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

Thinning affects negatively the quality of sawn timber of Douglas fir, Norway spruce, and Sitka spruce. The effect was confirmed in structural-sized boards and small clear samples, and on standing trees using longitudinal velocity. The loss of quality across the three species due to thinning rarely exceeds 20% and is in most cases smaller than 5%.

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. Sixty 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.

Thinning, Wood quality, Softwoods, Bayesian analysis

Krajnc, L., Farrelly, N. & Harte, A.M. Annals of Forest Science (2019) 76: 3.

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Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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