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.

Abstract Fire regimes in North American forests are diverse and modern fire records are often too short to capture important patterns, trends, feedbacks, and drivers of variability. Tree-ring fire scars provide valuable perspectives on fire regimes, including centuries-long records of fire year, season, frequency, severity, and size. Here, we introduce the newly compiled North American tree-ring fire-scar network (NAFSN), which contains 2562 sites, >37,000 fire-scarred trees, and covers large parts of North America. We investigate the NAFSN in terms of geography, sample depth, vegetation, topography, climate, and human land use. Fire scars are found in most ecoregions, from boreal forests in northern Alaska and Canada to subtropical forests in southern Florida and Mexico. The network includes 91 tree species, but is dominated by gymnosperms in the genus Pinus. Fire scars are found from sea level to >4000-m elevation and across a range of topographic settings that vary by ecoregion. Multiple regions are densely sampled (e.g., >1000 fire-scarred trees), enabling new spatial analyses such as reconstructions of area burned. To demonstrate the potential of the network, we compared the climate space of the NAFSN to those of modern fires and forests; the NAFSN spans a climate space largely representative of the forested areas in North America, with notable gaps in warmer tropical climates. Modern fires are burning in similar climate spaces as historical fires, but disproportionately in warmer regions compared to the historical record, possibly related to under-sampling of warm subtropical forests or supporting observations of changing fire regimes. The historical influence of Indigenous and non-Indigenous human land use on fire regimes varies in space and time. A 20th century fire deficit associated with human activities is evident in many regions, yet fire regimes characterized by frequent surface fires are still active in some areas (e.g., Mexico and the southeastern United States). These analyses provide a foundation and framework for future studies using the hundreds of thousands of annually- to sub-annually-resolved tree-ring records of fire spanning centuries, which will further advance our understanding of the interactions among fire, climate, topography, vegetation, and humans across North America.

Climate changes are expected to progressively increase extreme wildfire frequency in forests. Finding past analogs for periods of extreme biomass burning would provide valuable insights regarding what the effects of warming might be for tree species distribution, ecosystem integrity, atmospheric greenhouse gas balance, and human safety. Here, we used a network of 42 lake-sediment charcoal records across a ~2000 km transect in eastern boreal North America to infer widespread periods of wildfire activity in association with past climate conditions. The reconstructed fluctuations in biomass burning are broadly consistent with variations in ethane concentration in Greenland polar ice cores. Biomass burning fluctuations also significantly co-varied with Greenland temperatures estimated from ice cores, at least for the past 6000 years. Our retrospective analysis of past fire activity allowed us to identify two fire periods centered around 4800 and 1100 BP, coinciding with large-scale warming in northern latitudes and having respectively affected an estimated ~71% and ~57% of the study area. These two periods co-occurred with widespread decreases in mean fire-return intervals. The two periods are likely the best analogs for what could be anticipated in terms of impacts of fire on ecosystem services provided by these forests in coming decades.

In Canada, recent catastrophic wildfire events raised concern from governments and communities. As climate change is expected to increase fire activity in boreal forests, the need for a better understanding of fire regimes is becoming urgent. This study addresses the 1972–2015 spatial distributions of fire cycles, mean fire size (FireSz) and mean fire occurrence (mean annual number of fires per 100 000 ha, FireOcc) in eastern Canada. The objectives were to determine (1) the spatial variability of fire-regime attributes, (2) the capacity of FireSz and FireOcc to distinguish homogeneous fire zones and (3) the environmental factors driving FireSz and FireOcc, with some emphasis on lightning strikes. Fire cycles, FireSz and FireOcc greatly varied throughout the study area. Even within homogeneous fire zones, FireSz and FireOcc were highly variable. FireSz was controlled by moisture content in deep layers of the soil and by surficial deposits, whereas FireOcc was controlled by moisture content in top layers of the soil and by relief. The lack of a relationship between FireOcc and lightning-strike density suggested that the limiting effect of lightning-strike density on FireOcc could be operating only under certain circumstances, when interacting with other environmental factors.

Boreal forests subject to low fire activity are complex ecosystems in terms of structure and dynamics. They have a high ecological value as they contain important proportions of old forests that play a crucial role in preserving biodiversity and ecological functions. They also sequester important amounts of carbon at the landscape level. However, the role of time since fire in controlling the different processes and attributes of those forests is still poorly understood. The Romaine River area experiences a fire regime characterized by very rare but large fires and has recently been opened to economic development for energy and timber production. In this study, we aimed to characterize this region in terms of live aboveground biomass, merchantable volume, stand structure and composition, and to establish relations between these attributes and the time since the last fire. Mean live aboveground biomass and merchantable volume showed values similar to those of commercial boreal coniferous forests. They were both found to increase up to around 150 years after a fire before declining. However, no significant relation was found between time since fire and stand structure and composition. Instead, they seemed to mostly depend on stand productivity and non-fire disturbances. At the landscape level, this region contains large amounts of biomass and carbon stored resulting from the long fire cycles it experiences. Although in terms of merchantable volume these forests seemed profitable for the forest industry, a large proportion were old forests or presented structures of old forests. Therefore, if forest management was to be undertaken in this region, particular attention should be given to these old forests in order to protect biodiversity and ecological functions. Partial cutting with variable levels of retention would be an appropriate management strategy as it reproduces the structural complexity of old forests.

