem management.

Forest stands within the Quebec—Ontario paludification-prone Clay Belt are expected to converge to open unproductive black spruce (Picea mariana) stands regardless of initial tree composition with the prolonged absence of fire. We hypothesized that different surface deposits would display different stand transition characteristics, as recent research on the deglaciation history of the regions suggests that certain site conditions could exhibit different susceptibility to paludification. We quantified the rate and age of transitions of different succession stages for various surface deposits using a large spatio-temporal forest database. Our results suggest that a complete convergence to open and less productive black spruce stands can occur, but it may take a long time (i.e., more than 500 y), especially on surface deposits less prone to paludification, such as coarse-textured soils. We also observed that if succession pathways start with open and less productive black spruce stands, their capac...

While expansion of intolerant hardwoods, including trembling aspen, has been recognized as a problem by boreal forestmanagers in eastern Canada, the problem has mostly been associated with the boreal mixedwood and, with a few exceptions,the spatial aspects of the phenomenon have not been well documented in the scientific literature. The objective ofthis study is to evaluate how the density and size of aspen patches influence the change in density, size and persistence ofpatches following clearcutting of black spruce forests in the claybelt region of northwestern Quebec. To do this, we finelyinterpreted: 1) aerial photos taken between 1970 and 1979 of 12 mature black spruce-dominated stands containing varyingsizes and densities of aspen patches prior to being clearcut harvested and 2) aerial photos taken between 1992 and1995 of the 12 resulting clearcuts, 11 to 18 years after harvesting. Sketch maps of pre- and post-harvest aspen presencewere rasterized and changes in aspen patch size and cover were determined. As well, the probabilities of aspen colonizationwith increasing distance from pre-harvest patches were calculated for each site. Black spruce forests of the claybeltcontaining no aspen prior to harvesting are relatively resistant to aspen invasion. However, when forests do contain aspenpatches with cover values ≤25%, almost 60% of these patches increase to a higher density class. Aspen patches less than 2ha in area had about a 50% probability of disappearing following clearcutting whereas larger patches had about a 96%chance of persisting as a result of clearcutting. In the black spruce forest of the claybelt, thick organic layers appear to havea negative effect on aspen recruitment. Distances of regenerating aspen from pre-harvest patches suggest that recruitmentfrom seed is more important than previously thought. Key words: encroachment, trembling aspen, clearcutting, black spruce, claybelt, aerial photography

Questions: How does woody vegetation abundanceand diversity differ after natural disturbances caus-ing different levels of mortality?Location: Abies balsamea–Betula papyrifera borealmixed-wood stands of southeast Quebec, Canada.Methods: Woody vegetation abundance and diver-sity were quantified and compared among threedisturbance-caused mortality classes, canopy gap,moderate-severity disturbances, and catastrophicfire, using redundancy analysis, a constrained linearordination technique, and diversity indices.Results: Substantial changes in canopy tree speciesabundance and diversity only occurred aftercatastrophic fire. Shade-tolerant, late-successionalconifer species remained dominant after canopygap and moderate-severity disturbances, whereasshade-intolerant, early-successional colonizersdominated canopy tree regeneration after cata-strophic fire. Density and diversity of mid-tolerantand shade-intolerant understory tree and shrubspecies increased as the impact of disturbance in-creased. Highest species richness estimates wereobserved after catastrophic fire, with several speciesestablishing exclusively under these conditions. Re-lative abundance of canopy tree regeneration wasmost similar after canopy gap and moderate-sever-ity disturbances. For the sub-canopy tree and shrubcommunity, relative species abundances were mostsimilar after moderate-severity disturbances andcatastrophic fire. Vegetation responses to moder-ate-severity disturbances thus had commonalitieswith both extremes of the disturbance-caused mor-tality gradient, but for different regeneration layers.Conclusions: Current spatio-temporal parameters ofnatural disturbances causing varying degrees ofmortality promote the development of a complex,multi-cohort forest condition throughout the land-scape. The projected increase in time intervalsbetween catastrophic fires may lead to reduceddiversity within the system.Keywords: Abies balsamea–Betula papyriferaforests; Canopy gap disturbance; Catastrophic fire;Disturbance-caused mortality; Moderate-severitydisturbance; Natural regenerationNomenclature: Gleason & Cronquist (1991)Abbreviations: MSD, moderate-severity distur-bance; RDA5redundancy analysis; R5speciesrichness; H

