La saison des feux de forêt de 2023 au Québec, marquée par des conditions extrêmement chaudes et sèches, a établi de nouveaux records en brûlant 4,5 millions d'hectares. Cette situation est directement liée aux impacts persistants et en augmentation du changement climatique. Cette étude examine les conditions météorologiques exceptionnelles ayant mené aux feux et évalue leurs impacts significatifs sur le secteur forestier, la gestion des feux, les habitats du caribou boréal, et met particulièrement en lumière les répercussions profondes sur les communautés des Premières Nations. Les feux ont entraîné une baisse significative de la productivité des forêts et de l'approvisionnement en bois, submergeant les équipes de gestion des feux et nécessitant des évacuations massives. Le territoire et les communautés des Premières Nations ont été profondément affectés, confrontés à de graves problèmes de qualité de l'air et à des bouleversements considérables. Si l'impact sur l?habitat du caribou a été modeste dans l'ensemble de la province, les répercussions écologiques, économiques et sociales ont été considérables. Pour atténuer les impacts à venir des prochaines saisons de feux de forêt extrêmes, une avenue suggérée serait de modifier les pratiques d?aménagement forestier afin d'accroître la résilience et la résistance des forêts, d'adapter les structures industrielles aux nouvelles sources d'approvisionnement en bois et d'améliorer les stratégies de lutte contre les feux et la gestion des risques. De même, une approche globale

La saison des incendies forestiers au Canada en 2023 a été d?une telle ampleur qu?elle force une réflexion profonde au sujet de l?aménagement forestier tel qu?il est pratiqué actuellement dans la forêt boréale canadienne. Comme des événements similaires sont susceptibles de se reproduire dans les années à venir, nous devons réfléchir à des pratiques forestières permettant de mieux faire face à ces risques pour en atténuer les conséquences. En nous concentrant sur la situation du Québec, nous examinons six options générales en guise de contribution au débat à propos d?une stratégie d?adaptation à mettre en ?uvre face à l?augmentation du risque d?incendie de forêt. Afin d?atténuer les conséquences néfastes des futurs incendies dans la forêt boréale, nous suggérons que le maintien des processus naturels de résilience, la protection des communautés et des infrastructures clés et le déploiement d?un banc d?essai pour tester certaines pratiques d?adaptation sont deux options à considérer. Nous proposons aussi la prise en compte a priori du risque d?incendie dans la planification de l?approvisionnement en bois et dans les investissements sylvicoles ainsi que l?amorce d?une transition industrielle pour les communautés dépendantes de la forêt.

Mixed plantations are garnering increased attention due to their potential to provide a broader array of benefits compared to monocultures. Although numerous studies have indicated promising complementarity between black spruce (Picea mariana) and tamarack (Larix laricina), few have delved into individual tree growth interactions to thoroughly assess early growth complementarity. We sampled 119 planted black spruce and their immediate environment to quantify and qualify any differences between two conditions in young plantations: mixed tamarack (?mixtures?) and black spruce monocultures (?monocultures?) within young plantations. We investigated the effect of neighbouring under four perspectives: tree competition, microenvironment, foliar nutrients, and soil nutrients. Our results showed increased values for black spruce foliar nitrogen total concentration, soil pH, and canopy closure in mixtures compared to monocultures. Furthermore, black spruce stem volume was increased by 38.1% in mixture compared to monoculture. Black spruce stem volume was negatively affected (86% decrease) by the combined effect of shrubs and non-crop trees under high competition pressure, despite the plantation being mechanically released in 2017. Collectively, our results suggest that black spruce growing in mixtures holds a greater growth potential than black spruce in monocultures.

Little is known about the growth interactions of black spruce (Picea mariana) and tamarack (Larix laricina), two important commercial tree species of the Canadian boreal forest. We investigated growth relations between black spruce and tamarack in mature mixed stands. We sampled tree

Structural complexity generated by forest development processes and tree species compositional changes provide key habitat features for vertebrate communities that rely upon tree size and decay processes for foraging, denning or nesting. Complexity of forest structure in old stands could not only be key for harboring increased taxonomic species diversity but also greater functional diversity through more complexity in networks of tree cavity dependent species. Using a nest web approach that hierarchically links cavity-bearing trees with cavity formation agents (natural decay processes and avian excavators) and cavity users (non-excavator species), we compared network characteristics of nest webs along a time since fire gradient in a naturally disturbed boreal mixedwood forest landscape in eastern North America. Since 2003, twelve 24 to 40 ha plots ranging from 61 to more than 245 years after fire were surveyed at the Lake Duparquet Research and Teaching Forest in Abitibi, Quebec, Canada to detect active nesting, and denning cavities. We found that network complexity both in terms of number of vertebrate species and number of interactions among species, increased along the age gradient and was significantly higher in the older stands than predicted by chance. Whereas cavity-nesting communities in old forests used a higher diversity of tree species over a wide range of decay stages, trembling aspen remained a key cavity-bearing tree throughout the age gradient. Woodpeckers were the main cavity formation agents whereas less than 1% of cavities originated from natural decay. The structural development of older forests is thus a driver for functional diversity in cavity-using vertebrate communities through higher interaction richness in nest webs, among cavity-bearing trees, excavators and non-excavating users. The pivotal contribution of the entire gradient of old forest cover types to the overall complexity of nest webs in the boreal mixedwood zone is also a key for the resilience of the cavity-using vertebrate community to natural disturbances. We discuss how such resilience may be compromised by even-aged industrial timber harvesting with short rotations that shifts the age structure of boreal landscapes toward regenerating and young pole forests whereas old forest cover types become below their historical range of variability.

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.

Even-aged silviculture based on short-rotation clearcuts had severely altered boreal forests. Silvicultural alternatives (e.g. continuous cover or retention forestry) have the potential to restore and protect the habitats and functions of boreal forests. These alternatives are however often restricted to structurally complex old-growth forest, which are particularly threatened by anthropogenic disturbances. Increasing the use of alternatives to even-aged silviculture in early-successional stands could help recruit more structurally complex forests, with characteristics closer to the old-growth. In this article, we therefore evaluate the potential for silvicultural alternatives to even-aged management in boreal forests that burned less than a century ago.

We analysed 1085 field plots in a 243,000 km² area situated in the boreal forest of eastern Canada. These plots burned 30–100 years before the survey and had not been subjected to previous or subsequent anthropogenic disturbance; they hence represent young primary forests. The main patterns of tree diameter distribution variation within the plots were identified using k-means clustering. Stand structure, tree species composition and environmental variables that most explained the differences among the clusters were identified with a random forest model, and then compared using Kruskal–Wallis and Fisher's exact tests.

The majority (>75%) of the plots presented an irregular structure of stem diameters (i.e. non-normally distributed, with many small diameter trees). The understorey was generally dominated by black spruce (Picea mariana [Mill.] BSP), a shade-tolerant species. Irregular structures were observed in both forests of high and low productivity, implying that different processes (e.g. early regeneration, variable tree growth) can lead to observed early irregular structure. Regular structures were generally characterized by a higher productivity and abundance in hardwood species compared to the irregular structures.

Synthesis and applications. Many boreal forests of eastern Canada progress towards an irregular structure in the decades following the last stand-replacing fire. A substantial part of these early-successional forests may be suitable for alternatives to even-aged silviculture that better maintains habitats and functions of preindustrial boreal forests.

