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Australian Blackwood
Acacia melanoxylon

Useful farm tree with high value timber

Australian Blackwood: Worth growing right if your site is right.

Australian Blackwood grows naturally from the Adelaide Hills, through southern Victoria and Tasmania and up the east coast to the Atherton Tablelands in North Queensland. Internationally, it can be found growing in plantations in New Zealand, Chile, South Africa and China! With ample supply still coming from native forests in Tasmania there is a risk that Australia will be left behind in the research and production of this valuable species. Could it be that Blackwood goes the way of Eucalyptus Oil and Macadamias? If demand increases or supply from public native forests is reduced due to conservation pressures will we be importing Australian Blackwood timber in the future?

Seeing the writing on the wall back in 1997, a small group of Australian researchers, processors and growers teamed up with colleagues in New Zealand to form the Blackwood Industry Group. The group aims to share research, marketing and practical information about the establishment, management, harvesting and processing of Blackwood timber from both native forests and plantations. The verdict? Despite the risks and uncertainty, Blackwood is arguably one of the most challenging, yet rewarding, tree species to grow. As one of the few long-lived Australian wattle species this all-rounder is not only good for the environment, it also works well with farming and has the tantalising attraction of offering growers the opportunity to produce one of the world’s most highly regarded cabinet timbers.

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We grow Blackwood along our creek for erosion control and biodiversity - but prune it for sawlogs

A tree for all reasons

Blackwood is certainly a useful species of inclusion in farm plantings. As a component of shelterbelts it can be used to block the gaps below the canopy of taller eucalypts which tunnel deadly cold winds. Being a legume, Blackwood fixes nitrogen through a symbiotic relationship with Rhizobium leading to increased soil fertility and enhanced growth of neighbouring trees. Once established, the bark is resistant to stock damage making it an ideal paddock shade tree. It is also said to be a useful stock fodder.

As a locally native species Blackwood is a key component of many environmental plantings. Its propensity to sucker from exposed roots and spread by seed makes it an ideal tree to grow along an eroding creek or on unstable slopes for soil conservation. The dense foliage offers refuge for small birds and can help shade creek lines thereby improving their habitat value.

For farmers with an interest in timber production it is not a question of whether or not Blackwood is worth planting – the non-timber values should cover that - but whether it will survive and grow to a reasonable size. When farmers ask about this I suggest they look at the native Blackwoods in their area. Do they reach diameters of more than 40 cm or do they seem to die out well before that? They should also talk to the “old-timers”: was there ever large Blackwood logs harvested from the region? If it all looks promising the next step is site selection.

It is all about site selection

The best sawlogs did, and still, come from the cool wet forests of southern Victoria and northern Tasmania. In most other areas large trees are only found along creeks where there is ample fresh water and shelter from hot dry winds. On exposed sites Blackwoods are usually stunted and succumb to borers when stressed resulting in a premature death. Although the biggest and best trees grow in the freshwater swamps of north-western Tasmania, Blackwood does not necessarily like waterlogging. There must be oxygen in the water and the best indicator is water movement. Sites with stagnant water and heavy wet clays are usually unsuitable.

The next point is seasonal water stress. I’ve seen Blackwood growing well under irrigation as far inland as Wagga Wagga but if the water was turned off they would probably die over the next summer. When stressed, Blackwood becomes very susceptible to borers that attack the sapwood damaging the timber and eventually killing the tree. Even in high rainfall areas clay soils that restrict root growth can induce a drought response. Ideal sites are deep well drained soils below natural springs or along streams where there is ample fresh water underground.

The final point about site selection is exposure to drying winds. On a cool, well sheltered south-facing slope, height growth of Blackwood continues throughout spring and into summer. If exposed to drying winds the height growth is stunted resulting in poor stem form and heavy branching. This has led some to believe that Blackwood needs to be grown in a “light-well” amongst a dense forest of taller trees to nurse it into shape. My feeling is that it is shelter, not shading, that is most important and I encourage farmers to start by growing Blackwood in their most sheltered areas. If the site is exposed to the drying northerly or westerly winds, plant a dense belt of tall trees and shrubs on the windward side to shelter the timber trees.

