Acacia melanoxylon (Australian Blackwood)

Scientific name

Acacia melanoxylon R. Br.

Synonyms

Acacia arcuata Sieber ex Spreng.; Acacia melanoxylon R. Br. forma frutescens Hochr.; Acacia melanoxylon R. Br. forma melanoxylon; Acacia melanoxylon R. Br. var. arcuata (Sieber ex Spreng.) Ser.; Acacia melanoxylon R. Br. var. melanoxylon; Acacia melanoxylon R. Br. var. obtusifolia Ser.; Acacia melanoxylon (R.Br.) Poir.; Mimosa melanoxylon (R. Br.) Poir.; Racosperma melanoxylon (R. Br.) Pedley

Common names

Australian blackwood, black wattle, blackwood, blackwood acacia, blackwood wattle, hickory, mudgerabah, Paluma blackwood, Sally wattle, Tasmanian blackwood

Family

Fabaceae (Leguminosae): sub-family Mimosoideae

Origin

Acacia melanoxylon is a widespread and often common species that is native to large parts of eastern and south-eastern Australia.

Naturalised distribution (global)

Locations in which Acacia melanoxylon is naturalised include New Zealand, Brazil and Africa.

Introduced, naturalised or invasive in East Africa

Acacia melanoxylon is invasive in parts of Kenya (A.B.R. Witt pers. obs.) and in Tanzania (Henderson 2002 and Global Invasive Species Database). The species was introduced in Tanzania before the First World War

Habitat

In tropical Africa, Acacia melanoxylon is found in cool and wet upland regions. It grows best in deep, moist and fertile soils, but grows also on sandy and alluvial soils, and in wet nearly swampy places. In eastern Africa, it is also common along in riparian zones (banks of watercourses) and along roadsides, in open woodlands and shrublands, and in grasslands. It has also been recorded growing in coastal environs, disturbed sites, urban open spaces, forest plantations and wetlands. In Tanzania, it is found at altitudes of 1500-2500 m above sea level. In Uganda it is found on a small scale at similar altitudes ranges as for Tanzania in Muko and Mafuga Forest plantations in the south-western highlands where it was introduced. It is not regarded a very serious problem, but tends to be locally dominant in patches at the forest edges or in gaps associated with fires (D.L.N. Hafashimana pers. comm.).

Description

Acacia melanoxylon is a tree up to 20 m high, with a bole of about 150 cm in diameter. The bark on older trunks is dark greyish-black in colour, deeply fissured and somewhat scaly. Younger branches are ribbed, angular, or flattened towards their tips and are greenish in colour. These branchlets are usually mostly hairless (glabrous or glabrescent), but the stems of younger plants are sometimes more obviously hairy (densely pubescent).

On young plants, partially formed phyllodes can be seen which bear twice-compound (bipinnate) leaves at their tips. As the seedling grows, each new 'leaf' tends to have phyllodes that are more fully formed and the leaves at their tips eventually vanish altogether. However, these remnant twice-compound (bipinnate) leaves can occasionally be seen on the tips of the phyllodes of older plants (1-2 m or more tall). The dark green to greyish-green phyllodes (4-16 cm long and 6-30 mm wide) are quite variable in shape. However, they are generally somewhat elongated (narrowly elliptic to lanceolate) and usually about 4-12 times longer than they are wide. They vary from being relatively straight to slightly curved (sub-falcate) and are usually tapered towards the base. These phyllodes are hairless (glabrous) with a glossy appearance and a slightly leathery (coriaceous) texture. They usually have three to five prominent veins running lengthwise and rounded to pointed tips (obtuse to acute apices). Where the phyllodes join to the stem there is a short slightly thickened flexible joint (a pulvinus) about 2-4 mm long. There is also a small raised structure (gland) present on the margin of each phyllode, about 1-10 mm above its base.

The pale yellow, cream or whitish coloured flowers are fluffy in appearance due to the presence of numerous stamens. They are densely arranged into small rounded clusters (5-10 mm across), each containing numerous (30-56) flowers. Each individual flower in this cluster is stalkless (sessile) and has five relatively inconspicuous petals and sepals. The flower clusters are borne on stalks (peduncles) 5-14 mm long and are alternately arranged on a short branch (6-40 mm long) emanating from a 'leaf' fork (phyllode axil). These compound flower clusters (axillary racemes) generally contain only 2-8 of the small globular flower clusters. Flowering can occur throughout the year.

The fruit is an elongated and somewhat flattened pod (4-15 cm long and 3.5-8 mm wide) that is strongly curved, twisted or coiled. These pods are mostly hairless (glabrous) and only slightly constricted between each of the seeds. They are green and leathery in nature when young but turn brown or reddish-brown in colour and become more woody as they mature. Each pod contains several very distinctive seeds, and after opening to release these seeds they become twisted and contorted. The seeds are broadly oval (elliptic) in shape (3-5 mm long and 1.7-3 mm wide), glossy in appearance, and black in colour. They are almost encircled by a large pink, pinkish-red or dark red folded fleshy structure (aril).

