Trends in Genetic and Environmental Parameters for Height, Diameter, and Volume in a Multilocation Clonal Study with Loblolly Pine

Abstract Seedlings from 30 full-sib families (contained in 2, 4 x 4 factorials) of loblolly pine (Pinus taeda L.) were cloned and planted in three test sites in Georgia. Analyses were conducted on total height at ages 1 to 5 yr in the field, dbh at age 5, and individual tree volume at age 5. Four sources of genetic control were tested: male parent, female parent, male x female parent interaction, and clone within family. Differential growth responses due to test sites were present. Significant differences were detected among male parents for only one (age 5 height) of the seven traits in only one factorial. However, variation for height among female parents was found at ages 1 to 5 in only one of the two factorials accounting for 1% to 9% of the total variation. Significant effects of clone within family were found at all ages in one factorial and at ages 1, 3, and 4 in the other factorial for height but not for dbh or individual tree volume. None of the parental sources (male, female, or male x female) were interactive with test sites except one isolated case at age 2 in one factorial. However, the clone within family source of variation interacted significantly with site for height at ages 3 to 5 in factorial 1. Differences due to male or female parent effects were somewhat lower than has been found in other similar studies, possibly due to the relatively low number of parents in both factorials and hence, sampling effects. Future genetic studies should include more parents in the mating design but with approximately the same number of cloned individuals per cross in order to provide a better test of sources of variation. Trends in genetic and environmental variances and heritabilities were examined. Additive genetic variance (VA')for tree height displayed a steady increase from age 1 to 5. Dominance genetic variance (VD') for height also increased steadily over the same age range. The relationship between VA' and VD' differed between the two factorials. In factorial 1, VA' was larger than VD' for ages 1 to 4, then VD' became larger for age 5. The reverse pattern occurred in factorial 2. Epistatic genetic variance was detected only at age 1 for height in factorial 1 and at ages 1 and 3 in factorial 2. Dominance variance equaled or exceeded additive genetic variance for dbh and individual tree volume at age 5. Narrow-sense and broad-sense heritabilities for height were low to intermediate (0.05 to 0.37) from ages 1 to 5 and were more or less stable over ages. The importance of dominance genetic variance, at least to age 5, underscores the likelihood of additional genetic gains through a clonal tree improvement and deployment program beyond the gains achieved in a seed orchard/seedling based program. For. Sci. 42(1):87-98.