Interest in blackwood logs at a public auction.

Blackwood, used in the high quality furniture industry.

The alien invasive tree, Australian blackwood (Acacia melanoxylon), was actively introduced to southern Cape's Afromontane forests since the early 1900's and is well established today in and around the forests of the Garden Route National Park (GRNP). Even though it is an aggressive invader in open or disturbed forest, it does not aggressively invade closed, evergreen forests¹. In the Southern Cape, blackwood is used primarily in the high-quality furniture industry and makes up about 50% of the timber volume and revenue from timber sales from indigenous forests, managed by South African National Parks. Timber harvesting in the GRNP is currently under review, but blackwood as a resource would still be available to the furniture industry.

by Lizette Moolman, SANParks

Blackwood Management Plan
The invader qualities of blackwood, together with its economic value, form the basis of the Blackwood Management Plan which was developed in 1997. The plan aims to provide an ecological benefit by controlling blackwood as an alien invader, and an economic benefit by optimally using it as an available timber resource. Consequently, trees in the forest interior are not actively controlled but are allowed to grow to maturity to yield good quality timber, and are actively controlled in areas prone to invasion (forest margins, riverine areas, etc.). These management actions were justified on the basis of predictions extrapolated from research data, that the population in the forest interior would eventually stabilise. The actions were allowed on condition that the predictions are tested during a 10-year monitoring programme. The blackwood management system therefore involves a combination of zonation of the forest into different blackwood management zones and the monitoring programme.

Management zones
The management zones include two different zones with different management prescriptions and objectives for blackwood eradication and harvesting². In Zone A, active and intense controlling takes place with the aim to eradicate blackwood and prevent re-infestation. Utilisable trees are harvested once-off. Non-utilisable trees are culled and saplings and seedlings controlled with follow-up operations. This zone includes nature reserves and long-term forest dynamics research or monitoring sites. In Zone B, no control of blackwood in the forest interior is maintained and opportunistic harvesting of blackwood takes place.

Zone B is sub-divided into special sub-zones which include forest margins, riverine areas, areas of aesthetic importance and buffer zones, each zone with specific and different blackwood eradication objectives. For example, because rivers and streams are susceptible to high levels of infestation, blackwood control measures in riverine areas are aimed at preventing blackwoods from reaching sizes at which they could cause damage during floods when uprooted. Therefore, in the riverine sub-zone, all trees above 5 cm diameter are eradicated every five years.

A sound knowledge base
The 10-year monitoring programme, initiated in 1997, is set out to gather a sound knowledge base on which future blackwood management decisions will be based, when policies will be re-assessed. The monitoring and research includes investigating blackwood population dynamics, blackwood incidence of spread and success of control. Re-measurement of permanently set up plots commenced in 2009 and data analysis is already underway, a continuing process until the scheduled data collection will be completed in 2012.

Based on predictions, one would expect that blackwood populations in the forest interior would exhibit a bell-shaped size-class distribution curve that would flatten-out over time, that there will be a difference in incidences of spread (and size-class distribution plots) between the forest interior and forest margins or riverine areas, and that the population inside the forest would stabilise at low equilibrium densities over time, once utilisable trees have been removed. Once all the data has been gathered, these predictions will be tested, theory will be compared to reality, and the Blackwood Management Plan will be adapted where, or if, necessary.

References

1. Geldenhuys, C.J. (1996). The blackwood group system: its relevance to sustainable forest management in the southern Cape. South African Forestry Journal 177: 7-21.
2. Vermeulen, W.J. and Seydack, A.H.W. (2000) Management policy changes for blackwood (Acacia melanoxylon) in the Southern Cape In: Seydack, A.H.W., Vermeulen, W.J. and Vermeulen, C. (eds). Towards Sustainable Management Based on Scientific Understanding of Natural Forests and Woodlands. Proceedings: Natural Forests and Woodland Symposium II, Knysna, South Africa, 5-9 September 1999, p.45-52

Published in October 2011