Selected mechanical properties of fast-growing poplar hybrid clones.

Twenty-eight nine-year-old trees from ten clones of the hybrid Populus x euramericana from one site in Quebec were sampled to study the variation of selected mechanical properties within trees, within clones, and among clones. Four small and clear ASTM wood samples were taken from each tree at breast height, as well as a 10-mm diameter increment core. The parallel-to-grain compliance coefficient and ultimate crushing strength were evaluated on the ASTM samples for air-dry and green conditions. The dynamic compliance coefficient was measured on increment cores using an ultrasonic wave propagation method. Differences in all mechanical properties among clones were highly significant, while variation among trees was generally not significant. The dynamic compliance coefficient tended to be lowest near the pith, increased to a maximum at one third of the tree diameter, and then decreased outward towards the bark. There was also a highly significant correlation between mechanical properties of ASTM samples and dynamic compliance coefficients of increment cores. Mechanical properties were only moderately correlated to wood density. Finally, there was a significant but weak correlation showing that wood density and mechanical properties decreased with increasing growth rate.