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.

As a result of extreme weather conditions associated with anthropogenic climate change, fire regimes are expected to continue to change in the boreal forest over the 21st century and beyond. Consequently, changes in ecological attributes like stand composition, tree density and forest carbon stock can be expected. In the present study, we used an adjusted version of the CanFIRE model to project long-term (1971–2100) changes in burn rates, fire severity and fire-induced shifts in vegetation composition in response to anticipated scenarios of climate change, in the black spruce-feather moss subdomain of Western Quebec. The model provides decadal-scale estimates of the immediate physical effects of fire on forest communities by computing expected fire behavior and the resulting ecological effects. Changes in species composition of the forest is also computed based on mechanisms of succession in natural forest communities and fire-mediated vegetation transitions. Projections suggest an increase in potential burn rates across the study area under future weather conditions and also an overall reduction in percent tree mortality and total fuel consumption. This reduction is caused by negative feedback from vegetation composition that shifts to less-fire prone states. Although common forest communities will remain the same in the studied subdomain until 2100 (recurrence dynamics), significant losses of productive area (LPA) are projected, particularly in forest management units rich in forest communities dominated by black spruce or jack pine, as a result of regeneration failure due to very short intervals between successive fires. While remaining similar under moderate (RCP4.5) and high-end (RCP8.5) warming scenarios in all forest management units, LPA will vary from 25 to 36% of the percent cover by 2100 compared to 1970. These results provide insights to policy makers and land managers, and they attract attention to the pressing need to adjust management practices in the context of climate change.

Aim

Impacts of longitudinal gradients on species distributions have received less attention than latitudinal gradients. In Canada, precipitation varies longitudinally, with direct effects on plant growth and fire regimes. Despite the geographical extent of Canadian boreal forests, vascular plant diversity is relatively limited, with just under 300 species. Understorey communities comprise most of this diversity and play key roles in forests, affecting succession, nutrient cycling and wildlife habitat. Our objective was to evaluate the relative impacts of local and regional environmental conditions on vascular plant community composition and diversity in the boreal forest.

Location

Sampling took place in 33 trembling aspen (Populus tremuloides) stands along a transcontinental gradient in precipitation and fire regimes in boreal Canada.

Methods

We measured community composition and vegetation diversity indices of the vascular understorey in 400-m2 plots and measured environmental variables at the local (drainage, nutrient and light availability, site heterogeneity) and regional (annual temperature and precipitation, fire weather index, landscape heterogeneity) scales. Multivariate analyses and hierarchical model selection were used to analyze patterns.

Results

Species composition varied longitudinally, with western and eastern species pools. Western indicator species tended to be fire-adapted pioneer species, while indicator species at eastern sites were fire avoiders and late-successional species. Species richness and intra-site beta diversity seemed primarily driven by local variables and did not vary across the country. Species evenness, however, was slightly higher in the western region and decreased under higher precipitation, colder temperatures, and higher landscape-level heterogeneity.

Conclusion

Our results suggested that even for similar canopy composition environmental variables play a key role in the establishment of plant communities and structure local plant assemblages by selecting or eliminating species from the regional pool, which was also controlled by fire regimes.

Clear cut harvest simplifies and eliminates old growth forest structure, negatively impacting biodiversity. Partial cut harvest has been hypothesized (1) to have less impact on biodiversity than clear cut harvest, and (2) to encourage old growth forest structures. Long-term studies are required to test this hypothesis as most studies are conducted soon after harvest. Using epixylic bryophytes as indicators, this study addresses this knowledge gap. Fourteen years after harvest, we examined changes in epixylic bryophyte community composition richness and traits, and their microhabitats (coarse woody debris characteristics and microclimate) along an unharvested, partial cuts and clear cuts harvest treatment in 30 permanent plots established in the boreal black spruce (Picea mariana) forests of northwestern Quebec, Canada. Our results were compared to those of an initial post-harvest study (year 5) and to a chronosequence of old growth forests to examine species changes over time and the similarity of bryophyte communities in partial cut and old growth forests. Coarse woody debris (CWD) volume by decay class varied among harvest treatments with partial cuts and clear cuts recording lower volumes of early decay CWD. The epixylic community was richer in partial cuts than in mature unharvested forests and clear cuts. In addition, species richness and overall abundance doubled in partial and clear cuts between years 5 and 14. Species composition also differed among treatments between years 5 and 14. Furthermore, conditions in partial cut stands supported small, drought sensitive, and old growth confined species that are threatened by conditions in clear cut stands. Lastly, over time, species composition in partial cuts became more similar to old growth forests. Partial cuts reduced harvest impacts by continuing to provide favorable microhabitat conditions that support epixylic bryophytes. Also, partial cut harvest has the potential to encourage old growth species assemblages, which has been a major concern for biodiversity conservation in managed forest landscapes. Our findings support the promotion of partial cut harvest as an effective strategy to achieve species and habitat conservation goals.

The boreal forest is considered to be a low productivity forest due to its cold climate and poorly drained soils promoting paludification. These factors create conditions favouring accumulation of undecomposed organic matter, which causes declining growth rates of forest stands, ultimately converting mature stands into peatlands. Under these conditions, careful logging is conducted during winter, which minimizes soil disturbance in northwestern Quebec boreal forest. This results in water table rise, increased light availability and paludification. Our main objective was to evaluate the short-term effect of partial harvesting as an alternative method to careful logging in winter to mitigate water table rise on black spruce (Picea mariana [Mill.] B.S.P.) stands. We quantified tree stem diameter variation and daily variation in water table depth in mature spruce stands before and after partial harvest (basal area reduction of 40%) and girdling (same basal area reduction with delayed mortality) during 2016 and 2017 growing seasons. Water table variation prior to and following silvicultural treatments did not differ one year after treatment. Daily stem diameter variation in black spruce did not differ between treatments and control. Furthermore, temperature exerted a positive effect on variation in water table and on stem diameter. These results suggest that partial harvest could be more effective than clearcutting to mitigate negative effects of a high water table while limiting paludification.

Aims

In the boreal mixedwood, fire initiates forest succession; however, over time other disturbances such as insect outbreaks cause pulses of mortality and opportunities for recruitment of shade?intolerant species.

Questions

What are the respective roles of time since fire and insect outbreaks in driving directional, vs cyclical and retrogressive succession? Do assessments from direct measurements and chronosequence approaches converge? We hypothesise that the chronosequence approach will accurately characterise large?scale compositional patterns especially in younger stands and that direct measurements will better describe small?scale, non?directional changes in succession.

Location

Boreal mixed wood of northwestern Quebec (79°1? W, 48°30? N).

Methods

We sampled 469 plots over an 18?year interval (in 1991 and 2009) in mixed hardwood/conifer forests to observe in situ the changes in stands having originated from seven fires covering a 249?year chronosequence.

Results

The combination of the remeasurement and chronosequence analyses validates predictions of forest succession. Our results indicate that time since last fire is the dominant factor explaining forest succession for the first 150 years after fire and globally at the scale of the entire forest, although smaller?scale disturbances such as spruce budworm outbreaks can change the course of forest succession, especially at stand scales.

