Characterization of Knots in Plantation White Spruce
Keywords:Knot size, inclination, azimuth direction, taper, distribution
AbstractKnot size and distribution are of key importance to wood quality and lumber grade yield. This study characterized white spruce (Picea glauca [Moench] Voss) knots with computerized tomography images and knot mapping software. The characteristics included shape, number, diameter, inclination (angle between z-axis and the line connecting the knot's starting point at the pith with the end point), azimuth direction, and distribution inside the stem. There were on average 112 grade-important knots per tree in a 32-yr-old plantation white spruce tree, 7% of which could downgrade lumber from Select Structural to No. 2 and lower grades. A total of 3.6% were ramicorn knots, and 70% had an inclination angle between 60 and 80° with the tree axis. There were 24 more knots per tree with every 2-cm increase in tree diameter. Knot diameters had a positively skewed frequency distribution with an average of 156 mm. The majority of the knots had a taper between 0.18 and 0.38 mm/mm, which could have had a major impact on lumber mechanical properties. Knots grew steeply upward until reaching their greatest diameters and became more horizontal afterward. Butt logs had smaller but a higher number of knots and a lower percentage of knot volume (out of log volume) than those higher up. In the same height growth unit, inclination angle decreased with increasing height. Wider knots had a smaller inclination angle. White spruce has a weak self-pruning mechanism and is prone to forking or ramicorn branching. The widest knot in a height growth unit is more likely to be found in the south direction. Trees from wider spacing tend to have wider knots but not necessarily a higher number of knots at the same heights. The results suggest the need for pruning operations and including knot information in determining sawing strategies. Information from this study should be useful in forest management decision-making and improving wood use.
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