Old-growth forests provide many precious ecological, economic, cultural, recreational and spiritual ecosystem services (Wirth et al., 2009). These ecosystem services are generally specific in comparison to younger or managed forests, explaining the high ecological and social value we place on their conservation, especially considering worldwide threats to them from human activities. These pressures are expected to increase in the coming decades in the face of global change (Wright and Muller-Landau, 2006; Kuuluvainen and Gauthier, 2018; Sabatini et al., 2018). Understanding how to protect and restore these old-growth forests in a changing world has therefore become a major forest management issue (Puettmann et al., 2015).

Assessing the impacts of natural disturbance on the functioning of complex forest systems are imperative in the context of global change. The unprecedented rate of contemporary species extirpations, coupled with widely held expectations that future disturbance intensity will increase with warming, highlights a need to better understand how natural processes structure habitat availability in forest ecosystems. Standardised typologies of tree-related microhabitats (TreMs) have been developed to facilitate assessments of resource availability for multiple taxa. However, natural disturbance effects on TreM diversity have never been assessed. We amassed a comprehensive dataset of TreM occurrences and a concomitant 300-year disturbance history reconstruction that spanned large environmental gradients in temperate primary forests. We used nonlinear analyses to quantify relations between past disturbance parameters and contemporary patterns of TreM occurrence. Our results reveal that natural forest dynamics, characterised by fluctuating disturbance intervals and variable severity levels, maintained structurally complex landscapes rich in TreMs. Different microhabitat types developed over time in response to divergent disturbance histories. The relative abundance of alternate TreMs was maximised by unique interactions between past disturbance severity and elapsed time. Despite an unequal distribution of individual TreMs, total microhabitat diversity was maintained at constant levels, suggesting that spatially heterogeneous disturbances maintained a shifting mosaic of habitat types over the region as a whole. Our findings underscore the fundamental role of natural processes in promoting conditions that maximise biodiversity potential. Strict conservation and management systems that preserve natural disturbance outcomes, including associated biological legacies, may therefore safeguard biodiversity at large scales.

Sustainable management of forest ecosystems requires the use of reliable and easy to implement biodiversity and naturalness indicators. Tree-related microhabitats (TreMs) can fulfill these roles as they harbor specialized species that directly or indirectly depend on them, and are generally more abundant and diverse in natural forests or forests unmanaged for several decades. The TreM concept is however still recent, implying the existence of many knowledge gaps that can challenge its robustness and applicability. To evaluate the current state of knowledge on TreMs, we conducted a systematic review followed by a bibliometric analysis of the literature identified. A total of 101 articles constituted the final corpus. Most of the articles (60.3%) were published in 2017 or after. TreM research presented a marked lack of geographical representativity, as the vast majority (68.3%) of the articles studied French, German or Italian forests. The main themes addressed by the literature were the value of TreMs as biodiversity indicators, the impact of forest management on TreMs and the factors at the tree- and stand-scales favoring TreMs occurrence. Old-growth and unmanaged forests played a key role as a “natural” forest reference for these previous themes, as TreMs were often much more abundant and diverse compared to managed forests. Arthropods were the main phylum studied for the theme of TreMs as biodiversity indicators. Other more diverse themes were identified, such as restoration, remote sensing, climate change and economy and there was a lack of research related to the social sciences. Overall, current research on TreMs has focused on assessing its robustness as an indicator of biodiversity and naturalness at the stand scale. The important geographical gap identified underscores the importance of expanding the use of the TreMs in other forest ecosystems of the world. The notable efforts made in recent years to standardize TreM studies are an important step in this direction. The novelty of the TreM concept can partially explain the thematic knowledge gaps. Our results nevertheless stress the high potential of TreMs for multidisciplinary research, and we discuss the benefits of expanding the use of TreMs on a larger spatial scale.

