Acacia melanoxylon
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| iAustralian Blackwood | ||||||||||||||
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| Image:Acacia melanoxylon branch.jpg Flowering twig of Acacia melanoxylon
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| Acacia melanoxylon R.Br. | ||||||||||||||
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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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[edit] Description
Unarmed, evergree tree 8-15 (sometimes up to 45) m high; trunk straight, crown dense and pyramidal to cylindrical, sometimes with heavy spreading branches. Leaves: Bipinnate (feathery) leaves 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. Flowers: Pale yellow, globular flower heads. Fruits: 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.
[edit] 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.
[edit] 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 [succession]]al 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.
[edit] 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. 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.
[edit] 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.
[edit] 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
