Annual measurements on growth traits from a total of 275 parents and 690 full-sib families from 23 diallel tests of loblolly pine (Pinus taeda L.) in the Northern, Coastal and Piedmont

Annual measurements on growth traits from a total of 275 parents and 690 full-sib families from 23 diallel tests of loblolly pine (Pinus taeda L.) in the Northern, Coastal and Piedmont test regions of the southeastern U.S. were used to reveal the time trend of genetic parameters through age 8. Variance components were estimated from mixed model analyses on growth traits (height, DBH and volume) and were used to calculate genetic variance components, heritabilities and age-age genetic correlations. In three test regions, the dominance variance was found to be less than the additive variance. The range of the dominance variance fell within 20%~40% of the total genetic variance for all traits. Different trends of heritability estimates were found for three test regions, but the general trend was that heritability increased over time. The magnitude of heritabilities for DBH and volume was found to be comparable with the corresponding heritability for height. Age-age genetic correlations of early height with 8-year volume increased significantly in the first 3–4 years, and then leveled off after that. DBH and volume had higher age-age correlations than height. The trends of heritabilities and age-age correlations indicated that the optimum selection age could be as early as 3 for height and as early as 4 for DBH or volume. Early selection for volume at age 4–5 could maximize genetic gain.