Old-growth forests of different ages provide specific structures, habitats and ecosystem services. Methods to distinguish this internal diversity are still rare, especially in boreal forests. This research therefore aims to determine the ability of Airborne Laser Scanning (ALS) technology to identify age-related structural diversity in old-growth boreal forests. The study area was located in primary boreal forests in Quebec (Canada) dominated by black spruce (Picea mariana). This area contained 71.8 km2 of early old-growth forests (burned 110 years ago), 17.1 km2 of late old-growth forests (protected areas; unburned for at least 250 years) and 370 km2 of old-growth forests of unknown age (> 125-years-old). We divided the study area into 1 ha tiles, where we extracted seven ALS indices representing vertical and horizontal forest structure. We trained random forest models using an iterative approach to discriminate between early and late old-growth forests based on ALS indices. Model predictions were applied to the old-growth tiles of unknown age, and to 86 field plots (28 from provincial forest surveys and 58 from a dedicated survey of old-growth forests) to evaluate the predictive capacity of the models. The models very accurately distinguished early and late old-growth forests (error-rate = 4.9%). Old-growth survey plots confirmed model ability to discriminate early and late old-growth forests, but not provincial survey plots, possibly because of a lower reliability of these data when forest age exceeds 150 years. Model predictions for tiles of unknown age highlighted the presence of very large tracts of late old-growth forests within a matrix of old-growth forests of intermediate age (≈150–200 years). Overall, ALS-data can contribute to a finer structural age distinction and mapping of boreal old-growth forests. This enhanced knowledge of old-growth landscapes will greatly help to improve their protection, restoration and management. The scarcity of reliable field data for model evaluation is, however, a limitation to be addressed.

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.

In old-growth forests, natural disturbances form a complex mosaic of structures, providing a wide diversity of habitats and functions of great importance. Old-growth forests are still often seen as a homogeneous whole and few remote-sensing approaches have been tested to identify their structural diversity, especially in boreal forests. The aim of this study is to use a combination of airborne LiDAR and satellite imagery to identify and discriminate old-growth forest structures resulting from different disturbance histories. The study area, which was located in the mixed boreal forest of Quebec (Canada), is Monts Valin National Park and adjacent managed territories. Balsam fir (Abies balsamea (L.) Mill) is the dominant species in the study area, but hardwood species such as white birch (Betula papyrifera Marsh.) and trembling aspen (Populus tremuloides Michx.) can also be abundant in the early succession stages. Four forest classes were studied: second-growth (logged between 1970 and 1980); transition old-growth (burned in 1920); undisturbed old-growth (unburned for at least 125 years); and disturbed old-growth forest (unburned for at least 125 years, but severely disturbed by an insect outbreak around 1980). A multivariate Random Forest model was used to discriminate the classes on 6466 1 ha tiles, based on 11 complementary LiDAR and satellite-derived indices describing stand vertical and horizontal structure, together with “greenness” and disturbance history over the last 30 years. This model had high predictive efficiency (AUC = 94.2), with 81.8% of the tiles accurately classified. Interestingly, undisturbed old-growth forests exhibited intermediate characteristics compared to transition and disturbed old-growth forests. This emphasizes that some structural attributes recognized as important for the classification of temperate and tropical old-growth forests, such as high vertical complexity, are of lesser relevance for boreal old-growth forests. In comparison to undisturbed old-growth forests, transition old-growth forests had a taller canopy of high “greenness” due to a greater hardwood abundance; disturbed old-growth forests had a higher gap fraction and heterogeneity in tree size; second-growth forests exhibited a lower and more even canopy. Misclassified tiles were explained by spatial variation in disturbance severity or different levels of forest resistance and resilience to disturbance. These misclassifications are also of ecological interest, as they highlight the nuances in structural diversity that are rarely identified by disturbance mapping. A reasonable combination of LiDAR and satellite indices was effective not only in discriminating old-growth forests from second-growth forests, but also identifying their different structures, which result from specific disturbance histories. This method could contribute to effective monitoring of changes in the areas and characteristics old-growth forest that are caused by anthropogenic and natural disturbances.

