The effect of thinning on mechanical properties of Douglas fir, Norway spruce, and Sitka spruce

Context: The relationship between silvicultural management and the quality of timber produced is not entirely elucidated. Aims: The effects of thinning on structural grade-determining properties of wood (elastic modulus, bending strength and density) were studied on Douglas fir (Pseudotsuga menziesii (Mirb.) Franco), Norway spruce (Picea abies (L.) H. Karst) and Sitka spruce (Picea sitchensis (Bong.) Carr)). Methods: Acoustic velocity was measured in a total of 487 trees and their crown social status was recorded. 60 trees were selected and cut into structural-sized boards (N = 1343). The amount of knots in each board was quantified using the grading machine GoldenEye702. All boards were destructively tested in four-point bending, after which a small clear specimen was cut from each board and again tested in bending (N = 1303). Specific stiffness and specific strength were used to estimate the size of the effect accounting for differing influence of thinning across the before-mentioned properties. Results: Thinning reduces all three properties with the likelihood and magnitude of the effect varying between species. The loss of quality due to thinning rarely exceeds 20 % and is in most cases smaller than 5%. The effect of thinning and its size were also confirmed on the full sample of trees by using longitudinal velocity. Conclusion: The results give a clearer idea of what the trade-offs are between timber quality and silvicultural management.The first author was supported in undertaking this work by a Teagasc Walsh Fellowship. This work was also supported by grant aid from the Forest Sector Development Division of the Department of Agriculture, Food and the Marine, Ireland.2020-01-0