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

Regional analyses assessing the vulnerabilities of forest ecosystems and the forest sector to climate change are key to considering the heterogeneity of climate change impacts as well as the fact that risks, opportunities, and adaptation capacities might differ regionally. Here we provide the Regional Integrated Assessment of climate change on Quebec's forests, a work that involved several research teams and focused on climate change impacts on Quebec's commercial forests and on potential adaptation solutions. Our work showed that climate change will alter several ecological processes within Quebec's forests. These changes will result in important modifications in forest landscapes. Harvest will cumulate with climate change effects to further alter future forest landscapes, which will also have consequences on wildlife habitats (including woodland caribou habitat), avian biodiversity, carbon budget, and a variety of forest landscape values for Indigenous peoples. The adaptation of the forest sector will be crucial to mitigate the impacts of climate change on forest ecosystem goods and services and improve their resilience. Moving forward, a broad range of adaptation measures, notably through reducing harvest levels, should be explored to help strike a balance among social, ecological, and economic values. We conclude that without climate adaptation, strong negative economic and ecological impacts will likely affect Quebec's forests.

As global climate conditions continue to change, disturbance regimes and environmental drivers will continue to shift, impacting global vegetation dynamics. Following a period of vegetation greening, there has been a progressive increase in remotely sensed vegetation browning globally. Given the many societal benefits that forests provide, it is critical that we understand vegetation dynamic alterations. Here, we review associative drivers, impacts, and feedbacks, revealing the complexity of browning. Concomitant increases in browning include the weakening of ecosystem services and functions and alterations to vegetation structure and species composition, as well as the development of potential positive climate change feedbacks. Also discussed are the current challenges in browning detection and understanding associated impacts and feedbacks. Finally, we outline recommended strategies.

Regional analyses assessing the vulnerabilities of forest ecosystems and the forest sector to climate change are key to consider the heterogeneity of climate change impacts but also the fact that risks, opportunities and adaptation capacities might differ regionally. Here we provide the Regional Integrated Assessment of climate change on Quebec’s forests, a work that involved several research teams and that focused on climate change impacts on Quebec’s commercial forests and on potential adaptation solutions. Our work showed that climate change will alter several ecological processes within Quebec’s forests. These changes will result in important modifications in forest landscapes. Harvest will cumulate with climate change effects to further alter future forest landscapes which will also have consequences on wildlife habitat (including woodland caribou habitat), avian biodiversity, carbon budget and a variety of forest landscape values for Indigenous peoples. The adaptation of the forest sector, will be crucial to mitigate the impacts of climate change on forest ecosystem goods and services and improve their resilience. Moving forward, a broad range of adaptation measures, notably through reducing harvest levels, should be explored to help strike a balance among social, ecological and economic values. We conclude that without climate adaptation strong negative economical and ecological impacts will likely affect Quebec’s forests.

Warning: This article contains terms, descriptions, and opinions used for historical context that may be culturally sensitive for some readers.Background: Understanding drivers of boreal forest dynamics supports adaptation strategies in the context of climate change.Aims: We aimed to understand how burn rates varied since the early 1700s in North American boreal forests.Methods: We used 16 fire-history study sites distributed across such forests and investigated variation in burn rates for the historical period spanning 1700-1990. These were benchmarked against recent burn rates estimated for the modern period spanning 1980-2020 using various data sources.Key results: Burn rates during the historical period for most sites showed a declining trend, particularly during the early to mid 1900s. Compared to the historical period, the modern period showed less variable and lower burn rates across sites. Mean burn rates during the modern period presented divergent trends among eastern versus northwestern sites, with increasing trends in mean burn rates in most northwestern North American sites.Conclusions: The synchronicity of trends suggests that large spatial patterns of atmospheric conditions drove burn rates in addition to regional changes in land use like fire exclusion and suppression.Implications: Low burn rates in eastern Canadian boreal forests may continue unless climate change overrides the capacity to suppress fire.

