Interactions of genotype with site for height and stem straightness in Pinus taeda in Zimbabwe

The magnitude and importance of genotype × environment interactions (GXE) for stem height and straightness at 2, 10, 14 and 23 years of age were estimated for Pinus taeda L. planted at four sites in Zimbabwe. Genetic correlations between heights at different sites varied from low to high (0.18 to 0.95). They were not consistent across the different ages, but were generally high between all sites at younger ages and progressively diminished with age. These results suggest that early growth assessments may not be reliable for predicting GXE at maturity. Genetic correlations between stem straightness scores at different sites also varied from low to high (-0.13 to 0.95); those between sites A and C, and between sites B and D, were consistently high (>0.84) at ages greater that 2 years. On the basis of the magnitude of genetic correlations, site B was generally the best selection site for height. At 23 years, there was no single best selection site for height. There did not appear to be any single environmental factor that influenced GXE for height, making it difficult to classify environments. Since GXE interactions were unpredictable for height, the value of the multiple population breeding strategy adopted for P. taeda in Zimbabwe may not be high. Genetic correlations for straightness were high between sites A and C and between sites B and D, suggesting similarity within these two pairs of sites. On the basis of correlations between breeding values and weighted aggregate genetic merit, site A was the best single site for selection to improve the average performance across all four sites. Consequently, a single breeding population could be established at site A.