Tree mortality after experimental shelterwood and seed-tree cuttings in boreal forest

Abstract

Partial cuttings are considered as the best alternative to clear-cutting to achieve sustainability in forest management. The success of the approach is closely linked to the residual tree mortality by windthrow, a key factor to consider in forestry planning that may lead to strong economic and ecological implications. To better understand tree mortality by windthrow after partial cuttings, we developed a study following two main questions: 1) how many trees died by windthrow ten years after treatment? And 2) which factors are involved in this phenomenon? Six study blocks comprised of mature even-aged black spruce [Picea mariana (Mill.) B.S.P.] stands of the Canadian boreal forest, were submitted to experimental shelterwood and seed-tree treatments. Tree mortality was measured before and after cutting, and dead trees were classified in the following groups: standing dead, overturned or broken. Factors potentially influencing windthrow included stand, tree, topographic and geographic characteristics. Our results show that 60% of residual trees were dead 10 years after experimental seed-tree cutting and about half (30%) of that value in the case of experimental shelterwood cuttings. Windthrow represented 80% of residual tree mortality in black spruce stands. Broken trees were associated with young stands hosting a high density of specimens characterized by a small diameter and a short crown, located near adjacent cuts (< 200 m). Overturning was associated to trees with severe wound damages and large diameter, in treatments with high harvesting intensity (R2= 81%). Standing dead mortality was related to suppressed trees with small diameters and slow growth in untreated plots. This study demonstrated that understanding tree mortality by windthrow after cutting is essential to improve silvicultural planning in North-American boreal forests.