The effectiveness of cloning for the genetic improvement of Mexican white cypress Cupressus lusitanica (Mill.)

Two trials on Mexican cypress Cupressus lusitanica Miller in the North Island of New Zealand were assessed for diameter at breast height and at one site, subjective scores for branch size and stem canker (caused by Seiridium spp.) at age 6 from planting. The trials comprised 15 open-pollinated families, represented by both cloned and seedling progeny. Linear mixed model methodology, using a spatial model for the residuals, was applied to estimate genetic parameters. Estimated narrow-sense heritabilities were moderate to high for diameter at breast height (range from 0.46 to 0.62), stem canker (≈0.30) and branch size (range from 0.23 to 0.45) and did not appear to differ significantly between propagule types for all traits. Clonally replicated progeny led to an increase in accuracy of selection for additive genetic merit when compared with seedling testing, with the improvement being greater for traits with lower narrow-sense heritabilities. Estimated additive genetic correlations between cloned and seedling progeny were moderate to high (≥0.65) for diameter and branch size, indicating that selection decisions would not be substantially changed using either propagule type for progeny testing. All estimates of non-additive genetic variation based on the cloned progeny were non-significant. The use of spatial analysis was effective for diameter and branch size, but not for stem canker. No significant genotype by environment interaction was detected for diameter. Implications of the results for breeding and deployment of C. lusitanica are discussed.