Abstract Old-growth forests are optimal habitats for many woodpeckers, which are often themselves excellent indicators of deadwood-associated biodiversity. Old-growth forests are, however, heterogeneous ecosystems in terms of structure, composition, and deadwood characteristics, thus implying a varied use of these forests by woodpeckers. In boreal landscapes, old-growth stands are threatened by forest harvesting; however, there is little information in regard to the consequences for biodiversity with the loss of specific types of old-growth forests. This study aimed to assess how the black-backed woodpecker (Picoides arcticus), a biodiversity indicator species associated with old-growth forest attributes, uses different types of old-growth forests for its foraging needs. We identified woodpecker foraging marks in 24 boreal old-growth forest stands in eastern Canada that were dominated by black spruce (Picea mariana), located within the home range of eight black-backed woodpeckers. We identified the various old-growth forest types using a typology based on the structural attributes of old-growth stands. We classified the sampled stands into four old-growth forest types, corresponding to different successional stages (recent or old, relative to the onset of the old-growth stage), composition (pure black spruce or mixed black spruce–balsam fir [Abies balsamea]), and productivity (ongoing paludification or not). The black-backed woodpecker foraged in all types of old-growth forests, but favored dense old-growth forests that were not paludified and that showed a high temporal continuity (i.e., old-growth dynamics probably started more than a century ago). The temporal continuity of the old-growth state allows for the continuous supply of large, slightly decayed snags, the preferred foraging substrates of the black-backed woodpecker. The old-growth forest type most favored by this woodpecker is, however, also the forest type most often targeted first by logging operations. Protecting the biodiversity associated with recent deadwood in managed areas thus requires maintaining a sufficient area and density of dense, old-growth black spruce-dominated forests in managed areas.

Tree-related microhabitats (TreM) and deadwood are two forest attributes providing essential resources for biodiversity conservation and ecosystem services. Old-growth forests are generally defined by a high abundance and diversity of TreM and deadwood, but little is known about TreM and deadwood dynamics once the old-growth stage is reached, in particular in the boreal biome. In this context, knowledge on TreM and deadwood dynamics in old-growth forest stands is necessary to better understand how these forests contribute to biodiversity and ecosystem services. The aim of this study is thus to determine how TreM, and deadwood abundance and diversity vary within boreal old-growth forests. To reach this objective, we surveyed TreM and deadwood attributes, as well as structural and abiotic attributes, in 71 boreal old-growth forests situated in Quebec, Canada. We used hierarchical clustering analysis to identify TreM and deadwood abundance and diversity patterns in the studied stands. We identified five clusters of TreM and deadwood characteristics, which corresponded to three stages of old-growth forest succession: canopy break-up (beginning of the old-growth stage), transition old-growth stage (replacement of the first cohort by old-growth cohorts) and true old-growth stage (first cohort all or almost all gone). The peak in TreM richness and diversity was reached at the transition old-growth stage, whereas the peak for deadwood richness and diversity was reached at the true old-growth stage. Overall, true old-growth forests were defined by a combination of moderate to high TreM density and high deadwood volume, but these values significantly varied among stands depending on past secondary disturbances, stand structure and its composition (black spruce [Picea mariana Mill.] dominated vs mixed black spruce – balsam fir [Abies balsamea (L.) Mill.]). These results therefore underscore the importance of considering old-growth forests as dynamic rather than static ecosystems, as the composition of tree- and deadwood-related microhabitats in the same old-growth stand may markedly change over time. At landscape scale, these results also imply that the mosaic of habitats present in old-growth forests can vary greatly from one location to another, highlighting the importance of maintaining a diversity of old-growth forest structure and composition.

Tree-related microhabitats (hereafter, “TreMs”) are key components of forest biodiversity but they are still poorly known in North American hardwood forests. The spatial patterns of living trees bearing TreMs (hereafter, “TreM-trees”) also remain to be determined. As logging practices can lead to a loss of TreM-trees and of their associated biodiversity, it is essential to identify the factors explaining TreM occurrence to better integrate them into forest management. We therefore inventoried TreMs in 4 0.5-ha survey strips in northern hardwood forests in Quebec, Canada, while recording the spatial location of each tree. Two strips were located in unmanaged old-growth forests, and 2 were in forests managed under selection cutting. All 4 stands were dominated by sugar maple (Acer saccharum Marsh.) and American beech (Fagus grandifolia Ehrn.). Beech bark disease, an exotic pathology, was observed in all the strips.

