Compression wood in Scots pine and Norway spruce

This thesis gives a summary and discussion of results from five studies concerned with: the distribution of compression wood in relation to external stem and log geometry and the impact of the compression wood distribution on deformations of sawn timber. Moreover the straightening process of trees is discussed. In order to study the distribution of compression wood in stems and logs, 63 trees of Norway spruce and Scots pine representing different age and curvature categories were sliced into 621 discs. In order to study the impact of compression wood on deformations of sawn timber, 146 logs of Scots pine and Norway spruce were processed into studs and battens. Before crosscutting, the external geometry of stems and logs were assessed. A 3D-logscanner was used for the assessment of logs. Deformations such as bow, spring and twist were measured on studs and battens at green condition, at 18% moisture content and at 12% moisture content. Compression wood content in thin cross sections of discs, studs and battens were analysed using transmitted light and image analysis. The analyses showed that a majority of all discs and cross sections contained compression wood. In all studies, the pith eccentricity was significantly correlated to the discs´ compression wood content. For the 6-year-old Scots pine trees, the compression wood content was significantly correlated to the size of basal sweep but not to out-of-roundness. For the 22-year-old Scots pine trees, the compression wood content was not correlated to either bow height or size of basal sweep. For the 60-year-old Norway spruce trees, the compression wood content was significantly correlated to compression wood content in log ends, pith eccentricity and bow height. For sawn timber, the compression wood content and its position was significantly correlated to bow and spring but not twist. Twist was significantly correlated to grain angle measured on tangential faces of studs and spiral grain angle measured under bark on the surface of logs. Results indicate that eccentric growth and compression wood formation play major roles in the development of stem straightness. Moreover, young trees with basal sweep will be straighter over time. Consequently, the straightness of a log is not a reliable measure of the compression wood content within the stem. By combining data describing external geometry with information regarding compression wood content and pith eccentricity obtained from log ends it is possible to detect logs that are prone to contain compression wood. The results also indicate that the distribution of compression wood in sawn timber influences the direction and size of bow and spring.