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.

There is a pressing need for a better understanding of changing forest fire regimes worldwide, especially to separate the relative effects of potential drivers that control burned areas. Here we present a meta-analysis of the impacts of climate fluctuation and Euro-Canadian settlement on burned areas from 1850 to 1990 in a large zone (>100 000 km2) in northern temperate and boreal forests of eastern Canada. Using Cox regression models, we tested for potential statistical relationships between historical burned areas in 12 large landscapes (reconstructed with dendrochronological data) with climate reconstructions, changes in the Euro-Canadian population, and active suppression (all reconstructed at the decadal scale). Our results revealed a dominant impact of climate fluctuations on forest burned areas, with the driest decades showing fire hazards between 5 to 15 times higher than the average decades. Comparatively, the Euro-Canadian settlement had a much weaker effect, having increased burned areas significantly only during less fire-prone climate conditions. During periods of fire-prone climate, burned areas were maximum independent of fluctuations in Euro-Canadian populations. Moreover, the development of active fire suppression did not appear to reduce burned areas. These results suggest that a potential increase in climate moisture deficit and drought may trigger unprecedented burned areas and extreme fire events no matter the effects of anthropogenic ignition or suppression.

Globally, increasing drought-induced tree mortality rates under climate change are projected to have far-reaching effects on forest ecosystems. Among these forest systems, the boreal forest is considered a ‘tipping element’ of the Earth's climate system. This forest biome plays a critical role in ecosystem services, structures and functions while being highly sensitive to drought stress. Although process-based models are important tools in ecological research, very few have yet been developed that integrate advanced physiological mechanisms to simulate drought-induced mortality in boreal forests. Accordingly, based on the process-based TRIPLEX model, this study introduces the new TRIPLEX-Mortality submodule for the Canadian boreal forests at the stand level, that for the first time successfully incorporates two advanced drought-induced physiological mortality mechanisms (i.e., hydraulic failure and carbon starvation). To calibrate and validate the model, 73 permanent sample plots (PSPs) were selected across Canada's boreal forests. Results confirm a good agreement between simulated mortality and mortality observations (R2=0.79; P<0.01; IA=0.94), demonstrating good model performance in simulating drought-induced mortality in boreal forests. Sensitivity analysis indicated that parameter sensitivity increased as drought intensified, and the shape parameter (c) for calculating percentage loss of conductivity (PLC) was the most sensitive parameter (average SI = -3.51) to simulate tree mortality. Furthermore, the results of model input sensitivity analysis also showed that the model can capture changes in mortality under different drought scenarios. Consequently, our model is suitable for simulating drought-induced mortality in boreal forests while also providing new insight into improving model simulations for tree mortality and associated carbon dynamics in a progressively warmer and drier world.

Eastern Canadian boreal forests are mainly influenced by natural wildfires and forest management activities. To evaluate forest dynamics under possible interactions among fire and timber harvest in a future climate warming scenario (RCP 2.6, RCP 4.5 and RCP 8.5) the forest landscape model Landis II was used to simulate the dynamics of the 78000 km2 of boreal forests in eastern Canada. Forest management intensity scenarios were modeled considering the changes in the annual harvested area (0.5%, 1%, and 2%) and the age that conifers and hardwoods can be harvested (50 and 30 years, 70 and 50 years, and 90 and 70 years). The results of the 300-year model projections implied that both forest management intensity and climatic scenarios explained most of the variability in aboveground biomass, aboveground net primary productivity and forest composition. Forest management seems to be the most important factor that modified the landscape in the southern forests because there were scheduled stands with the age and composition required by each harvesting prescription to deal with the annual allowable cut volume. On the contrary, in the northern forests there was a mixed effect of climate change and forest management because many of the areas suitable for harvesting were previously burned limiting the amount of area available for harvesting. Thus, although it is expected an increase in wildfire area burned due to climate change, the intensification of forest management seems to be the most important driver of the increase of hardwoods and mixed stands and the decrease of conifers stands on the mixedwood boreal landscape, mainly in the southern forests. These results suggest that timber supply would be at risk in the Abitibi Plain, therefore, some strategies should be applied to adapt forest management to climate change.