Conclusion

While time since fire is the dominant driver of forest succession in younger forests, secondary disturbances, such as spruce budworm outbreaks, can advance or reverse forest succession in older forests. This study also serves as a validation of the overall patterns described in spatial chronosequence approaches at the landscape level in fire?dominated systems but highlights that local succession may deviate from the overall pattern especially in older forests or in forests affected by non?stand re?initiating intermediate disturbances.

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.

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.

Le Rapport du Comité scientifique chargé d’examiner la limite nordique des forêts attribuables (Ministère des Ressources naturelles du Québec 2013) distinguait des portions de la forêt boréale où le risque pour la pratique d’un aménagement durable était qualifié de modéré à élevé. Le présent projet portait sur deux de ces zones où l’aménagement peut être durable à condition d’y développer des stratégies d’aménagement adaptées selon les recommandations du rapport scientifique.  

Le projet a permis de prendre la mesure du risque que posent les accidents de régénération dans les territoires où les cycles de feu sont courts ou très courts. Après quelques décennies, des pertes de superficies productives s’accumuleront de manière significative. Ces pertes de territoire productif auxquelles s’ajoutent les diminutions de production sur d’autres territoires demeurés productifs affecteraient considérablement la production ligneuse attendue dans les prochaines décennies. Le projet a également permis de prendre la mesure du risque que pose la paludification dans les territoires situés sur le till de Cochrane, c’est-à-dire la partie nord-ouest de la ceinture d’argile qui parcourt l’Ontario et le Québec, entre le district de Cochrane et l’Abitibi-Témiscamingue. Dans la portion Ouest du territoire pilote, l’historique d’évitement des peuplements susceptibles à la paludification fait en sorte que des superficies récoltables susceptibles à la paludification occuperont une proportion significative des superficies récoltables au cours des 40 prochaines années. Les prochaines décennies apparaissent donc cruciales dans la gestion durable des peuplements susceptibles à la paludification. Le feu présente aussi un risque important pour la stabilité des approvisionnements. Au fil du temps, le feu prélèvera une quantité de bois qui devra être soustrait des approvisionnements. Comme à mesure du rajeunissement de la structure d’âge de la forêt, de plus en plus de peuplements jeunes et non marchands brûleront, les coupes de récupération offre peu d’options de mitigation. À long terme, la quantité de bois disponible à la récolte demeure similaire, peu importe le niveau de coupe choisi. Les aménagistes ne peuvent que décider du moment où cette même quantité de bois peut être récoltée le long de l’horizon de planification.  

Des solutions existent pour diminuer l’ampleur des problèmes documentés, mais leur efficacité est variable et elles mobiliseront des parts importantes des budgets sylvicoles et de voirie. De plus, les actions requérant des investissements accrus sont risquées en considérant les cycles de feu actifs sur le territoire. Les simulations montrent que les risques de voir les investissements sylvicoles anéantis par l’action du feu sont d’autant plus grands que le cycle de feu est court et que l’âge de maturité est élevé. Les résultats du projet aideront à la mise au point d’une approche de gestion du risque qui permettra de prendre des décisions d’aménagement en contrôlant les facteurs de risque qui peuvent compromettre le succès des stratégies d’aménagement. Il y aurait probablement avantage à canaliser les investissements de façon à valoriser davantage le territoire des zones à sensibilité faible. Toutefois, même si dans les autres portions de la forêt boréale les problèmes sont moins aigus, elles présentent, à des degrés variables, les mêmes risques identifiés dans le rapport. Les stratégies d’aménagement devraient prendre en compte les problèmes potentiels selon le niveau de risque que l’on peut leur associer dans toutes les portions de la forêt boréale.  

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.

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.

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.

À l’été 2018, le Ministère des forêts, de la faune et des parcs (MFFP) a tenu des consultations sur un projet de Stratégie nationale de production de bois. Dans ce document se trouvent les commentaires formulés aux noms de l’Institut de recherche sur les forêts (IRF) et de la Chaire en aménagement forestier durable (AFD). Ils sont structurés en fonction des questions contenues dans le formulaire fourni par le MFFP.

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 Québec (c. 483,100 km2) over the last 30 years (between ‘1984?89’ 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, non?migrant, 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 non?productive 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, which 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, suggested that cumulative effects of intensive logging and wildfires on mature forests pose a threat for forest?specialist species. Habitat suitability changes predicted by our course?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 Québec, together with the need to explicitly incorporate it into global change scenarios that better inform decision?makers on conservation and management.

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.

La maîtrise de la végétation concurrente est essentielle afin que les plantations présentent les rendements escomptés, mais ses effets dépendent de son moment d’application. Ainsi, notre objectif était d’évaluer l’effet, après 15–20 ans, d’un délai d’application du dégagement mécanique par rapport au moment requis selon les procédures opérationnelles au Québec (Canada) pour des plantations de Picea glauca et Picea mariana. Nous avons utilisé trois dispositifs expérimentaux de reboisement hâtif comprenant les traitements suivants : i) témoin, non dégagé; ii) dégagement mécanique l’année requise (Requis); iii) dégagement mécanique avec un délai d’une année par rapport à Requis (Requis+1); et, iv) dégagement mécanique avec un délai de deux années (Requis+2). Nos résultats montrent que dans un scénario de reboisement hâtif comprenant un traitement de nettoiement au stade gaulis, il n’y a pas d’impact significatif sur la croissance à retarder l’application d’un dégagement mécanique jusqu’à deux années après le moment requis. Cette marge opérationnelle devrait être utilisée afin d’assurer la réalisation des dégagements; l’omission du dégagement a un effet négatif marqué sur la production aux échelles de l’arbre et du peuplement. Le dégagement a en effet augmenté la hauteur et le diamètre des arbres plantés ainsi que la surface terrière totale des peuplements.

In northeastern Canadian boreal forests, a coarse-filter approach was adopted to provide sustainable ecosystem services in order to maintain a balance between biodiversity, ecosystem function and timber production. An old forest (>100 years) maintenance target was established considering the range of historical variability in the proportion of this forest stage. However, the estimation of the harvesting rate that maintains the target level in old forests did not consider explicitly the impact of current and future, i.e. possibly higher, fire frequency. In this context, we compared historical, current, and future age structures according to recorded or projected fire activity and the current level of harvesting in western Quebec's boreal forest. Results show that under the current rates of harvesting and fire, the proportion of old forests could reach a minimum level rarely seen in the natural landscape in the past. The situation could become even more critical with the projected increase in fire activity under climate change. Numerous forest and fire management solutions exist, such as increasing rotation length, implementing a diversified silviculture, using a fire-smart approach or reaching a better balance between intensive management and conservation. We advocate their rapid implementation to reverse the projected decrease in the proportion of old forests.

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. © 2017 Elsevier B.V.

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.

