Age trends in the genetic parameters of wood density and the relationship with growth rates in hybrid larch (Larix gmelinii var. japonica ¥ L. kaempferi) F 1 Abstract Age trends in variance components and heritability of overall wood density, earlywood and latewood density, and latewood proportion were investigated in 29-year-old trees of 19 full-sib families of hybrid larch (Larix gmelinii var. japonica × Larix kaempferi) F 1 . The age-age correlation and optimum selection age for these traits were also estimated and genetic and phenotypic correlations between wood density and radial growth rate were calculated for each growth ring. Intraring wood density data were obtained using X-ray densitometry. The coefficient of additive genetic variance was stable over all ages, whereas the coefficient of environmental variances gradually decreased with increasing age, resulting in increases in heritability estimates with age for overall density. The latewood proportion had the highest heritability estimates at all ages, ranging from 0.44 to 0.66. Overall density and its various components at 28 years of age showed strong genetic correlations with their respective traits at all younger ages. Optimum selection ages for the wood density traits ranged from 8 to 14 years, at which point maximum gain efficiencies per year were obtained. There were negative correlations between wood density and radial growth rate at early ages, although these relationships tended to be weaker with increasing age. These results suggest that selection at a young age is effective for wood density, but particular care must be taken in selecting trees with an improved radial growth rate because rapid growth will result in a low-density wood product, especially in the early growth period.

Hybrids generated by crossing Kuril larch (Larix gmelinii var. japonica) and Japanese larch (L. kaempferi) are expected to have high carbon accumulation ability because of fast growth and high wood density in Hokkaido, Japan. We estimated the amount of carbon accumulation of the hybrid larch in three progeny test plantations consisting of 21 full-sib families and compared the results to the carbon accumulation of open-pollinated progenies of Japanese larch plus-trees (improved Japanese larch). Geneenvironment interactions were not observed for tree height and diameter at breast height but were seen for wood density (area-weighted density, AWD). The amount of carbon accumulated per unit area (C stand ) positively correlated with the stand volume. The AWD did not correlate with the C stand ; therefore, families with high wood density can be selected independent of the C stand . The C stand of the best full-sib family, female parent half-sib family, and male parent half-sib family at three sites were 106.1, 84.6, and 93.2 Cton•ha −1 , respectively. All these values exceed the mean for the improved Japanese larch, which has a C stand value of 82.5 Cton•ha −1 .

We identified diagnostic chloroplast DNA and mitochondrial DNA markers that can (a) discriminate between Larix gmelinii var. japonica and L. kaempferi, and (b) determine the maternal and paternal species of hybrids between them by exploiting the difference in inheritance mode between the two genomes. We also investigated the hybridization rates at a site with two types of interspecific seed orchard-a new type with rows of a single maternal clone of L. gmelinii var japonica among rows of L. kaemferi and a "traditional" type with multiple, intimately mixed clones-in 2 years using chloroplast diagnostic DNA markers. The average hybridization rates in the single maternal clone (SMC) interspecific seed orchards [84.2% (±9.4%) in 2004 and 94.1% (±3.9%) in 2005) were higher than that in the traditional interspecific seed orchards [15.9% (±13.4%) in 2004 and 30.0% (±25.5%) in 2005] because of the self-incompatibility of the L. gmelinii var. japonica clone. We detected significant differences in hybridization rates between the orchard types in both investigated years (P<0.001, analysis of variance, ANOVA). This finding suggests that SMC interspecific seed orchards can reliably provide seeds with high proportions of hybrids. In the traditional interspecific seed orchard, there were significant differences in the proportions of hybrids among L. gmelinii var. japonica seeds between the two years (P<0.005, ANOVA), which may have been partly due to differences in the relative amounts of pollen cones produced by L. gmelinii var. japonica and L. kaempferi.

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