Les changements dans le régime des feux peuvent affecter le potentiel de régénération après perturbation des espèces d’arbres de la forêt boréale, modifiant ainsi la densité et la fermeture des peuplements forestiers. Cela pourrait nuire à la durabilité de la gestion forestière, en particulier dans les régions caractérisées actuellement par un cycle de feu court et une faible productivité. À titre d’étude de cas, nous utilisons un véritable paysage (1.3 Mha) de la forêt boréale du nord-ouest du Québec, caractérisé par une superficie annuelle brûlée élevée et pour laquelle l’activité des feux devrait augmenter fortement, pour modéliser l’effet des cycles de feux actuel et ceux induits par le climat, et le taux de récolte sur le potentiel de régénération des peuplements purs d’épinette noire (Picea mariana (Mill.) BSP) et de pin gris (Pinus banksiana Lamb.). Des simulations ont été effectuées sur une période de 50 ans selon trois seuils de maturité reproductive par espèce, représentant l’âge auquel un approvisionnement suffisant en semences est atteint afin d’assurer l’auto-remplacement du peuplement. Les résultats montrent une augmentation progressive de la superficie affectée par les accidents de régénération naturelle au cours de la période de simulation dans les deux scénarios climatiques, montrant une perte de 18.5 % (149?210 ha) de superficie productive sous le cycle de feu actuel et de 65.8 % (532?141 ha) sous les cycles de feux futurs. Les variations dans le cycle de feu ont eu le plus grand effet sur le taux des accidents de régénération, suivi par le seuil d’âge de régénération et le taux de récolte. Nous décrivons les pratiques proactives de gestion forestière et la planification stratégique qui inclut le risque de feu peuvent réduire la probabilité par des accidents de régénération après feu. Cela comprend la gestion intensive des peuplements et les stratégies de rétention variable après la récolte. De même un réseau de suivi des sites après feu aiderait à évaluer les accidents de régénération au fil du temps et serait utile pour valider à la fois les résultats du modèle et l’efficacité des stratégies visant à en minimiser la probabilité.

Quantifying fire regimes in the boreal forest ecosystem is crucial for understanding the past and present dynamics, as well as for predicting its future dynamics. Survival analyses have often been used to estimate the fire cycle in eastern Canada because they make it possible to take into account the censored information that is made prevalent by the typically long fire return intervals and the limited scope of the dendroecological methods that are used to quantify them. Here, we assess how the true length of the fire cycle, the short-term temporal variations in fire activity, and the sampling effort affect the accuracy and precision of estimates obtained from two types of parametric survival models, the Weibull and the exponential models, and one non-parametric model obtained with the Cox regression. Then, we apply those results in a case area located in eastern Canada. Our simulation experiment confirms some documented concerns regarding the detrimental effects of temporal variations in fire activity on parametric estimation of the fire cycle. Cox regressions appear to provide the most accurate and robust estimator, being by far the least affected by temporal variations in fire activity. The Cox-based estimate of the fire cycle for the last 300 years in the case study area is 229 years (CI95: 162–407), compared with the likely overestimated 319 years obtained with the commonly used exponential model.