Fire is the main disturbance in North American coniferous boreal forests. In Northern Quebec, Canada, where forest management is not allowed, the landscape is gradually constituted of more opened lichen woodlands. Those forests are discontinuous and show a low regeneration potential resulting from the cumulative effects of harsh climatic conditions and very short fire intervals. In a climate change context, and because the forest industry is interested in opening new territories to forest management in the north, it is crucial to better understand how and why fire risk varies from the north to the south at the transition between the discontinuous and continuous boreal forest. We used time-since-fire (TSF) data from fire archives as well as a broad field campaign in Quebec’s coniferous boreal forests along four north-south transects in order to reconstruct the fire history of the past 150 to 300 years. We performed survival analyses in each transect in order to (1) determine if climate influences the fire risk along the latitudinal gradient; (2) fractionate the transects into different fire risk zones; and (3) quantify the fire cycle—defined as the time required to burn an area equivalent to the size of the study area—of each zone and compare its estimated value with current fire activity. Results suggest that drought conditions are moderately to highly responsible for the increasing fire risk from south to north in the three westernmost transects. No climate influence was observed in the last one, possibly because of its complex physical environment. Fire cycles are shortening from south to north, and from east to west. Limits between high and low fire risk zones are consistent with the limit between discontinuous and continuous forests, established based on recent fire activity. Compared to the last 40 years, fire cycles of the last 150–300 years are shorter. Our results suggest that as drought episodes are expected to become more frequent in the future, fire activity might increase significantly, possibly leading to greater openings within forests. However, if fire activity increases and yet remains within the range of variability of the last 150–300 years, the limit between open and closed forests should stay relatively stable.

Stand structure and composition play a key role in maintaining the ecological integrity of the boreal forest. However, future changes in climate and disturbance regime could affect these forest attributes. Using provincial forest inventory datasets, we analysed stands dominated by aspen (?75% of the plot total basal area) distributed along a wide longitudinal gradient of environmental conditions across Canada. Stands were classified into three diameter structure types (inverted J, intermediate and advanced). There was no major difference in the distribution pattern of structural types of aspen-dominated stands between the western and eastern Canadian boreal mixedwood forests, despite a marked contrast in climatic conditions and fire regime. These results suggest that the predominance of juvenile structures in the western aspen forests is mainly related to the frequent recurrence of fires, while within eastern aspen forests, the longer fire cycle was not the controlling factor of stand structure. Anthropogenic activities would have strongly shaped the structure of aspen forests in eastern Canada. White spruce in the west and balsam fir in the east are among the main shade-tolerant conifer companion species associated with these stands. Although stand structure and composition were highly related to stand age and site productivity, regional climate and human activities, through their influence on disturbance regime, might have impacted these forest attributes.

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.

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.

Risk analysis entails the systematic use of historical information to determine the frequency, magnitude and effects of unexpected events. Wildfire in boreal North America is a key driver of forest dynamics and may cause very significant economic losses. An actuarial approach to risk analysis based on cumulative probability distributions was developed to reduce the adverse effects of wildfire. To this effect, we developed spatially explicit landscape models to simulate the interactions between harvest, fire and forest succession over time in a boreal forest of eastern Canada. We estimated the amount of reduction of timber harvest necessary to build a buffer stock of sufficient size to cover fire losses and compared it to an insurance premium estimated in units of timber volume from the probability of occurrence and the amount of damage. Overall, the timber harvest reduction we applied was much more costly than the insurance premium even with a zero interest rate. This is due to the fact that the insurance premium is directly related to risk while the timber harvest reduction is not and, as a consequence, is much less efficient. These results, especially the comparison with a standard indicator such as an insurance premium, have useful implications at the time of choosing a mitigation strategy to protect timber supplies against risk without overly diminishing the provision of services from the forest. They are also promoting the use of insurance against disastrous events in forest management planning.

Some regenerating stands of the boreal forest exhibit low juvenile growth after major disturbances, which compromise sustainable forest management objectives. In black spruce-feather moss stands of eastern Canada subject to paludification, careful logging methods could decrease stand productivity with time, by preventing a beneficial reduction in organic soil thickness. This project aimed at confirming decreases in juvenile growth between stands originating from careful logging and the former stands originating from old fires on the same sites. Stem analyses showed that stands originating from CPRS had significant better juvenile height growth than the former stands,but significant lower growth than post-recent fire stands, in the study region. If organic matter thickness apparently played a role in the growth differences observed between fire and harvesting, it was not the only factor determining stand productivity. The cohort status, the climatic regime and the quality of the residual organic matter are other factors that seem to drive productivity according to our results. Our results show that post-harvest management approaches (e.g., site preparation) should be used to increase yields after harvest for the sites to express their full growth potential.

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 m3/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 m3/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.

Il est reconnu que les feux de forêt d’origine naturelle ne peuvent pas et même ne doivent pas être éliminés de la forêt boréale nord-américaine. Les feux de forêt occasionnent des pertes immédiates de volume de bois, perturbent la conversion de la structure courante d’âge de la forêt vers une structure cible et empêchent l’approvisionnement planifié en bois (APB) d’être atteint de manière constante. Dans cet article, nous explorons dans quelle mesure les déficits périodiques en bois disponible causés par divers risques de feux peuvent être atténués par la coupe de récupération et par le degré de tolérance des gestionnaires forestiers face à ces déficits, et ceci en fonction de la structure d’âge des forêts. Les simulations sont faites en utilisant une représentation temporelle déterministe et stochastique des feux. Les résultats montrent que la fréquence des périodes en déficit de bois peut être réduite par la coupe de récupération et par l’introduction de mesures de tolérance à ces déficits, et que ce potentiel d’atténuation est influencé par la structure d’âge de la forêt initiale et par le niveau de pertes par le feu. Les résultats montrent également que même un taux de coupe de récupération à 100 % ne peut pas compenser entièrement les pertes de bois par le feu et éliminer les déficits périodiques qui en résultent. En outre, l’ajout de la variabilité interannuelle des feux réduit l’efficacité des deux mesures d’atténuation. Enfin, puisque l’APB n’est en fait jamais réalisé dans les forêts sujettes aux feux, le coût réel d’une réduction l’APB doit être estimé non pas par la différence l’APB, mais plutôt par la différence plus réaliste de récolte de bois réalisée.q

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.

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.

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.

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.

La ceinture d’argile québéco-ontarienne est susceptible à l’entourbement. Dans cette région, on s’attend à ce que les peuplements convergent vers des peuplements d’épinettes noires (Picea mariana) ouverts et peu productifs peu importe leur composition initiale. Cependant, des études sur l’Holocène et l’historique de la déglaciation suggèrent que la susceptibilité à l’entourbement pourrait varier en fonction du dépôt de surface et qu’ainsi les peuplements pourraient présenter des caractéristiques de succession différentes selon le dépôt. À l’aide d’une base de données forestières spatio-temporelles, nous avons quantifié le taux et l’âge de transition des peuplements entre différents stades de succession pour différents dépôts de surface de la ceinture d’argile. Nos résultats suggèrent qu’une convergence totale vers des peuplements d’épinettes noires ouverts et moins productifs peut se produire, mais qu’elle peut s’étaler sur une très longue période (plus de 500 ans), surtout dans le cas des dépôts peu susceptibles à l’entourbement comme les dépôts grossiers. De plus, nos résultats suggèrent que dans le cas des successions qui débutent avec des peuplements d’épinettes noires ouverts et moins productifs, leur capacité à atteindre un état plus productif est aussi liée au type de dépôt de surface. Sur la base des âges préférentiels de transition, des taux de transition et des types de peuplements impliqués dans la succession, nous suggérons un gradient croissant de susceptibilité à l’entourbement allant des dépôts grossiers aux dépôts fins argileux, et enfin, aux dépôts fins ayant subi une réavancée des glaces lors de la dernière grande déglaciation. Dans un contexte d’aménagement forestier, une bonne gestion de la couche organique du sol en fonction de la susceptibilité du dépôt à l’entourbement pourrait assurer le retour de peuplements productifs.

