Early Selection for Improving Volume Growth in Coastal Douglas-fir Breeding Programs

Measurements on growth traits up to 41 years of age from 68 progeny sites in eight first-generation breeding zones of coastal Douglas-fir (Pseudotsuga menziesii [MIRB.] FRANCO var. menziesii) in the US Pacific Northwest were used to investigate age trends of genetic parameters and to determine optimum age of selection. Heritabilities and age-age genetic correlations were estimated using univariate or bivariate mixed model analyses. Heritability estimates tended to increase with age for both total growth and growth increment traits. The estimates showed different age trends among breeding zones, but the differences were generally small. Age-age genetic correlations for total growth traits fitted Lambeth’s model surprisingly well, despite the data being collected from multiple breeding zones. Using rotation-age (i.e., 50yr) volume as the selection criterion, the greatest correlated gains per year were achieved by making family selection at juvenile ages (i.e., 9 for height, 13 for diameter, and 11 for volume). Similar results were obtained for within-family selection except that the optimum ages of selection were 2~4 years later than that from family selection, i.e., 11 for height, 15 for diameter and volume. Early selection on total height was always more efficient and had earlier optimum ages than on other growth traits. The optimum ages of early family selection on total growth were 4~11 years earlier than on the corresponding growth increment traits. It was also evident that the optimum ages of selection occurred later for slow-growth trials than for fast-growing trials.