The Blackwood Group System: its relevance for sustainable forest management in the southern Cape

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Abstract
The invasion status of Australian blackwood, Acacia melanoxylon, is assessed against its value as a commercial tree in the southern Cape forests. The species was introduced from Australian rain forests and planted extensively in the southern Cape forests since 1909. It has since become well established in the forests, and invaded open areas outside the forest. However, it is also a useful and valuable timber tree which contributes more than 60% of both the timber volume and revenue obtained from the forest. A Blackwood Group System was established in the forest since 1968 in which the tree was planted in pure stands in the forest gaps caused by the felling of large trees. This study comprised various investigations to assess the invasion status of the species in the forests and to determine under what conditions it poses a threat to the forest. The results show that A. melanoxylon occurs in all the main forests throughout the southern Cape. It has the characteristics of an aggressive invader plant, but it does not aggressively invade closed, evergreen forest. In dosed forest its population diameter distribution is bell-shaped, a curve typical of pioneering populations in maturing vegetation. Outside the forest it is a serious invader of open and disturbed sites, as is shown by its negatively exponential diameter distribution along rivers and forest margins, i.e. a curve typical of expanding populations. In the forest it requires large gaps to grow fast. In the larger logging gaps (but generally small gaps) mean diameter growth was 0,9 cm/a over 22 years. In the larger gaps (l 200 to 1 500 m2) of the Blackwood Group System, diameter growth averaged 1,6 to 1,9 cm and the largest tree of 58 cm averaged 2,6 cm/a diameter growth over 22 years. Volume growth was 10,5 to 13,4 m3/ha per annum. The surrounding forest had a suppressive effect on the diameter and height growth of A. melanoxylon trees on the gap margin. A wide range of indigenous forest canopy tree species, including the commercially valuable Ocotea bullata, Podocarpus falcatus and P. latifolius, regenerated and became established in the understorey of A. melanoxylon in the Blackwood Groups. Crown volume is significantly related to DBH and total tree height. The large emergent crown and very shallow root system make the large trees very susceptible to strong gusty winds. It is concluded that the ecological characteristics and requirements of the shade-intolerant blackwood should be considered in relation to the ecological processes of disturbance and recovery of the forest to reconcile its production from the forest as timber tree with control of its invasion potential. Recommendations are made to consider the species as a useful, fast-growing, shade-intolerant pioneer tree which requires large gaps for optimum timber production, but whose spread can be controlled by keeping canopy gaps to a minimum.