Visual assessment of tree vigour in Canadian northern hardwood forests: The need for a simplified system

Abstract

Northern hardwood forests include many degraded stands dominated by trees of low vigour due to past management. To facilitate the implementation of stand improvement, several classification systems have been developed to help tree markers visually assess tree vigour at time of harvest based on the presence of individual defects. Because very few studies have tried to empirically validate such systems, it remains uncertain whether many of these defects should be used to guide the tree marking process. In this study, we assess tree vigour using repeated measurements collected as part of long-term silvicultural trials conducted in 615 permanent plots throughout the northern hardwood forest of Quebec, Canada. We aimed to determine whether the defects that are commonly used for classification have a significant effect on both growth and survival over three decades, using 9,338 sugar maple and 1,316 yellow birch trees. We also conducted a retrospective analysis to quantify the rate at which vigorous trees develop defects. Our results confirmed that crown dieback is by far the best indicator of vigour for both sugar maple and yellow birch trees. Conversely, our results revealed that stem defects did not contribute much to explaining the variation in vigour, except for the presence of cankers and fungi, which had modest effects. Consequently, stem defects should not be used as the main indicators of tree vigour, and existing classification systems should be simplified by reducing the number of stem defects under consideration for this purpose. Lastly, our results showed that the rate that vigorous trees develop defects increased with increasing diameter, more so than the probability of surviving. Thus, assessing the risk of retaining large trees should not only be based on survival, but also on the risk of developing defects that reduce the growth and value of trees.