Accurate estimation of aboveground biomass (AGB) using remote sensing data is still challenging and an approach based on an understanding of forest disturbance and succession could help improve AGB estimation. In the boreal forest of North America, time since last fire (TSLF) is seen as a useful variable to explain post-fire successional change and aboveground biomass (AGB). Within a large study area (>200 000 km2) located in the northeastern American boreal forest, we compared remotely sensed biomass estimates of MODIS (Moderate Resolution Imaging Spectroradiometer), GLAS (Geoscience Laser Altimeter System) and ASAR (Advanced Synthetic Aperture Radar) with inventory-based estimates derived from ground plots, and forest maps at a spatial resolution of 2-km2. We identified that TSLF could explain the error observed in remotely sensed AGB estimates (MODIS (45%), GLAS (47%) or ASAR (23%)) when associated with surficial geological substrate information at that scale. Our results therefore show the importance of TSLF as a potential ancillary variable for improving the accuracy of remotely sensed AGB estimates in North American boreal forests.

On anticipe une augmentation de l’activité des feux de forêt au Canada à cause du changement climatique mais on ne tient généralement pas compte des rétroactions de la végétation. À l’aide de nouvelles informations concernant la sélectivité et l’évitement du feu en fonction de l’âge et de la composition des peuplements, nous avons utilisé des modèles de simulation simples qui tiennent compte des changements dans les matrices d’âge régional engendrés par le feu et la coupe pour prévoir les futurs taux de brûlage. Nous avons également prévu la vulnérabilité régionale estimée de l’approvisionnement en bois face aux feux de forêt en tenant compte de ces nouveaux taux de brûlage. L’inclusion de rétroactions reliées à l’âge devrait avoir un impact important sur l’augmentation prévue des taux de brûlage, surtout dans les zones soumises à un forçage climatique agressif où le feu est très actif. Les taux de brûlage devraient augmenter encore mais devraient être 50 % moins élevés en 2100 que s’ils étaient anticipés sans rétroaction biologique dans certaines zones. Les rétroactions négatives devraient être pratiquement inexistantes lorsque les taux de brûlage potentiels sont inférieurs à 1 %, tandis que les taux de brûlage effectifs devraient diminuer de plus de 0,5 point de pourcentage lorsque les taux potentiels de brûlage dépassent 2,5 %. L’inclusion des rétroactions entre le feu et la végétation n’a eu pratiquement aucun impact sur le volume total récolté. À mesure que le feu brûle davantage de vieux peuplements de conifères, des impacts légèrement négatifs sur les conifères récoltés sont prévus presque partout. Ces résultats font ressortir la nécessité d’incorporer les rétroactions entre le feu et la végétation lorsqu’on prévoit les taux futurs de brûlage. [Traduit par la Rédaction]

Strategies for increasing the mobilization of forest biomass supply chains for bioenergy production require continuous assessments of the spatial and temporal availability of biomass feedstock. Using remote sensing products at a 250-m pixel resolution, estimates of theoretical biomass availability from harvest residues and fire-killed trees were computed by combining Canada-wide maps of forest attributes (2001) and of yearly (2002–2011) fires and harvests. At the national scale, biomass availability was estimated at 47 ± 18 M ODT year?1 from fire-killed trees and at 14 ± 2 M ODT year?1 from harvest residues. Mean biomass densities in burned and harvested pixels were estimated at 34 ± 3.0 ODT ha?1 and at 24 ± 1.2 ODT ha?1, respectively. Mean biomass densities also varied dramatically among ecozones, from 14 ODT ha?1 to 206 ODT ha?1 and from 6 ODT ha?1 to 63 ODT ha?1 for burned and harvested pixels, respectively. Spatial averaging with a 100-km radius window shows distinct hotspots of biomass availability across Canada. The largest hotspots from fire-killed trees reached 3.6 M ODT year?1 in the Boreal Shield and the Boreal Plains ecozones of northern Alberta and Saskatchewan, where fires are large and frequent. The largest hotspots from harvest residues reached 1.2 M ODT year?1 in the Montane Cordillera ecozone of British Columbia. The use of spatially explicit remote sensing products yields estimates of theoretical biomass availability that are methodologically consistent across Canada. Future development should include validations with on-the-ground forest inventories as well as the factoring in of environmental, technical and economic considerations to implement operational biomass supply chains.

