Validation of a stem straightness scoring system for Sitka spruce (Picea sitchensis (Bong.) Carr.)

This study describes the refinement and testing of a prototype scoring system for the visual assessment of stem straightness in Sitka spruce. The original system comprised a six-point scoring system from 1 (least straight) to 6 (straightest) based on an estimate of straight log lengths in the first 6 m of the stem. A longer log length category and additional score were added to the system to increase relevance to industry practice. The use of aids to measurement to improve the objectivity and accuracy of the visual assessment was tested. A purely visual assessment was as accurate as the measurement tools and was considerably quicker and easier to complete. A study to establish appropriate sampling levels indicated that between 60 and 100 trees should be assessed to obtain an acceptable estimate of the median and interquartile range of straightness scores for a stand. An investigation of the relationship between stem straightness score and yield of logs of different quality suggested that the scoring system may be used to differentiate between stands of different log quality.