In this study, we characterised the composition and configuration of post-fire residual habitats belonging to two physiographic zones of the black spruce–moss domain in western Quebec. Thirty-three large fires (2000–52 000 ha) were selected and extracted on classified Landsat satellite imagery. The results show that a minimum of 2% and a maximum of 22% of burned areas escaped fire, with an overall average of 10.4%. The many forest patches that partially or entirely escaped fire formed residual habitats (RHs). It was found that although the area of RHs follows a linear relationship with fire size, their proportion appears relatively constant. Spatial analyses showed that the fires could be separated into two groups depending on the physiographic zones (East-Canadian Shield v. West-Clay Belt Lowlands). Fires in the west zone generate less RHs and appear to be associated with more extreme weather conditions. In most cases there was no association with water or wetlands; in some fires the presence of RHs is associated with the proximity of water bodies. The failure to find an association between RHs and wetlands suggests that this type of environment is part of the fuel. Coniferous woodland with moss appears particularly overrepresented within RHs. Our results suggest that the local and regional physiographic conditions strongly influence the creation of RHs; therefore, it is important to consider those differences when applying ecosystem-based management.

En forêt boréale québécoise, l’aménagement forestier écosystémique tend à s’inspirer de la dynamique naturelle des écosystèmes afin de réduire les écarts entre les forêts aménagées et les forêts non aménagées. La variabilité dans la taille, la sévérité et la fréquence des feux de forêts créent une mosaïque paysagère hétérogène formée de peuplements d’âges, de composition et de structures différentes. Au sein des superficies brûlées, certaines parcelles rescapées, les îlots résiduels, jouent un rôle prépondérant dans le fonctionnement à long terme de ces milieux perturbés en assurant le maintien d’habitat pour de nombreuses espèces et en contribuant à la recolonisation de la matrice brûlée. Les coupes à rétention variable sont des traitements sylvicoles intéressants qui permettent de produire des îlots forestiers dans les secteurs de coupe. Afin de définir les lignes directrices nécessaires aux aménagistes pour émuler de tels îlots résiduels, la caractérisation de la structure, du fonctionnement et de la dynamique de ces véritables « Arches de Noé » naturelles a été réalisée par le biais de différentes études regroupées sous le projet ILOTS.

En forêt boréale de l’est du Canada, le terme enfeuillement est généralement réservé au phénomène d’augmentation de la proportion relative de feuillus de lumière, notamment du peuplier faux-tremble (Populus tremuloides), qui survient après coupe forestière. Il est perçu comme un processus qui, bien qu’ayant cours à une échelle locale, a des répercussions à l’échelle régionale. L’objectif de cette étude était de caractériser l’augmentation du couvert de peuplier suite à la coupe totale dans la plaine argileuse de l’Abitibi, dans le nord-ouest du Québec. Les données de 3 inventaires forestiers provinciaux (1970, 1980 et 1990) du ministère des Ressources naturelles et de la Faune du Québec ont été analysées pour 2 régions écologiques, la plaine de l’Abitibi (sous-domaine de la sapinière à bouleau blanc de l’Ouest) et la plaine du lac Matagami (sous-domaine de la pessière à mousses de l’Ouest). Un bilan des transformations qu’a subi le couvert forestier sous l’influence des coupes totales sur près de 3 décennies a été dressé en fonction du temps écoulé depuis la coupe, du dépôt/drainage et de la région écologique. Les résultats montrent que les argiles mésiques sont particulièrement sensibles à l’enfeuillement par le peuplier et que le taux global d’enfeuillement dans la sapinière et la pessière est d’environ 30 % entre les années 1970 à 1990. Par ailleurs, malgré une plus grande abondance de milieux humides et organiques peu propices à l’établissement du peuplier dans la région de la pessière, les 2 régions comparées semblent démontrer la même sensibilité (ou propension) à l’enfeuillement. Sur les sites considérés sensibles, la pessière noire devrait donc faire l’objet de pratiques d’aménagement adaptées visant à limiter l’augmentation de la composante feuillue.

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 Québec 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.

Abstract

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.

Résumé

Le rajeunissement de la forêt boréale canadienne est maintenant d’avantage lié aux activités forestières qu’à l’action des feux de forêt. Les différences majeures dans les patrons spatiaux créés par ces régimes de perturbations suscitent donc des inquiétudes quant aux pratiques forestières actuelles. Dans le cadre d’une approche d’aménagement écosystémique qui s’inspire des perturbations naturelles, nous proposons d’analyser la variation temporelle des patrons d’aire en régénération créés par le feu (brûlis de 25 ans ou moins) et d‘élaborer des objectifs stratégiques d’espacement et de taille des chantiers de coupe. Les variations dans la proportion de forêt équienne (peuplements de moins de 100 ans) ont d’abord été évaluées et comparées à des valeurs attendues pour plusieurs régimes de feu (sept cycles de feux : 50 à 400 ans, et trois tailles moyennes : 3000, 15 000 et 60 000 ha). Ensuite, la variabilité temporelle liée à la distribution de taille et à la dispersion des aires en régénération a été évaluée. Telle qu’attendue la proportion de forêt de moins de 100 ans est inversement proportionnelle à la longueur du cycle de feu. Cependant, la variabilité temporelle associée à cette proportion serait davantage influencée par la taille moyenne des feux. La taille moyenne des feux serait aussi le principal agent qui influence la taille et l’espacement des aires en régénération. Suite à ces observations nous proposons des lignes directrices d’aménagement qui précisent les proportions de forêt aménagée de façon équienne, leur distribution quant à la superficie des chantiers de coupe et leur dispersion ou espacement minimal.

Abstract

The aim of this study was to evaluate if and how the previous land use (here: agriculture versus forest) has influenced the floristic composition now observed in the understory of jack pine plantations of Abitibi-Temiscamingue (North-Eastern Canada). Floristic inventories were conducted in jack pine plantations which had been established on forest sites after logging and on sites previously utilized for agriculture. These were compared to one another and to inventories conducted in natural forests and on agricultural sites which had not (not yet) been planted with trees (old fields). The vegetation community composition of the sites is analysed using several multivariate techniques. Results show that the flora of plantations on old agricultural fields is markedly different than that of plantations established on sites which were previously forested and that the two types of plantation are unlikely to become floristically the same before the anticipated harvest time. Comparing old and young plantations and old fields in light of natural forests sites, we show that the previous land-use is a key determinant of future compositional differences in plantations. This must be considered when we evaluate the impact of establishing plantations on sites with different land-use histories.

Résumé

Le but de cette étude est d’évaluer dans quelle mesure l’utilisation du sol a pu influencer la composition floristique observée aujourd’hui dans le sous-bois de plantations de pin gris en Abitibi-Témiscamingue (nord-ouest du Québec). Des inventaires floristiques ont été effectués dans des plantations établies sur d’anciennes friches agricoles et sur d’anciennes friches forestières. Une comparaison (à l’aide d’analyses multivariées) est faite entre les sous-bois de chaque type de plantation, ainsi qu’avec ceux de sites présentement en friche et de forêts naturelles. Nos résultats montrent que la flore du sous-bois de plantations réalisées sur d’anciennes terres agricoles est très différente du sous-bois de plantations réalisées en sites forestiers. Les deux types de sites ont ainsi peu de chances de se ressembler dans le futur, surtout compte tenu des délais d’exploitation. L’historique d’utilisation du sol est donc un facteur déterminant qui explique les différences de composition floristique dans le sous-bois des plantations, ce qui doit être pris en compte dans l’évaluation des impacts de plantations réalisées dans ces environnements fort différents.© 2007 Elsevier B.V. All rights reserved.

