The relationship among stress-wave-based wood stiffness and growth traits revealed by planting density trial with multiple genotypes in Cryptomeria japonica

Abstract Understanding the genetic and environmental factors influencing wood stiffness is essential for improving wood quality in plantation forestry. In this study, we investigated the phenotypic, genetic, and environmental variation in wood stiffness, measured as stress-wave-based modulus of elasticity (swMOE), in Cryptomeria japonica. Six genotypes were planted at three planting densities (7000, 3000, and 1700 trees/ha) in a field trial, and growth traits such as tree height, diameter at breast height (DBH), and bole height were measured at stand age 40. Using linear mixed models, we estimated variance components, broad-sense heritabilities, and correlations between swMOE and growth traits. We compared linear mixed models incorporating growth traits as covariates and examined their predictive accuracy for swMOE. Our results showed that swMOE was under relatively strong genetic control and was less influenced by planting density compared to growth traits. Among the growth traits tested, form ratio (tree height to diameter) exhibited the strongest positive correlation with swMOE at the phenotypic, genetic, and residual levels. Including form ratio as a covariate improved model fit and increased prediction accuracy across replications. These findings suggest that form ratio is both a biologically meaningful and practically useful trait for explaining and predicting wood stiffness of C. japonica. This approach has potential applications in genetic selection and wood property prediction in operational breeding and forest management.