Genetic parameter estimates for growth, stem-form and branching traits of Casuarina junghuhniana clones grown in Thailand

•Broad-sense heritability estimates were moderate for growth and form traits.•Early selection at age-3 years appears appropriate.•Several novel clones significantly outperformed the commercial benchmark.•Prospects for genetic improvement of Casuarina junghuhniana are good.

Casuarina junghuhniana is an emerging tropical plantation species that is increasingly being planted on tropical lowland and mid-altitude sites with moderate dry seasons. Field trials have been established to investigate genetic variation in the species with a view to identifying genetically improved planting material. In this study genetic parameters were estimated for nine growth traits using 29 novel clones of Casuarina junghuhniana planted in Thailand. Survival, height, diameter and stem volume were recorded annually for six consecutive years and compared with a commercially-deployed, Thai benchmark clone. Stem form (axis persistence and stem straightness) and branching habit (branch density, branch thickness and branch angle) were scored when the trees were 4 years of age. Marked variation among clones was observed for all traits at each measurement. While the stand had attained a mean annual increment of 20.8 m3 ha−1 y−1 by year 6, the most productive clone was double the average and four times greater than the benchmark. Broad-sense heritability estimates (Ĥ2) for most traits were moderate (0.13-0.50), and there was a tendency for Ĥ2 to increase with age for survival, diameter and stem volume. There were no correlations between growth and stem form, but growth was negatively correlated with branch thickness and positively correlated with branch angle. Age-age correlation for growth was strong, suggesting that selection for stem volume need not be delayed beyond 3 years. Based on the growth and stem form traits, up to 11 clones are considered suitable for commercial plantation establishment.