Practical challenges when updating tree marking direction to prioritize crown dieback

Abstract

In Ontario’s Great Lakes – St. Lawrence Forest region, professional tree markers are often relied upon to select trees for harvest, based on the presence or absence of various defects. Conventional tree marking guidelines do not currently make any distinction between defects which affect tree vigour, and those that affect tree quality. It is desirable to retain trees that are high vigour, and therefore likely to continue to provide seed and ecosystem services to the residual stand for at least another 20 years, while harvesting enough high-quality timber to make these stand improvement cuts economically viable. For this reason, it is important that the distinction between vigour and quality be taken into consideration in tree marking prescriptions. Defects which have been shown to indicate that a tree is low vigour include crown dieback (CDBK), fungi, and cankers. Defects which indicate that a tree is low quality include fungi, cankers, cracks, cavities, and decay. CDBK is one of the most useful indicators that a tree is lower vigour when affecting at least 15% of the crown. This indicator could be given considerably more importance when tree marking. There is currently no official threshold used in Ontario, but CDBK will usually only be considered as a defect if it is affecting at least 50% of a tree’s canopy. The purpose of this study was to test out a proposed, simplified tree marking system which reflects these advances in our understanding of defects as indicators of vigour and quality and prioritizes the removal of low vigour/high quality trees (with ≥ 15% CDBK but no quality affecting defects). We compared this new prescription with two conventional prescriptions by having experienced tree markers use all three in eight different plots. We were expecting the new prescription to result in 1) improved residual stand vigour (because it prioritizes the removal of trees with CDBK) and 2) increased consistency between tree markers (because the defect list they were using was shorter than that of the conventional systems). Neither of these desired outcomes were achieved in practice – we observed no significant difference in CDBK marked or consistency between the three prescriptions. This study represents an important first try at updating tree marking direction. Subsequent trials will be necessary to refine the new prescription, to achieve desired management outcomes in the field.