In the eastern Canadian boreal forest, the term hardwood expansion or encroachment (enfeuillement in French) generally refers to the phenomenon of increasing cover of intolerant hardwoods, notably aspen (Populus tremuloides), following clearcutting. Such expansion is perceived as a local (stand-level) process that is reflected at a larger, regional scale. The objective of this study was to characterize the increase in aspen cover following clearcutting in the clay belt of Abitibi, in northwestern Quebec. Regional data from 3 province-wide forest inventories (1970, 1980, and 1990) conducted by the Quebec Ministry of Natural Resources and Wildlife were analyzed for 2 ecological regions, the Abitibi Plain (western balsam fir-white birch bioclimatic subdomain) and the Matagami Lake Plain (western black spruce-feathermoss bioclimatic subdomain). An assessment was done of forest cover transformations following clearcutting over 3 decades based on time since harvesting, surface deposit/drainage, and ecological region. Results show that mesic clay sites are particularly susceptible to aspen encroachment and that the overall rate of encroachment from the 1970s to the 1990s in the black spruce and balsam fir regions has been about 30%. Moreover, despite the greater abundance of poorly drained and organic sites, less favourable to hardwood establishment, in the more northerly black spruce region, the 2 regions appear to present the same sensitivity (or propensity) to intolerant hardwood expansion. On sensitive sites, management of black spruce forests should include adaptive practices to limit increases of the hardwood component.

L’etude proposee part du contraste saisissant entre la facture classicisante de l’opera de Schreker (progression reguliere de l’intrigue selon le schema exposition / peripetie / catastrophe, structure equilibree des trois actes, respect de l’unite d’action, cadre rassurant de la Renaissance italienne) et la psychologie hors norme des principaux personnages. Pour entrer dans le livret et par consequent dans l’univers des Stigmatises, le lecteur/auditeur doit accepter certaines donnees plus ou moins invraisemblables, sous peine de rester definitivement a l’exterieur du drame. Nous essaierons de montrer que, loin d’etre une faiblesse, ce desequilibre est pleinement assume par Schreker. Les invraisemblances et les outrances du livret ne sont qu’un moyen de montrer jusqu’ou peut aller la pulsion destructrice et autodestructrice de la psyche humaine quand celle-ci n’est plus irriguee par la relation authentique a l’Autre. Les Stigmatises meritent plus que tout autre le qualificatif d’« opera limite » ; c’est l’opera de l’enfermement narcissique a l’ere de l’individualisme moderne.

It is now recognized that in the Canadian boreal forest, timber harvesting activities have replaced wildfires as the main stand-replacing disturbance. Differences in landscape patterns derived from these two sources of disturbance have, however, raised concerns that the way forest harvesting has been dispersed is potentially shifting patterns away from the natural range. In the context of natural disturbance-based management, we used a spatially explicit model designed to capture general fire regimes in order to quantify temporal variability associated with regenerating areas (burnt areas of 25 years or younger), and to develop strategic objectives for harvest agglomeration sizes and dispersion. We first evaluated temporal variability in the proportion of stands younger than 100 years (assumed to be even-aged stands) for various fire regimes (seven fire cycles: 50 to 400 years, and three mean fires sizes: 3000, 15 000 and 60 000 ha). Secondly, we quantified the size distribution and dispersion of regenerating areas for each fire regime. As expected by theoretical fire frequencies and size distributions, the importance of even-aged stands at the forest management unit level was found to decrease with longer fire cycles. However, the temporal variability associated with these proportions is shown to increase with mean fire size. It was also observed that the size distribution and dispersion of regenerating areas was primarily influenced by mean fire size. Based on these observations, natural disturbance-based management objectives were formulated, providing guidelines on harvest agglomeration size and dispersion. Key words: temporal variability, boreal forest, fire regime, forest management, age distribution, fire size distribution, clearcut agglomeration size distribution