Large primary forest residuals can still be found in boreal landscapes. Their areas are however shrinking rapidly due to anthropogenic activities, in particular industrial-scale forestry. The impacts of logging activities on primary boreal forests may also strongly differ from those of wildfires, the dominant stand-replacing natural disturbance in these forests. Since industrial-scale forestry is driven by economic motives, there is a risk that stands of higher economic value will be primarily harvested, thus threatening habitats, and functions related to these forests. Hence, the objective of this study was to identify the main attributes differentiating burned and logged stands prior to disturbance in boreal forests. The study territory lies in the coniferous and closed-canopy boreal forest in Québec, Canada, where industrial-scale logging and wildfire are the two main stand-replacing disturbances. Based on Québec government inventories of primary forests, we identified 427 transects containing about 5.5 circular field plots/transect that were burned or logged shortly after being surveyed, between 1985 and 2016. Comparative analysis of the main structural and environmental attributes of these transects highlighted the strong divergence in the impact of fire and harvesting on primary boreal forests. Overall, logging activities mainly harvested forests with the highest economic value, while most burned stands were low to moderately productive or recently disturbed. These results raise concerns about the resistance and resilience of remnant primary forests within managed areas, particularly in a context of disturbance amplification due to climate change. Moreover, the majority of the stands studied were old-growth forests, characterized by a high ecological value but also highly threatened by anthropogenic disturbances. A loss in the diversity and functionality of primary forests, and particularly the old-growth forests, therefore adds to the current issues related to these ecosystems. Since 2013, the study area is under ecosystem-based management, which implies that there have been marked changes in forestry practices. Complementary research will be necessary to assess the capacity of ecosystem-based management to address the challenges identified in our study.

Tree-related microhabitats (hereafter “TreMs”) play a key role in forest biodiversity. However, harvesting may cause their erosion. In North America, knowledge about TreMs is still lacking but defect surveys are largely available in managed forests. The objectives of our study were: (1) to demonstrate that defect surveys can be a reliable resource to identify TreMs; and, (2)to evaluate the capacity of silvicultural treatments to maintain TreM abundance and diversity according to a harvest gradient.To achieve these objectives, we identified TreMs from a defect survey performed the year a harvest gradient was applied to20 plots, including uncut control, shelterwood treatments removing 50%, 43% and 36% of basal area, and clearcut (4 plots/treatment). The density and composition of TreMs were then compared based on treatments. Overall, 38% of defectsactually corresponded to TreMs, confirming that tree-defects can be used as TreM proxies. Bark loss was the most abundantTreM. While there was practically no TreM in clearcuts, all shelterwood treatments initially maintained TreM diversity anddensity at the same values found in uncut control plots. Shelterwood systems, especially those maintaining a continuouscover, could therefore prove helpful to sustain TreMs and their biodiversity in managed forests.

L’érosion des superficies des vieilles forêts boréales est actuellement l’un des enjeux majeurs de l’aménagement forestier; cependant, la résolution de ce problème nécessite des inventaires précises. Ainsi, l’objectif de cette étude était de déterminer si les précédents inventaires forestiers aériens identifiaient correctement les vieilles forêts dans les paysages boréaux du Québec, au Canada. Nous avons comparé les stades de succession (forêt équienne ou vieille forêt) de deux inventaires aériens réalisés en 1968 (inventaire aérien préindustriel) et en 2007 (inventaire aérien moderne) sur un territoire de 2200 km2. Nous avons aussi comparé les résultats de l’inventaire aérien moderne avec ceux obtenus à partir de 74 placettes de terrain échantillonnées entre 2014 et 2016. Les deux inventaires aériens étaient très incohérents : 80,8 % des peuplements non-perturbés identifiés comme « vieilles forêts » par l’inventaire préindustriel étaient classés comme « équiennes » par l’inventaire moderne et 60 % des placettes de terrain identifiées comme « vieilles forêts » étaient aussi classées « équiennes » par l’inventaire aérien moderne. Le manque d’attributs de vieilles forêts évidents ainsi que l’utilisation de critères inadaptés (c.-à-d. nécessitant une forte complexité verticale et d’importants changements de composition en espèces arborescentes durant la succession forestière) étaient les principaux éléments expliquant ces erreurs. Il est ainsi possible que la majorité des vieilles forêts boréales du Québec ne soient pas identifiées comme telles, limitant l’efficacité des stratégies de gestion durable.