Fire is a dominant mechanism of forest renewal in most of Canada’s forests and its activity is predicted to increase over the coming decades. Individual fire events have been considered to be non-selective with regards to forest properties, but evidence now suggests otherwise. Our objective was therefore to quantify the effect of forest properties on fire selectivity or avoidance, evaluate the stability of these effects across varying burn rates, and use these results to map local fire risk across the forests of Canada. We used Canada-wide MODIS-based maps of annual fires and of forest properties to identify burned and unburned pixels for the 2002–2011 period and to bin them into classes of forest composition (% conifer and broadleaved deciduous), above-ground tree biomass and stand age. Logistic binomial regressions were then used to quantify fire selectivity by forest properties classes and by zones of homogeneous fire regime (HFR). Results suggest that fire exhibits a strong selectivity for conifer stands, but an even stronger avoidance of broadleaved stands. In terms of age classes, fire also shows a strong avoidance for young (0 to 29 year) stands. The large differences among regional burn rates do not significantly alter the overall preference and avoidance ratings. Finally, we combined these results on relative burn preference with regional burn rates to map local fire risks across Canada.

In existing carbon budget models, carbon stocks are not explicitly related to forest successional dynamics and environmental factors. Yet time-since-last-fire (TSLF) is an important variable for explaining successional changes and subsequent carbon storage. The objective of this study was to predict the spatial variability of aboveground biomass carbon (ABC) as a function of TSLF and other environmental factors across the landscape at regional scales. ABC was predicted using random forest models, both at the sample-plot level and at the scale of 2-km2 cells. This cell size was chosen to match the observed minimum fire size of the Canadian large fire database. The percentage variance explained by the empirical sample-plot level model of ABC was 50%. At that scale, TSLF was not significantly related to ABC. At the 2-km2 scale, ABC was influenced mainly by the proportions of cover density classes, which explained 83% of the variance. Changes in cover density were related to TSLF at the same 2-km2 scale, indicating that the increase in cover density following fire disturbance is a dominant mechanism through which TSLF acts upon ABC at the scale of landscapes.

Les impacts des changements climatiques (CCs) sur la forêt sont déjà observés et iront en s'amplifiant dans le futur. Dans ce contexte, il importe de comprendre comment les communautés et l'industrie forestière seront affectées. Des professionnels du secteur forestier québécois ont été consultés lors d'un atelier afin de recueillir leurs perceptions des impacts potentiels des CCs et des possibles mesures d'adaptation. Les préoccupations touchaient les écosystèmes naturels, ainsi que les collectivités et l'industrie forestière. Les impacts identifiés incluaient l'augmentation des évènements météorologiques extrêmes et des perturbations naturelles, une diminution quantitative et qualitative du bois, ainsi que de plus grandes difficultés d'accès aux territoires et des coûts additionnels pour les opérations. Les mesures d'adaptation pourraient comprendre de nouvelles règlementations, une meilleure sensibilisation aux enjeux, et des ajustements locaux et régionaux dans la gestion et les opérations. Les barrières à l'adaptation identifiées incluaient une faible compréhension des enjeux au niveau des intervenants du milieu forestier, ainsi qu'un manque de soutien scientifique, de transfert des connaissances, et de leadership à l’échelle régionale. Cette synthèse aidera à orienter les besoins en matière de planification et de gestion et à identifier des solutions pour augmenter la capacité d'adaptation du secteur forestier.