Aim Climate is often regarded as the primary control determining the location of an ecotone between two vegetation zones. However, other ecological factors may also be important, especially when the northern limit of the dominant species of a vegetation zone extends further than the limit of the zone itself. This study aimed to identify the ecological variables explaining the transition between two zones within the boreal biome in Quebec (eastern Canada): the southern mixedwood forests dominated by balsam fir (Abies balsamea) and white birch (Betula papyrifera), and the northern coniferous forests dominated by black spruce (Picea mariana). Location Quebec (eastern Canada). Methods Data from 5023 sampling plots from the ecological inventory of the Quebec Ministry of Natural Resources distributed throughout the two bioclimatic zones were used in logistic regressions to determine the relationships between the presence or absence of balsam fir stands and different abiotic and biotic variables, at both stand and landscape scales. Results The presence of balsam fir stands was negatively related to the thick organic horizons, coarse xeric deposits and low positions on the slope, whereas stands were favoured by high elevations, steep slopes and moderate drainage. These results defined the suitable conditions for the development of balsam fir stands. In the coniferous zone these suitable conditions were less abundant. Furthermore, the saturation level of suitable sites was lower, as well as the incidence of balsam fir stands in unsuitable sites (overflow). Balsam fir stands were mostly located near lakes and rivers. All significant variables at both the stand and landscape scales explained between 34 and 42% of the location of the potential northern distribution limit of the mixedwood zone. Main conclusions Our results suggest the important role of historical factors related to post-glacial vegetation and past disturbances in determining the relative abundance of balsam fir in both zones of the boreal biome.

This study examined the relative importance of soil, stand development and climate hypotheses in driving productivity for a species that is widely distributed in North America. Inventory plots, 3548 of such, either dominated by aspen or made up of species mixture of which aspen occurs in dominant canopy position were sampled along a longitudinal gradient from Quebec to British Columbia. Site index (SI), was used as a measure of productivity, and soil, climate and stand attributes were correlated with site index in order to determine their effects on productivity. Results show a decline in productivity with high moisture deficit. Soil humus correlates significantly with SI but does not sufficiently capture differential rates of litter deposition and decomposition effects over the long-term. Consequently, aspen composition, stand ageing, and stand structural changes dominate variability in productivity. Within the context where deciduous cover has being increasing, there are implications for forest productivity.

The past decade has seen an increasing interest in forest management based on historical or natural disturbance dynamics. The rationale is that management that favours landscape compositions and stand structures similar to those found historically should also maintain biodiversity and essential ecological functions. In fire-dominated landscapes, this approach is feasible only if current and future fire frequencies are sufficiently low compared with the preindustrial fire frequency, so a substitution of fire by forest management can occur without elevating the overall frequency of disturbance. We address this question by comparing current and simulated future fire frequency based on 2 × CO2 and 3 × CO2 scenarios to historical reconstructions of fire frequency in the commercial forests of Quebec. For most regions, current and simulated future fire frequencies are lower than the historical fire frequency, suggesting that forest management could potentially be used to maintain or recreate the age-class distribution of fire-dominated preindustrial landscapes. Current even-aged management, however, tends to reduce forest variability by, for example, truncating the natural age-class distribution and eliminating mature and old-growth forests from the landscape. Therefore, in the context of sustainable forest management, silvicultural techniques that retain a spectrum of forest compositions and structures at different scales are necessary to maintain this variability and thereby allow a substitution of fire by harvesting.

We built two models to follow clonal species genotypic diversity (G/N) over long periods of time at the stand and landscape levels. The models were then validated with empirical data from trembling aspen (Populus tremuloides) populations in Quebec’s boreal forest. Data was collected using a chronosequence approach in seven sites that burned in 1717, 1760, 1797, 1823, 1847, 1944, and 1916. Genetic identification was done by using four microsatellite loci. At the stand scale, simulations were repeated for a genet size of 5, 25, 50 and 100 ramets each. At the landscape level, we simulated the cumulative genet survival rate under different fire cycles (5–500 years) for 500 years after fire. Stand simulations indicated that ramet mortality within genets rather than genet mortality accounts for the increase in G/N with time since fire. Both the initial genet size and the recurrent suckering of some genets (or ramet recruitment) play an important role in maintaining high G/N levels for long periods of time. In general, the larger the number of ramets per genet, the longer the genet survives under a gap disturbance regime and a minimum of 100 ramets per genet is required to maintain aspen genet survival for 500 years. At the landscape level, genet loss increases as the fire cycle gets longer. In Quebec’s boreal forest, short rotation even-aged management practices seem to maintain a genet survival rate similar to that produced by the natural succession regime.

