Age trends in genotypic variation of wood density and its intra-ring components in young poplar hybrid crosses

Age related dynamics of genotypic, phenotypic, and environmental variation, clonal repeatability, and genotypic correlations for wood density and its intra-ring components were analyzed in four poplar hybrid crosses, Populus deltoides x P. nigra, P. trichocarpa x P. deltoides, P. maximowiczii x P. balsamifera, and P. balsamifera x P. nigra, as well as P. deltoides. Using X-ray densitometry, measurements were taken on increment cores sampled in four clonal trials at 10 and 12 years of age from ramets of 19 clones. Wood density of all hybrid crosses was highest at the pith and decreased with increasing cambial age. The significance of the hybrid cross effect increased with age for mean wood density, dry fiber weight, and ring width. The coefficient of genotypic variation of cumulated mean wood density was rather stable over the 10-year period at all three sites, and ranged from 4.8-6.8%. Clonal repeatability increased with age from 0.46 to 0.79, mainly because of decreasing random variation. Corresponding genotypic parameters for individual rings varied greatly with age and across sites. Significance of the site effect on wood density tended to decrease with age. Significant negative genotypic correlations between ring width and wood density were found at only two of the four sites and they weakened with age. Age-age genotypic correlations between wood densities at ages 10 and younger were strong and significant from age 6 and over. This trend suggests that selection before this age would be unreliable. © INRA, EDP Sciences, 2006.