The impact of traditional even-aged forest management on landscape age structure, tree composition, and connectivity has been well documented. Very little, however, is known about the impact on stand structural diversity. This study aims to compare the structural and abiotic characteristics of forest stands disturbed by clearcut logging and by stand-replacing fire in Quebec’s boreal landscapes. We hypothesized that unlike fire, logging specifically targeted stands having a higher economic value, i.e., merchantable volume, leaving altered forest characteristics on post-harvested landscapes. We compared two aerial forest surveys of a 2200 km2 study area, one survey completed before any logging activity (preindustrial survey; 1980s), and the second survey collected >10 years after logging activity (modern survey; 2000s). Forest stands at the time of the preindustrial survey were primary forests. We identified stands as either burned, logged, or left aside after forest management of the area (remaining stands) between the two surveys and compared their structural and abiotic characteristics using logistic regression. The structural and abiotic characteristics of burned and logged stands differed significantly. Relative to the burned stands, logged stands were older, denser, and marked by poorer drainage and a higher proportion of black spruce; therefore post-harvest and post-burn landscapes differed in terms of their structural diversities. Traditional even-aged forest management has significantly altered the boreal forest landscape by targeting specific stands having higher economic value and leaving behind stands of lower economic value. Remaining high economic stands should be protected, and a more balanced approach to harvesting must be used in the context of ecosystem-based management.

Research Highlights: Radial growth patterns of trees growing in old-growth boreal forests in eastern Canada can be grouped into a small number of simple patterns that are specific to different old-growth forest types or successional stages. Background and Objectives: Identifying the main radial growth trends in old-growth forests could help to develop silvicultural treatments that mimic the complex dynamics of old-growth forests. Therefore, this study aimed to identify the main radial growth patterns and determine how their frequencies change during forest succession in old-growth forests, focusing on boreal landscapes in eastern Canada. Materials and Methods: We used dendrochronological data sampled from 21 old-growth stands in the province of Quebec, Canada. Tree-ring chronologies were simplified into chronologies of equal length to retain only primary growth trends. We used k-means clustering to identify individual growth patterns and the difference in growth-pattern frequency within the studied stands. We then used non-parametric analyses of variance to compare tree or stand characteristics among the clusters. Results: We identified six different growth patterns corresponding to four old-growth forest types, from stands at the canopy breakup stage to true old-growth stands (i.e., when all the pioneer cohort had disappeared). Secondary disturbances of low or moderate severity drove these growth patterns. Overall, the growth patterns were relatively simple and could be generally separated into two main phases (e.g., a phase of limited radial increment size due to juvenile suppression and a phase of increased radial increment size following a growth release). Conclusions: The complexity of old-growth forest dynamics was observed mainly at the stand level, not at the tree level. The growth patterns observed in true old-growth forests were similar to those observed following partial or stem-selection cuts in boreal stands; thus, these silvicultural treatments may be effective in mimicking old-growth dynamics.

