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Five years after spacing a young, 11 m tall paper birch (Betula papyrifera Marsh.) stand, we examine relationships between growth of understory white spruce (Picea glauca (Moench) Voss), subalpine fir (Abies lasiocarpa (Hook.) Nutt.), and level of birch retention. Our objectives were to evaluate the effectiveness of selected measures of competition for estimating the effects of the residual birch and to evaluate the influence of residual birch densities on growth responses of naturally regenerated subalpine fir and white spruce. Basal area of Scouler willow (Salix scouler iana Barratt) and birch were found to effectively predict light transmittance (diffuse noninterceptance). The best models for predicting 4-year volume growth of subalpine fir and white spruce incorporate initial crown volume of the subject trees and transmittance as independent variables. Lorimer's index and Hegyi's index gave similar results to those obtained using basal area and transmittance, suggesting that there is little benefit in including measurements of proximity in a competition index. For both species, the correlation between basal area increment and light was substantially stronger than observed for volume increment. However, height increment of both subalpine fir and spruce was only weakly correlated with measured light levels. The ratio of height increment to volume increment decreased with both increasing initial height and transmittance for subalpine fir and white spruce.

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Seedlings of four geographically diverse paper birch (Betula papyrifera Marsh.) populations native to British Columbia were grown under low and high levels of water and nitrogen in a greenhouse for 3 months to examine the relative sensitivity of each population to water and nutrient availability. Nine sequential measurements on height and basal diameter growth were made, and then the seedlings were harvested for final biomass measurements. Under optimal conditions the highest elevation population ceased height growth, while the low-elevation coastal population continued to increase in height under all treatment conditions. Plants of all populations grown under high N conditions grew faster and had larger total biomass and lower root/shoot ratios than plants grown under low N conditions. Relative growth rate was significantly correlated with shoot and foliage biomass, leaf area, and root weight ratio. All populations generally responded more to low N level than to low moisture level in terms of height growth, relative growth rate, total biomass, and root/shoot ratio. This suggests that the populations observed in this study are better adapted to drought than poor soil nutrient availability. Therefore, consideration of site quality, which includes soil moisture regime and soil nutrient regime, should be a primary concern when allocating birch seed lots in a planting program.Résumé : Des semis de bouleau blanc (Betula papyrifera Marsh.) ont été cultivés en serre, pendant 3 mois, sous des régimes de faibles et fortes teneurs en eau et en azote, en vue d'examiner leur sensibilité à la disponibilité en eau et en éléments nutritifs. Ces semis étaient originaires de quatre populations indigènes de Colombie-Britannique, provenant de différents lieux géographiques. Neuf mesures séquentielles de l'accroissement en hauteur et en diamètre à la base ont été effectuées, avant que les semis soient récoltés en vue de la détermination finale de la biomasse. Sous conditions optimales, la population provenant de la plus haute altitude a cessé son accroissement en hauteur, alors que la population côtière de basse altitude continuait sa croissance en hauteur sous toutes les conditions de traitement. Les plantules, de toutes les populations, qui avaient poussé en présence d'une teneur élevée en azote, se sont accrues plus rapidement et possédaient une plus grande biomasse, ainsi que des ratios racine/tige plus bas que les plantules ayant poussé dans un milieu pauvre en azote. Le taux de croissance relatif était corrélé significativement avec la biomasse de la tige et du feuillage, ainsi qu'avec la surface foliaire et avec le ratio racine/poids. Toutes les populations répondaient, généralement, davantage au bas niveau d'azote qu'au bas niveau d'humidité, en termes d'accroissement, de la biomasse totale et du ratio racine/tige. Cela suggère que les populations observées dans cette étude sont mieux adaptées à la sécheresse qu'à une faible disponibilité en éléments nutritifs. Par conséquent, la considération de la qualité du site, qui incl...