Boreal forest management in Eastern Canada has caused depletion and fragmentation of oldgrowth ecosystems, with growing impacts on the associated biodiversity. To mitigate impacts of management while maintaining timber supplies, ecosystem management aims to narrow the gap between natural and managed landscapes. Our study describes the fire history and associated natural old-growth forest proportions and distribution of a 5000 km2 area located in the black spruce-feather moss forest of central Quebec. We reconstructed a stand-origin map using archival data, aerial photos and dendrochronology. According to survival analysis (Cox hazard model), the mean fire cycle length was 247 years for the 1734–2009 period. Age-class distribution modelling showed that old-growth forests were present on an average of 55% of the landscape over the last 275 years. The mean fire size was 10 113 ha, while most of the burned area was attributable to fires larger than 10 000 ha, leading to old-growth agglomerations of hundreds of square kilometres. In regards to our findings, we propose ecosystem management targets and strategies to preserve forest diversity and resilience.

Les feux de forêts constituent l'un des processus les plus importants de la forêt boréale en établissant les fondements d'une mosaïque dynamique de peuplements forestiers à l'intérieur de laquelle une multitude d'autres processus interagissent. En initiant une succession secondaire, ils déterminent en partie la composition, la structure et la répartition spatiale des différents types d'habitats rencontrés en forêt boréale. Voilà pourquoi il est souvent suggéré que les impacts des perturbations anthropiques (e.g. coupes) sur les paysages aménagés seront atténués si celles-ci émulent le mieux possible les patrons et processus normalement générés par les perturbations naturelles (e.g. feux de forêt). Le cycle des feux, défini comme le temps nécessaire à ce qu'une superficie cumulée égale au territoire à l'étude ait brûlé de nouveau, est un paramètre important du régime des feux puisqu'il détermine la proportion des classes d'âge à l'échelle du paysage. Or, les cycles des feux relativement longs documentés dans l'est du Canada suggéraient que les paysages forestiers produits au terme de la première rotation forestière industrielle seraient amputés d'une portion importante de la variété d'habitats qui caractérisent les paysages produits par les perturbations naturelles. L'objectif principal de cette thèse était donc de documenter le cycle des feux sur la Côte-Nord, une région caractérisée par une présence particulièrement importante de vieilles forêts, ainsi que ses répercussions sur la dynamique successionelle des principales espèces d'arbres et les implications sur l'aménagement forestier en forêt boréale de l'est du Canada. Dans un premier chapitre, nous avons évalué le cycle des feux sur la Côte-Nord ainsi que l'incertitude qui lui est associée à l'aide d'une approche par modélisation. Nous y comparons aussi trois méthodes d'analyses de survie pouvant être utilisées pour estimer le cycle des feux. Il s'est avéré que l'approche non-paramétrique, la régression de Cox, permet l'obtention d'une estimation plus robuste aux variations temporelles de l'activité des feux, la source de biais potentiel la plus importante. À l'aide de cette méthode, nous avons pu estimer à environ 227 ans le cycle des feux récent dans le territoire à l'étude, une valeur à laquelle est toutefois associée un intervalle de confiance à 95% de ±60 à 70 ans. Dans un second chapitre, nous avons isolé les facteurs responsables d'une hétérogénéité spatiale de la fréquence des feux sur la Côte-Nord, qui se sont avérés dépendre fortement de l'échelle spatiale à laquelle ils sont décrits en raison du caractère contagieux des feux de forêt. Il s'est avéré qu'au sein d'un même paysage, certaines grandes zones dominées par des versants exposés au sud étaient 2 à 6 fois plus susceptibles que d'autres, affectant ainsi la répartition de peuplements forestiers distincts au niveau de la composition et/ou de la structure. Nous avons ensuite testé l'influence de cette hétérogénéité sur la dynamique des peuplements au moyen d'analyses multivariées des communautés végétales. Nous avons ainsi tenté d'isoler l'influence du temps depuis le dernier feu en tant que tel de l'appartenance à un contexte où les feux sont plus ou moins fréquents. De façon générale, c'est seulement dans les zones à faible fréquence des feux que le principal spécialiste de fin de succession, le sapin baumier, arrive à supplanter l'espèce globalement la plus abondante en forêt boréale de l'est du Canada, l'épinette noire, en raison du temps depuis le dernier feu généralement plus long. Nos résultats suggèrent aussi que la succession de P. mariana vers A. balsamea peut se produire longtemps après ce qui est généralement couvert par les reconstitutions dendroécologiques de l'historique des feux dans ce type de paysages boréaux (>200-300 ans). En dernier lieu, nous nous sommes intéressés à un autre paysage forestier boréal, situés à l'extrême ouest du Québec en partie sur la portion nord de l'Abitibi et au sud de la Jamésie. Nous avons examiné la variabilité à plus long terme (6800 ans) de l'activité des feux au moyen d'une reconstitution paléoécologique basée sur les fragments de charbon enfouis dans les sédiments de lac stratifiés. Cette analyse nous a permis de décrire ce que nous soutenons être une plage de variabilité naturelle pertinente pour l'établissement de cibles d'aménagement, qui fut comparée à l'état actuel du paysage étudié. Nos résultats confirment les appréhensions selon lesquels l'aménagement néglige une importante proportion des paysages boréaux naturels, i.e. les vieilles forêts, puisque celles-ci semblent avoir occupé une portion importante de ce paysage au cours de l'ensemble de son histoire post-glaciaire, une réalité qui a rapidement été altérée au cours des trois dernières décennies de récolte extensive.

