Abstract
Saplings of blackwood ( Acacia melanoxylon) responded positively to release from competing young Eucalyptus obliqua in native forests of north-west Tasmania. In the treated area, 6 years after establishment of the regrowth forest on a cleared and burned seedbed, the young emergent eucalypts were culled by stem injection of herbicide. The dense sub-canopy (principally Pomaderris apetala) was also removed in two gap treatments (3.6 and 7.2 m diameter); each treatment applied to 27 single tree plots. During the 6 years following silvicultural treatment, periodic annual increment (PAI) of blackwood stem diameter increased from 0.8 cm per year in the untreated area to 1.4 cm per year where eucalypts (only) had been removed. The additional removal of the sub-canopy increased blackwood PAI (diameter) to 1.6 cm per year in the small gaps and 2.1 cm per year in the larger gaps. These are statistically significant increases of 75, 100 and 160%, respectively. However, the larger sub-canopy gaps produced heavier, broader blackwood crowns, increased branch size and retention and reduced the length of branch-free bole. Maximum blackwood diameter and volume gains were therefore achieved at the expense of tree form and future log quality. In contrast, the removal of competing eucalypts (only) produced a smaller stem diameter response, but the remaining dense sub-canopy maintained excellent stem form and clear bole.
Published Version
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