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& Key message An increase in the stiffness of lumber with a decrease in initial tree spacing was confirmed for Pinus patula Schiede ex Schltdl. & Cham. The underlying properties of microfibril angle, wood density and knot frequency explained 71% of the variation in lumber stiffness. Tree spacing also influenced wood properties independent of radial growth rate. & Context Rapid growth rates and reduced harvesting ages of South African-grown pine plantations have caused a reduction in the stiffness of structural lumber, which accounts for about 75% of all sawn wood. Microfibril angle and wood density are known to influence wood stiffness, which may be manipulated by the growing space of trees. & Aims The objective of this study was to evaluate the effect of slower growth rates, caused by narrow tree spacing, on the suitability of Pinus patula Schiede ex Schltdl. & Cham. wood for structural lumber. & Methods An 18-and a 19-year-old spacing experiment with four levels of initial tree spacing (1.83 m × 1.83 m, 2.35 m × 2.35 m, 3.02 m × 3.02 m and 4.98 m × 4.98 m) were sampled. Linear and non-linear mixed-effects models were developed to examine the effect of tree spacing on the quality of wood and lumber as defined by the modulus of elasticity, modulus of rupture and knot frequency of 208 boards and the ring-level microfibril angle and wood density of 86 radial strips. & Results Wood and lumber quality improved with decreasing spacing, and only the narrowest spacing had lumber that conformed to the requirements of the lowest South African structural grade. Microfibril angle, wood density and knot frequency explained 71% of the variation of lumber stiffness. After accounting for ring width differences, there remained a significant effect of initial spacing on the parameters of models predicting microfibril angle and wood density. & Conclusion Wide initial spacing is discouraged if higher strength grades are desired for Pinus patula lumber.

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