Natural ecosystems have developed within ranges of conditions that can serve as references for setting conservation targets or assessing the current ecological integrity of managed ecosystems. Because of their climate determinism, forest fires are likely to have consequences that could exacerbate biophysical and socioeconomical vulnerabilities in the context of climate change. We evaluated future trends in fire activity under climate change in the eastern Canadian boreal forest and investigated whether these changes were included in the variability observed during the last 7000 years from sedimentary charcoal records from three lakes. Prediction of future annual area burned was made using simulated Monthly Drought Code data collected from an ensemble of 19 global climate model experiments. The increase in burn rate that is predicted for the end of the 21st century (0.45% year^–1 with 95% confidence interval (0.32, 0.59) falls well within the long-term past variability (0.37 to 0.90% year^–1). Although our results suggest that the predicted change in burn rates per se will not move this ecosystem to new conditions, the effects of increasing fire incidence cumulated with current rates of clear-cutting or other low-retention types of harvesting, which still prevail in this region, remain preoccupying.

Determination of the direct causal factors controlling wildfires is key to understanding wildfire–vegetation–climate dynamics in a changing climate and for developing sustainable management strategies for biodiversity conservation and maintenance of long-term forest productivity. In this study, we sought to understand how the fire frequency of a large mixedwood forest in the central boreal shield varies as a result of temporal and spatial factors. We reconstructed the fire history of an 11,600-km2 area located in the northwestern boreal forest of Ontario, using archival data of large fires occurring since 1921 and dendrochronological dating for fires prior to 1921. The fire cycle decreased from 295 years for the period of 1820–1920 to approximately 100 years for the period of 1921–2008. Spatially, fire frequency increased with latitude, attributable to higher human activities that have increased fragmentation and fire suppression in the southern portion of the study area. Fire frequency also increased with distance to waterbodies, and was higher on Podzols that were strongly correlated with moderate drainage and coniferous vegetation. The temporal increase of fire frequency in the central region, unlike western and eastern boreal forests where fire frequency has decreased, may be a result of increased warm and dry conditions associated with climate change in central North America, suggesting that the response of wildfire to global climate change may be regionally individualistic. The significant spatial factors we found in this study are in agreement with other wildfire studies, indicating the commonality of the influences by physiographic features and human activities on regional fire regimes across the boreal forest. Overall, wildfire in the central boreal shield is more frequent than that in the wetter eastern boreal region and less frequent than that in the drier western boreal region, confirming a climatic top-down control on the fire activities of the entire North American boreal forest.

