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Canadian boreal woodlands and forests cover approximately 3.09 x [10.sup.6] k[m.sup.2], located within a larger boreal zone characterized by cool summers and long cold winters. Warming since the 1850s, increases in annual mean temperature of at least 2°C between 2000 and 2050 are highly probable. Annual mean temperatures across the Canadian boreal zone could be 4-5°C warmer than today's by 2100. All aspects of boreal forest ecosystem function are likely to be affected. Further, several potential 'tipping elements'--where exposure to increasing changes in climate may trigger distinct shifts in ecosystem state--can be identified across the Canadian boreal zone. Approximately 40% of the forested area is underlain by permafrost, some of which is already degrading irreversibly, triggering a process of forest decline and re-establishment lasting several decades, while also releasing significant quantities of greenhouse gases that will amplify the future global warming trend. Warmer temperatures coupled with significant changes in the distribution and timing of annual precipitation are likely to cause serious tree-killing droughts in the west; east of the Great Lakes, however, where precipitation is generally nonlimiting, warming coupled with increasing atmospheric carbon dioxide may stimulate higher forest productivity. Large wildfires, which can cause serious economic losses, are expected to become more frequent, but increases in mean annual area burned will be relatively gradual. The most immediate threats could come from endemic forest insect pests that have the potential for population outbreaks in response to relatively small temperature increases. Quantifying the multiple effects of climate change will be challenging, particularly because there are great uncertainties attached to possible interactions among them, as well as with other land-use pressures. Considerable ingenuity will be needed from forest managers and scientists to address the formidable challenges posed by climate change to boreal ecosystems and develop effective strategies to adapt sustainable forest management practices to the impending changes. Key words: boreal forest, climate change, disturbance, feedback, permafrost, drought. Les milieux boises et les forets boreales canadiennes couvrent approximativement 3,09 x [10.sup.6] k[m.sup.2], localises au sein d'une vaste zone boreale caracterisee par des etes frais et de longs hivers froids. Avec le rechauffement en marche depuis les annees 1850, des augmentations de la temperature annuelle moyenne d'au moins 2 °C entre 2000 et 2050 apparaissent comme tres probables, et les temperatures moyennes sur l'ensemble de la zone boreale canadienne pourraient s'averer de 4-5 °C plus chaudes que les presentes vers 2100. Tous les aspects du fonctionnement des ecosystemes de la foret boreale sont susceptibles d'etre affectes. De plus, on peut identifier sur l'ensemble de la zone boreale canadienne des 'elements declencheurs' ou l'exposition a des changements accrus du climat peut provoquer des modifications de l'etat des ecosystemes. Environ 40 % des surfaces forestieres vivent sur le pergelisol, dont certaines connaissent presentement une degradation irreversible, enclenchant un processus de declin forestier et de retablissement qui s'etend sur plusieurs decennies, tout en relachant des quantites significatives de gaz a effet serre amplifiant la tendance du rechauffement global. Des temperatures plus chaudes couplees a des changements significatifs de la distribution spatio-temporelle de la precipitation causeront vraisemblablement des secheresses destructrices pour les arbres dans l'ouest; cependant, a l'est des Grands Lacs, ou la precipitation est generalement non limitative, le rechauffement couple a une augmentation du C[O.sub.2] pourrait favoriser une plus forte productivite forestiere. On peut s'attendre a ce que de vastes incendies de foret pouvant causer des pertes economiques enormes deviennent plus frequents, mais les augmentations annuelles moyennes des surfaces brulees se feront relativement graduellement. La menace la plus immediate pourrait venir d'insectes forestiers ravageurs ayant la capacite de connaitre des epidemies en reaction a des augmentations relativement faibles des temperatures. La quantification des multiples effets du changement climatique presente un defi, surtout compte tenu des grandes incertitudes sur les interactions possibles entre elles, aussi bien qu'avec les autres pressions venant de l'utilisation des terres. Il faudra beaucoup d'ingeniosite de la part des amenagistes et des scientifiques pour rencontrer les formidables defis poses par le changement climatique dans les ecosystemes boreaux et pour developper des strategies efficaces afin d'adapter des pratiques d'amenagement forestier durables aux changements imminents. [Traduit par la Redaction] Mots-cles: foret boreale, changement climatique, perturbation, reaction, pergelisol, secheresse., 1. Introduction The Intergovernmental Panel on Climate Change (IPCC 2007) provides a large consensus of scientific expertise that the global climate is now warming. This trend is primarily due to [...]

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Canadian boreal woodlands and forests cover approximately 3.09 × 106 km2, located within a larger boreal zone characterized by cool summers and long cold winters. Warming since the 1850s, increases in annual mean temperature of at least 2 °C between 2000 and 2050 are highly probable. Annual mean temperatures across the Canadian boreal zone could be 4-5 °C warmer than today's by 2100. All aspects of boreal forest ecosystem function are likely to be affected. Further, several potential 'tipping elements' - where exposure to increasing changes in climate may trigger distinct shifts in ecosystem state - can be identified across the Canadian boreal zone. Approximately 40% of the forested area is underlain by permafrost, some of which is already degrading irreversibly, triggering a process of forest decline and re-establishment lasting several decades, while also releasing significant quantities of greenhouse gases that will amplify the future global warming trend. Warmer temperatures coupled with significant changes in the distribution and timing of annual precipitation are likely to cause serious tree-killing droughts in the west; east of the Great Lakes, however, where precipitation is generally nonlimiting, warming coupled with increasing atmospheric carbon dioxide may stimulate higher forest productivity. Large wildfires, which can cause serious economic losses, are expected to become more frequent, but increases in mean annual area burned will be relatively gradual. The most immediate threats could come from endemic forest insect pests that have the potential for population outbreaks in response to relatively small temperature increases. Quantifying the multiple effects of climate change will be challenging, particularly because there are great uncertainties attached to possible interactions among them, as well as with other land-use pressures. Considerable ingenuity will be needed from forest managers and scientists to address the formidable challenges posed by climate change to boreal ecosystems and develop effective strategies to adapt sustainable forest management practices to the impending changes. [ABSTRACT FROM AUTHOR]