Abstract
The two main objectives of this study were: (1) to determine how early is it possible to undertake selection to improve the stiffness of corewood; (2) to determine if the selection based on corewood stiffness could also improve outerwood stiffness, and vice versa. Breastheight data from two progeny trials of Pinus radiata D. Don were used. In the first trial (age 30 years), data on Silviscan predicted stiffness (MoE) was obtained for each growth ring on each core sample from 50 open-pollinated families. In the second trial (age 14 years), data on static-bending MoE was obtained using clearwood sticks (300 × 20 × 20 mm) cut from each tree from 18 control-pollinated families. MoE varied from 3.5 GPa in rings 1-5 to about 17 GPa in rings 21-25. Coefficients of variation of corewood and outerwood MoE were about 20-30% and 15-20% respectively. Estimates of narrowsense heritability for MoE were generally higher (0.50-0.70) in the corewood compared with the outerwood (0.15-0.30). Early selection for MoE could yield substantial gain in corewood MoE but only small gains, if any, in outerwood MoE (especially for rings 21-30). Estimated genetic correlations between density and stiffness appeared moderate in the corewood zone, but high in the outerwood zone. Selection based on density (using 5-mm cores) and acoustic stiffness (using standing tree tools), assessed at age 6-7 years, appeared to be a good option to improve both corewood and outerwood stiffness.