Corewood in South African pine: necessity and opportunities for improvement

Abstract

South Africa has made excellent progress in breeding plantation trees fulfilling the primary demands of improved volume growth, stem and branching characteristics, tolerance to pests and diseases, and general adaptability. This has had marked positive impacts on wood quality, as straight stems normally contain no or very little reaction wood, while problems associated with pith eccentricity and wandering pith are kept to a minimum. The breeding of trees with small, well-distributed branches, forming large angles with the vertical axis of the tree stem, reduced performance problems associated with knots and knot-associated features. However, tree breeding and intensive silviculture are causing the average corewood diameter to increase, creating a wood resource that is increasing in variability, especially when combined with earlier harvesting. Reducing the size and improving the properties of the corewood zone have therefore become of utmost importance to ensure that the South African pine timber resource continues to meet quality demands related to strength, stiffness and stability. Breeding programmes to date have created sound bases for further wood quality improvement of the various pine species, especially of the corewood zone. Opportunities for success are excellent as large variation in corewood properties still exists within the current breeding populations, combined with operationally important wood properties, such as wood density, microfibril angle, spiral grain and transitional age, all being under moderate to strong genetic control. Corewood properties seem to be inherited largely independently of tree and growth characteristics, making it possible to combine improvement in these characteristics with desirable wood properties. This paper discusses results from several young field trials, which have shown marked family, species and site differences in wood stiffness. This suggests that there is good opportunity to increase the dynamic modulus of elasticity of corewood by early selection and breeding, using highly reliable non-destructive acoustic technology. The results also proved that vegetative propagation through cuttings has no detrimental effect on corewood stiffness. The latter may even increase with increasing physiological age of the cutting.