A study on the variation of knot width in Larix olgensis based on a Mixed-Effects model

Abstract

Knots are common internal defects in wood that significantly affect its mechanical strength and visual quality. Controlling knot size is an effective approach to improving wood quality, and knot width is a key indicator for measuring knot size. This study investigated 27 plantation-grown Larix olgensis trees from the Mengjiagang Forest Farm in Heilongjiang Province, China. Variables at both the tree and knot levels were incorporated to develop fixed-effects and mixed-effects models to simulate changes in knot width. The results showed that the mixed-effects model exhibited better fitting performance compared to the fixed-effects model. Additionally, the study evaluated the impact of four different sampling strategies on the predictive accuracy of the models. The findings indicated that the Type 2 sampling strategy, which involves selecting seven knot samples from the upper trunk, yielded the best predictive performance. The study also revealed that knot width increased with greater branch insertion height and angle, but decreased with higher height-diameter ratios, peaking at around the 10th year. These findings provide scientific evidence for optimizing pruning strategies, effectively controlling knot size, and increasing the proportion of knot-free timber, offering significant practical value.