Estimation of optimal timing of early selection based on time trends of genetic parameters in Abies sachalinensis

Early selection is an imperative in ongoing forest tree breeding. This study estimated the optimal timing of early selection in Abies sachalinensis (F. Schmidt) Mast. based on time trends of genetic parameters obtained from two test sites. Tree height (H_t) at 5, 10 (11), 15, 20, 30, and 40 years of age and diameter at breast height (DBH) at 20, 30, and 40 years of age were analyzed. The efficiency of early selection per year (E) for performing early indirect selection relative to performing direct selection at the earliest rotation age (40 years of age) was estimated based on narrow-sense heritability (h²) and age–age genetic correlation (r). The h² of H_t peaked at 10 or 15 years of age (0.52–0.71), and that of DBH was the highest at 20 years of age (0.19 or 0.22). The age–age genetic correlation between tree heights or between tree height and DBH at different ages decreased with increasing differences between ages (regression coefficients were −0.011 and −0.007, respectively). The E values were highest at 10 or 15 years of age (0.84-1.74 and 1.42-2.24 for H_t and H_t-DBH, respectively), indicating the optimum selection timing. In Japapnese forestry, the initial growth rate is considered important for reducing weeding costs. Selection at 10 or 15 years of age had more than in >65 % indirect genetic gain relative to the direct genetic gain at 5 years of age; thus, selection at 10-15 years of age is appropriate considering the initial and mature phases of tree growth.