No. 20 Australian blackwood (Acacia melanoxylon)
NEW ZEALAND FARM FORESTRY ASSOCIATION - INFORMATION LEAFLET
From the first days of European colonisation the Australian blackwood
has had a place among the world’s great decorative timbers. It’s
natural range in Australia is extensive, from Tasmania to Queensland –
however commercial extraction is largely confined to Tasmania and
southern Victoria.
Blackwood occupies disturbed sites, growing fromground-stored seeds
and sucker shoots. It is commonly associated with eucalypts and
occupies gaps among rain forest species. It is a major component of the
‘blackwood swamps’, a series of river flood plains in N.W. Tasmania.
Blackwood has been established in New Zealand for well over 100 years
and is now securely naturalised.
More extensive plantings since 1980 followed its recognition by the
Forest Research Institute as a species worth special attention for its
superb timber qualities. It has the potential to fill a gap once
occupied by rimu, is suited to riparian planting, and contributes to
diversity in our forest landscapes, but it has had a mixed press, in
view of its variable growth rates and commonly poor form. Correct
siting and silviculture can resolve both problems.
Establishment
Siting
The form problem
Interplanting
Pruning
Form pruning
Clearwood pruning
Crown management
Thinning and spacing
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| The Te Awa 6 clone of Acacia melanoxlon
in the PVR trial at five years.
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Establishment
Seedlings can be either bare-rooted or container grown, with little
growth difference between them provided they have been correctly grown.
The seed source is not critical. Clonal selections are available, and
have been trialled (contact Wade Cornell, 247 Forest Hill Rd, Waitakere
City 1208, ph (09) 814-9724).
Siting
Blackwood has a capacity to survive across a range of site conditions.
However, to produce useful timber it has very specific site
requirements – in particular it needs good shelter, and
soil moisture
during summer growth. It is best suited to the sides of damp gullies
and lower valley slopes. It grows poorly on dry exposed sites, in frost
pockets, and in stagnant swamps (the “blackwood swamps” are seasonally
flooded, but dry underfoot in summer). On good soils, additional
fertilisers are not required, but phosphate is advised if soils are
deficient. Blackwoods respond to release from weed competition in the
first summer.
The form problem
Blackwood has a plastic growth habit, adapting its form to its
environment:
it produces a straight stem when competing for light, but a short bole
with massive branches when open-grown. The primary aim in its
silviculture is to produce a straight stem of 5 to 6 metres. This can
be achieved by either:
- interplanting with other species
- form pruning
Interplanting
The use of a nurse species which will force the blackwood to compete
for light is sound in theory, but difficult in practice; it has
resulted in a few successes, and many failures. Basic principles must
be followed:
- the nurse should be established before the blackwood is
planted, or
be much faster growing in the first few years.
- the aim is to impose lateral shading, but allow light from
above.
This requires periodic visits for branch reduction.
- the method does not eliminate the need for some form
pruning.
- the nurse should be considered expendable, and removed when
it has
served its purpose.
- retaining the nurse as a crop species is unlikely to be
successful.
The method is best suited to interplanting blackwoods in native scrub
(especially manuka and kanuka).
(top)
Pruning
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| Two-and-a-half-year-old blackwoods, planted in groups
of three without a nurse crop, and grazed by sheep at two years.
The modified ARS pruner allows leader training to 5 metres. When the
tree is small, early attention can be given with easy hand/pocket tools
such as secateurs. |
There has been an increasing acceptance that pruning is essential
in
managing blackwood. The method has no resemblance to pruning radiata
pine; it is not difficult, but requires a commitment to visit the trees
annually during the establishment of the 6 metre stem. There are two
stages, which overlap:
- form pruning
- clearwood pruning
Form pruning
Blackwoods have poor apical dominance. Periods of stem growth are
interrupted by abortion of the shoot tip. When growth resumes, several
shoots compete for leadership. A blackwood stem is therefore formed by
segments of straight growth which are interrupted by zones of
disturbance which contain double or multiple leaders. The aim of form
pruning is to identify and remove these competing leaders while they
are still small (< 3cm diameter), and this requires an annual
pruning visit during the formation of the 6 metre butt log.
Long-handled pruners are useful.
Clearwood pruning
The aim is to confine the defect core. Clearwood pruning is done
annually, starting at about year 3, and is complete by about year 8. It
is carried out in stages, in which the largest branches on the stem are
removed first, using a 3cm calliper. The trunk is then pruned to the
diameter of your defect core (10 to 12cm). No more than a third of the
foliage is removed at one visit. It is recommended that about 3 metres
of the crown is left after each lift. Ladders that grip the trunk
should not be used.
A straight pruned butt log of 6 metres is ideal, but any log over 4
metres is acceptable, and slight sinuosity can add character to the
grain.
(top)
Crown management
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Typical log out-turn from a
35 year-old blackwood. Ministry of Forestry
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This is a neglected aspect of blackwood silviculture. The crown should
be even, stable, and live at its base. Three common problems
are:
- Small
high crown This is caused by delayed thinning, and
results in
slow diameter growth. Lateral shading is useful during stem
development, but the crown should be allowed to expand in open space.
- Asymmetric
crown Two crowns in close proximity will
interact, causing
asymmetry, and produce tensionwood in the trunks. Spacing between final
crop trees should therefore be fairly even.
- Unstable
crown The problem is breakage in a major fork at
the base of
the crown. It can be prevented by shortening a competing leader, and by
resisting the temptation to prune up to a fork.
Thinning and
spacing
This remains under trial, but the best bet currently is to aim for a
final crop of 200 trees per hectare, i.e. 7 to 8 metres apart. Plant
800 to 1000 per hectare in rows, or in groups at final spacing, and
thin progressively before the crowns interact. On good sites,
silvilcultural work is normally completed by 8
to 10 years, and a rotation of about 35 years can be expected.
(top)
Photographs reproduced from the February 1997, and February 2002
issues
of the New
Zealand Tree Grower.
Graphic reproduced from Special Purpose Timber
Species – no.1 in the
Small Forest Management series, jointly
produced by the Ministry of Forestry and the NZ Forest Research
Institute. © Crown Copyright Ministry of Forestry, May 1995.
AMIGO
The Acacia
Melanoxylon Interest Group is an action group within the
Farm Forestry Association, and is the main source of information for
blackwood growers. Members include growers, researchers, nurserymen,
and commercial users. All blackwood growers who want access to up to
date information on silviculture and marketing are strongly encouraged
to join. We publish a newsletter twice yearly, and hold a major
national field day each year. Annual subscription is $15. For
membership information click here or
contact the NZ Farm Forestry Association
National Office, PO Box 1122, Wellington.
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