To evaluate the influence of long-distance transport of charcoal particles on the detection of local wildfires from lake sediment sequences, we tracked three consecutive years of charcoal deposition into traps set within seven boreal lakes in northeastern Canada. Peaks in macroscopic charcoal accumulation (>150?µm) were linked to both local (inside the watershed) and regional wildfires. However, regional fires were characterized by higher proportions of small particles (<0.1?mm2) in charcoal assemblages. We conclude that the analysis of particle size distribution is useful to discriminate “true” local fires from regional wildfires.

Les feux de forêt constituent une importante perturbation en forêt boréale. Le climat, la température, la topographie, la végétation, les dépôts de surface et les activités humaines représentent autant de facteurs pouvant les influencer. À l’inverse, les feux de forêt affectent le climat par l’émission de gaz et d’aérosols et modifient l’albédo de surface, les processus édaphiques et la dynamique de la végétation. On ne connaît pas encore très bien l’effet net de ces facteurs, mais ils semblent avoir exercé une rétroaction négative sur le climat au cours du XXe siècle. Cependant, on prédit une augmentation des feux de forêt vers la fin du XXIe siècle susceptible de modifier leurs effets sur le climat vers une rétroaction positive capable d’exacerber réchauffement planétaire. Cette synthèse présente (1) une revue d’ensemble des régimes des feux et de la succession de la végétation en forêts boréales, (2) les effets sur le climat des émissions de combustion et des changements du fonctionnement des écosystèmes après feu, (3) les effets des variations du régime de feu sur le climat, particulièrement sur les changements de stock de carbone et d’albédo de surface, (4) une approche intégrée des effets des feux sur la dynamique du climat et (5) l’implication de l’augmentation de l’activité des feux sur le réchauffement planétaire en calculant le forçage radiatif de plusieurs facteurs vers 2100 en région boréale, avant de discuter les résultats et d’exposer les limites des données. Généralement, les pertes en carbone occasionnées par les feux de forêt en région boréale augmenteront dans le futur et leur effet sur les stocks de carbone (0,37 W/m2/décennie) sera plus grand que l’effet du feu sur l’albédo de surface (−0,09 W/m2/décennie). L’effet net des émissions d’aérosols venant des feux en forêt boréale causera vraisemblablement une rétroaction positive sur le réchauffement planétaire. Cette synthèse met l’accent sur l’importance des rétroactions entre les feux et le climat en forêt boréale. Elle présente les limites et les incertitudes à aborder dans les prochaines études, surtout en relation avec les effets de la fertilisation par le CO2 sur la productivité forestière, laquelle pourrait contrebalancer ou atténuer les effets du feu.

Fire frequency is expected to increase in boreal forests over the next century owing to climate change. In Quebec (Canada), the location of the northern limit of commercial forests (c. 51 °N) was established in 2000 taking into account mainly forest productivity and fire risk. The location of the limit is currently under debate and is being re-evaluated based on a more extensive survey of the territory. We characterized the natural variability of fire occurrence (FO) in the area surrounding the northern limit, and these results are a useful contribution to discussions on the re-evaluation of its location. Regional FO over the last 7000 years was reconstructed from sedimentary charcoal records from 11 lakes located in three regions surrounding the northern limit (i.e. south, north and near the limit). Holocene simulated precipitation and temperature from a general circulation model (GCM) were used to identify the long-term interactions between climate and fire. Fire histories displayed similar trends in all three regions, with FO increasing from 7000 calibrated years before present (cal. years BP) to reach a maximum at 4000-3000 cal. years BP, before decreasing during the late-Holocene. This trend matches the simulated changes in climate, characterized by drier and warmer conditions between 7000 and 3500 cal. years BP and cooler and moister conditions between 3500 and 0 cal. years BP. Northern ecosystems displayed higher sensitivity to climate change. The natural variability of FO was narrower in the southern region compared with the limit and northern regions. An abrupt decrease in FO was recorded close to and north of the limit at 3000 cal. years BP, whereas the decrease was more gradual in the south. Synthesis and applications. We reconstructed the natural variability in fire activity over the last 7000 years near the current location of the northern limit of commercial forests in Quebec. Fire occurrences were more sensitive to climate change near to and north of the limit of commercial forestry. In the context of predicted increase in fire activity, the lower resilience of northern forests advocates against a northern repositioning of the limit of commercial forests. © 2014 The Authors.

We compared fire episodes over the past 150 years reconstructed using charcoal particles retrieved from well-dated sediment deposits from two small lakes in the eastern Canadian boreal forest, with dendrochronological reconstructions of fire events from the corresponding watersheds. Fire scars and age structure of living trees highlighted three fire events (AD 1890, 1941, and 1989). To explore the ability to detect these fire events based on sedimentary charcoal records, we explored the influence of two user-determined parameters of a widely used peak-detection algorithm (the CharAnalysis software): (1) the temporal resolution used to interpolate charcoal series and (2) the width of the smoothing window used to model background noise. The signal-to-noise index (SNI) is often used to evaluate the ability to detect peaks in sedimentary charcoal records, which can be related to fire events. SNI values >3 identify records appropriate for peak detection. Selecting standard settings in paleoecological studies (median temporal resolution of the entire sequence and 500- to 1000-year window width) yielded higher global SNI values but failed to detect most recent fire events. Instead, selecting a shorter reference period (the past ~150 years) to determine the temporal resolution to interpolate the charcoal series and a narrower smoothing window (100 years) best matched the tree-ring data despite lower SNI values (often <3.0). However, Holocene fire history differed markedly when reconstructed using different smoothing window widths (100–150 years vs >300 years). Consequently, we suggest using the smallest window width yielding a SNI >3. Practitioners must not necessarily focus on obtaining the highest possible SNI, usually related to wide smoothing windows. We also suggest that fire history reconstructions should focus on core sections presenting fairly constant sedimentation rates. Alternatively, sediments could be subsampled after age–depth models have been obtained.