The biology and silviculture of pruning planted eucalypts for clear wood production—a review

Interest in producing higher value products from the 13,000,000 ha of planted Eucalyptus species has developed due to the declining availability of hardwood products from native forests and an oversupply of hardwood fibre. We review the biological and silvicultural basis for producing clear wood from planted eucalypts. The canopy dynamics of eucalypts influences both the quantity and quality of clear wood produced. The size of the canopy determines the growth and hence volume of wood produced, while canopy dynamics affect wood quality via the process of branch senescence, ejection and occlusion. Despite the “self-pruning” nature of many eucalypts species, the amount of clear wood able to be produced by untended stands is limited. Sawing studies on planted eucalypts indicate that knots are the major grade limiting factor. Pruning, by containing branch related defects to a central knotty core, increases the proportion of clear wood. To maximise the benefits from pruning, the evidence suggests that pruning should be undertaken while branches are small and alive. Pruning of trees while young has the potential to reduce tree growth, particularly if pruning occurs prior to canopy closure. Coinciding pruning with canopy closure allows a greater proportion of the green crown to be removed without impacting on tree growth. Pruning may increase the risk of wood decay. Pruning associated decay has been reported in a number of species. A major unresolved question is whether decay can spread across ‘barrier zones’ into new clear wood, after gaining access to the stem via pruned branch stubs. Minimising decay entry by ensuring branches are small when pruned is an important control measure. There would appear to be fundamental problems in converting pulpwood plantations to higher value sawlog regimes. To increase the volume of clear wood, silvicultural interventions need to occur early to constrain the knotty core. Pulpwood stands may be to old to benefit significantly from silvicultural interventions. Furthermore, wood properties of pulpwood may not produce quality solid wood products. In untended stands, very low recoveries of high value products are observed. Pruning in conjunction with thinning can substantially increase the volume of clear wood produced.