The influence of growth rate on the wood and stem properties of silver birch (Betula pendula Roth.)
The consequences of using low stocking density as a management tool to increase individual tree growth rate of silver birch, and thereby shorten rotation length, were investigated. Low stocking density was found to result in greater diameter and basal area increment but may also result in trees with larger branches, a shorter length of clear bole, and possibly a reduced total height. The rapid growth had little effect on wood properties. Grain angle was unaffected, longitudinal shrinkage increased slightly, but conversely transverse and volumetric shrinkage declined. Density was not affected directly, but the increased proportion of juvenile wood in rapidly grown trees was estimated to result in a 5% reduction in density compared with conventionally managed trees. While fast growth resulted in only a slight decline in the strength and stiffness of small clear wood specimens, a reduction of about 30% in these properties was found for large beams. This reduction in strength was correlated with high knot and grain angle values for the timber. Modulus of rupture was found to be correlated with modulus of elasticity for both small clear specimens and for beams, suggesting the possibility of machine stress grading of birch. Hardness was generally unaffected by growth rate. All the timber tested was suitable for typical domestic flooring. The use of low stocking density to reduce rotation lengths therefore appears to be technically feasible, allowing the production of 30 cm trees in as little as 40 years, although an economic appraisal of the practice was not performed. If, however, knot free timber is required then pruning will be necessary to remove the large branches that otherwise would persist on widely spaced trees. Following the removal of branches, rapid occlusion of branch scars can be expected and the production of a large volume of clear timber.