Abstract: 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.

Résumé :Au cours de la dernière décennie, un intérêt grandissant pour le développement d'approches d'aménagement basées sur notre compréhension de la dynamique historique des perturbations naturelles s'est manifesté. Ces approches reposent sur l'idée qu'un aménagement favorisant une composition des paysages et une structure des peuplements similaires à celles créées dans les forêts passées devrait aussi maintenir la diversité biologique et les fonctions écologiques essentielles de ces mêmes paysages et peuplements. Dans les paysages contrôlés par les feux, cette approche est possible seulement si les fréquences de feux actuelles et futures sont suffisamment faibles lorsque comparées aux fréquences pré-industrielles, cela afin de permettre de substituer le feu par la coupe forestière. Nous évaluons cette possibilité en comparant les fréquences de feux actuelles et futures aux fréquences historiques à partir d'études réalisées dans la forêt commerciale québécoise. Les fréquences actuelles et futures des feux, simulées en utilisant deux scénarios de concentration de CO2 (2× et 3× la concentration actuelle), sont plus faibles que les fréquences passées pour la majorité du territoire, suggérant que l'aménagement forestier pourrait potentiellement être utilisé afin de recréer la structure d'âge de la forêt soumise à un régime de feux sévères. Les aménagements équiennes actuels tendent toutefois à réduire la variabilité naturelle du système: par exemple, un aménagement équienne amputera, à terme, la structure d'âge de la forêt naturelle éliminant ainsi les forêts surannées et anciennes du paysage. Le développement de techniques de sylviculture permettant le maintien d'un spectre de compositions et structures forestières à différentes échelles de paysage est une des avenues proposées afin de maintenir cette variabilité. ©2006 NRC Canada

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 identifi- cation 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 indicate that seed tree 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 seedlings preferentially established on mineral soil, and that woody debris seems to be a good substrate for germination and survival. Seedlings establish more frequently and grow better where thickness of residual organic matter is lowest. Crown fire severity, combined with severity at ground level may therefore be a good indicator of regeneration success in coniferous stands.

En forêt boréale, bien que les régimes de feux soient dominés par des feux de cime couvrant de grandes superficies, ceux-ci brûlent rarement la forêt entièrement et uniformément. Par conséquent, il en résulte une mosaïque de végétation brûlée à divers degrés de sévérité. Cette étude avait pour objectifs de caractériser 1) la régénération arborescente dans des peuplements non récupérés et composés principalement de pins gris et d’épinettes noires dans quatre feux de 6 et 7 ans selon le degré de sévérité du feu au niveau de la couronne des arbres, 2) les préférences d’établissement des semis sur les différents types de litière mis en place à la suite du passage du feu. Des régressions logistiques ont indiqué que l’abondance des semenciers influençait le succès de la régénération. La relation entre la sévérité du feu au niveau de la couronne et le succès de la régénération n’était pas significative, mais le recrutement des semis semblait être limité dans les zones où la sévérité au niveau de la couronne était de légère à modérée. Des régressions de Poisson ont montré que les semis se sont établis préférentiellement sur le sol minéral et que les débris ligneux semblent être un bon substrat de germination et de survie. Une analyse de variance a montré que les semis semblent mieux s’établir et mieux croître là où l’épaisseur de matière organique résiduelle est la moins importante. La sévérité au niveau de la couronne combinée à la sévérité au sol peuvent donc être de bons indicateurs de la régénération dans les peuplements conifériens.

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. © 2005 Elsevier B.V. All rights reserved.

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 increasingly 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-m² 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 replacement 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 implications of our findings for stand management and conservation are discussed.

Les auteurs ont étudié le développement et la mortalité de communautés d'arbres en peuplements mixtes à plusieurs échelles spatiales dans une chronoséquence de 236 ans au lac Duparquet, Québec. Les relations spatiales, les associations d'espèces et les patrons de remplacement entre les espèces du couvert dominant et celles du sous-bois ont été étudiés en utilisant des statistiques spatiales, des indices d'agglomération, des tests de G et des matrices de transition. Les résultats de ces analyses montrent que les communautés d'arbres intolérants et tolérants à l'ombre forment un nombre croissant d'agglomérations de petites tailles à l'étape du milieu de la succession. La formation de ces associations de petites tailles semble partiellement attribuable, d'une part, aux tiges opprimées qui sont libérées à l'intérieur d'un couvert qui s'éclaircit lentement et, d'autre part, au recrutement de conifères tolérants à l'ombre. Dans des parcelles de 1 ha, la richesse en espèces et l'équitabilité ont culminé dans des sous-parcelles de 25, 100 et 400 m² de peuplements parvenus à l'étape du milieu de la succession. La diversité du type de couvert forestier estimée à partir de photographies aériennes a aussi culminé sur des territoires parvenus à l'étape de milieu de succession. La plus vieille placette de 1 ha était dominée par des agglomérations monospécifiques de thuya occidental (Thuja occidentalis L.), l'espèce la plus susceptible de remplacer les arbres morts et vivants et l'espèce la plus abondante dans les nombreuses sous-parcelles occupées par des arbres dont le DHP est inférieur à 8 cm. Malgré cette dominance, les espèces à forte longévité que sont le bouleau à papier (Betula papyrifera Marsh.) et les semis résiduels de sapin baumier (Abies balsamea (L.) Mill.) peuvent maintenir la mixité d'espèces de ce type de peuplement dans le futur. Les implications de nos résultats sont discutées en regard de l'aménagement et de la conservation des forêts. ©2005 NRC Canada

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.

Le changement climatique pose de nombreux défis pour l’aménagement forestier durable car le climat influence les processus naturels à de multiples échelles spatiales et temporelles. Parmi ces enjeux, l’intégration du risque de feu à l’aménagement forestier est de plus en plus discutée.Nous proposons ici une évaluation de la capacité adaptative de l’aménagement forestier aux changements de régimes de feu sous l’influence du changement climatique en forêt boréale au Québec. L’adaptation commence par une réinterprétation de certaines pratiques forestières courantes qui traitent déjà de la variabilité climatique. Parmi elles, la suppression des feux et l’amélioration de la régénération peuvent contribuer à atténuer certains impacts du changement climatique. Cependant, il y a une demande grandissante pour des approches plus intégratives et spatialement explicites afin de diminuer la vulnérabilité de l’aménagement forestier face aux changements du risque de feu. Certaines stratégies en développement, comme l’aménagement du combustible et l’approche et de la triade (système de zones de conservation, d’aménagement intensif et extensif) présentent un potentiel intéressant pour intégrer le risque de feu à la planification forestière. Alors que l’aménagement du combustible et la suppression des feux sont indiqués pour les régimes de feu particulièrement sévères, la protection des forêts contre les insectes et le maintient d’un cycle de perturbation plus court par l’aménagement forestier sont des options d’adaptation privilégiées lorsque le cycle de feu s’allonge sous l’influence du changement climatique.

