Within- and between-tree variations in the wood quality of radiata pine
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This study was conducted to determine the within- and between-tree variations in the physical and mechanical properties of Pinus radiata (radiata pine). Forty eight trees from a 25-year-old plantation on the Canterbury plains near Dunsandel in the South Island of New Zealand were felled and cross-cut to give three 3.6 meter logs. Each log was identified by tree number and position up the height of the tree (butt, middle and top log). At the sawmill the logs were sawn, first by removing 40 mm thick slices known as flitches from opposite sides of the trunk until a 100 mm thick plank known as a cant was left at the centre. The flitches were re-cut at the breast bench circular saw to yield timbers of nominal dimensions 100x40 mm. In re-cutting the 100 mm wide cant gave 3-5 boards depending on the diameter of the log. The position of every board was recorded relative to the pith and numbered. A total of 915 boards from the 48 trees (144 logs) were obtained. The boards were filleted (i.e. stacked with uniform and sufficient spacing between each layer both in the vertical and horizontal directions so as to ease air circulation) and air-dried to approximately 12% moisture content. After drying the boards were dressed to 90x35 mm and grouped into .one of the four Australian structural grades (F4, F5, F8 and F11) as each board passed through a stress grading machine. The modulus of elasticity of the boards was measured both in flatwise bending and axial tension. The strength of the boards was determined by destructive testing in tension and compression parallel to the grain. After failure in tension short clear planks (i.e. planks with no knots and any other natural defects) were cut from each board. From these short planks small clear specimens were prepared for the determination of stiffness, bending strength and compression strength parallel to the grain. The investigation of density, stiffness and strength in relation to the vertical and radial positions within a tree revealed that there is a significant variation in all properties with changes in radial positions across the diameter, and a significant variation in strength properties, but not stiffness with change in vertical position up the height of the tree. Regarding between-tree variation, all properties changed significantly. With reference to the production of structural framing timber, stiffness and density were compared as criteria for sorting trees and identifying superior material within logs. This analysis revealed that stiffness is a better criterion for selecting superior trees within the natural population of a forest stand, to improve the value of mill production and to achieve a better outturn in higher value grades (F5 and above). A regression analysis between the properties of the in-grade timber and clearwood showed that there is a very strong relationship between the modulus of elasticity of clearwood and that of the in-grade timber. As expected there was a general decrease in strength and stiffness of the graded timber as the grade value decreased from F11 to F4. Strength and stiffness values in tension, bending and compression have been compared with the current New Zealand, Australian and European code design values, generally giving good recovery of higher value grades (F5 and above), especially for strength.