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.

Old-growth forests play a decisive role in preserving biodiversity and ecological functions. In an environment frequently disturbed by fire, the importance of old-growth forests as both a carbon stock as well as a source of emissions when burnt is not fully understood. Here, we report on carbon accumulation with time since the last fire (TSF) in the dominant forest types of the Clay Belt region in eastern North America. To do so, we performed a fuel inventory (tree biomass, herbs and shrubs, dead woody debris, and duff loads) along four chronosequences. Carbon emissions by fire through successional stages were simulated using the Canadian Fire Effects Model. Our results show that fuel accumulates with TSF, especially in coniferous forests. Potential carbon emissions were on average 11.9 t·ha?1 and 29.5 t·ha?1 for old-growth and young forests, respectively. In conclusion, maintaining old-growth forests in the Clay Belt landscape not only ensures a sustainable management of the boreal forest, but it also optimizes the carbon storage.

Abstract

Questions

High moisture levels and low occurrences of wildfires have contributed during recent millennia to the accumulation of thick layers of organic soil and to a succession into open black spruce (Picea mariana)–Sphagnum-dominated forests in the Clay Belt boreal landscapes of eastern North America. In these forests, the anticipated increase in drought frequency with climate change could lead to a shift in forest structure and composition via increased fire disturbance. Here, we quantify the expected changes in fire behaviour, biomass burning and vegetation composition in the Clay Belt forest of North America that could arise under climate change over the next century.

Location

A managed forest unit in the Clay Belt boreal forest of eastern North America.

Methods

The impact of a changing climate from 1971 to 2100 on fire regime characteristics (i.e. rate of spread, fuel consumption, fire intensity, type of fire and depth of burn) and vegetation dynamics (mortality and recruitment) was investigated using the Canadian Fire Effects Model (CanFIRE). Vegetation dynamics were governed by the fire danger and behaviour that affect tree mortality and post-fire recruitment of species, and by long-term successional pathways that are driven by post-fire recruitment and forest age. An ensemble of two climate models forced by three scenarios of greenhouse gas emissions was used to drive CanFIRE simulations.

Results

Results from multiple scenarios suggested that fire danger will increase significantly during the 21st century in the Clay Belt forest. The burn rate was projected to change from 4.2% decade−1 during 1971–2000 to 18.6% decade−1 during 2071–2100. Stand mortality, fire intensity and areas affected by crown fires were also projected to increase. A shift in forest composition did not occur over the simulation period across most of our fire regime scenarios. Dominance of open black spruce–Sphagnum forests was projected to remain in future landscapes.

Conclusions

Moist and cool conditions in these forests prevent high depth of burn and contribute to the ecological resistance of these forests to increasing fire danger.

Ce doctorat s‘insère dans un contexte d‘aménagement adaptatif des ressources forestières boréales à l‘est du Canada en prévision de conditions climatiques futures plus propices à l‘activité des feux.

L‘importance des feux dans la dynamique végétale boréale a mené au développement de pratiques sylvicoles qui s‘inspirent du régime actuel des feux. Il s‘agit de cette manière de recréer des structures en âge et en composition similaires que celles observées dans les paysages naturels. Dans la région de la Ceinture d‘Argile, à l‘est du Canada, la récolte de bois et le retrait de couches organiques peuvent être justifiées par une activité actuelle de feux moins importante que celle connue dans le passé. Le régime des feux dépend des conditions météorologiques, le réchaufemment climatique attendu pour ces prochaines années induira des conditions de sécheresse plus propices aux feux et affecteront la dynamique forestière dans la région d‘étude. Les pratiques sylvicoles pourraient engendrer ainsi des effets cumulatifs aux impacts des changements climatiques et par conséquent influencer la capacité de la forêt à s‘adapter.

Dans une optique finale de permettre aux gestionnaires d‘adapter leurs pratiques sylvicoles en vue d‘anticiper les impacts des changements climatiques, l‘objectif principal de ce doctorat consiste en l‘évaluation de l‘impact des changements climatiques sur l‘activité des feux (occurence et profondeur de brûlage) et la végétation de la forêt boréale de la Ceinture d‘Argile au Québec. Ce doctorat se divise en trois objectifs présentés en chapitres: (1) évaluer les impacts des changements climatiques sur l‘occurrence des feux dans les prochaines décennies pour l‘ensemble du Québec à une échelle appropriée pour l‘aménagement (paysage), (2) projeter la réponse aux changements climatiques de la profondeur de brûlage dans des pessières à mousses et des pessières à sphaignes, les peuplements dominant des paysages de la forêt boréale de la Ceinture d‘Argile (3) simuler les modifications potentielles des paysages forestiers de la Ceinture d‘Argile du Québec en réponse aux changements des régimes des feux obtenus lors de la réalisation des objectifs précédents.