Abstract: The differences between boreal forest landscapes produced by natural disturbance regimes and landscapes produced by harvesting are important and increasingly well documented. To continue harvesting operations while maintaining biodiversity and other ecosystem services, government policies and certification processes are pushing for practices that preserve landscape features within their range of natural variability. One major shortcoming in the implementation of such a strategy is the lack of complete spatial or temporal information about these landscape features, such as the proportion of old stands, which are believed to act as a coarse filter for conservation if they remain representative enough of natural conditions. The objective of this study was to quantify the proportion of old stands in a very large landscape by combining fragmentary knowledge from two different sources, i.e., a provincial forest inventory and existing fire history reconstructions using a Bayesian Belief Network. This study was conducted over a 6.5 Mha landscape located within the Clay Belt of the province of Ontario, Canada, and suggests that more than 72.4% of this area is occupied by stands where no fire occurred during the last 150 years. The implications for management and potential for future research are discussed.

Résumé : Les différences entre les paysages de forêt boréale engendrés par les régimes de perturbations naturelles et ceux qui sont engendrés par la coupe sont importantes et de mieux en mieux documentées. Afin de poursuivre les opérations de récolte tout en maintenant la biodiversité et les autres services rendus par l’écosystème, les politiques gouvernementales et les processus de certification prônent l’utilisation de pratiques qui conservent les caractéristiques du paysage à l’intérieur d’écarts correspondant à la variation naturelle de ces caractéristiques. Un inconvénient majeur associé à la mise en œuvre d’une telle stratégie est l’absence d’une information spatiale et temporelle complète au sujet de ces caractéristiques du paysage, telles que la proportion de vieux peuplements qui agissent vraisemblablement comme brut grossier pour la conservation s’ils demeurent suffisamment représentatifs des conditions naturelles. L’objectif de cette étude consistait à quantifier la proportion de vieux peuplements dans un très vaste paysage en combinant des bribes de connaissance provenant de deux sources différentes, c.-à-d. un inventaire forestier provincial et les reconstitutions de l’historique des feux à l’aide d’un réseau de croyances bayésien. Cette étude a été réalisée dans un paysage de 6,5 Mha situé dans la ceinture d’argile de la province d’Ontario, au Canada. Les résultats indiquent que 72,4 % de ce territoire est occupé par des peuplements qui n’ont subi aucun feu au cours des 150 dernières années. La discussion porte sur les implications pour l’aménagement et le potentiel pour de futurs travaux de recherche.

Fire is fundamental to the natural dynamics of the North American boreal forest. It is therefore often suggested that the impacts of anthropogenic disturbances (eg logging) on a managed landscape are attenuated if the patterns and processes created by these events resemble those of natural disturbances (eg fire). To provide forest management guidelines, we investigate the long-term variability in the mean fire interval (MFI) of a boreal landscape in eastern North America, as reconstructed from lacustrine (lake-associated) sedimentary charcoal. We translate the natural variability in MFI into a range of landscape age structures, using a simple modeling approach. Although using the array of possible forest age structures provides managers with some flexibility, an assessment of the current state of the landscape suggests that logging has already caused a shift in the age-class distribution toward a stronger representation of young stands with a concurrent decrease in old-growth stands. Logging is indeed quickly forcing the studied landscape outside of its long-term natural range of variability, implying that substantial changes in management practices are required, if we collectively decide to maintain these fundamental attributes of the boreal forest.