En forêt boréale nord-américaine, les feux constituent la perturbation principale. Ils contrôlent de nombreux attributs et processus de la forêt comme la dynamique forestière, la structure des paysages et la composition des peuplements. La variabilité spatiale et temporelle des régimes de feux à travers la zone boréale nord-américaine est très importante, et résulte de divers facteurs environnementaux. Dans la forêt boréale québécoise, les pessières à lichens, des milieux forestiers ouverts et peu productifs, dominent au nord. Plus au sud, les pessières à mousses fermées sont les plus représentées. Les pessières à lichens sont souvent le résultat d’un climat difficile et de perturbations en rafales, principalement les feux, qui mènent à des accidents de régénération. Cependant, la variabilité spatiale et les facteurs déterminants des régimes de feux passés et contemporains sont encore mal connus au nord et à l’interface de ces forêts fermées et ouvertes. De plus, si l’on sait que les feux contrôlent majoritairement le fonctionnement de la forêt boréale, les liens entre le temps depuis feux et la dynamique forestière sont très peu documentés dans les zones soumises à une activité faible des feux.

En forêt boréale nord-américaine, les feux constituent la perturbation principale. Ils contrôlent de nombreux attributs et processus de la forêt comme la dynamique forestière, la structure des paysages et la composition des peuplements. La variabilité spatiale et temporelle des régimes de feux à travers la zone boréale nord-américaine est très importante, et résulte de divers facteurs environnementaux. Dans la forêt boréale québécoise, les pessières à lichens, des milieux forestiers ouverts et peu productifs, dominent au nord. Plus au sud, les pessières à mousses fermées sont les plus représentées. Les pessières à lichens sont souvent le résultat d’un climat difficile et de perturbations en rafales, principalement les feux, qui mènent à des accidents de régénération. Cependant, la variabilité spatiale et les facteurs déterminants des régimes de feux passés et contemporains sont encore mal connus au nord et à l’interface de ces forêts fermées et ouvertes. De plus, si l’on sait que les feux contrôlent majoritairement le fonctionnement de la forêt boréale, les liens entre le temps depuis feux et la dynamique forestière sont très peu documentés dans les zones soumises à une activité faible des feux.

L’objectif général de cette thèse est d’améliorer les connaissances des régimes de feux passés et contemporains de la forêt boréale résineuse québécoise, à l’interface entre les forêts fermées et les forêts ouvertes, en termes de variabilité spatiale, de facteurs déterminants et de conséquences des feux. La thèse se décline en trois chapitres visant chacun à répondre à une partie de cet objectif général. Le premier chapitre caractérise la variabilité latitudinale des risques de feux des 150-300 dernières années le long de quatre transects orientés nord-sud dans la zone boréale résineuse du Québec. Il fait également le lien entre la variabilité latitudinale du risque de feux et le climat. Le deuxième chapitre cherche à démontrer la performance d’une méthode statistique spatialement explicite dans l’analyse de taux de brûlage. Cette méthode est utilisée pour déterminer la contribution relative du climat, de l’environnement physique et de la végétation à la variabilité spatiale des taux de brûlage contemporains. Le troisième chapitre se concentre sur une zone soumise à un régime de feux caractérisé par des feux rares mais souvent de grande taille, dans le secteur de la rivière Romaine. Ce chapitre vise à caractériser ce territoire en termes de biomasse aérienne, de volume marchand, de structure et de composition des peuplements, ainsi qu’à faire le lien entre ces attributs et le temps depuis feux.

Les implications de nos résultats dans un contexte de changements climatiques sont importantes. En effet, dans le futur l’augmentation des précipitations ne devrait pas permettre de compenser l’augmentation prévue des températures. Ainsi, on pourrait s’attendre à ce que le système climat-feux redevienne contrôlé par les températures, engendrant une augmentation importante des taux de brûlage et de l’occurrence des feux. Ce phénomène est déjà observé dans l’ouest du Canada, et serait peut-être déjà en cours dans le nord-ouest du Québec. En effet, l’activité des feux y a récemment augmenté, ce qui pourrait se propager au reste du territoire dans le futur. Cependant, nous avons montré que malgré les changements temporels de régimes de feux, leur zonation restait stable. Cette zonation correspondait relativement bien à la limite nordique des forêts attribuables qui sépare les forêts ouvertes et naturelles au nord des forêts fermées et aménagées au sud. Si la variabilité future des régimes de feux se maintient à l’intérieur de la variabilité passée, cette zonation devrait ainsi maintenir son inertie. Il serait toutefois important de surveiller l’ouverture des peuplements dans le nord, puisque la transformation de pessières à mousses en pessières à lichens est considérée irréversible naturellement.

