Age–Age Correlations and Early Selection for Height in a Clonal Genetic Test of Norway Spruce

Abstract Heights of rooted cuttings from 40 clones of Norway spruce (Picea abies [L.] Karst) were measured at ages 3, 5, 8, 10, 13, and 17 from rooting at seven test environments in northern Germany. To determine the optimal age for juvenile selection for height, age-related changes in variance components, clone-mean heritability, and age–age correlations were estimated. Partitions of variance components were essentially constant after age 8. Clone-mean heritabilities were highest at age 3 (0.94) and were essentially stable after age 5. Age–age correlations were generally high between similar ages but declined as the difference between pairs of measurement ages increased. Genetic correlations were greater than corresponding phenotypic correlations but were similar in value. Trait–trait correlations between height and diameter (rG = 0.83) and height and volume (rG = 0.90) at age 17 were also high. Simple linear regressions of genetic and phenotypic age–age correlations on LAR (natural log of the ratio of younger age to older age) were significant (R2 ≥ 0.76). Predicted efficiencies of selection suggested that for a rotation age of 60 years, the optimum selection age would be as early as 13 years. Furthermore, gains per year would be increased by about 1½ times if selections were made at about age 15 rather than age 60.