We examined the impacts of a severe, regional drought (2001)(2002) on trembling aspen (Populus tremuloides Michx.) forests in the western Canadian interior. A total of 150 plots were established in 25 study areas as part of a regional-scale study (CIPHA). Aspen health and mortality were assessed annually during 2000-2005, and changes in stem biomass were estimated using tree-ring analysis and plot-based measurements. Net mean increment in living biomass for all plots was 2.2 tÁha -1 Áyear -1 during 2000-2002 but subsequently decreased to near zero. This collapse was driven by a more than two-fold increase in stem mortality and a 30% decrease in regional stem growth during and following the drought. The analysis showed that spatial variation in aspen productivity and biomass across the region was positively related to multiyear mean values of a climate moisture index and mineral soil silt content but was negatively related to levels of insect defoliation and wood-boring insects. In contrast, mortality and dieback was best correlated with minimum annual climate moisture index, which provided a measure of short-term drought severity. The results support previous studies showing that aspen forests are moisture limited in this region, which poses concerns for the future under a changing climate.Résumé : Nous avons étudié les impacts d'une sécheresse sévère (2001)(2002) à l'échelle régionale sur les forêts de peuplier faux-tremble (Populus tremuloides Michx.) de la région intérieure dans l'ouest du Canada. Au total, 150 parcelles ont été établies dans 25 zones d'étude dans le cadre d'une étude régionale (CIPHA). La santé et la mortalité du peuplier faux-tremble ont été évaluées annuellement de 2000 à 2005 et les changements dans la biomasse de la tige ont été estimés à l'aide de l'analyse des cernes annuels et du mesurage des parcelles. L'accroissement net moyen de la biomasse vivante pour l'ensemble des parcelles était de 2,2 tÁha -1 Áan -1 de 2000 à 2002, mais il a par la suite diminué pour devenir presque nul. Cet effondrement était le résultat de la mortalité des tiges qui avait plus que doublé et d'une diminution de la croissance régionale de la tige de 30% pendant et après la sécheresse. Les analyses ont montré que la variation spatiale dans la biomasse et la productivité du peuplier faux-tremble dans la région était positivement reliée aux valeurs moyennes sur plusieurs années de l'indice d'humidité du climat et de la teneur en limon du sol minéral. Par contre, elle était négativement reliée à la sévérité des défoliations causées par les insectes et des dégâts causés par les insectes perceurs. À l'inverse, la mortalité et le dépérissement étaient le mieux corrélés au minimum annuel de l'indice d'humidité du climat qui fournit une mesure de la sévérité de la sécheresse à court terme. Les résultats concordent avec ceux des études précédentes qui montraient que l'humidité est le facteur limitant dans les forêts de peuplier faux-tremble de cette région, ce qui soulève des craintes pour l'avenir dans le contexte des ...

Trembling aspen (Populus tremuloides Michx.) is the most important deciduous tree in the Canadian boreal forest, with >1000 Tg of carbon stored in the aboveground biomass of this species. Since the early 1990s, aspen dieback has been noted over parts of the southern boreal forest and aspen parkland in western Canada. In this study, tree-ring analysis and forest health assessments were conducted in 18 aspen stands near Grande Prairie, Alta., to examine causes of reduced growth and dieback. Defoliation histories were reconstructed based on light-colored ("white") tree rings and records of past insect outbreaks. The results indicated that several factors contributed to the observed dieback. Defoliation by forest tent caterpillar (Malacosoma disstria Hbn.) and drought in the 1960s and 1980s led to reduced growth and predisposed some stands to secondary damage by wood-boring insects and fungal pathogens. Thaw–freeze events during a period (1984–1993) of unusually light snow cover in late winter may have also contributed to the observed dieback. Under global change, the severity of these stressors may increase, which would pose a serious concern for the future health, productivity, and carbon sequestration of aspen forests in the region.

The circumpolar boreal zone is one of the world’s major biogeoclimatic zones, covering much of North America and Eurasia with forests, woodlands, wetlands, and lakes. It regulates climate, acts as a reservoir for biological and genetic diversity, plays a key role in biogeochemical cycles, and provides renewable resources, habitat, and recreational opportunities. Poor agreement exists amongst scientists regarding this zone’s delimitation and the areal extent of boreal forests, even though the zone has been well-studied. This paper reviews the literature on the phytogeography of the zone and makes use of a geographic information system (GIS) and published maps to delineate a current map of the North American boreal zone and the hemiboreal subzone, which is a transitional area lying immediately to the south of the boreal zone that is usually included in the boreal zone by Europeans but excluded by North Americans. On the basis of the map described here, the boreal zone covers about 627 million ha, or 29% of the North American continent north of Mexico. If the hemiboreal subzone, at 116 million ha, is included, then 34% of the same area is covered. Forests and other wooded land (362 million ha) cover 58% of the North American boreal zone on the basis of current forest inventory data. With forests and other wooded land of the hemiboreal subzone (68 million ha) factored in, this percentage remains basically unchanged. Values reported in this paper are compared with other published statistics. Important sources of error contributing to differences in areal statistics are discussed.

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.Résumé : Le secteur de la zone boréale et ses écosystèmes fournissent de nombreux services dans les domaines de l'approvisionnement, de la régularisation, de la culture et du soutien. Compte tenu de ses ressources et de son potentiel hydroélectrique, la zone boréale est importante pour l'économie du Canada qui est basée principalement sur les ressources. La région présentement occupée par la zone boréale canadienne a connu des changements drastiques au cours des derniers trois millions d'années; un refroidissement du climat et des glaciations répétées ont affecté à la fois le biote et le paysage. Au cours des 7000 dernières années, le climat, les feux, les insectes, les maladies et leurs interactions ont constitué les forces naturelles les plus importantes de la dynamique des écosystèmes de la zone boréale, incluant la régénération, le cycle biogéochimique, le maintien de la productivité et la variabilité des paysages. À ces agents naturels, il faut ajouter les changements causés par les habitants et le développement ainsi que ceux reliés au changement climatique d'origine anthropique. Les effets de ces agents varient de façon spatio-temporelle et l'augmentation globa...