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.

Over the past decades, there has been an increasing interest in the development of forest management approaches that are based on an understanding of historical natural disturbance dynamics. The rationale for such an approach is that management to favor landscape compositions and stand structures similar to those of natural ecosystems should also maintain biological diversity and essential ecological functions. In fire-dominated landscapes, this approach is possible only if current and future fire frequencies are sufficiently low, comparing to pre-industrial fire frequency, that we can substitute fire by forest management. We address this question by comparing current and future fire frequency to historical reconstruction of fire frequency from studies realized in the Canadian boreal forest. Current and simulated future fire frequencies using 2 and 3 x CO2 scenarios are lower than the historical fire frequency for many sites, suggesting that forest management could potentially be used to recreate the forest age structure of fire-controlled pre-industrial landscapes. There are however, important limitations to the current even-age management.

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% and 29% of the total area burned, respectively. With regard to ignition sources, lightning was responsible for 38% of the fires while humans were the ignition agent for 62% of fires. The fire regime parameters (burn rate, fire occurrence, and size) were subjected to a stepwise regression analysis on the basis of regional landscape units. Models indicate that climatic factors, particularly summer precipitation and maximum temperatures, play a primary role in forest fire dynamics, regardless of the ignition source. Fire occurrence models were the most predictable with R-2 values of 0.79 and 0.60 for lightning fires and human-caused fires, respectively. Models of burned areas reached an R-2 value of 0.63 for lightning but only 0.22 for human-caused fires; on the other hand, the fire-size model for human-caused fires showed an R-2 value of 0.57 but only 0.24 for lightning fires. In the case of human-induced fires, the density of the road network and sand deposits were important in fire occurrence and burned areas models. Once characterized, landscape units tend to group together naturally, forming extensive areas in which the fire regime is relatively homogeneous. The results of the regionalization based on lightning fire regimes are discussed from the standpoint of sustainable forest management.

In Canada, there are still extensive tracts of boreal forest consisting of stands that have resulted from natural disturbances. The country's forests are a mosaic made up to a large extent of old-growth forest that is beyond commercial harvesting age, especially in the boreal forest of eastern Canada. As areas of boreal forest under management steadily expand, as demand for forest products continues to grow and as rotation periods become shorter in response to silvicultural practices, the forest cover will inevitably become younger, causing changes to the structure and composition of the mosaic of forest stands that will affect the aspect of entire landscapes. These changes may have an adverse impact on biological diversity. Forest birds are one group of living organisms that may respond quickly to the advent of younger forest landscapes, thereby acting as a biological indicator. In this paper, we discuss some of the problems that birds face as a result of the truncation of the age-class distribution of managed forest landscapes in eastern Canada's coniferous boreal forest, using data obtained from our research in the Clay Belt region of Quebec and Ontario. More specifically, we look at how birds respond to changes in forest structure and composition in terms of time since natural disturbances, and to variation in dead trees availability. We then consider the impact of the prospective rejuvenation of the forest cover in managed forest landscapes, and possible solutions aimed at mitigating that impact through new management strategies based on the maintenance of forest ecosystem diversity. The ability of these new management strategies to maintain the ecological integrity of bird communities provides an indication of their potential as tools for contributing to the maintenance of biological diversity in a broader sense.

Au Canada, la forêt boréale comporte encore des superficies importantes de peuplements issus de perturbations naturelles. Une forte proportion de la mosaïque forestière est constituée de vieilles forêts qui dépassent l’âge commercial d’exploitation forestière, notam-ment dans la forêt boréale de l’est du Canada. Avec l’importance grandissante des territoires aménagés en forêt boréale, les besoins toujours croissants pour la matière ligneuse et le recours à une sylviculture réduisant les périodes de rotation, des changements dans la structure et la composition des mosaïques de peuplements forestiers liés au rajeunissement du couvert forestier sont à anticiper à l’échelle des paysages. Ces changements peuvent causer des préjudices à la diversité biologique. L’avifaune représente l’un des groupes d’organismes qui peut répondre rapidement au rajeunissement des paysages forestiers et servir ainsi d’indicateur biologique. Dans cet article, nous exposons certains des problèmes auxquels est confrontée l’avifaune face au rajeunissement anticipé des mosaïques forestières aménagées de la forêt boréale résineuse de l’est du Canada à partir de nos travaux réalisés dans la région de la ceinture d’argile du Québec et de l’Ontario. Plus spécifiquement, nous examinons la réponse des oiseaux aux changements de structure et de composition des forêts en fonction du temps depuis les dernières perturbations dans des mosaïques forestières naturelles, ainsi qu’en rapport avec la varia-tion de la densité d’arbres morts disponibles dans ces écosystèmes. Nous discutons ensuite de l’effet du rajeunissement du couvert foresti-er qui est anticipé dans les mosaïques forestières aménagées de ces écosystèmes et des solutions envisageables pour atténuer cet effet au moyen des nouvelles stratégies d’aménagement fondées sur le maintien de la diversité des écosystèmes forestiers. La capacité de ces nouvelles stratégies d’aménagement à maintenir l’intégrité écologique des communautés d’oiseaux forestiers donne une indica-tion de leur potentiel à contribuer plus largement au maintien de la diversité biologique.

Boreal forest ecosystems are adapted to periodic disturbance, but there is widespread concern that conventional forest practises degrade plant communities. We examined vegetation diversity and composition after clearcut logging, mechanical and chemical site preparation in eight 5- to 12-yr old studies located in southern boreal forests of British Columbia and Quebec, Canada to find useful indicators for monitoring ecosystem integrity and to provide recommendations for the development and testing of new silvicultural approaches. Community-wide and species-specific responses were measured across gradients of disturbance severity and the results were explained in terms of the intermediate disturbance hypothesis and a simple regeneration model based on plant life history strategies. Species richness was 30 to 35% higher 5 to 8 years after clearcut logging than in old forest. Total and vascular species diversity generally peaked on moderately severe site treatments, while non-vascular diversity declined with increasing disturbance severity. On more-or-less mesic sites, there was little evidence of diversity loss within the range of conventional silvicultural disturbances; however, there were important changes in plant community composition. Removing soil organic layers caused a shift from residual and resprouting understory species to ruderal species regenerating from seeds and spores. Severe treatments dramatically increased non-native species invasion. Two important challenges for the proposed natural dynamics-based silviculture will be 1) to find ways of maintaining populations of sensitive non-vascular species and forest mycoheterotrophs, and 2) to create regeneration niches for disturbance-dependent indigenous plants without accelerating non-native species invasion.

