We studied growth, crown architecture, and specific leaf area acclimation of a shade-intolerant species, Pinuscontorta Dougl. ex Loud. var. latifolia Engelm., and a moderately shade-tolerant species, Pseudotsugamenziesii var. glauca (Beissn.) Franco, using naturally regenerated saplings along a wide range of light conditions. Study sites were located within a dry-summer, cool-temperate climate represented by the Dry Cool Interior Douglas-fir biogeoclimatic subzone near Williams Lake, British Columbia, Canada. In an open light environment, Pinuscontorta had a greater growth increment in both terminal and lateral shoots than did Pseudotsugamenziesii. With decreasing light availability (i) terminal increment, mean lateral increment, and total lateral increment decreased in both species (although Pinuscontorta reduced lateral growth significantly faster than Pseudotsugamenziesii); (ii) the ratio of mean lateral to terminal increment in Pinuscontorta did not change, but significantly increased in Pseudotsugamenziesii; (iii) the ratio of total lateral to terminal increment in Pinuscontorta decreased, but increased in Pseudotsugamenziesii; and (iv) specific leaf area in both species increased (in Pseudotsugamenziesii, it was always higher and increased marginally faster than in Pinuscontorta). We concluded that the less shade-tolerant Pinuscontorta was less plastic in crown architecture and specific leaf area than the more shade-tolerant Pseudotsugamenziesii.

Pacific silver fir and subalpine fir, both typically inhabiting high-elevation forests in northwestern North America, were considered shade-tolerant species, the former more tolerant than the latter. To determine their relative shade tolerance, established advance regeneration was sampled along a light gradient ranging from open areas to inside a forest stand, and analysis of irradiance, growth, and leaf measures was obtained. Relationships between the percentage of above-canopy light (in the photosynthetically active wavelengths) associated with each study tree and its 1991 height increment, 1991 lateral increment, caliper at the base of the 1991 leader, and specific leaf area were examined for individual sites, and sites were grouped according to soil moisture. There were strong, consistent, and similar relationships between irradiance, growth performance, and specific leaf area for both species. As irradiance decreased, growth performance decreased and specific leaf area increased. In spite of ecological differences between the study sites for each species, it was concluded that on fresh sites, Pacific silver fir and subalpine fir are very tolerant of shade. Both are equally well adapted to survive under high shade and snowpack by allocating more resources to caliper and lateral growth than to height growth and by increasing specific leaf area. Key words: shade tolerance, advance regeneration, irradiance, growth performance, specific leaf area, Pacific silver fir, subalpine fir.

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In spring 2007, young planted and natural conifers suffered extensive needle and bud injury near Hearst and Kapuskasing in northeastern Ontario. Damage was observed on all species of conifers up to 20 years old and 8 m in height. Taller trees, especially those in the overstory, and young understory trees protected by a closed canopy had less damage. The damage was caused by earlier than normal loss of cold hardiness followed by late spring frosts according to damage observations, weather station data, and calculated thermal time requirements for budbreak and cold hardiness of conifers. During May 19 to 21, 2007, minimum temperatures were recorded between -8°C and -9°C, and before this period, temperatures were warm enough to induce early dehardening and loss of trees' cold hardiness. According to the historical weather data for Kapuskasing, estimated budbreak time has become earlier since 1918 and freezing temperatures during budbreak and shoot elongation (between growing degree days 100 and June 15) have become more frequent since 1980. If the trend towards earlier budbreak is due to climate change, then the type of frost damage we observed in northeastern Ontario in 2007 could become more common. We discuss implications of such events and suggest research needed to understand the risk of frost damage with climatic warming and to reduce damage.Key words: conifer frost damage, mature needle and bud (or needle/bud) mortality, early dehardening and budbreak (or dehardening/budbreak), climatic warming RÉSUMÉAu printemps 2007, de jeunes conifères plantés et naturels ont subi des dommages importants aux aiguilles et aux bourgeons dans la région de Hearst et de Kapuskasing dans le nord-est de l'Ontario. Les dégâts ont été observés sur toutes les espèces de conifères âgées de moins de 20 ans et atteignant 8 m de hauteur. Les arbres plus grands, notamment ceux dominant le couvert, ainsi que les jeunes arbres en sous-étage protégés par la fermeture du couvert ont subi moins de dégâts. Les dégâts ont été causés par une perte plus hâtive que normale de la résistance au froid suivie de gels printaniers tardifs selon les observations des dégâts, les données des stations météorologiques et les besoins en degrés-jours calculés pour provoquer le débourrement et l' endurcissement au froid chez les conifères. Du 19 au 21 mai 2007, les températures minimales enregistrées ont atteint entre -8°C et -9°C et, avant cette période, les températures avaient été suffisamment chaudes pour amorcer un désendurcissement hâtif et une cessation des mécanismes de résistance au froid. Selon l'historique des données météorologiques de Kapuskasing, la période du débourrement est de plus en plus hâtive depuis 1918 et les gels tardifs au cours du débourrement et de la croissance de la pousse terminale (entre le degré-jour 100 et le 15 juin) sont de plus en fréquents depuis 1980. Si la tendance vers un débourrement plus hâtif est la conséquence des changements climatiques, alors ce type de dégâts par le gel que nous avons observé dans le nord-...

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