The influence of silvicultural practices on mechanical properties of softwood timber
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The increase in the use of structural timber in construction over recent years has been one of the main drivers behind the renewed interest in optimizing forest management for timber quality. Past research and forest management have been more focused on producing larger quantities of timber in a shorter amount of time, while the relationship between forest management and timber quality is not well understood. A better understanding of that relationship is needed in order to successfully manipulate the quality of the timber produced, especially in fast growing species currently planted and grown in Ireland and elsewhere in Europe. This research study examined the relationships between tree characteristics, forest management and timber quality in three softwood species, grown in Ireland. A relatively large-scale study was performed, using trees from six different stands of Douglas fir, Norway spruce and Sitka spruce. Half of the stands was regularly managed with thinning and the other half was not managed. A total of 600 standing trees of different species was included, and their crown and stem properties were assessed. Acoustic velocity was measured in all trees of the three main softwood species, after which the trees were felled and a sub-sample of them sawn into structural-sized timber. Over 1300 structural-sized boards were assessed non-destructively and tested destructively in four-point bending. Small clear specimens were extracted as close to the fracture as possible, evaluated non-destructively and tested in three-point bending. The data were analysed using a novel, multi-level approach using Bayesian data analysis. The analysis was conducted on the full sample of standing trees using acoustic velocity, on the sub-sample of trees cut into structural-sized boards using mechanical properties and on small clear specimens using mechanical properties. This innovative approach enabled a comparison of the analyzed relationships across three different measurement levels instead of only using one, as commonly done by past studies. The results indicate that thinning negatively affects mechanical properties overall, with the size of the loss dependent on the species. In Sitka spruce, the loss was negligible and thinning was recommended for maximizing volume production with a negligible loss in mechanical properties. Tree characteristics were shown to have a significant impact on the mechanical properties of the timber. Crown social class was found to have an effect, as did crown projection area and stem slenderness. Several other measured variables (crown ratio, crown eccentricity or crown roundness) were found to have no influence on mechanical properties. This study also examined the use of acoustic velocity in standing trees for prediction of timber quality. The results indicate that acoustic velocity cannot be used reliably in larger-diameter trees for prediction of the mechanical properties of timber. While this appears to be possible in smaller-diameter trees, alternative methods of evaluating timber of standing trees are needed to achieve the same in trees of larger diameters. This study also examined the relationships between mechanical properties of boards and small clear specimens cut from those boards. Overall, the relationships were poor-to-moderate. However, this study has successfully demonstrated that in order to evaluate mechanical properties on the level of an individual tree, either specimen size could potentially be used. In both specimen sizes the non-destructive evaluation of mechanical properties reflected the mechanical properties relatively well, suggesting that non-destructive evaluation of timber specimens can present a suitable replacement to destructive testing of specimens under certain conditions.
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