Les prédictions courantes suggèrent une augmentation de la fréquence des feux au Canada, ce qui pourrait affecter la disponibilité en bois pour des fins industrielles. Nous avons donc évalué la vulnérabilité de l’approvisionnement en bois au risque de feux actuel et futur grâce à des calculs simplifiés impliquant les taux de récolte et la croissance historiques de la forêt, ainsi que les régimes de feux actuels et projetés. Les calculs ont été effectués au niveau des unités d’aménagement forestier (UAF) compris dans les écozones boréales et montagnardes du Canada. Pour certaines UAF, l’analyse suggère une vulnérabilité élevée à extrême de l’approvisionnement en bois au feu d’ici le milieu du siècle. Pour ces UAF, les augmentations de croissance des arbres nécessaires à atténuer ces risques sont généralement irréalistes. Une diminution modeste de la croissance de l’arbre au fil du tempsserait cependant suffisante pour augmenter de faible à modérée la vulnérabilité de nombreuses autres UAF. Des biais connus dans l’analyse suggèrent que notre évaluation pourrait sous-estimer le niveau de vulnérabilité dans toutes les UAF. Les autres perturbations naturelles ne sont pas incluses dans l’analyse, mais leurs impacts sur l’approvisionnement en bois pourraient être additifs à celui du feu. Certaines mesures d’adaptation pour faire face à ces risques croissants sont présentées.

The boreal forest, one of the largest biomes on Earth, provides ecosystem services that benefit society at levels ranging from local to global. Currently, about two-thirds of the area covered by this biome is under some form of management, mostly for wood production. Services such as climate regulation are also provided by both the unmanaged and managed boreal forests. Although most of the boreal forests have retained the resilience to cope with current disturbances, projected environmental changes of unprecedented speed and amplitude pose a substantial threat to their health. Management options to reduce these threats are available and could be implemented, but economic incentives and a greater focus on the boreal biome in international fora are needed to support further adaptation and mitigation actions.

Il est reconnu que les feux de forêt d’origine naturelle ne peuvent pas et même ne doivent pas être éliminés de la forêt boréale nord-américaine. Les feux de forêt occasionnent des pertes immédiates de volume de bois, perturbent la conversion de la structure courante d’âge de la forêt vers une structure cible et empêchent l’approvisionnement planifié en bois (APB) d’être atteint de manière constante. Dans cet article, nous explorons dans quelle mesure les déficits périodiques en bois disponible causés par divers risques de feux peuvent être atténués par la coupe de récupération et par le degré de tolérance des gestionnaires forestiers face à ces déficits, et ceci en fonction de la structure d’âge des forêts. Les simulations sont faites en utilisant une représentation temporelle déterministe et stochastique des feux. Les résultats montrent que la fréquence des périodes en déficit de bois peut être réduite par la coupe de récupération et par l’introduction de mesures de tolérance à ces déficits, et que ce potentiel d’atténuation est influencé par la structure d’âge de la forêt initiale et par le niveau de pertes par le feu. Les résultats montrent également que même un taux de coupe de récupération à 100 % ne peut pas compenser entièrement les pertes de bois par le feu et éliminer les déficits périodiques qui en résultent. En outre, l’ajout de la variabilité interannuelle des feux réduit l’efficacité des deux mesures d’atténuation. Enfin, puisque l’APB n’est en fait jamais réalisé dans les forêts sujettes aux feux, le coût réel d’une réduction l’APB doit être estimé non pas par la différence l’APB, mais plutôt par la différence plus réaliste de récolte de bois réalisée.q

Fire plays an important role for boreal forest succession, and time since last fire (TSLF) is therefore seen as a useful covariate to devise forest management strategies, but TSLF information is currently either spatially or temporarily limited. We therefore developed a TSLF map for an extensive region in eastern Canada (217,000 km2) by generalizing the empirical relationships that exist between regional historical records of fire (1880–2000) with forest inventory data and biophysical variables. Two random forest models were used to predict TSLF at the scale of 2-km2 cells. These cells were first classified into TSLF ? 120 years and >120 years and TSLF was then estimated by decade for cells classified as younger than 120 years. Overall, both models showed a substantial agreement at the scale of both the study area and landscape units, but the accuracy remained fairly low at the scale of individual cells. Results show that the decades between 1920 and 1940 were characterized by widespread fire activity covering approximately 28% of the study region. Studies have reported a doubling of the burn rate from 1970 to 2000, but our longer-term analysis suggests that the 1970–2000 burn rate (4.3% decade?1) is lower than the one detected between 1920 and 1940 (16.4% decade?1) and provides a relevant context for interpreting the recent increases in area burned observed since 1970. These results highlight the importance of lengthening the historical records of fire history maps in order to provide a better perspective of the actual changes of fire regime.

