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Clonal repeatabilities on individual tree ( $$ H_i^2 $$ ) and clonal mean ( $$ H_{{\overline C }}^2 $$ ) bases for growth (14-year height and volume), wood quality traits (latewood proportion, wood density, fiber length, and microfibril angle), and genotypic correlations among the traits were estimated, using 30 white spruce (Picea glauca [Moench] Voss) clones from six full-sib families (five per family). These families were selected from a clonally replicated test to represent different early growth categories: fast, moderate, and slow. Wood increment cores of the 30 clones were collected from two contrasting sites at age 19 years. For growth traits, in contrast to most wood quality traits, more genetic variation was accounted for by clone within family than by family within growth category. Both growth and wood quality traits appear to be under moderate genetic control, with $$ \widehat{H}_i^2 = 0.20 - 0.36 $$ and $$ \widehat{H}_{{\overline C }}^2 = 0.70 - 0.83 $$ . The only exception was microfibril angle ( $$ \widehat{H}_i^2 = 0.10\;{\text{and}}\;\widehat{H}_{{\overline C }}^2 = 0.34 $$ ). Generally, faster growth resulted in a significantly lower latewood proportion and lower overall wood density. Selection for faster growth does not appear to impact on either fiber length or microfibril angle. Among the wood quality traits, significant genotypic association was observed only between latewood proportion and wood density. Despite the generally negative association between growth and wood density among families, several fast-growing clones maintained above-average density. This implies that, by adopting multiclonal forestry, one can simultaneously improve growth and wood density.

Maximizing genetic gain at an acceptable diversity level is an ideal outcome of selection and deployment. Based on this criterion, this study investigated the efficiency of unequal clonal deployment strategies for clonal forestry and compared them with truncation selection and equal deployment (truncation deployment). Two unequal deployment strategies were considered: (1) deploying the clones in linear relationship to their genetic values (linear deployment) and (2) optimizing genetic gain at a given diversity level using an algorithm (optimal deployment). All strategies were applied to candidate clone sets constructed from two clonal tests of spruces with one having a complex relationship of clone, half-sib, and full-sib and the other having a simpler relationship of clone and half-sib. At a constant diversity level, substantially more expected gains were obtained by the unequal deployment strategies than by truncation deployment. Optimal deployment was at least equal to linear deployment. Optimal deployment’s superiority was more evident when the candidate clones in the set were more closely related, having less available diversity for selection, and/or when higher diversity levels were demanded, but diminished when the candidate clones were unrelated or equally related. We recommend using optimal deployment for clonal forestry, although in some cases, linear deployment might be a near-optimal alternative. As current clonal tests are based on advanced breeding cycles, candidate clones for selection are inevitably related to some degree, so optimal deployment is likely to become preferred.

When selecting a clonal mixture for clonal forestry, a common practice is to specify a minimum acceptable genetic diversity for the mixture, and under that constraint, to maximize its genetic gain. Three selection methods—combined index selection (CIS), clonal mean selection (GMS), and family plus within-family clonal selection (FWFGS)—together with various restrictions on family contributions (family restrictions) were used to estimate gain at a given effective population size (N e ) of clonal mixtures selected from a clonally replicated genetic test of white spruce. The designated target trait for improvement was individual tree volume at age 14 after planting. Regardless of selection method, all genetic gains were >30% at given N e from five to 20, suggesting that implementing clonal forestry was a very effective deployment strategy. Genetic gain at a specified N e could be enhanced by using appropriate selection methods: CIS was the most effective, followed by GMS and FWFGS. With an N e of 15, CIS resulted in an average gain of 43.2%, which corresponded to an increase by 8.1% and 17.3% relative to GMS and FWFGS, respectively. Imposing family restriction increased gain at an N e . Compared with the respective unrestricted selection, family restrictions increase genetic gain by 3.3%, 7.7%, and 54% for CIS, GMS, and FWFGS, respectively. The optimal family restriction level for each selection method varied with the specified N e .

Genetic variances and selection efficiencies for growth traits of white spruce (Picea glauca (Moench) Voss) were estimated from clonally replicated full-sib progeny tests established both in nursery and field environments in New Brunswick, Canada. The available data included heights at 4, 5, and 6 years in the nursery test; height at 9 years, height, DBH, and volume at 14 years in the field test. Estimated variance components were interpreted according to an additive-dominance-epistasis model. For heights in the nursery test, while both non-additive and additive variances were important sources of genetic variation, the former decreased but the latter increased with age; among the non-additive genetic variance, the epistatic variance was much more important than the dominance variance. Different from the nursery traits, for traits in the field test, additive variance accounted for an average of 81% of the total genetic variance, whereas dominance variance explained most of the remaining genetic variance. Genetic parameters and selection efficiencies for three vegetative deployment strategies: deploying half-sib families (VD_FAMHS), full-sib families (VD_FAMFS), and multi-varietal forestry (MVF), were compared. Heritability estimates were moderate for VD_FAMHS and VD_FAMFS (0.61–0.72), high for MVF (>0.82) for the nursery heights, and high (>0.79) for the field traits for all strategies. Genetic correlations of volume at age 14 in the field test, the target trait for improvement, were strong (>0.85) with other field traits. Genetic correlations of VOL14 with the nursery heights were also strong (>0.71) at the half-sib and full-sib family levels, but were only moderate (>0.59) for MVF. Overall, practicing MVF is the most effective deployment strategy, yielding the highest genetic gains, followed by VD_FAMFS and VD_FAMHS, regardless of traits and selection methods. Furthermore, early selections for HT9 or for HT4–HT6 were very encouraging, resulting in higher gain in volume at age 14 on a per year basis.

Major advances have been achieved in somatic embryogenesis (SE) of loblolly pine, making it a promising method for the implementation of clonal forestry. However, the frequency of initiation of SE cultures, which is highly variable among loblolly pine families, needs improvement to further advance the implementation of this technology in conjunction with tree breeding. Genetic control of SE initiation was investigated using a diallel mating design with six parent trees. The results showed that SE initiation is under the control of strong genetic additive effects, as 42% of the total variance was explained by the variation due to general combining ability effects. The variation due to maternal effects explained a moderate proportion of the total variance, whereas other components of variance had small but significant effects. The conclusions regarding the strong genetic control of SE initiation were drawn from two independent experiments in which consistent results were obtained with seed from the same controlled pollinations but using entirely different procedures. Practical implications for breeding and clonal propagation were tested in independent experiments with targeted matings. Our results indicated that large improvement in SE culture initiation could be achieved in a predictable manner by selecting the most favorable female parent, or in some cases, a favorable male parent.