Blackwood (Acacia melanoxylon R. Br.) plantation silviculture: a review

Abstract

Summary In Australia, blackwood is logged from native forest stands. The limited size of this resource, and its capacity to meet the demand for blackwood timber in the longer term, has resulted recently in the establishment of plantations in Australia. Plantation silviculture of blackwood has a longer history in New Zealand, South Africa and Brazil. A major problem in plantation establishment is that blackwood has poor apical dominance, producing large numbers of branches and trees of poor stem form when grown in full light. Blackwood has two types of foliage, bipinnate compound leaves and vertically-oriented phyllodes. A phase change from leaves to phyllodes occurs, though this is reversible. The type foliage present can be a useful indicator of the prevailing light conditions. Apical dominance is associated with conditions of low light shade also reduces the extent of branch development, encouraging good form. Blackwood is susceptible to frost and defoliation by pests though both are a function of the type of foliage present. Leaves are more efficient than phyllodes at producing photosynthate per unit carbon invested in foliage, though both types of foliage appear reasonably well adapted to a range of light environments. Application of phosphorus fertiliser at planting is generally associated with improved early growth and this may be allied to increased numbers of root nodules. Addition of nitrogen fertiliser appears unnecessary in the absence of N deficiency. Whether grown in pure or mixed stands, form pruning is necessary to produce good stem form, and lift pruning of live branches to produce clearwood. Various approaches to form pruning have been used but all are designed to prevent the development of large branches. Crown shape, which is variable in blackwood, can determine whether lift pruning reduces subsequent growth. Establishment at high stockings and thinning to a final crop of between 100 and 250 stems ha−1 is used commercially. Nurse crops have been used, particularly in Tasmania, to induce good form and to minimise the requirement for form pruning. This practice has met with mixed success as the use of eucalypts or Pinus radiata has led to overtopping and suppression of the blackwood. In New Zealand, the use of nurse crops has led to little improvement in stem form.