Leaf photosynthetic characteristics could be determinant factors to identify the most productive clones of hybrid poplars (Populus spp.) and willows (Salix spp.). Photosynthetic acclimation of hybrid poplars and willows was studied under greenhouse conditions. Seven Populus and five Salix clones were grown for 3 months at three spacings [20 × 20, 35 × 35, and 60 × 60 cm] and two nitrogen (N) levels (20 and 200 ?g g-1). There were no significant spacing effects on leafless aboveground biomass per tree (AGBT) and height. Clonal acclimation to higher density was associated to increases in leaf area index (LAI) by 347% and specific leaf area (SLA) by 13% despite decreased leaf N content per unit leaf area (Narea) by 31%. There were no changes in net CO2 assimilation rate (A) and photosynthetic N-use efficiency in the ambient light condition (PNUEamb) within different spacings. The N addition alleviated competition effects by maximizing leaf area (LA) and SLA. Compared with less productive clones, more productive clones had 28% greater SLA, greater LA and AGBT per unit of increase in Narea over all treatments. The increased development of LA and SLA under high planting density is a key indicator of more productive clones. © 2017 Université Laval.

Les caractéristiques photosynthétiques des feuilles peuvent être déterminantes pour identifier les clones les plus productifs de peupliers (Populus spp.) et de saules (Salix spp.) hybrides. L’acclimatation photosynthétique de clones de peupliers et de saules hybrides a été étudiée en serre. Sept clones de Populus et cinq clones de Salix ont cru pendant trois mois selon trois espacements [20×20, 35 × 35 et 60 × 60 cm] et deux niveaux d’azote (20 et 200 ?g g?1). Il n’y avait pas d’effet significatif de l’espacement sur la biomasse aérienne sans feuilles par arbre (AGBT) et sur la hauteur. L’acclimatation des clones à des densités plus élevées était associée à une augmentation de 347% de l’indice de surface foliaire (LAI) et de 13% de la surface foliaire spécifique (SLA) malgré une diminution de 31% du contenu foliaire en azote par unité de surface foliaire (Narea). Il n’y a pas eu de changement du taux net d’assimilation de CO2 (A) et de l’efficacité d’utilisation photosynthétique de l’azote sous conditions de lumière ambiante (PNUEamb) et selon différents espacements. L’ajout d’azote a diminué les effets de la compétition en maximisant la surface foliaire (LA) et la SLA. Comparativement aux clones moins productifs, les clones plus productifs avaient des SLA 28% plus élevées, ainsi que des LA et AGBT plus élevées par unité d’augmentation de Narea dans tous les traitements. L’augmentation de LA et de SLA dans les plantations à forte densité est un indicateur clé de clones plus productifs. © 2017 Université Laval.

We developed allometric equations for small-diameter woody species growing on mixed forest marginal lands, which are potential sources of biomass for bioenergy. Eleven species of trees and shrubs were sampled from a site located in eastern Canada. Equations derived in this study generally performed better than equations from the literature. Also, fixed-area plots (FAP) and line-intersect sampling (LIS) methods using both random or systematic selection of sampling units were compared to determine which method required the lowest number of measurements to estimate stand biomass for the same precision.

The fixed-area plots method was successfully used to estimate relatively accurately oven-dry biomass per hectare. Results indicated that potentially harvestable woody biomass (oven dry basis) varied between 33-41 and 12–13 t ha?1 for the most and least productive marginal sites respectively. On the most productive site, LIS estimates (between 20 and 42 t ha?1) were usually lower than those obtained using different FAP sampling methods (i.e. systematic or random, small (50 m2) or large (100 m2) plots), but similar on the more open sites (between 10 and 14 t ha?1). Small FAP resulted in a plot without measurements in one case. Moreover, estimates based on small FAP were generally higher, even if not significantly different from larger plot estimates. We therefore suggest using FAP with 100 m2 plots to estimate small-diameter woody biomass on marginal lands with dense vegetation, while LIS, even if promising for open stands, needs further evaluation before recommendation.

Morphological characteristics of poplar and willow clones were determined in order to identify main characteristics leading to superior growth under increased plant competition with low or high nitrogen (N) availability. Seven hybrid poplar (Populus spp. including one hybrid aspen) and five willow (Salix spp.) clones were grown under greenhouse conditions for 13 weeks at three spacings (20 × 20, 35 × 35, and 60 × 60 cm) and two N levels (20 and 200 mg kg?1). The decrease in spacing from 60 to 20 cm reduced leaf area by 50% but clones had similar aboveground biomass per tree under all spacings, with increasing their height per unit leaf area. More productive clones had greater leaf area (+102%), leaf area per unit plant biomass (+12%) and lower root-to-shoot ratios (?27%) compared to less productive clones. There were positive relationships between leaf area and above-ground biomass per tree for both more and less productive clones. Compared to low N level and 60 cm spacing, trees growing in high N level and 20 cm spacing reached similar root collar diameter, crown width, and leaf area values and even greater height, suggesting that an addition of N could help mitigate negative effects of tree competition.

Il y a un intérêt pour les plantations d'espèces à croissance rapide de peupliers hybrides et de saules au Canada. Cet intérêt s'explique à la fois par le taux de croissance rapide et le potentiel élevé de rendement de ce type de système de production. Cependant, leur établissement et leur entretien requièrent des investissements considérables. Les efforts de recherche sur les comparaisons de productivité de différents clones ou espèces, les croisements, la physiologie, la nutrition, la sylviculture, la gestion des ravageurs, les méthodes de récolte, les aspects socio-économiques et la transformation industrielle contribuent à maximiser la production de fibre et à minimiser les coûts de production. L'objectif de cet article est de décrire brièvement des travaux de recherche en cours ou récemment complétés sur les plantations d'espèces à croissance rapide de peupliers hybrides et de saules et de discuter de leur utilité et de leurs impacts à des fins d'applications pratiques.