Par ailleurs, nos résultats apportent des connaissances utiles pour l’aménagement forestier. En particulier, nous confirmons de précédentes études démontrant que la végétation jouait un rôle important dans le contrôle des taux de brûlage. Ainsi, l’aménagement pourrait se servir de la végétation moins inflammable, principalement les espèces feuillues, pour diminuer les risques de feux et ainsi réduire les impacts des changements climatiques sur les possibilités forestières. Dans le secteur de la rivière Romaine, la biomasse aérienne et le volume marchand sont assimilables à ceux de forêts boréales commerciales. Cependant, puisque l’activité des feux y est faible, les stocks de carbone y sont importants et la proportion de forêts présentant des attributs de vieilles forêts y est considérable. Ces écosystèmes sont extrêmement importants pour le maintien de la biodiversité et des fonctions écologiques. Ainsi, il est indispensable d’implanter des stratégies d’aménagement – possiblement davantage basées sur la structure que sur l’âge des peuplements – adaptées aux vieilles forêts ou aux forêts présentant des attributs de vieilles forêts. D’autre part, le développement économique de cette région, tant d’un point de vue forestier qu’énergétique, doit tenir compte des importants stocks de carbone contenus dans ces forêts.

Wildland fires are the main natural disturbance shaping forest structure and composition in eastern boreal Canada. On average, more than 700?000?ha of forest burns annually and causes as much as CAD 2.9 million worth of damage. Although we know that occurrence of fires depends upon the coincidence of favourable conditions for fire ignition, propagation, and fuel availability, the interplay between these three drivers in shaping spatiotemporal patterns of fires in eastern Canada remains to be evaluated. The goal of this study was to reconstruct the spatiotemporal patterns of fire activity during the last century in eastern Canada's boreal forest as a function of changes in lightning ignition, climate, and vegetation. We addressed this objective using the dynamic global vegetation model LPJ-LMfire, which we parametrized for four plant functional types (PFTs) that correspond to the prevalent tree genera in eastern boreal Canada (Picea, Abies, Pinus, Populus). LPJ-LMfire was run with a monthly time step from 1901 to 2012 on a 10?km2 resolution grid covering the boreal forest from Manitoba to Newfoundland. Outputs of LPJ-LMfire were analyzed in terms of fire frequency, net primary productivity (NPP), and aboveground biomass. The predictive skills of LPJ-LMfire were examined by comparing our simulations of annual burn rates and biomass with independent data sets. The simulation adequately reproduced the latitudinal gradient in fire frequency in Manitoba and the longitudinal gradient from Manitoba towards southern Ontario, as well as the temporal patterns present in independent fire histories. However, the simulation led to the underestimation and overestimation of fire frequency at both the northern and southern limits of the boreal forest in Québec. The general pattern of simulated total tree biomass also agreed well with observations, with the notable exception of overestimated biomass at the northern treeline, mainly for PFT Picea. In these northern areas, the predictive ability of LPJ-LMfire is likely being affected by the low density of weather stations, which leads to underestimation of the strength of fire–weather interactions and, therefore, vegetation consumption during extreme fire years. Agreement between the spatiotemporal patterns of fire frequency and the observed data across a vast portion of the study area confirmed that fire therein is strongly ignition limited. A drier climate coupled with an increase in lightning frequency during the second half of the 20th century notably led to an increase in fire activity. Finally, our simulations highlighted the importance of both climate and fire in vegetation: despite an overarching CO2-induced enhancement of NPP in LPJ-LMfire, forest biomass was relatively stable because of the compensatory effects of increasing fire activity.

Context
Wildfires play a crucial role in maintaining ecological and societal functions of North American boreal forests. Because of their contagious way of spreading, using statistical methods dealing with spatial autocorrelation has become a major challenge in fire studies analyzing how environmental factors affect their spatial variability.

Objectives
We aimed to demonstrate the performance of a spatially explicit method accounting for spatial autocorrelation in burn rates modelling, and to use this method to determine the relative contribution of climate, physical environment and vegetation to the spatial variability of burn rates between 1972 and 2015.

Methods
Using a 482,000 km² territory located in the coniferous boreal forest of eastern Canada, we built and compared burn rates models with and without accounting for spatial autocorrelation. The relative contribution of climate, physical environment and vegetation to the burn rates variability was identified with variance partitioning.

Results
Accounting for spatial autocorrelation improved the models' performance by a factor of 1.5. Our method allowed the unadulterated extraction of the contribution of climate, physical environment and vegetation to the spatial variability of burn rates. This contribution was similar for the three groups of factors. The spatial autocorrelation extent was linked to the fire size distribution.

Conclusions
Accounting for spatial autocorrelation can highly improve models and avoids biased results and misinterpretation. Considering climate, physical environment and vegetation altogether is essential, especially when attempting to predict future area burned. In addition to the direct effect of climate, changes in vegetation could have important impacts on future burn rates.

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