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hybrid poplar, structural timber, timber preservation, mechanical properties, building code, end-splitting, timber grading, pruning wounds, knots, stiffness, strength
Background: While poplar (Populus spp.) is an important source of raw materials for the wood processing sector in many regions of the world, in New Zealand it has a reputation for producing poor grade recoveries of sawn timber that is unsuitable for structural applications. However, the ‘Kawa’ poplar clone (Populus deltoides Marshall x P. yunnanensis Dode), which has relatively high wood density, could yield structural timber. This, along with evidence demonstrating building code compliance, would improve utilisation options for this species in New Zealand.
Methods: Sawn timber conversion and grade recovery were quantified for a 28-year-old pruned stand of ‘Kawa’ poplar grown in Northland, New Zealand. A sample of 90 mm x 45 mm structural boards were tested to determine their mechanical properties and the resulting strength class. Boron preservative retention and penetration were measured to determine whether timber could be treated to the level required under New Zealand’s building standards. Density, modulus of elasticity and modulus of rupture were assessed on small defect-free specimens taken from different radial and vertical positions within trees to determine intra-stem and inter-stem variation in these properties.
Results: The overall conversion of logs to sawn timber was 53%, with approximately 94% of this recovery consisting of graded timber. The most common sources of downgrade were knots, pruning wounds, and end-splits. Approximately 70% of the sawn boards were graded as clears, with smaller recoveries of cladding and structural boards. The average length of clear section was approximately 2.5 m. Mechanical testing of structural boards demonstrated that they have characteristic values sufficient to meet the requirements for the SG10 strength class. Preservative treatment achieved the H1.2 specification. Density, modulus of elasticity and modulus of rupture were all higher in specimens cut from the outside of the log compared with those taken from near the pith at all heights up the stem.
Conclusions: Mechanical properties and boron treatment results indicate suitability for structural applications in accordance with New Zealand’s building code. ‘Kawa’ poplar also produced high grade recoveries suggesting potential for commercial sawn timber production, especially for structural appearance products.