& Key message 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.

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Field measurements were carried out to assess the feasibility of ultrasound velocity and damping for the non-invasive testing of standing trees. A total of 87 trees of European beech (Fagus sylvaticaL.) and 68 trees of Norway spruce (Picea abiesKarst.) were measured in the field, felled and assessed individually for the presence of red heartwood or butt rot. The field assessment of these internal structural defects at the stump level (SL) of the trees was compared with the ultrasound measurements recorded at two tree heights [(i.e. at the SL and 0.5 m above the stump level (ASL)] and in several directions, all perpendicular (PP) to the tree stem. Lower ultrasound velocity and higher damping were found in both species with the presence of internal defects in both the radial and tangential directions of the tree stem. The diameter at breast height (DBH) had a varying effect on both ultrasound velocity and damping. A binary logistic regression was used to test the potential of ultrasound velocity and damping to predict the presence of internal defects. Both the approaches offer similar levels of prediction accuracy (0.72 and 0.76 in beech, and 0.83 and 0.82 in spruce). Due to the significant reduction in measuring time when using ultrasound damping only, this principle is recommended for the detection of red heartwood in beech trees and butt rot in spruce trees.

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Research on the mechanical and physical properties of wood is commonly carried out on either small clear specimens or structural-sized boards. The first approach was more frequently utilized in the past, while the latter is more commonly used nowadays. However, there is very little information on how the two approaches relate with one another. This study aimed to quantify the relationships between the mechanical [modulus of elasticity (MOE) and bending strength] and physical properties (density) of both specimen sizes. A total of 1376 structural-sized boards from three different species (Douglas-fir, Norway spruce and Sitka spruce) were tested in bending, after which a small clear specimen was extracted from the undamaged portion of each board and re-tested in bending. Prior to destructive testing, all boards and clear specimens were evaluated using non-destructive technology. Poor-to-moderate relationships were found between all measured mechanical and physical properties of structural-sized timber and small clear specimens. In both specimen sizes, the properties correlated with one another within the same specimen size, as well as across the two sizes. The strength of correlations appears to be somewhat species dependent. Relatively good relationships were identified when comparing the mean tree values of the properties examined, suggesting either method can be used for a tree-level comparison. The non-destructive evaluation of specimens was shown to reflect the measured properties moderately well, with the relationships changing significantly depending on which measured property was being predicted.