Variable and sometimes high levels of mortality in lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) planted in western Alberta have raised awareness of the need to forecast and ameliorate risks to reforestation success. Mortality and health of regeneration during the 12 years following harvest were monitored and evaluated in relation to climate, site and silvicultural treatments. Results suggest that physiological stress related to evapotranspiration is the most prevalent cause of overall juvenile mortality and susceptibility to Armillaria root disease in planted pine. Mortality and disease not only increase at higher rates of drying during the growing season, but an opposite effect is also demonstrated whereby they decrease with increasing spring temperatures. Mechanical site preparation aimed at countering mortality and disease of planted stock with improved soil conditions appears to have good potential for ameliorating adverse climatic effects in juvenile stands.Keywords: climate, juvenile mortality, Armillaria root disease, lodgepole pine, reforestation RÉSUMÉ Les taux de mortalité variables et parfois élevés constatés dans les plantations de pin tordu (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) de l' ouest de l' Alberta font ressortir l'importance de prévoir et d'améliorer les chances de succès des reboisements. L' étude cherche à mettre en lien la mortalité et la santé de la régénération durant les 12 années suivant la coupe avec le climat, la qualité de station et les pratiques sylvicoles. Les résultats laissent voir que c' est le stress physiologique lié à l' évaporation qui est responsable de la mortalité juvénile et de la susceptibilité au pourridié-agaric (Armillaria) chez les pins en plantation. Non seulement la mortalité et la maladie augmentent-elles lorsque la sécheresse est plus importante au cours de la saison de croissante, mais on observe également un effet contraire et donc une diminution lorsque la température printanière augmente. La scarification mécanique visant à contrer la mortalité et la maladie chez les semis en améliorant la préparation du sol semble une option prometteuse pour améliorer les conditions climatiques adverses dans les jeunes peuplements.

No abstract

A large long-term field trial was established in the Upper and Lower Foothills sub-regions of the Canadian boreal forest to monitor the effects of planting, vegetation management, and pre-commercial thinning on the development of lodgepole pine stands following harvesting. Data collected at the end of the trial’s regeneration phase, 17 to 20 years after its establishment, were tested for treatment effects and projected to rotation age. Planting generally improved stocking and increased projected growth and yield of lodgepole pine. On modal sites, planted trees were often greatly outnumbered by natural regeneration; however, on others, typically with either poorer or richer soils, satisfactory restocking was not achieved without planting. Control of competing vegetation by herbicide application facilitated regeneration of pine where it was otherwise difficult or impossible on sites with excessive herbaceous or hardwood competition. Pre-commercial thinning accelerated the growth of individual trees and was projected to shorten rotations in dense stands. Responses to the treatments varied depending on environmental factors. Particular treatments may be effective to meet management objectives under some site conditions but unnecessary or counterproductive elsewhere.