Modeling knot geometry from branch angles in Douglas-fir (Pseudotsuga menziesii)

Lumber and veneer recovery from Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees depend on the size and distribution of knots. Two approaches have been used to simulate the effect of knots on recovery of these products: (i) prediction of recovery based on mill studies and (ii) simulated milling of virtual trees. A benefit of the latter approach is that different milling configurations may be tested. Knots in virtual logs are usually based on data from X-ray scanning. A novel approach was used in this study to model knot geometry by inferring the development of a branch knot over time from a chronosequence of branch angle and diameter measurements. Branch angle was modeled from a database of 17 953 branch measurements on 412 trees sampled in 16 Douglas-fir plantations. Branch angles from tree tip to crown base were assumed to represent a chronosequence describing the change in branch angle. Knot pith curvature was then derived from this chronosequence of branch angles and modeled as a first-degree inverse polynomial, conditioned on tree size and position within the tree bole. Knot pith curvature was predicted to follow a linear path near the tree tip and became more curved with increasing age and depth into the crown.