Ecosystem-based management (EBM) of forests is gaining acceptance for its focus on the maintenance of the long-term integrity of ecosystem processes, but climate change challenges this view because of its impacts on these very processes. We have therefore evaluated the robustness of EBM to projected climate change, considering the role of climate on forest growth and fire regime in a boreal forest of eastern Canada. A climate sensitive growth index model was calibrated for three commercial species and used to project the evolution of merchantable volume for two climate scenarios (B1 and A2) under conventional and EBM strategies. Current burn rate and burn rates under future climate scenarios were also considered. Under the most extreme projected climate scenario, the periodic timber supply could be reduced by up to 79% through direct (growth reduction) and indirect (fire) effects. However, ecological indicators show that EBM is a more robust forest management strategy than conventional one demonstrating its adaptation potential to climate change at least in the short term.

The 20th century was a pivotal period at high northern latitudes as it marked the onset of rapid climatic warming brought on by major anthropogenic changes in global atmospheric composition. In parallel, Arctic sea ice extent has been decreasing over the period of available satellite data records. Here, we document how these changes influenced vegetation productivity in adjacent eastern boreal North America. To do this, we used normalized difference vegetation index (NDVI) data, model simulations of net primary productivity (NPP) and tree-ring width measurements covering the last 300 years. Climatic and proxy-climatic data sets were used to explore the relationships between vegetation productivity and Arctic sea ice concentration and extent, and temperatures. Results indicate that an unusually large number of black spruce (Picea mariana) trees entered into a period of growth decline during the late-20th century (62% of sampled trees; n = 724 cross sections of age >70 years). This finding is coherent with evidence encoded in NDVI and simulated NPP data. Analyses of climatic and vegetation productivity relationships indicate that the influence of recent climatic changes in the studied forests has been via the enhanced moisture stress (i.e. greater water demands) and autotrophic respiration amplified by the declining sea ice concentration in Hudson Bay and Hudson Strait. The recent decline strongly contrasts with other growth reduction events that occurred during the 19th century, which were associated with cooling and high sea ice severity. The recent decline of vegetation productivity is the first one to occur under circumstances related to excess heat in a 300-year period, and further culminates with an intensifying wildfire regime in the region. Our results concur with observations from other forest ecosystems about intensifying temperature-driven drought stress and tree mortality with ongoing climatic changes.

In spite of the many factors that are occurring and known for positively affecting the growth of forests, some boreal forests across North America have recently felt the adverse impacts of environmental changes. Knowledge of causes for productivity declines in North American boreal forests remains limited, and this is owed to the large spatial and temporal scales involved, and the many plant processes affected. Here, the response of pristine eastern boreal North American (PEBNA) forests to ongoing climatic changes is examined using in situ data, community ecology statistics, and species-specific model simulations of carbon exchanges forced by contemporary climatic data. To examine trends in forest growth, we used a recently acquired collection of tree-ring width data from 252 sample plots distributed in PEBNA forests dominated by black spruce (Picea mariana [Mill.] B.S.P.) and jack pine (Pinus banksiana Lamb.). Results of linear trend analysis on the tree growth data highlight a dominating forest growth decline in overmature forests (age > 120 years) from 1950 to 2005. In contrast, improving growth conditions are seen in jack pine and mature (70–120 years) black spruce stands. Multivariate analysis of climate and growth relationships suggests that responses of PEBNA forests to climate are dependent on demographic and species traits via their mediation of temperature and water stress constraints. In support of this hypothesis, the simulation experiment suggests that in old-growth black spruce stands the benefit to growth brought on by a longer growing season may have been low in comparison with the increasing moisture stress and respiration losses caused by warmer summer temperatures. Predicted increases in wildfire frequency in PEBNA forests will likely enhance the positive response of landscape-level forest growth to climate change by shifting the forest distribution to younger age classes while also enhancing the jack pine component.