Forest ecosystem management, based partly on a greater understanding of natural disturbance regimes, has many variations but is generally considered the most promising approach to accommodating biodiversity concerns in managed forested regions. Using the Lake Duparquet Forest in the southeastern Canadian boreal forest as an example, we demonstrate an approach that attempts to integrate forest and stand-level scales in biodiversity maintenance. The concept of cohorts is used to integrate stand age, composition and structure into broad successional or stand development phases. Mean forest age (MFA), because it partly incorporates historic variability of the regional fire cycle, is used as a target fire cycle. At the landscape level, forest composition and cohort objectives are derived from regional natural disturbance history, ecosystem classification, stand dynamics and a negative exponential age distribution based on a 140 year fire cycle. The resulting multi-cohort structure provides a framework for maintaining the landscape in a semi-natural age structure and composition. At the stand level, the approach relies on diversifying interventions, using both even-aged and uneven-aged silviculture to reflect natural stand dynamics, control the passage (‘‘fluxes’’) between forest types of different cohorts and maintain forest-level objectives. Partial and selective harvesting is intended to create the structural and compositional characteristics of mid- to late-successional forest types and, as such, offers an alternative to increasing rotation lengths to maintain ecosystem diversity associated with over-mature and old-growth forests. The approach does not however supplant the necessity for complementary strategies for maintaining biodiversity such as the creation of reserves to protect rare, old or simply natural ecosystems. The emphasis on maintaining the cohort structure and forest type diversity contrasts significantly with current even-aged management in the Canadian boreal forest and has implications for stand-level interventions, notably in necessitating a greater diversification of silvicultural practices including more uneven-aged harvesting regimes. The approach also presents a number of operational challenges and potentially higher risks associated with multiply stand entries, partial cutting and longer intervals between final harvests. There is a need for translating the conceptual model into a more quantitative silvicultural framework. Silvicultural Forest ecosystem management, based partly on a greater understanding of natural disturbance regimes, has many variations but is generally considered the most promising approach to accommodating biodiversity concerns in managed forested regions. Using the Lake Duparquet Forest in the southeastern Canadian boreal forest as an example, we demonstrate an approach that attempts to integrate forest and stand-level scales in biodiversity maintenance. The concept of cohorts is used to integrate stand age, composition and structure into broad successional or stand development phases. Mean forest age (MFA), because it partly incorporates historic variability of the regional fire cycle, is used as a target fire cycle. At the landscape level, forest composition and cohort objectives are derived from regional natural disturbance history, ecosystem classification, stand dynamics and a negative exponential age distribution based on a 140 year fire cycle. The resulting multi-cohort structure provides a framework for maintaining the landscape in a semi-natural age structure and composition. At the stand level, the approach relies on diversifying interventions, using both even-aged and uneven-aged silviculture to reflect natural stand dynamics, control the passage (‘‘fluxes’’) between forest types of different cohorts and maintain forest-level objectives. Partial and selective harvesting is intended to create the structural and compositional characteristics of mid- to late-successional forest types and, as such, offers an alternative to increasing rotation lengths to maintain ecosystem diversity associated with over-mature and old-growth forests. The approach does not however supplant the necessity for complementary strategies for maintaining biodiversity such as the creation of reserves to protect rare, old or simply natural ecosystems. The emphasis on maintaining the cohort structure and forest type diversity contrasts significantly with current even-aged management in the Canadian boreal forest and has implications for stand-level interventions, notably in necessitating a greater diversification of silvicultural practices including more uneven-aged harvesting regimes. The approach also presents a number of operational challenges and potentially higher risks associated with multiply stand entries, partial cutting and longer intervals between final harvests. There is a need for translating the conceptual model into a more quantitative silvicultural framework. SilviculturalForest ecosystem management, based partly on a greater understanding of natural disturbance regimes, has many variations but is generally considered the most promising approach to accommodating biodiversity concerns in managed forested regions. Using the Lake Duparquet Forest in the southeastern Canadian boreal forest as an example, we demonstrate an approach that attempts to integrate forest and stand-level scales in biodiversity maintenance. The concept of cohorts is used to integrate stand age, composition and structure into broad successional or stand development phases. Mean forest age (MFA), because it partly incorporates historic variability of the regional fire cycle, is used as a target fire cycle. At the landscape level, forest composition and cohort objectives are derived from regional natural disturbance history, ecosystem classification, stand dynamics and a negative exponential age distribution based on a 140 year fire cycle. The resulting multi-cohort structure provides a framework for maintaining the landscape in a semi-natural age structure and composition. At the stand level, the approach relies on diversifying interventions, using both even-aged and uneven-aged silviculture to reflect natural stand dynamics, control the passage (‘‘fluxes’’) between forest types of different cohorts and maintain forest-level objectives. Partial and selective harvesting is intended to create the structural and compositional characteristics of mid- to late-successional forest types and, as such, offers an alternative to increasing rotation lengths to maintain ecosystem diversity associated with over-mature and old-growth forests. The approach does not however supplant the necessity for complementary strategies for maintaining biodiversity such as the creation of reserves to protect rare, old or simply natural ecosystems. The emphasis on maintaining the cohort structure and forest type diversity contrasts significantly with current even-aged management in the Canadian boreal forest and has implications for stand-level interventions, notably in necessitating a greater diversification of silvicultural practices including more uneven-aged harvesting regimes. The approach also presents a number of operational challenges and potentially higher risks associated with multiply stand entries, partial cutting and longer intervals between final harvests. There is a need for translating the conceptual model into a more quantitative silvicultural framework. Silvicultural trials have been established to evaluate stand-level responses to treatments and operational aspects of the approach. © 2002 Elsevier B.V. All rights reserved.

The combination of certain features of fire disturbance, notably fire frequency, size and severity, may be used to characterize the disturbance regime in any region of the boreal forest. As some consequences of fire resemble the effects of industrial forest harvesting, conventional forest management is often considered as a disturbance that has effects similar to those of natural disturbances. Although the analogy between forest management and are disturbance in boreal ecosystems has some merit, it is important to recognise that it also has its limitations. Short fire cycles generally described for boreal ecosystems do not appear to be universal rather, important spatial and temporal variations have been observed in Canada. These variations in the fire cycle have an important influence on forest composition and structure at the landscape and regional levels. Size and severity of fires also show a large range of variability. In regions where the natural matrix of the boreal forest remains relatively intact, maintenance of this natural variability should be targeted by forest managers concerned with biodiversity conservation. Current forest management tends to reduce this variability: for example, fully regulated, even-aged management will tend to truncate the natural forest age distribution and eliminate over-mature and old-growth forests from the landscape. We suggest that the development of strategic-level forest management planning approaches and silvicultural techniques designed to maintain a spectrum of forest compositions and structures at different scales in the landscape is one avenue to maintain this variability. Although we use the boreal forest of Quebec for our examples, it is possible to apply the approach to those portions of the boreal forest where the fire regime favours the development of even-aged stands in burns.

Variation in canopy composition can influence ecosystem processes, such as nutrient cycling and light transmittance, even when environmental soil conditions are similar. To determine whether forest cover type influences species composition of the understory vegetation (herbs and shrubs), the composition of this layer was studied on two different surface deposits, clay and till, and under four different forest cover types dominated, respectively, by Populus tremuloides Michx. (aspen), Betula papyrifera Marsh. (white birch), Pinus banksiana Lamb. (jack pine), and Picea glauca (Moench) Voss - Abies balsamea (L.) Mill. (spruce-fir) over similar environmental conditions. Detrended correspondence analysis and analysis of variance performed on the ordination scores revealed that understory plant composition was highly affected by surface deposit and forest cover. The gradient observed in the correspondence analysis proceeds from aspen, white birch, spruce-fir, to jack pine. Indicator species were identified for each surface deposit and cover type, and most of them were associated with either jack pine or aspen. The richness, evenness, and diversity of the understory vegetation did not vary between cover types, but were affected by surface deposit. By controlling ecosystem processes such as light transmittance and nutrient cycling, forest cover influences understory composition.