Despite the recognized importance of fire in North American boreal forests, the relative importance of stochastic and determinist portions of intra-regional spatial variability in fire frequency is still poorly understood. The first objective of this study is to identify sources of spatial variability in fire frequency in a landscape of eastern Quebec’s coniferous boreal forest. Broad-scale environmental factors considered included latitude, longitude, human activities and belonging to a given bioclimatic domain, whereas fine-scale factors included slope, position on the slope, aspect, elevation, surficial deposit and drainage. The average distance to waterbodies was also considered as a potential intermediate-scale source of variability in fire frequency. In order to assess these environmental factors’ potential influence, they were incorporated into a proportional hazard model, a semi-parametric form of survival analysis. We also used a digital elevation model in order to evaluate the dominant aspect within neighborhoods of varying sizes and successively incorporated these covariates into the proportional hazard model. We found that longitude significantly affects fire frequency, suggesting a maritime influence on fire frequency in this coastal landscape. We also found that position on the slope was related to fire frequency since hilltops and upperslopes were subject to a lower fire frequency. Dominant aspect was also related to fire frequency, but only when characterized within a neighborhood delimited by 4,000 to 10,000-m radii (5,027–31,416 ha). A 2–6-fold variation in fire frequency can be induced by geographic and topographic contexts, suggesting a substantial intra-regional heterogeneity in disturbance regime with potential consequences on forest dynamics and biodiversity patterns. Implications for forest management are also briefly discussed.

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

Old-growth forests make up a substantial proportion of the forest mosaic in the Clay Belt region of Ontario and Quebec, Canada, despite fire cycles that are presumed to be relatively short. Two hypotheses have been suggested as explanations for this phenomenon: (1) the old-growth forests in question are located on sites that are protected from fire or (2) the fire hazard is just as great there as elsewhere, and that part of the mosaic is simply the tail of the distribution, having been spared from fire merely by chance. The tree-ring method has proven inadequate as a means of determining the date of the most recent fire in these old-growth forests, as the time that has elapsed since that date probably exceeds the age of the oldest trees. Accordingly, a paleoecological study was conducted with a view to determining the date of the last fire in these forests. Charcoal horizons were located and radiocarbon dated in six old-growth forests. The possibility that these forests have never burned at all is ruled out by the fact that macroscopic charcoal fragments were found at all sites. The proximity of potential firebreaks has a significant influence in the survival model, suggesting fire-cycle heterogeneity throughout the landscape. However, the proportion of old-growth forests observed is in agreement with what would be expected assuming that fire hazard is independent of stand age. Old-growth stands could thus be incorporated into natural disturbance based management, although the great variability of the intervals between catastrophic disturbances should be carefully considered.

Les forêts anciennes constituent une proportion importante de la mosaïque forestière de la ceinture d'argile (Ontario et Québec, Canada) en dépit de cycles de feu que l'on présume relativement courts. Deux hypothèses ont été avancées pour expliquer ce phénomène : (1) Les forêts anciennes sont localisées sur des sites protégés des feux ou (2) le risque d'incendie y est équivalent et cette portion de la mosaïque ne constitue que la queue de la distribution, épargnée par le feu simplement par hasard. La dendrochronologie s'est avérée inadéquate pour dater le dernier feu dans ces forêts anciennes puisque le temps écoulé depuis le dernier feu est probablement supérieur à l'âge des plus vieux arbres. Une étude paléoécologique a donc été effectuée dans le but de dater le dernier feu dans ces forêts. Les horizons contenant du charbon de bois ont été localisés et datés au carbone 14 dans six forêts anciennes. Le fait que des fragments de charbon macroscopiques aient été retrouvés dans tous les sites indique que ces forêts ont bel et bien été affectées par les feux dans le passé. La proximité de coupe-feu potentiels accroît la durée du cycle des feux. Toutefois, la proportion de forêts anciennes observée indique que le risque d'incendie est indépendant du temps écoulé depuis le dernier feu. Les forêts anciennes pourraient donc être intégrées à des aménagements forestiers inspirés par la dynamique des perturbations naturelles. Toutefois, la grande variabilité de l'intervalle entre deux perturbations catastrophiques devrait être soigneusement considérée. ©2005 NRC Canada