Abstract: In many northern forest ecosystems, soil organic matter accumulation can lead to paludification and forest productivity losses. Paludification rate is primarily influenced by topography and time elapsed since fire, two factors whose influence is often confounded and whose discrimination would help forest management. This study, which was conducted in the black spruce (Picea mariana (Mill.) BSP) boreal forest of northwestern Quebec (Canada), aimed to (1) quantify the effect of slope and time since fire on paludification rates, (2) determine whether soil organic layer depth could be estimated by surface variables that can potentially be remotely sensed, and (3) relate the degree of paludification to tree productivity. In this study, soil organic layer depth was used as an estimator of the degree of paludification. Slope and postfire age strongly affected paludification dynamics. Young stands growing on steep slopes had thinner organic layers and lower organic matter accumulation rates compared with young stands growing on flat sites. Black spruce basal area and Sphagnum cover were strong predictors of organic layer depth, potentially allowing mapping of paludification degree across the landscape. Tree productivity was negatively related to organic layer depth (R² = 0.57). The equations developed here can be used to quantify forest productivity decline in stands that are undergoing paludification, as well as potential productivity recovery given appropriate site preparation techniques.

Résumé : Dans plusieurs écosystèmes forestiers nordiques, l'accumulation de matière organique peut mener à la paludification des sols et entraîner des pertes de productivité forestière. Le taux de paludification est principalement influencé par la topographie et le temps écoulé depuis le dernier feu, deux facteurs dont l'influence est souvent confondue et dont la séparation aiderait à l'aménagement forestier. Cette étude réalisée dans la pessière noire (Picea mariana (Mill.) BSP) du nord-ouest du Québec (Canada) avait comme objectifs : (1) de quantifier l'effet de la pente et du temps depuis le dernier feu sur le taux de paludification, (2) de déterminer si l'épaisseur de la couche organique pouvait être estimée à partir de deux variables de surface quantifiables au moyen de la télédétection, et (3) d'établir la relation entre le degré de paludification et la productivité des arbres. Dans cette étude, l'épaisseur de la couche organique du sol a été utilisée pour estimer le degré de paludification. La pente et l'âge des peuplements après feu influencent fortement la dynamique de paludification. La couche organique était plus mince et avait un taux d'accumulation plus faible dans les jeunes peuplements établis sur des pentes fortes que dans les peuplements d'âge similaire établis sur terrain plat. La surface terrière en épinette noire et le recouvrement de sphaignes avaient un fort pouvoir de prédiction de l'épaisseur de la couche organique, ce qui pourrait permettre de cartographier le degré de paludification à l'échelle du paysage. Enfin, la productivité des arbres était négativement reliée à l'épaisseur de la couche organique (R² = 0,57). Les équations développées dans ce travail peuvent être utilisées pour quantifier le déclin de productivité forestière dans les peuplements sujets à la paludification, ainsi que le potentiel de récupération de productivité à la suite d'une préparation de terrain appropriée.

Long-term forest productivity decline in boreal forests has been extensively studied in the last decades, yet its causes are still unclear. Soil conditions associated with soil organic matter accumulation are thought to be responsible for site productivity decline. The objectives of this study were to determine if paludification of boreal soils resulted in reduced forest productivity, and to identify changes in the physical and chemical properties of soils associated with reduction in productivity. We used a chronosequence of 23 black spruce stands ranging in postfire age from 50 to 2350 years and calculated three different stand productivity indices, including site index. We assessed changes in forest productivity with time using two complementary approaches: (1) by comparing productivity among the chronosequence stands and (2) by comparing the productivity of successive cohorts of trees within the same stands to determine the influence of time independently of other site factors.

Charcoal stratigraphy indicates that the forest stands differ in their fire history and originated either from high- or low-severity soil burns. Both chronosequence and cohort approaches demonstrate declines in black spruce productivity of 50–80% with increased paludification, particularly during the first centuries after fire. Paludification alters bryophyte abundance and succession, increases soil moisture, reduces soil temperature and nutrient availability, and alters the vertical distribution of roots. Low-severity soil burns significantly accelerate rates of paludification and productivity decline compared with high-severity fires and ultimately reduce nutrient content in black spruce needles. The two combined approaches indicate that paludification can be driven by forest succession only, independently of site factors such as position on slope. This successional paludification contrasts with edaphic paludification, where topography and drainage primarily control the extent and rate of paludification. At the landscape scale, the fire regime (frequency and severity) controls paludification and forest productivity through its effect on soil organic layers. Implications for global carbon budgets and sustainable forestry are discussed. ©2007 ESA