La modélisation a été le principal outil utilisé pour la réalisation de ce projet. L‘originalité réside dans l‘intégration des effets rétroactifs de la végétation sur l‘activité des feux dans un contexte de changements climatiques. Des modèles empiriques de l‘occurrence de feux et de la profondeur de brûlage ont été développés à partir de données climatiques de feux et d‘attributs de végétation. Ces modèles ont été ensuite appliqués à des données climatiques de feux futurs simulés à l‘aide de modèles climatiques globaux ou régionaux selon les chapitres pour projeter le nombre de feu et la profondeur de brûlage potentiels pour la fin du 21ème siècle. Les relations empiriques ont été ensuite intégrées à un modèle basé sur des processus afin de calibrer la dynamique de la végétation de la Ceinture d‘Argile en réponse à la profondeur de brûlage et de l‘aire brûlée. Il a finalement été appliqué à des données de risque futur de feux afin de simuler la réponse de la végétation aux changements futurs de l‘activité des feux.

Les résultas principaux de cette étude ont révélé que les changements climatiques induiront une augmentation de l‘activité des feux dans la région de la Ceinture d‘Argile à l‘Est du Canada. La potentielle expansion des essences de feuillues limitées plus au sud ainsi que des pessières dominées par des sphaignes auront pour effet de réduire les impacts du climat sur les feux. La mise en place de pratiques sylvicoles qui modifient la composition et la structure des paysages est ainsi proposée afin de diminuer la vulnérabilité des forêts aux feux. Cette étude constitue une première tentative de simulation de la dynamique de végétation et des effets des changements climatiques futurs dans la pessière a mousse de la Ceinture d‘Argile. L‘intégration des effets de rétroaction de la végétation sur les feux permet de comprendre les interactions futures entre le climat, la végétation et le feu. Les résultats de ce doctorat ont des implications importantes pour l‘aménagement forestier, car ils ouvrent le débat sur de nouvelles statégies d‘aménagement.

The characterization of the fire regime in the boreal forest rarely considers spatial attributes other than fire size. This study investigates the spatial attributes of fires using the physiography of the landscape as a spatial constraint at a regional scale. Using the Canadian National Fire Database, the size, shape, orientation and eccentricity were assessed for 1,136 fires between 1970 and 2010 in Quebec's boreal forest and were summarized by ecodistrict. These spatial metrics were used to cluster 33 ecodistricts into homogeneous fire zones and then to determine which environmental variables (climate, topography, hydrography, and surficial deposits) influence the spatial attributes of fires. Analyses showed that 28 out of 33 ecodistricts belonging to a given fire zone were spatially contiguous, suggesting that factors driving the spatial attributes of fire are acting at a regional scale. Indeed, the orientation and size of fires vary significantly among the zones and are driven by the spatial orientation of the landscape and the seasonal regional climate. In some zones, prevailing winds during periods conducive to fire events parallel to the orientation of the landscape may favour the occurrence of very large fires (>100,000 ha). Conversely, an orientation of the landscape opposite to the prevailing winds may act as a natural firebreak and limit the fire size and orientation. This study highlights the need to consider the synergistic relationship between the landscape spatial patterns and the climate regime over the spatial attributes of fire at supra-regional scale. Further scale-dependant studies are needed to improve our understanding of the spatial factors controlling the spatial attributes of fire. © 2014 Her Majesty the Queen in Right of Canada.

High moisture levels and low frequency of wildfires have contributed to the accumulation of the organic layer in open black spruce (Picea mariana)–Sphagnum dominated stands of eastern boreal North America. The anticipated increase in drought frequency with climate change could lead to moisture losses and a transfer of the stored carbon back into the atmosphere due to increased fire disturbance and decomposition. Here we studied the dynamics of soil moisture content and weather conditions in spruce–feather moss and spruce–Sphagnum dominated stands of the boreal Clay Belt of eastern Canada during particularly dry conditions. A linear mixed model was developed to predict the moisture content of the organic material according to weather, depth and site conditions. This model was then used to calculate potential depth of burn and applied to climate model projections to determine the sensitivity of depth of burn to future fire hazards. Our results suggest that depth of burn varies only slightly in response to changes in weather conditions in spruce–Sphagnum stands. The reverse holds true in spruce–feather moss stands. In conclusion, our results suggest that spruce–Sphagnum stands in the boreal Clay Belt may be resistant to an increase in the depth of burn risk under climate change.

