Genetic control of growth and shoot phenology in juvenile loblolly pine (Pinus taeda L.) clonal trials

Southern pine genetic improvement programs have selected for faster early growth which has often increased yields over unimproved material, and some of this improvement is likely attributable to variation in growth phenology among genotypes. However, the genetics of shoot growth phenology traits are not well characterized. Loblolly pine cuttings and seedlings from parents originating in the Atlantic coastal plain (ACP) and Florida and grown on sites established in Palatka, FL and Cuthbert, GA were assessed for shoot phenology and growth traits during the second year and for growth in year 6. Individual-tree clonal repeatability in different growth and shoot phenology traits varied from 0.09 to 0.79 in cuttings, and was lower in Palatka than Cuthbert. Non-additive components of heritability were lower, with a few exceptions, than additive effects. Additive and genotypic correlations across sites were high (> 0.6) for all traits measured in cuttings and for most seedling traits, suggesting low genotype x environment interactions between these two sites. Compared with progeny from crosses between ACP parents, progeny of Florida parents started growth earlier in the season and ended later. Strong genetic correlations were observed between second-year phenology traits and sixth-year height and diameter. This suggests some two-year traits could be useful for early selection of high-performing genotypes.