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Copyright of Environmental Reviews is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The boreal zone and its ecosystems provide numerous provisioning, regulating, cultural, and supporting services. Because of its resources and its hydroelectric potential, Canada's boreal zone is important to the country's resource-based economy. The region presently occupied by Canada's boreal zone has experienced dramatic changes during the past 3 million years as the climate cooled and repeated glaciations affected both the biota and the landscape. For about the past 7000 years, climate, fire, insects, diseases, and their interactions have been the most important natural drivers of boreal ecosystem dynamics, including rejuvenation, biogeochemical cycling, maintenance of productivity, and landscape variability. Layered upon natural drivers are changes increasingly caused by people and development and those related to human-caused climate change. Effects of these agents vary spatially and temporally, and, as global population increases, the demands and impacts on ecosystems will likely increase. Understanding how humans directly affect terrestrial and aquatic ecosystems in Canada's boreal zone and how these effects and actions interact with natural disturbance agents is a prerequisite for informed and adaptive decisions about management of natural resources, while maintaining the economy and environment upon which humans depend. This paper reports on the genesis and present condition of the boreal zone and its ecosystems and sets the context for a detailed scientific investigation in subsequent papers published in this journal on several key aspects: carbon in boreal forests; climate change consequences, adaptation, and mitigation; nutrient and elemental cycling; protected areas; status, impacts, and risks of non-native species; factors affecting sustainable timber harvest levels; terrestrial and aquatic biodiversity; and water and wetland resources. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Forest Research is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Canadian Journal of Forest Research is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Environmental Reviews is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Environmental Reviews is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Environmental Reviews is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Intergovernmental Panel on Climate Change recommends that countries that use advanced (Tier 3) models to meet their international reporting obligations on forest greenhouse gas emissions and removals evaluate model predictions against independent field data. Unfortunately, estimates of total ecosystem C stocks and stock changes are scarce and consequently the recommended evaluations are rarely completed. The Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) is the core model of Canada's National Forest Carbon Monitoring, Accounting, and Reporting System that implements an Intergovernmental Panel on Climate Change Tier 3 approach. It accounts for biomass, dead organic matter, and soil C pools as affected by natural and anthropogenic disturbances. We used data from a recent study of total ecosystem C stocks for black spruce (Picea mariana (Mill.) BSP) and balsam fir (Abies balsamea (L.) Mill.) boreal forest chronosequences of different disturbance origins in Newfoundland, Canada, to evaluate C stock and stock change predictions from the CBM-CFS3. Results indicated that the accuracy of the CBM-CFS3 is high for landscape-scale estimation of C stocks. Comparison of estimates stratified by lead species or disturbance type indicated that model accuracy could be improved at finer scales by increasing specific model parameters such as the snag fall rate and woody debris decay rates relative to default parameters. Le Groupe d'experts intergouvernemental sur l'évolution du climat recommande que les pays qui utilisent des modèles avancés (niveau 3), pour s'acquitter de leurs engagements internationaux en matière de présentation de rapports sur les émissions et l'élimination des gaz à effet de serre par les forêts, évaluent les prédictions des modèles à partir de données terrain indépendantes. Malheureusement, les estimations des stocks totaux de C des écosystèmes et des variations de ces stocks sont peu fréquentes et les évaluations recommandées sont par conséquent rarement effectuées. Le modèle du bilan du carbone du secteur forestier canadien (MBC-SFC3), qui utilise une approche de niveau 3 telle que définie par le Groupe d'experts intergouvernemental sur l'évolution du climat, est au cœur du Système national de surveillance, de comptabilisation et de production de rapports concernant le C des forêts du Canada. Cette approche tient compte de la biomasse, de la matière organique morte, des réservoirs de C dans le sol et de la façon dont ils sont influencés par les perturbations d'origine naturelle et anthropique. Nous avons utilisé des données provenant d'une étude récente des stocks totaux de C de l'écosystème réalisée dans des chronoséquences de forêt boréale composée d'épinette noire (Picea mariana (Mill.) BSP) et de sapin baumier (Abies balsamea (L.) Mill.) issues de différentes perturbations à Terre-Neuve, au Canada, pour évaluer les prédictions du modèle MBC-SFC3 concernant les stocks de C et ses variations. Les résultats indiquent que le modèle MBC-SFC3 a une grande précision pour estimer les stocks de C à l'échelle du paysage. La comparaison des estimations stratifiées sur la base des principales espèces ou des types de perturbation indique que la précision du modèle peut être améliorée en développant certains paramètres du modèle tels que les taux de renversement des chicots et de décomposition des débris ligneux relativement aux paramètres implicites. [ABSTRACT FROM AUTHOR]

Postfire succession in the Alaskan boreal forest follows several different pathways, the most common being self-replacement and species-dominance relay. In self-replacement, canopy-dominant tree species replace themselves as the postfire dominants. It implies a relatively unchanging forest composition through time maintained by trees segregated within their respective, ecophysiological niches on an environmentally complex landscape. In contrast, species-dominance relay involves the simultaneous, postfire establishment of multiple tree species, followed by later shifts in canopy dominance. It implies that stand compositions vary with time since last fire. The relative frequencies of these and other successional pathways are poorly understood, despite their importance in determining the species mosaic of the present forest and their varying, potential responses to climate changes. Here we assess the relative frequencies of different successional pathways by modeling the relationship between stand type, solar insolation, and altitude; by describing how stand age relates to species composition; and by inferring successional trajectories from stand understories. Results suggest that >70% of the study forest is the product of self-replacement, and tree distributions are controlled mainly by the spatial distribution of solar insolation and altitude, not by time since last fire. As climate warms over the coming decades, deciduous trees will invade cold sites formerly dominated by black spruce, and increased fire frequency will make species-dominance relay even rarer. La succession après feu dans la forêt boréale de l’Alaska adopte différents modes, les plus communs étant le retour des mêmes espèces et la dominance successive de différentes espèces. Dans le cas du retour des mêmes espèces, les espèces d’arbres qui dominent la canopée sont remplacées par les mêmes espèces qui deviennent dominantes après un feu. Cela implique que la composition de la forêt qui demeure relativement stable dans le temps soit maintenue par des arbres qui sont restreints à leur niche écologique respective dans un paysage complexe du point de vue environnemental. Au contraire, la dominance successive de différentes espèces suppose l’établissement après feu de plusieurs espèces d’arbres suivi par des changements ultérieurs de dominance dans la canopée. Cela signifie que la composition du peuplement change avec le temps après un feu. Les fréquences relatives de ces modes de succession et d’autres sont peu connues malgré leur importance dans la détermination de la mosaïque d’espèces de la forêt actuelle et de leurs différentes réactions potentielles aux changements climatiques. Dans cet article, nous évaluons la fréquence relative de différents modes de succession en modélisant la relation entre le type de peuplement, l’ensoleillement et l’altitude; en décrivant comment la composition en espèces est reliée à l’âge du peuplement et en déduisant le mode de succession à partir du sous-étage d’un peuplement. Les résultats indiquent que plus de 70 % de la forêt étudiée est le résultat du retour des mêmes espèces et que la distribution des arbres est régie principalement par la distribution spatiale de l’ensoleillement et l’altitude, non par le temps écoulé depuis le dernier feu. À mesure que le climat se réchauffera au cours des prochaines décennies, les espèces décidues vont envahir les stations froides autrefois dominées par l’épinette noire et l’augmentation de la fréquence des feux va faire en sorte que la dominance successive de différentes espèces sera encore plus rare. [ABSTRACT FROM AUTHOR]