Can high-quality sawn timber be produced from young Paulownia trees plantation? Evaluation of stem-stress-wave velocity and timber qualities

Fast-growing trees have an important role in sustainable forestry. Paulownia species are considered fast-growing trees, although their wood is characterised by low wood density. The object of this study is to produce high-quality sawn timber from young Paulownia species. Ten 5-year-old trees were harvested (stem diameter at 1.3 m above the ground = 21.1 ± 3.3 cm, tree height = 15.0 ± 1.3 m) for producing timber (38 × 89 × 1820 mm, without pith cavity). Before harvesting, the stress-wave velocity of stems was measured to non-destructively estimate the mechanical properties of the timber. A static bending test was conducted on the timber to determine the modulus of elasticity (MOE) and the modulus of rupture (MOR). The mean values of crook, bow and twist were 0.30%, 0.08% and 1.41 degree, respectively. The mechanical properties at 15% moisture content were 4.17 GPa in MOE ₁₅ and 19.2 MPa in MOR ₁₅ . The MOE ₁₅ and MOR ₁₅ apparently increased compared to timber with a pith cavity in the previous study. The 5% lower tolerance limit with a 75% confidence level (TL _75%,95% ) of MOR ₁₅ was 11.6 MPa. At the standing tree-level ( n  = 10), stress-wave velocity of stems was correlated with air-dry density of timber ( r  = 0.644, p  = 0.045) and MOE ₁₅ ( r  = 0.768, p  = 0.009). Based on these results, high-quality sawn timber can be produced from young Paulownia trees by selecting suitable trees at the standing-tree level using stem stress-wave velocity and eliminating the pith cavity during sawing.