Perspectives for genetic improvement in heartwood size and extractive content in relation to natural durability and aesthetics in interspecific hybrid larch (Larix x eurolepis)

Larch heartwood is the only valuable part of timber when natural durability and aesthetics are requested for end-use products. Therefore, improvement in both quantitative and qualitative properties of heartwood will benefit industry. Genetic variation of heartwood size, extractive content in relation to decay resistance and of colour was studied in 40 full-sib progenies of hybrid larch (Larix x eurolepis) at age 21. Expected genetic gains from various selection scenarios were then investigated. Among all traits studied, extractive contents (total phenols, taxifolin and DHK) showed the highest variability both at the individual (e.g. for taxifolin 3-42 mg g(-1) DW) and mean family (e.g. for taxifolin 10-21 mg g(-1) DW) levels with heritabilities over 0.66. In contrast, CIELAB colour parameters together with ring density were the least variable, but their heritabilities were close to those of the extractives. Heartwood diameter showed a FS-family mean heritability close to that of radial growth (0.68). European larch (female parent) seemed to play a major role in the genetic control of extractives content, colour and density in the hybrid, while Japanese larch (male) contributed most to the variability for radial growth and heartwood size. Direct selection for heartwood extractive content and heartwood size looked promising with expected gains over 24 and 14 %, respectively (i = 2.043), but it was much less efficient for colour parameters (6 % at most) whatever the selection methods tested. Our study showed that it is still possible to jointly improve radial growth, heartwood size and its extractive content with some success (gains around 10 %) without costly chemical analysis.