• Strategic introduction of less flammable broadleaf vegetation into landscapes was suggested as a management strategy for decreasing the risk of boreal wildfires projected under climatic change. However, the realization and strength of this offsetting effect in an actual environment remain to be demonstrated.
• Here we combined paleoecological data, global climate models and wildfire modelling to assess regional fire frequency (RegFF, i.e. the number of fires through time) in boreal forests as it relates to tree species composition and climate over millennial time-scales.
• Lacustrine charcoals from northern landscapes of eastern boreal Canada indicate that RegFF during the mid-Holocene (6000–3000 yr ago) was significantly higher than pre-industrial RegFF (ad c. 1750). In southern landscapes, RegFF was not significantly higher than the pre-industrial RegFF in spite of the declining drought severity. The modelling experiment indicates that the high fire risk brought about by a warmer and drier climate in the south during the mid-Holocene was offset by a higher broadleaf component.
• Our data highlight an important function for broadleaf vegetation in determining boreal RegFF in a warmer climate. We estimate that its feedback may be large enough to offset the projected climate change impacts on drought conditions.

In this era of climate change, understanding past and predicting future fire activity are scientific challenges that are central to the development of sustainable forest management practices and policies. Such objectives, however, are difficult to achieve for several reasons. Uncertainties about future fire activity can be superimposed on the short time period covered by existing meteorological data and fire statistics, from which a historical range of variability can be determined. Regional fire activity is also tremendously variable over time, such that contemporary fire records cannot provide information on the full range of fire activity variability a given forest experienced and adapted to. This factor is increasingly important when it comes to determining the resilience of boreal forests to changes in climate and disturbance regimes. In this paper, we present a synthesis of past, present and future trends in seasonal fire danger and fire activity based on data gathered in eastern Canadian boreal forests over the last 20 years, and we provide a critical assessment of the ability to conduct sustainable forest management over the 21st century. The data synthesis provides compelling evidence of a synchronous pattern of decreasing fire-conducive climatic conditions and activity of large fire seasons over the last 2000 years in the eastern coniferous boreal forest. Model simulations suggest that the climate will become drier in upcoming decades, driving future fire activity close to the upper bound of the pre-industrial range of variability. The effects of increasing fire incidence cumulated with forest harvesting may thus pose a risk to forest resilience in the future. This ecological knowledge should help us to define forest management strategies and practices considering future fire activity changes forecasted under climate change. Development of alternative silvicultural interventions that would emulate secondary disturbances (e.g. wind, insects) rather than fire would be necessary to maintain pre-industrial forest characteristics (e.g. composition and age class distribution), and associated forest resilience.

There is general consensus that wildfires in boreal forests will increase throughout this century in response to more severe and frequent drought conditions induced by climate change. However, prediction models generally assume that the vegetation component will remain static over the next few decades. As deciduous species are less flammable than conifer species, it is reasonable to believe that a potential expansion of deciduous species in boreal forests, either occurring naturally or through landscape management, could offset some of the impacts of climate change on the occurrence of boreal wildfires. The objective of this study was to determine the potential of this offsetting effect through a simulation experiment conducted in eastern boreal North America. Predictions of future fire activity were made using multivariate adaptive regression splines (MARS) with fire behavior indices and ecological niche models as predictor variables so as to take into account the effects of changing climate and tree distribution on fire activity. A regional climate model (RCM) was used for predictions of future fire risk conditions. The experiment was conducted under two tree dispersal scenarios: the status quo scenario, in which the distribution of forest types does not differ from the present one, and the unlimited dispersal scenario, which allows forest types to expand their range to fully occupy their climatic niche. Our results show that future warming will create climate conditions that are more prone to fire occurrence. However, unlimited dispersal of southern restricted deciduous species could reduce the impact of climate change on future fire occurrence. Hence, the use of deciduous species could be a good option for an efficient strategic fire mitigation strategy aimed at reducing fire propagation in coniferous landscapes and increasing public safety in remote populated areas of eastern boreal Canada under climate change.

Wildfire activity in North American boreal forests increased during the last decades of the 20th century, partly owing to ongoing human-caused climatic changes. How these changes affect regional fire regimes (annual area burned, seasonality, and number, size, and severity of fires) remains uncertain as data available to explore fire–climate–vegetation interactions have limited temporal depth. Here we present a Holocene reconstruction of fire regime, combining lacustrine charcoal analyses with past drought and fire-season length simulations to elucidate the mechanisms linking long-term fire regime and climatic changes. We decomposed fire regime into fire frequency (FF) and biomass burned (BB) and recombined these into a new index to assess fire size (FS) fluctuations. Results indicated that an earlier termination of the fire season, due to decreasing summer radiative insolation and increasing precipitation over the last 7.0 ky, induced a sharp decrease in FF and BB ca. 3.0 kyBP toward the present. In contrast, a progressive increase of FS was recorded, which is most likely related to a gradual increase in temperatures during the spring fire season. Continuing climatic warming could lead to a change in the fire regime toward larger spring wildfires in eastern boreal North America.