Improving tree selection for partial cutting through joint probability modelling of tree vigor and quality

Abstract

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.Résumé : Les systèmes de classification des arbres sont généralement conçus pour prévoir l'approvisionnement en bois de haute qualité des industries de transformation et pour marquer les arbres défectueux à prélever dans le but d'augmenter la vigueur et la qualité des futurs peuplements. Bien que ces deux objectifs soient généralement contradictoires, leur prise en compte simultanée pourrait permettre de définir des critères robustes visant à améliorer la sélection des arbres à prélever lors de coupes partielles dans des peuplements de feuillus nordiques en ciblant les arbres de faible vigueur (FV) et de haute qualité (HQ). Dans cette étude, nous avons utilisé une approche par copule pour modéliser la distribution de la probabilité conjointe des arbres caractérisés par leur vigueur et leur qualité qui tient compte de la dépendance statistique entre les classes des deux systèmes de classification. La relation entre la probabilité qu'un arbre se situe dans chaque catégorie conjointe et son diamètre à hauteur de poitrine (DHP) était assez similaire pour les trois espèces étudiées : le bouleau jaune (Betula alleghaniensis Britt.), l'érable à sucre (Acer saccharum Marsh.) et le hêtre à grandes feuilles (Fagus grandifolia Ehrh.). De façon particulière, la probabilité d'occurrence des arbres FV-HQ était caractérisée par une courbe parabolique dont la valeur maximale était atteinte près du milieu de la gamme des DHP, alors que celle des arbres FV et de faible qualité augmentait avec le DHP. Ces...

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