Fire regimes in the boreal forest are dominated by crown fires that burn over large areas. However, these fires rarely burn forest stands entirely and with the same intensity throughout, resulting in a mosaic of vegetation burnt to varying degrees of severity. The objectives of this study were to: (1) assess regeneration of tree species six or seven years after fire in relation to crown fire severity in non-salvaged jack pine and black spruce stands, and (2) assess establishment preferences of seedlings on the different types of germination beds created by fire. Logistic regressions indicated that seed trees abundance influenced regeneration success. The relationship between fire severity at the crown level and regeneration success was not significant, although seedling recruitment appeared limited in areas where severity was light to moderate. Poisson regressions showed that seedlings preferentially establish on mineral soil, and that woody debris seem to be a good substrate for germination and ...

Single-tree selection cutting is sometimes believed to be similar to the natural gap disturbance regime of hardwood forests, but few studies have specifically compared the compositional and structural characteristics of old-growth hardwood stands, undergoing natural gap dynamics and hardwood stands previously subjected to partial cuts. This study characterized and compared the composition (saplings and trees) and structure (gaps, foliage distribution, tree diameter and density, snags and coarse woody debris) of old-growth stands (OG), 12-year-old selection cuts (SC), and 28-33-year-old diameter-limit cuts (DLC) in sugar maple (Acer saccharum)-dominated northern hardwood stands. Results showed marked structural differences between OG and harvested stands, with stronger differences between DLC and OG than between SC and OG. The synchronized formation of numerous canopy openings in harvested stands induced a massive post-harvest recruitment of advance regeneration in both SC and DLC that created a dense foliage layer in the understory. Large living trees (dbh > 39.1 cm) and defective trees were less numerous in SC than OG, which can have a detrimental impact on species dependent on these structural elements, and on the future availability and characteristics of coarse woody debris. Relatively few compositional differences were noticed among stand types, although a greater proportion of mid-tolerant species was found in the post-harvest recruitment cohorts of harvested stands compared to OG, and a lower proportion of beech (Fagus grandifolia Ehrh.) saplings was observed in DLC compared to OG and SC. We argue that even if selection cutting is closer to the natural disturbance regime of hardwood forests than diameter-limit cutting, and therefore representing progress toward the development and implementation of a natural-disturbance-based management, a recurring application of selection cutting might lead to a homogenization of forest structure and composition, a reduction of key structural features and a reduction in biological diversity at both the stand and landscape scales. Some management recommendations are proposed.

Climate influences natural processes at multiple spatial and temporal scales. Consequently, climate change raises many challenges for sustainable forest management; among them, the integration of fire and forest management is increasingly discussed. We propose here an evaluation of the adaptive capacity of forest management under changing forest fire regimes under climate change in the boreal forest of Quebec. Adaptation begins by reinterpreting current practices dealing with climatically driven variability. Among them, fire suppression, and regeneration enhancement can contribute to coping with some impacts of climate change. However, there is an increasing need to develop more integrative and spatially explicit management strategies to decrease the vulnerability of forest management to changing fire risk. Some developing management strategies, such as fuel management or the triad approach (zoning system for conservation, intensive, and extensive forest management), present an interesting potential for integrating the fire risk in management plans. While fuel management and fire suppression are indicated for particularly severe fire regimes, protection against insects, and maintaining a shorter disturbance cycle using forest management represent the preferred adaptation options where the fire cycle is lengthening under climate change. Key words: forest fire, fire risk, climate change, vulnerability, impacts, adaptation strategy, adaptation options, sustainable forest management, fire management

