The effect of planting density on Pinus patula stem form, wood properties and lumber strength and stiffness

[摘要] ENGLISH ABSTRACT: Faster growth and reduced harvesting age are causing a reduction in the stiffness of structural lumber from South African-grown pine plantations. A number of studies have shown the positive effects of high planting densities as a tool to improve the mean modulus of elasticity (MOE) of structural lumber. The objective of this study was to investigate the effect of planting density on stem form, wood properties and the strength and stiffness of structural lumber of young Pinus Patula – the most important and extensively planted commercial softwood in South Africa.In the first part of this study, four different planting density treatments (403, 1097, 1808 and 2981 stems/ha) from an 18-year old P. patula spacing trial located in Mpumalanga, South Africa were sampled non-destructively. Stem slenderness, stem curvature, and the dynamic modulus of elasticity (MOEfak) were measured on 171 standing trees. Increment cores were removed from 40 trees for measurement of density, microfibril angle (MFA) and ring width using the Silviscan 3 technology. Planting density had a significant effect on stem curve with the lowest planting density having the highest mean stem curve. Planting density also had a highly significant effect on stem slenderness. The MOEfak increased greatly with increases in planting density. MFA was significantly influenced by both planting density and year ring number and the interaction between them. The mean MFA at similar ring numbers decreased significantly from the 403 stems/ha treatment toward the higher planting densities (1808 and 2981 stems/ha). Planting density had a limited effect on wood density. MFA seems to be the mechanism through which the tree compensates for the instability caused by a high slenderness ratio. Density, on the other hand, did not correlate with slenderness at all and was probably mostly influenced by environmental and growth factors.In the second part of this study, a total of 37 trees from two commercial compartments, planted at different densities, were processed into 71 logs, cant-sawed into lumber, and tested for static MOE, modulus of rupture (MOR), density, and warp. The first compartment was 18 years old, planted at 1334 stems/ha and thinned to 827 stems/ha at age 11. The second compartment was 17 years old, planted at 1667 stems/ha and was unthinned. Lumber from the 1667 stems/ha compartment had a mean MOE of 8967 MPa compared to a mean MOE of 7134 MPa for the 1334/827 stems/ha compartment. Based on this evidence and results from previous studies, it seems as if planting density has a large effect on the stiffness of young P. patula lumber and that planting density may be used as a practical management intervention to increase the stiffness of lumber.

[发布机构] Stellenbosch University