Relevance of stem and crown defects to estimate tree vigour in northern hardwood forests

The visual assessment of tree vigour before partial cutting is a key element of the long-term sustainability of managed hardwood forests. Several classification systems have been developed and applied to perform this task during the tree marking process. They segregate trees in different vigour classes based on the presence and severity of stem and crown defects. Yet, the relevance of using these defects to assess tree vigour has rarely been empirically validated. In this study, we analysed the relationships between quantitative vigour indicators and a full range of individual defects using 336 sugar maple and 84 yellow birch trees. Among the tested defects, the tree crown density showed the best ability to identify slow growing, non-vigorous sugar maple trees, regardless of their size. However, none of the stem-related defects, such as bark aspect, the presence of cambial necrosis or conks and stromata were strongly related to our quantitative vigour indicators. For yellow birch, none of the stem defects and crown conditions were found to be related to our vigour indicators. We conclude that, pending longitudinal studies that will provide a full empirical validation of classification systems, crown density should be used to assess recent growth and vigour of sugar maple trees and guide tree marking.