Spiral grain is the angular arrangement of fibres in a tangential plane with reference to the pith or vertical tree axis. Spiral grain angles exceeding 5° can cause wood to twist, which may result in a considerable amount of waste and degrade. We assessed spiral grain at breast height in two related progeny tests of radiata pine (Pinus radiata D. Don) aged 8 and 9 years established at two different sites in Australia. Radial trends for grain angle at the two sites were similar. Mean spiral grain (MSG) across the two trials was 4.3° with a standard deviation of 1.5° and a range of 0.8–10°. Estimates of individual tree heritabilities on a single-site basis for individual rings and MSG suggested that spiral grain is lowly to highly inherited (h2 = 0.11 ± 0.08 to 0.66 ± 0.21 for individual rings and 0.44 ± 0.12 for MSG). Additive genotypic correlations between individual rings grain angle and MSG were generally high, above 0.71, suggesting a favourable expected correlated response of mean grain angle in the juvenile wood to selection for grain angle of individual rings. Selection to reduce spiral grain on any of rings 2–4 (at a selection intensity of 1.755, i.e., selecting the best 10% of trees) would result in a predicted correlated genetic gain in MSG of 1.0°. Our results suggest that selection could be performed in any of the individual rings 2, 3, or 4 (equivalent to ages 4–6) and still achieve at least 75% of the genetic gain possible from selection on the mean of all rings 1–5 (MSG). This suggests that there is an optimum stage (rings 2–4) in which selection for this trait should take place. Our results suggest that a reduction in spiral grain angle in the juvenile core is one strategy to reduce the amount of lower grade timber owing to twist.
On the causes of spiral grain in corewood of Radiata pine