Tree classification systems are generally designed to predict the supply of high-quality logs for the wood processing industries and to mark defective trees for removal with the aim of improving both the vigor and quality of future stands. Although these two objectives are generally inversely related, their joint consideration could enable the development of robust criteria for improved tree selection in partial cuttings of northern hardwood stands by targeting low-vigor (LV) trees of high quality (HQ). In this study, we used a copula approach to model the joint probability distribution of trees characterized by both vigor and quality, which accounts for the statistical dependence between the different classes of both classification systems. The relationships between the probability of occurrence of a tree in each joint category and tree diameter at breast height (DBH) were quite similar among the three studied species: yellow birch (Betula alleghaniensis Britt.), sugar maple (Acer saccharum Marsh.), and American beech (Fagus grandifolia Ehrh.). In particular, the probability of occurrence of LV–HQ trees was characterized by a parabolic curve with a maximum value attained at the mid-DBH range, whereas that of LV and low-quality (LQ) trees increased with increasing DBH. This suggests that instead of harvesting large and mostly LV–LQ trees, partial cutting should target smaller LV–HQ trees, so that the volume of harvested HQ trees would increase without compromising the silvicultural objective of improving the vigor of future stands.

The objective of this study was to establish whether the relationships between wood density, growth rate and ring number from the pith were equivalent in the contrasting conditions of a natural forest and an intensively managed plantation of northern red oak (Quercus rubra Liebl.). The radial X-ray densitometry profiles of 14 natural forest trees and 18 plantation trees growing in the southwest of Québec, Canada, were studied in detail using a linear mixed-effects modelling approach. In the natural stand, the effect of faster growth on overall ring density changed from a slightly negative relationship at a ring number 5 to a strongly positive effect at ring 40. In the plantation trees, the overall ring density remained almost constant along the range of observed ring widths. For the range of ring numbers common to both stand types (i.e. rings 1–15) the relationship was almost equivalent. Overall, our results suggest that because trees are harvested at a target diameter, silvicultural interventions specifically designed to improve radial growth rate in northern red oak will not be detrimental to overall wood density.