Effect of growth rate on wood specific gravity and selected mechanical properties in individual species from distinct wood categories

Abstract

This study examined the relationships of wood specific gravity and selected mechanical properties (MOR, MOE and Cmax) with growth rate in 16 timber species from four distinct wood categories: 1) first softwood category (FSC); 2) second softwood category (SSC); 3) diffuse-porous wood category (DPC); and 4) ring-porous wood category (RPC). And genetic, silvicultural and environmental influence on the relationships was briefly discussed. Statistical results show that the relationships of specific gravity and the mechanical properties with growth rate vary remarkably with both the wood property and the wood category. In general, the mechanical properties in the FSC species decrease remarkably with increasing growth rate, while they appear to be less influenced in the SSC species. Compared with the softwoods studied, the physico-mechanical properties in the hardwoods studied are remarkably less influenced. In the DPC species, growth rate generally has very a little influence on both specific gravity and the mechanical properties. In the RPC species, the physico-mechanical properties appear not to decrease with increasing growth rate, and in some species they even tend to increase. Among the three mechanical properties studied, MOE is remarkably less influenced by growth rate than MOR and Cmax. Compared with specific gravity, however, the mechanical properties are generally more influenced by growth rate. Therefore, the impact of growth rate on wood mechanical properties in a species can not be estimated exactly through the relationship of wood specific gravity with growth rate. Path analysis reveals that growth rate has a large effect on the mechanical properties which can be accounted for by the affected specific gravity. In addition to this indirect effect through specific gravity, growth rate still has an additional effect on the mechanical properties which can not be explained by specific gravity. In the SFC species, such effect is significant, and this, to a lesser extent, applies to the DPC species. However, this effect is not remarkable in the SSC species and may be negligible in the RPC species.