Reproduction and dispersal

Acacia melanoxylon reproduces  by seed, which are known to germinate prolifically after fire. It also sprouts profusely from root suckers, particularly when the roots are damaged, and readily coppices from damaged stems. It is a very fast growing species.

The seeds are spread by animals, particularly birds, and they may also be dispersed in dumped garden waste and contaminated soil. The seed-containing pods are also known to float on water. Root suckers can spread laterally some distance and enable the formation of dense clumps or thickets from a single plant.

A. melanoxylon is a pioneer species and seedlings are intolerant to shade.

Similar species

In early years of growth Acacia melanoxylon can be very difficult to distinguish from Acacia implexa (lightwood), which has with similar pointed phyllodes. However, phyllodes in Acacia implexa are usually more sickle shaped than those of A. melanoxylon.

Economic and other uses

Acacia melanoxylon is cultivated in forestry plantings in eastern Africa (including Kenya and Ethiopia), South Africa and Zimbabwe. It is used for lumber, fuelwood and also in amenity plantings. The wood is used for light construction, tool handles, turnery and fence posts. It is used as a nurse tree in the rehabilitation of disturbed natural forests.

Environmental and other impacts

Though not yet considered to be a major environmental weed in East Africa it is regarded to be a potentially serious woody weed of forest, shrubland and grassland in this region. It is causing serious problems in southern Africa and other parts of the world. In South Africa it is a major invader of forests and is a particularly serious threat to 'fynbos' shrubland and grassland areas. It is known to transform these communities by replacing the native non-tree vegetation. It is considered to be difficult to control because of its fast growth rate, vigorous regrowth from root suckers, and prolific regeneration from seed.
A. melanoxylon has been included in the Global Invasive Species Database (GISD 2003). It has been listed as a Category 2 invader in South Africa (invaders with certain qualities, e.g. commercial use or for woodlots, animal fodder, soil stabilisation, etc. These plants are allowed in certain areas under controlled conditions).

Management

The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.

The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Controlling the weed before it seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

Small plants can be uprooted but it is important to remove the roots completely as A. melanoxylon reproduces vegetatively from root suckers. Mature trees can be cut and herbicide applied to the stump to limit resprouting. Basal bark methods (painting herbicide onto the bark) can also be effective. Large trees can be killed by ring barking (Tunison, 1991). Foliar sprays can be used on young plants. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert. A seed predating weevil Melanterius acaciae was first released in South Africa in 1985 to control A. melanoxylon. Reports state that the weevils achieve more than 90% seed predation thus reducing the number of seeds that accumulate in the soil (ARC, 2000).

Legislation

Not listed as a noxious weed by any state or government authorities in Kenya, Tanzania or Uganda.

References

ARC (2000). Biocontrol agents against alien invasive plants in fynbos. Agricultural Research Council, Plant Protection Research Institute (ARC-PPRI), South Africa. www.cabi.org/ISC. Accessed March 2011.

Bekele-Tesemma, A. (2007). Useful trees of Ethiopia. identification, propagation and Management in 17 Agrecological zones. Nairobi, Relma in ICRAF projects.

CABI invasive species compendium online data sheet. Acacia melanoxylon (Australian blackwood). CABI Publishing 2011. www.cabi.org/ISC. Accessed March 2011.

GISD (2003). Global Invasive Species Database online data sheet. Acacia melanoxylon (tree). Global Invasive Species Database. www.issg.org/database. Invasive Species Specialist Group. Accessed March 2011.

Henderson, L. (2001).  Alien weeds and invasive plants. A complete guide to declared weeds and invaders in South Africa.  Plant Protection Research Institute Handbook No. 12, 300pp. PPR, ARC South Africa.

Henderson, L. (2002). Problem plants in Ngorongoro Conservation Area. Final Report to the NCAA.

Pacific Island Ecosystems at Risk (PIER). Acacia melanoxylon R.Br. ex Aiton, Fabaceae (Leguminosae): plant threats to Pacific ecosystems. www.hear.org/pier/species/acacia_melanoxylon.htm. Institute of Pacific Islands Forestry, Hawaii, USA. Accessed January 2011.

PROTA database. Prota 7(1): Timbers/Bois d'œuvre 1. Record display: Acacia melanoxylon R.Br. database.prota.org/PROTAhtml/Acacia%20melanoxylon_En.htm. Accessed January 2011.

Tunison, T. (1991). Element Stewardship Abstract for Acacia melanoxylon - Blackwood Acacia. tncweeds.ucdavis.edu/esadocs.html. The Nature Conservancy, Arlington, Virginia, USA.

Editors

Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.

Acknowledgments

This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).

Contact

BioNET-EAFRINET Regional Coordinator: eafrinet@africaonline.co.ke