This study aims to understand the phenotypic and genotypic correlations among wood anatomical, physical, and mechanical properties of hybrid poplar clones. Samples were taken from seven clones grown on three sites in Southern Quebec, Canada. Five trees per clone were randomly sampled from each site to measure anatomical (fiber length, fiber proportion, vessel proportion, fiber wall thickness, tension wood), physical (basic density, volumetric, longitudinal, tangential, and radial shrinkage), and mechanical wood properties (flexural modulus of elasticity (MOE), modulus of rupture (MOR), ultimate crushing strength parallel to the grain). The observed phenotypic and genotypic correlations between these wood properties were moderate to strong, except for fiber length and vessel proportion. Genotypic correlations for all wood properties were higher than for corresponding phenotypic correlations. Furthermore, fiber length showed weak correlations, whereas, vessel proportion showed strongly negative correlations with all other properties. Strong correlations were also found among fiber proportion, fiber wall thickness, basic density, and mechanical properties. Furthermore, results from this study show close genotypic and phenotypic correlations between fiber proportion, fiber wall thickness, and wood density, which consequently affect the mechanical performance of wood products. These findings indicate that there is a substantial opportunity to improve wood quality by selecting several wood properties for different end uses. © 2018 by the authors.

To meet increasing wood demand with a dwindling forested land base, there has been a progressive interest in developing productive short rotation tree plantations. In Canada, hybrid poplar plantations are of particular interest because short rotation forestry can contribute to a stronger, more stable and renewable supply of fiber. Moreover, tree breeding is considered one of the best options to accelerate fiber production and improve wood quality. The main objective of this study was to assess the anatomical, physical and mechanical properties of various hybrid poplar clones in a perspective of evaluating the potential of their wood for various end-uses. This study contributes to the Quebec hybrid poplar-breeding program by adding information on wood quality of fast growing clones. For this study, we analysed wood quality of seven hybrid clones from three sites (Pointe-Platon, Saint-Ours, and Windsor) from southern Quebec. Five trees per clone were randomly sampled from each site to measure selected wood anatomical (fiber length, fiber width, fiber proportion, vessel proportion, fiber wall thickness, tension wood, cell wall area percentage), physical (basic density, volumetric, radial, tangential and longitudinal shrinkage), and mechanical (bending and compression strength) properties.

Statistical analyses revealed that the effect of site on the studied properties was highly significant, and was explained by environmental conditions and site quality. All anatomical properties of hybrid poplar wood showed significant clonal variation, indicating the possibility of identifying clones with superior wood properties. All anatomical properties showed variation in their magnitude with increasing cambial age. The variation in radial pattern was characterized by a rapid increase in the first few years in fiber length, fiber width, fiber proportion, wall thickness, and percent cell wall area; in contrast vessel proportion decreased. Pith to bark trends showed that wood density traits increased with cambial age. Heritability values of ring density traits had a tendency to increase with cambial age, and were generally lower near the pith. In contrast, results of ring width traits showed a decreasing trend with cambial age.

All physical and mechanical properties of hybrid poplar wood showed significant interclonal variation, especially for density, flexural modulus of rupture, and ultimate crushing strength. Genetic correlations between all wood properties were higher than the phenotypic correlations. All ring density traits were highly correlated with each other. Strong correlations were also found among fiber proportion, fiber wall thickness, basic density, and mechanical properties. Results from this study further show close genotypic and phenotypic correlations between fiber proportion, fiber wall thickness, and wood density. The genotypic correlations among mechanical properties and density were very strong. As a result, the inclusion of wood density into tree breeding programs can lead to an improvement of mechanical properties. These high genotypic correlations with MOE and MOR make density a strong candidate for direct genetic improvement of general wood quality. Consequently, the use of this property can ultimately benefit solid wood and fiber-based wood products.

High heritability values for the studied properties indicated that these properties are under moderate to high genetic control. The clonal variation, heritability, and genetic gain values for the properties investigated in this study should help poplar-breeding programs that aim to optimize hybrid poplar clones for solid wood and fiber-based products. However, a future challenge will be to determine whether breeding objectives are compatible with industrial objectives by improving wood properties.

The anatomical properties of seven hybrid poplar clones grown in three sites in southern Quebec, Canada were investigated. Radial and longitudinal variations in selected anatomical properties of wood were measured by image analysis of transverse sections and by fiber quality analysis. Results indicate that all measured anatomical properties varied significantly across sites. Clonal variation was highly significant for all anatomical properties studied, and broad-sense heritability ranged from 0.10 (average vessel lumen area) to 0.76 (cell wall area percentage). Genetic gain was positive for all anatomical properties. The variation in radial pattern was characterized by a rapid increase in the first few years in fiber length, width, and proportion, wall thickness, and percent cell wall area. Ray proportion remained constant, whereas the vessel lumen area and proportion decreased with cambial age.