The effect of planting density on the wood quality of South African-grownEucalyptus grandis

Abstract

This paper presents the results of a wood property and sawn board quality study performed on disc samples and sawlogs taken from a 23-year-old Eucalyptus grandis Nelder 1a spacing trial at J.D.M. Keet plantation near Tzaneen. Ten trees from each of four markedly different planting densities were chosen to provide contrasting material to study the effects of growth rate on the quality of the wood produced. Spacing treatment diameter means ranged from 18.9 to 57.9 cm. An inverse relationship was found between planting density and wood density. Typical pith-to-bark density profiles for trees at this age occurred in the trees with rapid growth, i.e. steep gradients from the pith towards the bark during the early stages of growth, followed by a gradual transition into more constant mature wood density values. Trees of which the growth had been curbed by higher planting densities, on the other hand, showed little sign of levelling off. As a result, the faster growing trees contained significantly larger proportions of relatively homogeneous wood compared to the slower grown trees, giving them an obvious quality advantage over slower grown trees. Accelerated growth thus seems to yield trees that can be utilized more effectively and converted into products with more reliable performance characteristics. Fast growth seemed to promote interlocking grain, but the correlation with interlocking grain severity was too poor to be of any practical significance. Mean splitting showed a downward trend with decreasing planting density, but statistically the differences were non-significant. A statistically significant correlation was found between the amount of splitting in logs and the proportional length of sawn boards affected by splits immediately after wet-mill processing. At the end of the sawmilling process, however, such correlation did not exist any longer, suggesting that the boards responded differently to drying stresses and impacts during handling and transport with regard to further splitting. No significant effect of planting density on sawn volume losses due to splitting or collapse could be detected.