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 ...

Drought-induced, regional-scale dieback of forests has emerged as a global concern that is expected to escalate under model projections of climate change. Since 2000, drought of unusual severity, extent, and duration has affected large areas of western North America, leading to regional-scale dieback of forests in the southwestern US. We report on drought impacts on forests in a region farther north, encompassing the transition between boreal forest and prairie in western Canada. A central question is the significance of drought as an agent of large-scale tree mortality and its potential future impact on carbon cycling in this cold region. We used a combination of plot-based, meteorological, and remote sensing measures to map and quantify aboveground, dead biomass of trembling aspen (Populus tremuloides Michx.) across an 11.5 Mha survey area where drought was exceptionally severe during 2001–2002. Within this area, a satellite-based land cover map showed that aspen-dominated broadleaf forests occupied 2.3 Mha. Aerial surveys revealed extensive patches of severe mortality (>55%) resembling the impacts of fire. Dead aboveground biomass was estimated at 45 Mt, representing 20% of the total aboveground biomass, based on a spatial interpolation of plot-based measurements. Spatial variation in percentage dead biomass showed a moderately strong correlation with drought severity. In the prairie-like, southern half of the study area where the drought was most severe, 35% of aspen biomass was dead, compared with an estimated 7% dead biomass in the absence of drought. Drought led to an estimated 29 Mt increase in dead biomass across the survey area, corresponding to 14 Mt of potential future carbon emissions following decomposition. Many recent, comparable episodes of drought-induced forest dieback have been reported from around the world, which points to an emerging need for multiscale monitoring approaches to quantify drought effects on woody biomass and carbon cycling across large areas.

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