Influence de la densité de peuplement et de la hauteur dans l'arbre sur les propriétés physico-mécaniques du bois d'épinette noire (Picea mariana (Mill.) B.S.P.)

Trente-six arbres d’épinette noire ont été récoltés dans la région de Chibougamau au nord de Québec et répartis dans trois catégories de densité de peuplement après avoir déterminé leur densité de peuplement locale. Les arbres ont été débités en trois billes de 2,7 m de longueur dont la partie supérieure de 30 cm de longueur a été coupée et utilisée comme support de l’échantillonnage. Le travail a consisté à étudier les variations des propriétés du bois avec la densité de peuplement à différentes hauteurs de l’arbre. Ces propriétés, estimées comme les critères de qualité du bois les plus importants, sont la masse volumique du bois, le taux de croissance, le retrait, la résistance mécanique et la structure anatomique en coupe transversale. De plus, ces caractéristiques anatomiques, physiques et mécaniques ont été étudiées sur des échantillons de même provenance permettant ainsi de mettre en évidence les relations entre celles-ci. La masse volumique du bois et la structure anatomique ont été déterminées respectivement par densitométrie aux rayons X et à l’aide du logiciel WinCell, cerne par cerne, afin d’obtenir un profil radial dépendant de l’âge cambial. L’angle des microfibrilles a été mesuré par le procédé SilviScan au CSIRO Forestry and Forest Products Department, Australie, et les propriétés mécaniques ont été mesurées par essais de flexion statique sur des éprouvettes tirées de la moelle vers l’écorce. Les résultats montrent en premier lieu de très grandes variations radiales de la moelle vers l’écorce des principales caractéristiques du bois dont certaines (surface des cernes, masse volumique maximale des cernes, angle des microfibrilles, module d’élasticité) caractérisent la transition entre le bois juvénile et le bois mature. Ils montrent aussi que la plupart des variations significatives des caractéristiques étudiées concernent les variations en hauteur dans l’arbre plutôt que les variations causées par la densité de peuplement, et que ces variations sont très accentuées dans le bois juvénile. On constate également que la densité de peuplement a un effet plus important sur les caractéristiques de croissance de l’arbre que sur la masse volumique du bois. L’étude de l’âge de transition du bois juvénile au bois mature, calculé à partir du profil radial de la surface des cernes et de la masse volumique maximale des cernes, met en évidence un effet significatif de la hauteur dans l’arbre dont l’effet est de raccourcir la période juvénile de 17 ans en bas de l’arbre à 12 ans en haut de l’arbre. Toutefois, la proportion en volume de bois juvénile, semble présenter peu de variations avec la hauteur dans l’arbre. L’effet de la densité de peuplement n’est pas significatif sur l’âge de transition, par contre on observe une augmentation de la proportion de bois juvénile avec la densité de peuplement. Une analyse statistique utilisant le modèle mixte a montré qu’il y a une grande influence de l’angle des microfibrilles et de la masse volumique des cernes sur les propriétés mécaniques du bois et que la largeur des cernes n’a pas d’influence sur ces propriétés.Thirty-six black spruce trees were collected in the Chibougamau area, north of Québec City and assigned into three stand density groups according to their local stand density. The trees were cut into three logs of 2.7-m in length from which the upper 30-cm long bolt was sawn and used as the sampling material. The aim of the study was to determine the variations of the wood properties in relation to stand density at different sampling heights. These properties, considered as determinant for wood quality, are wood density, growth rate, shrinkage, mechanical properties and anatomical structure. Moreover, these anatomical, physical and mechanical properties were studied on the same sample in order to highlight the relations among them. X-ray densitometry and the WinCell software were used to determine wood density and the anatomical structure, for each growth ring, in order to obtain a radial profile in relation to cambial age. The microfibril angle was measured by the SilviScan technology at CSIRO Forestry and Forest Products Department, Australia, and the mechanical properties were determined by static bending on specimens taken from pith to bark. The results show a high radial variation of the main wood characteristics from pith to bark where some of them (ring area, maximum ring density, microfibril angle, modulus of elasticity) characterise the juvenile wood to mature wood transition. The results show that most of the significant variation of wood characteristics occurs with sampling height rather than with stand density, these variations being more important in juvenile wood. One observes that stand density has more impact on growth traits than on wood density traits. The study of the transition age from juvenile wood to mature wood calculated from the radial pattern of ring area and maximum ring density, highlights a significant effect of sampling height. This effect results in a reduction of the juvenile period from 17 to 12 years at the top of the tree. Nevertheless, the juvenile wood volume proportion seems to present little variation with sampling height. The effect of stand density on transition age is not significant but it seems that the proportion of juvenile wood is higher in the high stand density group. A statistical analysis by a mixed model showed a strong effect of microfibril angle and ring density on mechanical properties but no significant effect of ring width

