From inside to outside: CT scanning as a tool to link internal knot structure and external branch diameter as a prerequisite for quality assessment

Branches are as essential for tree growth as knots are detrimental from the wood quality point of view. To bridge the gap between tree growth and the quality toward end-use, this study aims to establish a relationship between internal and external diameters of Douglas-fir whorl branches. The data comprised 102 trees of a wide age range (30–80 years old) from nine study sites in Southwest Germany. External branch measurements were performed in the field following an established protocol. Logs were scanned on a MiCROTEC CT.LOG, and knots were detected by applying an automated algorithm. Obvious detection artefacts by the CT algorithm were excluded to reveal the relationship between inner-outer branch diameters as clear as possible. Results showed a significant mean difference of 13.8 (± 10.0) mm between the methods (external diameter being larger), with a model indicating an offset of 9.75 mm and angular shift of 0.53 (RMSE = 7.12 mm; R2 = 0.57) between the methods. Separate calculations of sound and dead datasets did not reveal a statistically significant difference. By linking the internal knot structure to external branch measurements, the findings of this study constitute a first step toward the incorporation of CT data into growth models, providing a meaningful prediction of the maximum internal knot diameter at an early stage in the wood supply chain.