Old-growth forests play a major role in conserving biodiversity, protecting water resources, and sequestrating carbon, as well as serving as indispensable resources for indigenous societies. Novel silvicultural practices must be developed to emulate the natural dynamics and structural attributes of old-growth forests and preserve the ecosystem services provided by these boreal ecosystems. The success of these forest management strategies depends on developing an accurate understanding of natural regeneration dynamics. Our goal was therefore to identify the main patterns and drivers involved in the regeneration dynamics of old-growth forests with a focus on boreal stands dominated by black spruce (Picea mariana (L.) Mill.) and balsam fir (Abies balsamea (L.) Mill.) in eastern Canada. We sampled 71 stands in a 2 200 km2 study area located within Quebec’s boreal region. For each stand, we noted tree regeneration (seedlings and saplings), structural attributes (diameter distribution, deadwood volume, etc.), and abiotic (slope and soil) factors. The presence of seed-trees located nearby and slopes having moderate to high angles most influenced balsam fir regeneration. In contrast, the indirect indices of recent secondary disturbances (e.g., insect outbreaks or windthrows) and topographic constraints (slope and drainage) most influenced black spruce regeneration. We propose that black spruce regeneration dynamics can be separated into distinct phases: (i) layering within the understory, (ii) seedling growth when gaps open in the canopy, (iii) gradual canopy closure, and (iv) production of new layers once the canopy is closed. These dynamics are not observed in paludified stands or stands where balsam fir is more competitive than black spruce. Overall, this research helps explain the complexity of old-growth forest dynamics, where many ecological factors interact at multiple temporal and spatial scales. This study also improves our understanding of ecological processes within primary old-growth forests and identifies the key factors to consider when ensuring the sustainable management of old-growth boreal stands.

La conservation des vieilles forêts, c'est-à-dire les peuplements dynamisés par les perturbations naturelles secondaires, dans les territoires aménagés du biome boréal est désormais un enjeu environnemental majeur. L'efficacité des mesures de gestion prises à leur encontre nécessite néanmoins une bonne connaissance des écosystèmes que l'on regroupe sous le terme« vieille forêt». L'objectif de cette thèse est donc d'améliorer nos connaissances quant à la diversité, la dynamique et la conservation des vieilles forêts boréales résineuses dans les territoires aménagés de l'Est du Canada. Cette étude se concentre sur un territoire d'étude de 2200km2 situé dans le domaine bioclimatique de la pessière noire à mousse du Québec, Canada. Les caractéristiques structurelles de 7 4 peuplements, matures à surannés, y ont été échantillonnées. Des analyses dendrochronologiques ont aussi été réalisées pour 21 de ces sites. Enfin, nous avons utilisé les données des inventaires forestiers décennaux réalisés par le gouvernement du Québec depuis les années 60 pour observer l'évolution des vieilles forêts dans le territoire d'étude ainsi que l'acuité avec laquelle elles sont identifiées. Nos résultats montrent que les vieilles forêts boréales sont structurellement très diversifiées. Onze différentes structures ont ainsi pu être identifiées. Cette diversité résulte pour partie de la topographie mais augmente aussi avec le temps depuis la dernière perturbation majeure. La dynamique de perturbation secondaire de ces écosystèmes gagne aussi en complexité au fur et à mesure que la première cohorte, apparue suite à la dernière perturbation majeure, est remplacée par de nouvelles cohortes, renforçant cette diversité structurelle. Au début du processus de transition vers l'état de vieille forêt, les perturbations de sévérité faible dominent mais les perturbations de sévérité modérée gagnent progressivement en importance, jusqu'à atteindre un niveau similaire à celui des perturbations de sévérité faible. Les épidémies de tordeuse des bourgeons de l'épinette ( Choristoneura fumiferana, Clemens) apparaissent alors comme un élément clé de cette dynamique de perturbations secondaires, indépendamment de leur sévérité. En comparaison des feux de forêt, l'aménagement forestier a récolté une plus grande proportion de vieilles forêts ( se définissant ici par un âge supérieur à 100 ans) sur ce territoire, notamment caractérisées par un volume de bois marchand plus important. L'influence de l'exploitation forestière sur la diversité structurelle du territoire diffère donc de celle du feu. Par ailleurs, le plus récent inventaire photographique aérien réalisé par le gouvernement du Québec sous-estimait fortement l'abondance des vieilles forêts sur le territoire d'étude. La majorité de ces peuplements, se définissant sur le terrain par une structure complexe, étaient identifiés comme des peuplements équiens (c'est-à-dire structurellement simples) par l'inventaire aérien. Des seuils inadaptés à la forêt boréale ainsi que l'absence d'éléments de vieilles forêts particulièrement visibles par télédétection ou suivant une observation superficielle du peuplement peuvent expliquer ces résultats. Une fois le stade de vieille forêt atteint, les peuplements forestiers boréaux continuent donc de suivre une dynamique complexe, ce qui peut mener à de nombreux changements structuraux dans le temps. La diversité des vieilles forêts boréales doit donc être reconnue dans l'aménagement forestier, car impliquant la présence de nombreux types d'habitats nécessaires à la biodiversité. Nos résultats montrent cependant que les peuplements récoltés en priorité sont les vieilles forêts avec la plus forte valeur marchande. Ignorer cette diversité - par exemple en se restreignant à un simple pourcentage de vieilles forêts à conserver, qu'importe leur type -pourrait donc mener à une réduction de la diversité structurelle des vieilles forêts en territoire aménagé, causant à terme une perte d'habitat. Dans l'état actuel des choses, les inventaires photographiques aériens ne permettent pas de reconnaître la diversité des vieilles forêts, ce qui empêche la mise en place de stratégies efficaces. Intégrer de manière durable les vieilles forêts boréales dans l'aménagement forestier demande par conséquent non-seulement la reconnaissance de la richesse et de la complexité de ces écosystèmes, mais aussi le développement d'outils permettant d'assurer l'identification des nombreuses formes sous lesquelles elles peuvent se présenter. 

