Acacia melanoxylon

The Australian Blackwood (Acacia melanoxylon) is an Acacia species native in eastern Australia. Known to some as Tasmanian Blackwood, this tree grows fast and tall, up to 45 m height. It has a wide ecological tolerance, occurring over an extensive range of soils and climatic conditions, but develops better in colder climates. Control of its invasion of natural vegetation, commercial timber plantations and farmland in several host counties incur considerable costs, but its timber value and nursing of natural forest succession provides a positive contribution.

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Description

Acacia melanoxylon grows as an unarmed, evergreen tree 8-15 (sometimes up to 45) m high, with a straight trunk and dense and pyramidal to cylindrical crown , sometimes with heavy spreading branches. The leaves are bipinnate (feathery) on seedlings and coppice shoots turn into phyllodes. Phyllodes are 7-10 cm long, greyish turning dark dull-green, straight to slightly curved, with 3-7 prominent longitudinal veins and fine net-veins between; often bipinnate on young plants and coppice shoots. Pale yellow, globular flower heads are followed by Reddish-brown pods, narrower than phyllodes, slightly constricted, twisted; flat roundish shiny black seeds 2-3 mm long, seeds almost encircled by pinkish-red seed stalks (aril)" (Henderson, 1995. In PIER, 2002). It has a shallow root system with dense, surface feeder roots.

Reproduction

Seed dispersal: The pink-red aril attracts birds for dispersal of the seed. Once birds in host-countries become adapted to feeding on the pink-red aril around the seed, the seed is dispersed widely, as in South Africa. It is possible that in host countries where the species has not become invasive, birds and/or other frugivores were not forced by food shortages (as result of drought or other natural phenomena) to switch to this food source. Soil-stored seed banks develop that can remain viable for many years. Seeds germinate easily when placed in hot (boiling water) over night, or when soil-stored seeds are heated by the sun (in disturbed or exposed sites), or after fire (Hill, 1982). Acacia melanoxylon reproduces prolifically after fire.

Seed can be dispersed by the following methods

• Digestion/excretion: Birds (ingest small seeds with pink-red aril), Primates (ingest seeds with pods).
• For ornamental purposes (local): Nursery trade, Landscaping, Tree seed distributors.
• Garden escape/garden waste
• Road vehicles
• Water currents: Seeds with pods, floating vegetation/debris. (Geldenhuys, pers.comm. 2003)
• Wind

It can also multiply by vegetative methods, coppice shoots develop from cut and damaged stems, and from damaged roots.  

Ecology and habitat

It is native to rainforests in Australia, from the Atherton Tableland (17°S) in Queensland above 500 m above sea level to central Tasmania (43°S) between sea level and 1000 m above sea level (Farell and Ashton, 1978; Jennings, 2002). In these areas, it occurs as an understorey tree in wet eucalypt forests, as a pioneer to co-dominant trees in riverine rainforest and as a dominant tree in blackwood/tea tree swamps in northwest Tasmania. It is best adapted to cooler moist sites.

It tolerates drought, poor drainage, any soil, salt air, gusty, steady or cold winds if grown in open, fog, smog, temperature extremes, sun, or shade. Occurs in agricultural areas, coastland, disturbed areas, estuaries, natural forest, planted forests, range/grasslands, riparian zones, scrub/shrublands, urban areas, wetlands.

This fast growing perennial tree is a successional species. It lives for 15 – 50 years, regularly producing large numbers of well-dispersed seeds. Seed viability is sufficiently long to bridge the time between successive seedling stages.

In South-east Queensland it is an important host plant for a number of indigenous butterfly larvae, including Tailed Emperor (Polyura sempronius); Silky Hairstreak (Pseudalmenus chlorinda); Imperial Hairstreak (Jalmenus evagoras evagoras); Stencilled Hairstreak (Jalmenus ictinus) & Large Grass-yellow (Eurema hecabe hecabe).

Invasive species

Replaces native non-tree vegetation, such as grassland and shrubland, and transforms such habitats. It invades the understorey of relatively open pine and eucalypt plantations. Tree stands facilitate the establishment of natural evergreen forest species and the development of regrowth forest. Windfalls obstruct water flow along invaded streams and rivers. Root suckering, it may require root barriers when planted for landscaping in built-up areas.

It has been introduced to many countries for forestry plantings and as an ornamental tree. It now is present in Africa, Asia, Europe, Indian Ocean, the Pacific Ocean, South America and the United States. It is a declared noxious weed species in South Africa. It was also recently listed by the California Invasive Plant Council (Cal-IPC) as an invasive weed that may cause limited impact (Knapp 2003). Its use as a street tree is being phased out in some locales because of the damage it often causes to pavements and underground plumbing.

Management

Preventative measures: In general, blackwood is either recognized as an invader species in some areas, or it does not invade in other areas (although its potential to invade is recognized), or its invasion status is not yet recognized. South Africa provides information on the management of areas where blackwood invasion has become a problem. In areas where blackwood is not yet an invasion problem or where the species is in an early stage of invasion, the following options could be followed:

• Be careful with the introduction of Acacia melanoxylon into natural areas or area where the species is not present because of the potential of the species to become invasive.
• Production of viable seed should be monitored.
• Seedling recruitment should be monitored in natural ecosystems and along drainage lines.

Plants in natural ecosystems should be removed before they flower and produce seed.

Uses

Indigenous Australians derive an analgesic from the tree.^[2]

The wood is very good for many uses including furniture, tools, boats, and wooden kegs. It is of about the same quality as walnut and it is well-suited for shaping with steam. The bark has a tannin content of about 20%.^[3]

The tree's twigs and its bark are used to poison fish as a way of fishing.^[4]

Plain and Figured Australian Blackwood is used in musical instrument making, and in recent years has become increasingly valued as a substitute for koa wood.

References

Notes

1. ^ ILDIS LegumeWeb
2. ^ Analgesic Plants Australian New Crops Newsletter
3. ^ Google Books Select Extra-tropical Plants Readily Eligible for Industrial Culture Or Naturalization By Ferdinand von Mueller
4. ^ A. Melanoxylon

General references

Public Domain Information From:

• The IUCN/SSC Invasive Species Specialist Group (ISSG) (http://www.issg.org), Global Invesive Species Database: http://www.issg.org/database/welcome/,
• Related Disclaimer: http://www.issg.org/database/welcome/disclaimer.asp
• Cal-IPC Plant Assessment Form (http://portal.cal-ipc.org/files/PAFs/Acacia%20melanoxylon.pdf)

• http://www.ellisguitars.com/AustralianBlackwood.html Musical instrument maker utilizing Acacia melanoxylon

• Management of Blackwood in Plantations [1]