Depuis un peu plus d’une dizaine d’années, un paradigme motive une grande proportion des recherches effectuées en sciences forestières. Ce paradigme consiste en l’hypothèse selon laquelle nos aménagements forestiers auront des effets négatifs moindres sur l’intégrité des écosystèmes forestiers s’ils s’inspirent le plus possible de la dynamique des perturbations naturelles. Une certaine proportion de la recherche en sciences forestières porte sur le passé de ces écosystèmes, il s’agit donc de paléoécologie forestière. Cet essai tente d’évaluer la pertinence de telles recherches, portant sur des écosystèmes forestiers passés, lorsque venu le temps d’établir des référentiels (benchmarks) pour les modèles d’aménagements forestiers, inspirés de la dynamique des perturbations naturelles. La paléoécologie semble être un outil utile et même parfois essentiel à l’étude de certains processus agissant à des échelles temporelles particulièrement longues, ce qui est souvent le cas pour les écosystèmes forestiers. Ceci est particulièrement le cas en ce qui a trait aux interactions climat-végétation-perturbations, qui constituent des enjeux importants au sein de nos aménagements forestiers avec les changements climatiques prévus dans un avenir relativement proche. Ayant su souligner le caractère changeant des populations, communautés, écosystèmes et paysages à travers le temps, la paléoécologie semble aussi offrir un cadre de référence intéressant pour l’élaboration de réseaux d’aires de conservation bien adaptés aux modifications environnementales à venir. Quelques aspects propres à la paléoécologie doivent par contre être considérés soigneusement lors de l’interprétation des résultats provenant d’études paléoécologiques, notamment en raison du caractère indirect des observations et de l’incertitude associées à ce type d’observations. Somme toute, la paléoécologie forestière semble être en mesure d’occuper une place importante dans développement de modèles d’aménagement forestier inspirés de la dynamique des perturbations naturelles. Quelques actions concrètes, issues de réflexions apportées par la paléoécologie et visant à ce que nos aménagements forestiers s’inspirent davantage de la dynamique des perturbations naturelles sont suggérées. © 2003 UQAM tous droits réservés.

Les forêts anciennes constituent une proportion importante de la mosaïque forestière du nord de l'Abitibi en dépit de cycles des feux relativement courts. Deux hypothèses ont été avancées pour expliquer cela: 1- Les forêts anciennes sont localisées sur des sites protégés des feux, ou 2- le risque de brûler y est équivalent et cette portion de la mosaïque ne constitue que la queue de la distribution, épargnée du feu par simple hasard statistique. Pour que la première éventualité s'avère exacte, un intervalle de temps sans feu exceptionnellement long devrait être observé et ces sites pourraient être intimement liés à des barrières topographiques ou hydrologiques les protégeant du feu. Pour que la deuxième éventualité s'avère exacte, la portion la plus ancienne de la mosaïque constituerait la queue d'une courbe de survie suivant une distribution exponentielle négative. La dendrochronologie s'est avérée inadéquate pour dater le dernier feu dans ces forêts anciennes puisque le temps écoulé depuis le dernier feu est probablement supérieur à l'âge des plus vieux arbres. Une étude paléoécologique a donc été effectuée dans le but de dater le dernier feu dans ces forêts. Trois monolithes de tourbe ont été prélevés dans six forêts anciennes. Les horizons de charbon ont été localisés avec une précision de 1 cm et datés au ¹⁴C. Le fait que des fragments de charbon macroscopiques aient été retrouvés dans tous les sites élimine la possibilité que ces forêts anciennes n'aient jamais brûlé dans le passé. Ces forêts anciennes semblent s'inscrire dans le prolongement d'une courbe de survie suivant le modèle exponentiel négatif. Aussi, aucune influence de la distance moyenne aux coupe-feu potentiels n'a été observé dans le modèle de survie. Cela suggère que la susceptibilité au feu des forêts anciennes est équivalente à celle des forêts plus jeunes. Par conséquent, le territoire occupé par les forêts anciennes étudiées semble faire partie de la mosaïque forestière dynamique régulée par les feux et pourrait être inclus dans un aménagement forestier s'inspirant des perturbations naturelles. © 2001 UQAM tous droits réservés.