 

Both low- and moderate-severity secondary disturbances are drivers of eastern Canadian boreal old-growth forests dynamics. Moderate-severity disturbances reflect mainly spruce budworm outbreaks. Low-severity disturbances are produced by both spruce budworm outbreaks and random events such as windthrow. Each level of disturbance severity has a specific impact on stand dynamics, and both shape the diversity of boreal old-growth forests.

Context

A regular succession of low-severity disturbances is seen as determining the dynamics of the old-growth stage (gap dynamics); however, recent studies suggest that moderate-severity secondary disturbances also play an important role in the dynamics of eastern Canadian boreal forests.

Aims

This study aims to determine if eastern Canadian boreal old-growth forests are driven by a combination of low- and moderate-severity secondary disturbances.

Methods

We reconstructed the 200-year disturbance history of 20 boreal old-growth stands using dendrochronological analysis. We discriminated low- from moderate-severity disturbances based on their respective influence on mean stand growth.

Results

The secondary disturbance regime of eastern Canadian boreal old-growth forests varies highly over time, reflected by disturbance peaks in the chronological record. Most peaks occurred during spruce budworm outbreaks related to both low- and moderate-severity disturbances. Between each peak, low-severity disturbances dominate. Each level of disturbance severity has specific consequences for stand dynamics.

Conclusion

Both low and moderate secondary disturbances are drivers of forest dynamics in eastern Canadian boreal old-growth stands and shape the structural diversity of these stands. The complexity of these dynamics should be recognized in management planning to ensure the efficiency of old-growth forest conservation policies.

Old-growth stands are considered as key components of boreal forest diversity and their preservation is largely integrated into management plans. However, while the differences between old-growth and young forests have largely been studied, little is known about the diversity of boreal old-growth forests. In managed landscapes, the efficacy of old-growth conservation plans may be reduced depending on how these old-growth forests are considered: as a single, homogeneous and steady-state forest type or as multiple, diverse and dynamic forest types. To fulfil this gap, our objectives were: (1) to create a typology of old-growth boreal structures; (2) to observe how these structures are influenced by environmental and temporal parameters; and (3) to elaborate a succession model of old-growth structural dynamics along temporal and environmental gradients. Seventy-one mature and overmature stands were sampled within a 2200?km2 territory situated in Eastern Canada. Cluster analysis divided the sampled stands into two even-aged types, three transition old-growth types and six true old-growth types. Slope, minimum time since last fire and organic horizon depth were the three environmental and temporal parameters influencing the old-growth structures. Paludification-related productivity decline was present in only one old-growth forest type, while the other sites remained productive. These results allowed the creation of three succession models of the dynamics of old-growth stands in the boreal forest of eastern Canada. Boreal stands can undergo numerous structural changes once the old-growth succession process is initiated. An increase in structural diversity when the true old-growth stage is reached, coupled with a variety of secondary disturbance characteristics, favours multiple pathways of structural evolution of these ecosystems over time. Therefore, forest management planning should incorporate this complexity to improve the preservation of old-growth forests in managed territories.