Genetic control of wood density and bark thickness, and their correlations with diameter, in pure and hybrid populations of Eucalyptus grandis and E-urophylla in South Africa

Tree diameter under and over bark at breast height (dbh), wood density and bark thickness were assessed on samples from control-pollinated families of Eucalyptus grandis, E. urophylla, E. grandis x E. urophylla and E. urophylla x E. grandis. The material was planted in field trials in the coastal Zululand region of South Africa. At 75 months, between three and seven of the best trees per family were felled and wood samples collected. Genetic parameters for wood density, bark thickness and bark percentage (ratio of double bark thickness to overbark diameter) and the inter-trait correlations for the different species and hybrids were calculated. Genetic parameter estimates for wood density, bark thickness and bark percentage in the E. urophylla x E. grandis hybrids showed these traits to be under total additive genetic control. This was confirmed by the intermediate hybrid means for these traits relative to those of the parental species. There was a very low correlation between dbh and wood density for the E. urophylla x E. grandis hybrids (r(G) = -0.07 and r(P) = 0.064). Amongst the E. urophylla families there was a moderate positive and significant phenotypic correlation between wood density and bark thickness (r(P) = 0.391), and between wood density and bark percentage (r(P) = 0.442).