Genetic parameters, including heritabilities, trait-trait correlations, across-site and acrossgeneration correlations, in four Corymbia citriodora subsp. variegata clonal trials in northern New South Wales, Australia and four Eucalyptus urophylla open-pollinated progeny trials (two first generation and two second generation trials) in Guangxi, China were estimated. The additive and non-additive variances were calculated separately from the total genetic variances in C. citriodora subsp. variegata clonal trials to allow the estimation of broad- and narrow- sense heritabilities. Efficient selection strategies using selection indices are also presented in discussion section.
For C. citriodora subsp. variegata clonal trials, Quambalaria shoot blight damage at the ages of 1.0-1.7 years and 3.5-3.8 years was under predominantly non-additive genetic control (H2 = 0.02-0.46, h2 = 0-0.17). Growth traits were, overall, under moderate genetic control (H2 = 0.32-0.54, h2 = 0-0.54). For tree height, nearly 100% of the genetic variances were contributed by the non-additive component. For diameter at breast height and individual tree volume, the dominant genetic variances were non-additive (66-80%) at the Coffs Harbour and Grafton sites, whilst the additive variances (88-100%) became dominant at the other two sites. Bark thickness and percentage (H2 = 0.45-0.81) and non-destructive wood properties (H2 = 0.57-0.87) were under moderate to high genetic control, with 45-100% of the genetic variances being additive for bark traits and basic density and 76-100% of the genetic variances being non-additive for acoustic stiffness (represented by acoustic velocity squared).
Growth traits were strongly correlated with Quambalaria damage, especially for the second assessment (rg = 0.53-0.85). Correlations between growth and bark traits showed that larger trees tended to have thicker bark (rg = 0.50-0.79), but did not necessarily have higher bark proportion (rg = -0.48 to 0.26). There was no significant relationship between growth traits and wood properties (Pilodyn penetration, basic density or acoustic velocity squared), but strongly negative correlations between Pilodyn penetration and acoustic velocity squared (rg = -0.76 to -0.53). Basic density and Pilodyn penetration were highly negatively correlated (rg = -0.95 to -1.00).
Low across-site correlations for Quambalaria damage (rg = 0.13-1.00) were observed, indicating genotype by environment interactions (GEIs) occurred. Across-site correlations for growth traits were reasonably high, with most estimates above 0.8. Relatively lower correlations (under 0.7), however, were also found between some pairs of sites. These lower correlations for growth traits were probably due to the disease effects or due to some degree of GEL By contrast, across-site correlations for wood properties (rg = 0.84-0.99) and bark traits (rg = 0.83-0.96) were notably high, especially those for basic density, indicating no significant GEI.
For E. urophylla progeny trials, growth traits (h2 = 0.05-0.39) were under low to moderate additive genetic control, whereas bark traits (h2= 0.22-0.38) and wood properties (h2= 0.35- 0.66) were under moderate to strong additive genetic control. Narrow-sense heritabilities for growth traits and basic density appeared to decrease over generations.
The larger trees tended to have thicker bark (rg = 0.48 to 0.57) but did not necessarily have higher bark percentage (rg = -0.40 to 0.32). Correlations between growth traits and basic density (say diameter at breast height and Pilodyn penetration) were favourable strong in the first generation (rg = 0.26 and 0.48) but were not significantly different from zero in the second generation (rg = -0.06 and 0.20). There was strong negative correlation (rg = -1.00) between basic density and Pilodyn penetration. Acoustic velocity squared was highly positively correlated with basic density (rg = 0.63) (except Pilodyn penetration, rg = -0.04) and growth traits (rg = 0.49-0.63).
Across-site correlation for diameter at breast height within the inland Dongmen region (rg = 0.81) indicated that there was no GEI. However, GEIs for growth traits were significant between Dongmen and Shankou (rg = 0.37-0.79). No GEI for basic density was present either within the Dongmen region (rg = 1.00) or between Dongmen and Shankou (rg = 0.99). Traits with higher heritabilities (e.g. wood property traits) in the first generation had higher across-generation correlations (rg = 0.63-1.00). On the contrary, traits with lower heritabilities (e.g. growth traits) had lower across-generation correlations (rg = -0.26 to 0.83). The estimates of across-generation correlations between Dongmen and Shankou obtained in this study were somewhat lower, indicating some degree of GEI.