We studied community development and mortality among tree species at multiple spatial scales in a 236- year mixedwood chronosequence at Lake Duparquet, Quebec. Spatial relationships, species associations, and understory-overstory replacement patterns were studied using spatial statistics, patch indices, G tests, and transition matrices. Results of these analyses showed that shade-intolerant and shade-tolerant tree communities formed increas- ingly fine-grained patches in midsuccession. These fine-grained associations appear to be partly due to suppressed stems being released into a slowly thinning canopy and partly due to new recruitment of shade-tolerant conifers. In 1- ha plots, tree species richness and evenness peaked in 25-, 100-, and 400-m 2 subplots during midsuccession. Forest cover type diversity also peaked in midsuccessional landscapes assessed using aerial photographs. The oldest 1-ha plot was dominated by monospecific patches of eastern white cedar (Thuja occidentalis L.), which was the most likely re- placement for live and dead trees and the most abundant species in the numerous subplots that were occupied by trees smaller than 8 cm DBH. In spite of this dominance, long-lived paper birch (Betula papyrifera Marsh.) and residual fir (Abies balsamea (L.) Mill.) seedlings may maintain the mixedwood character of this stand into the future. The implica- tions of our findings for stand management and conservation are discussed. Resume : Les auteurs ont etudie le developpement et la mortalite de communautes d'arbres en peuplements mixtes a plusieurs echelles spatiales dans une chronosequence de 236 ans au lac Duparquet, Quebec. Les relations spatiales, les associations d'especes et les patrons de remplacement entre les especes du couvert dominant et celles du sous-bois ont ete etudies en utilisant des statistiques spatiales, des indices d'agglomeration, des tests de G et des matrices de transi- tion. Les resultats de ces analyses montrent que les communautes d'arbres intolerants et tolerants a l'ombre forment un nombre croissant d'agglomerations de petites tailles a l'etape du milieu de la succession. La formation de ces associa- tions de petites tailles semble partiellement attribuable, d'une part, aux tiges opprimees qui sont liberees a l'interieur d'un couvert qui s'eclaircit lentement et, d'autre part, au recrutement de coniferes tolerants a l'ombre. Dans des parcel- les de 1 ha, la richesse en especes et l'equitabilite ont culmine dans des sous-parcelles de 25, 100 et 400 m 2 de peu- plements parvenus a l'etape du milieu de la succession. La diversite du type de couvert forestier estimee a partir de photographies aeriennes a aussi culmine sur des territoires parvenus a l'etape de milieu de succession. La plus vieille placette de 1 ha etait dominee par des agglomerations monospecifiques de thuya occidental (Thuja occidentalis L.), l'espece la plus susceptible de remplacer les arbres morts et vivants et l'espece la plus abondante dans les nombreuses sous-parcelles occupees par des arbres dont le DHP est inferieu ra8c m. Malgre cette dominance, les especes a forte longevite que sont le bouleau a papier (Betula papyrifera Marsh.) et les semis residuels de sapin baumier (Abies balsa- mea (L.) Mill.) peuvent maintenir la mixite d'especes de ce type de peuplement dans le futur. Les implications de nos resultats sont discutees en regard de l'amenagement et de la conservation des forets.

1 We investigated recovery following small-scale disturbance, i.e. selection cutting, by determining how understorey light profiles vary over time in temperate deciduous forests in Quebec (Canada). 2 We measured light availability (% PPFD, photosynthetic photon flux density) 0.2, 1, 2 and 5 m above the forest floor, as well as the density of saplings < 5 m in height, in seven Acer saccharum-Betula alleghaniensis-Fagus grandifolia stands that had been subjected to selection cutting 1-13 years before the study, and in adjacent uncut plots. 3 In the most recent cut (1 year old), mean % PPFD was 3. 5 to 5 times higher (depending on height) than in the uncut plot. Light availability rapidly decreased over time following selection cutting, especially near the forest floor. By about 13 years after cutting, light availability was similar to levels observed in the uncut plots. 4 Light profiles were used to assess the temporal pattern of recovery of the understorey after selection cutting, and four recovery phases could be identified. Uncut stands were characterized by profiles with low light near the forest floor and with a rather slow increase in light with increasing height, and recent cuts (1-4 years old) were characterized by J-shaped light profiles with relatively high % PPFD at all heights. Intermediate-age cuts (7-8 years old) were characterized by reverse J-shaped profiles that had a high % PPFD (13-46%) at 5 m, and very dark conditions (< 2%) near the forest floor, and were associated with high abundance of saplings. The relative frequency of the various profiles found in older cuts (11-13 years old) was generally similar to that observed in the uncut stands, except that the reverse J-shaped profiles were slightly more frequent. 5 As the microsuccession that follows canopy disturbance is very much influenced by local understorey structure and composition, forestry practices should consider such microscale forest characteristics in their harvest planning to regenerate the desired tree species.

The forest fire regime was characterized for the boreal forest of western Quebec using the provincial government’s digital databases (1945-1998). Lightning- and human-caused fires account for 71% a...