Preliminary results from a structural conversion thinning trial in eastern Canada

Abstract

In 2008, a thinning trial consisting in the removal of competitors around high growth potential stems (crop trees, CTs) was initiated as the first step of a structural conversion to transform even-aged stands into uneven-aged stands. Two intensities of thinning by CT release and thinning from below were tested in white spruce (Picea glauca [Moench] Voss) plantations and in balsam fir (Abies balsamea [L.] Mill)-dominated naturally regenerated stands. Unlike thinning from below, which aims to remove poor quality stems to improve the growth of residual trees, thinning by CT release aims to reduce competition between dominant trees. Trees thinned by CT release showed a significantly higher diameter at breast height increment than those in the control plots. CT release did not significantly increase stand structure, nor did it reduce stand growth, which helps to address the concerns of regional forest stakeholders. CT release may offer other benefits in the long term, but subsequent interventions are necessary to evaluate if this treatment meets the desired objectives. Study Implications: In Eastern Canada, the forest landscape has changed in the past century from uneven-aged to even-aged stands because of heavy harvesting. The Sustainable Forest Act that went into effect in 2013 in the province of Quebec now promotes forest management inspired by ecosystem natural dynamics, even in plantations. To meet ecosystem-based management requirements, we used crop tree (CT) release to reintroduce structural complexity in managed forests as a first step toward the structural conversion of even-aged stands to uneven-aged stands. CT release frees high-quality stems from their surrounding competitors, compared to thinning from below, which focuses on harvesting suppressed trees and stems with defects. Thinning by CT release might be a harvesting strategy that reconciles sustainable development with economic profitability, since the initial harvest entry in the long process of structural conversion can provide quality stems, unlike thinning from below. Moreover, our results indicate that growth and yield are not affected by CT release, at least not in the short term, which can help address the concerns of regional forest stakeholders. CTs showed a significantly higher diameter at breast height increment than control trees as well as a higher diameter at breast height increment than their closest neighbors. These trees play a structuring role, as they are intended to be the framework of the developing irregular stand. Moreover, no final cut is planned. Thinning by CT release represents the first step toward structural conversion with several subsequent thinning entries required to meet this long-term objective.