Mechanical Properties in Relation to Selected Wood Characteristics of Black Spruce

The relation between ring width, ring density, microfibril angle, and bending properties was analyzed at 2.4-m height on twelve 80-year-old black spruce trees. The moduli of elasticity and rupture were measured in the southernmost radial direction on extracted specimens of size 10 x 10 x 150 mm3 from pith to bark. Ring density and ring width were measured by X-ray densitometry, and microfibril angle was measured by the Silviscan technology. The impact of these three traits on the moduli of elasticity and rupture was evaluated by explicitly separating the radial variation from the variation among trees using a mixed model analysis. The results obtained show first that the modulus of elasticity is negatively correlated to microfibril angle. This result supports the assumption that the relation between modulus of elasticity and microfibril angle is not dependent on radial growth rate. Secondly, ring density has a lower contribution in predicting the modulus of elasticity than the modulus of rupture. In both cases, ring width was not a significant factor of variation of the moduli of elasticity and rupture

Influence of Stand Density on Ring Width and Wood Density at Different Sampling Heights in Black Spruce (Picea Mariana (Mill.) B.S.P.)

Thirty-six black spruce sample trees were collected from an 80-year-old stand to examine the influence of stand density on selected wood quality attributes and their variation with sampling height. The stand, naturally regenerated from fire in 1906, was located in Chibougamau, 400 km north of Québec. Each tree was assigned a local stand density ranging from 1390 to 3590 stems/ha, calculated from the number of neighboring trees. The trees were grouped into three stand density categories (1790, 2700, and 3400 stems/ha). Each sample tree was analyzed by X-ray densitometry, and various ring features including ring width and wood density were measured for each ring from pith to bark, at three heights (2.4, 5.1, and 7.8m) and ring area and earlywood proportion were computed. For all features studied, the variation due to sampling height was larger than that due to stand density. The longitudinal variations for ring density and earlywood density depend largely upon the wood type (juvenile wood or mature wood). A variation of ring density with sampling height in the stem from 425 to 458 kg/m3 was observed in juvenile wood, but variations with stand density in all the growth ring features studied were small. Notably, it was observed that stand density had more influence on ring width features than on ring density features

Characterization of juvenile wood to mature wood transition age in black spruce (Picea mariana (Mill.) B.S.P.) at different stand densities and sampling heights

The radial pattern of both maximum ring density and ring area of 36 black spruce trees were used to determine the transition age from juvenile wood to mature wood. The data were obtained by X-ray densitometry and both segmented linear and polynomial regressions were used to point out the age of the juvenile wood boundary. Three stand densities (1,790, 2,700 and 3,400 stems/ha) and three sampling heights (2.4, 5.1 and 7.8 m) were studied. Although maximum ring density and ring area presented similar radial patterns, they gave two significantly different results of transition ages. The maximum ring density over-estimated the transition age (17.6 years) in contrast to ring area (14 years). The results show that the transition from juvenile wood to mature wood occurred after 12 years at 7.8 m (versus 13.1 years at a height of 5.1 m, and 17.6 years at 2.4 m). Although transition age occurred later in the high stand density group (21 years), the difference was not significant between the three stand density groups. Nevertheless, transition age remains difficult to determine since no standard definition exists. The transition occurs over years, and most probably a transition wood exists between juvenile wood and mature wood. Estimation of the juvenile wood proportion in volume shows that it remains constant along the stem and it increases with stand density

Patterns of distribution of longitudinal and tangential maturation stresses in Eucalyptus nitens plantation trees

International audienceContext: Tree orientation is controlled by asymmetric mechanical stresses set during wood maturation. The magnitude of maturation stress differs between longitudinal and tangential directions, and between normal and tension woods. * Aims: We aimed at evaluating patterns of maturation stress on eucalypt plantation trees and their relation with growth, with a focus on tangential stress evaluation. * Methods: Released maturation strains along longitudinal and tangential directions were measured around the circumference of 29 Eucalyptus nitens trees, including both straight and leaning trees. * Results: Most trees produced asymmetric patterns of longitudinal maturation strain but more than half of the maturation strain variability occurred between trees. Many trees produced high longitudinal tensile stress all around their circumference. High longitudinal tensile stress was not systematically associated with the presence of gelatinous layer. The average magnitude of released longitudinal maturation strain was found negatively correlated to the growth rate. A methodology is proposed to ensure reliable evaluation of released maturation strain in both longitudinal and tangential directions. Tangential strain evaluated with this method was lower than previously reported. * Conclusion: The stress was always tensile along longitudinal direction and compressive along tangential direction, and their respective magnitude was positively correlated. This correlation does not result from a Poisson effect but may be related to the mechanism of maturation stress generation