Inheritance of growth and solid wood quality traits in a large Norway spruce population tested at two locations in southern Sweden

Unfavorable genetic correlations between growth and wood quality traits are one of the biggest challenges in advanced conifer breeding programs. To examine and deal with such correlation, increment cores were sampled at breast height from 5,618 trees in 524 open-pollinated families in two 21-year-old Norway spruce progeny trials in southern Sweden, and age trends of genetic variation, genetic correlation, and efficiency of selection were investigated. Wood quality traits were measured on 12-mm increment cores using SilviScan. Heritability was moderate (similar to 0.4-0.5) for wood density and modulus of elasticity (MOE) but low (similar to 0.2) for microfibril angle (MFA). Different age trends were observed for wood density, MFA, and MOE, and the lower heritability of MFA relative to wood density and MOE in Norway spruce contrasted with general trends of the three wood quality traits in pine. Genetic correlations among growth, wood density, MFA, and MOE increased to a considerably high value from pith to bark with unfavorable genetic correlations (-0.6 between growth and wood density, -0.74 between growth and MOE). Age-age genetic correlations reached 0.9 after ring 4 for diameter at breast height (DBH), wood density, MFA, and MOE traits. Early selections at ring 10 for diameter and at ring 6 or 7 for wood quality traits had similar effectiveness as selection conducted at reference ring 15. Selection based on diameter alone produced 19.0 % genetic gain in diameter but resulted in 4.8 % decrease in wood density, 9.4 % decrease in MOE, and 8.0 % increase in MFA. Index selection with a restriction of no change in wood density, MOE, and MFA, respectively, produced relatively lower genetic gains in diameter (16.4, 12.2, and 14.1 %, respectively), indicating such index selection could be implemented to maintain current wood density. Index selection using economic weights is, however, recommended for maximum economic efficiency.