Blackwood is easy to grow from seed and can also be propagated from root cuttings of selected individuals. This technique may allow for the production of clones with known timber properties but unfortunately we don’t yet know if characteristics such as colour or fiddleback grain are under genetic or environmental influence. Until we know more seedlings are the most cost effective. Many farmers will find that wallabies love their young trees. Whilst it is possible to cull the wallabies I prefer to use tall guards to protect the trees. Blackwood seems to thrive in the humid tubes and soon gets above their reach.

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Carefully pruned Blackwood provides a framework on which to grow high quality cabinet timber

Once established Blackwood must be managed

Inherent genetic variability in stem form and its susceptibility to exposure suggests that growers should consider planting many more than they hope to harvest so that they can select individuals of good form to manage. The aim, for high quality timber, is a straight thick stem with no branches for at least 3 or 4 metres. I’m aiming for a pruned log of 6 metres and a stem diameter at breast height (1.3m) of more than 50cm.

Pruning begins when the trees are young. New Zealand researchers have found that regularly removing any branch over about 2.5cm (1 inch) will not only improve the form of the tree but also reduce subsequent stem pruning costs.  In addition to removing all large branches up to 5 or 6 metres I prune all the branches below the point where the diameter of the stem is 10cm. This combination of form pruning and stem pruning is repeated every year until the full log length is achieved. It is then a case of maintaining tree health and sustaining diameter growth over the pruned stem.

Blackwood does not like competition. Without intervention, forests can become overstocked resulting in very low diameter increments.  One of the most important research findings arising out of the recent meeting was that wood quality, in terms of both colour and density, are not reduced by rapid diameter growth. In fact, it may be that the faster the trees grow the better the wood colour and density. This means that we can safely thin plantations to encourage diameter growth and reduce the time it takes to grow large logs without risking quality.

So, how much space does each tree require to maintain diameter growth? This was the main topic of debate in the 14 year old plantation at Lardner Park in Gippsland during the field trip. Once the experts had marked the stand for thinning it was clear that reducing the stocking such that the Basal Area (in m2/ha) was less than half the Mean Tree Diameter (in cm) was well supported. The Basal Area is the cross-sectional area of all the trees at 1.3m above the ground. In an unthinned forest the Basal Area increases as the trees grow larger. Thinning reduces competition by providing more space for each remaining tree.

What this level of thinning means in terms of stocking rates or distance between trees in a monoculture plantation of Blackwood is shown in the Table 1. Whilst it may seem wasteful to establish trees that will later be cut to waste, experience has shown that being able to select the best trees for pruning dramatically improves productivity. Once pruning is complete there might be around 300 trees per hectare. This makes allowances for future losses due to wind and disease. Further thinning is still required and could involve the harvest of small sawlogs once they reach 40cm in diameter.

By leaving well spaced straight healthy individuals at a final stocking of about 150 trees per hectare the owner has the option of selectively harvesting large sawlogs and replanting into the gaps or waiting to clear fell. Bear in mind that you don’t have to plant Blackwood in large regular plantations. I grow Blackwood along a creek with the space between the maturing trees planted to understorey for biodiversity. Rather than clearfell a block of trees I expect to selectively harvest high quality logs while encouraging regeneration for the next rotation. On steep slopes I graze between the trees.

Table 1. Suggested stocking rates based on regularly thinning to a Basal Area equal to half the Mean Tree Diameter. Small diameter or poorly formed trees are cut to waste.

Mean tree Diameter

Appropriate Basal Area

Approximate

stocking Rate

Approximate distance between trees

Management

(cm)

(m2/ha)

(trees/ha)

(metres)

 

10

5

650

4

Successful establishment of at least 650 trees/ha

20

10

300

6

Selection of the 300 or so best trees for pruning over 4 or 5 years

30

15

200

7

Harvesting of small sawlogs and some natural losses

40

20

150

8

50

25

125

9

Harvesting of large sawlogs

60

30

100

10

 Download my paper on Blackwood silviculture: