Acacia mearnsii

Acacia mearnsii

name = "Acacia mearnsii"

image_width = 300px
image_caption =
regnum = Plantae
divisio = Magnoliophyta
classis = Magnoliopsida
ordo = Fabales
familia = Fabaceae
genus = "Acacia"
species = "A. mearnsii"
binomial = "Acacia mearnsii"
binomial_authority =De Wild.
synonyms =
*"Acacia decurrens auct. non" Willd.
*"Acacia decurrens" Willd. var. "mollis auct. non" Lindl.
*"Acacia decurrens" Willd. var. "mollis" Lindl.
*"Acacia mollissima sensu auct."
*"Albizia mearnsii"
*"Racosperma mearnsii" (De Wild.) [ [ ILDIS LegumeWeb] ]

"Acacia mearnsii" is a fast-growing leguminous tree native to Australia. Common names for it include Black Wattle, "Acácia-negra" (Portuguese), Australian Acacia, "Australische Akazie" (German), "Swartwattel" (Afrikaans), "Uwatela" (Zulu).

"A. mearnsii" has been introduced to numerous parts of the world, and in those areas is often used as a commercial source of tannin or a source of fire wood for local communities. In areas where it has been introduced it is often a weed, and is seen as threatening native habitats by competing with indigenous vegetation, replacing grass communities, reducing native biodiversity and increasing water loss from riparian zones. The species is named after Edgar Alexander Mearns, who collected the type from a cultivated specimen in East Africa. [ [ Royal Botanic Gardens Sydney] ]


The trees are unarmed, evergreen and grow six to 20 meters high. The branchlets are shallowly ridged; all parts finely hairy; growth tips golden-hairy. Leaves dark olive-green, finely hairy, bipinnate; leaflets short (1.5 - 4 mm) and crowded; raised glands occur at and between the junctions of pinnae pairs. Flowers pale yellow or cream, globular flower heads in large, fragrant sprays. Fruits dark brown pods, finely hairy, usually markedly constricted.Henderson, L. 1995. Plant invaders of Southern Africa. Agriculture Research Council, ARC/LNR, Pretoria, South Africa. 55 pp.] PIER 2003 Pacific Island Ecosystems At Risk [ web] ] De Wit, M.P., Crookes, D.J. and Van Wilgen, B.W. 2001. Conflicts of Interest in Environmental Management: Estimating the Costs and Benefits of a Tree Invasion, Biological Invasions: 3 167 - 178.]

Habitat description

In its native range "A. mearnsii" is a tree of tall woodland and forests in subtropical and warm temperate regions. In Africa the species grows in disturbed areas, range/grasslands, riparian zones, urban areas, water courses, and mesic habitats at an altitude of between 600-1700m. In Africa it grows in a range of climates including warm temperate dry climates and moist tropical climates. "A. mearnsii" is reported to tolerate an annual precipitation of between 6.6 – 22.8 dm (mean of 6 cases = 12.6), an annual mean temperature of 14.7 – 27.8°C (mean of 6 cases = 2.6°C), and a pH of 5.0 – 7.2 (mean of 5 cases = 0.5). Duke, J. A. 1983. Acacia mearnsii. Handbook of Energy Crops. Unpublished. [ web] ] "A. mearnsii" does not grow well on very dry and poor soils.Franco, J.A. 1943. Dendrologia Florestal. Lisboa.]

Weed impacts outside of native range

The invasiveness of this species is partly due to its ability to produce large numbers of long-lived seeds (which may be triggered to germinate "en masse" following bush fires), and the development of a large crown which shades other vegetation. Its leaves and branches may have allelopathic properties. "A. mearnsii" competes with and replaces indigenous vegetation. It may replace grass communities to the detriment of the grazing industry and grazing wildlife. By causing an increase in the height and biomass of vegetation "A. mearnsii" infestations increase rainfall interception and transpiration, which causes a decrease in streamflow. Soil under "A. mearnsii" becomes desiccated more quickly (than it does under grass). "A. mearnsii" stands also destabilise stream banks and support a lower diversity of speciesSankaran, K. V. (2002). Black Wattle Problem Emerges in Indian Forests. CABI Biocontrol News March 2002 23(1) [ web page] ] Adair, R. (2002). Black Wattle: South Africa Manages Conflict of Interest. CABI Biocontrol News March 2002, Volume 23 No. 1. [ web] ] Samways, M. J., Caldwell, P. M., Osborn, R. 1996. Ground-living invertebrate assemblages in native, planted and invasive vegetation in South Africa. Agriculture, Ecosystems and Environment 59: 19-32.] Le Maitre, D. C., Scott, D.F. and Colvin, C. (1999). A review of information on interactions between vegetation and groundwater. Water SA 25 (2): 137-152.]

Commercial plantations and invasive stands of "A . mearnsii" in South Africa reduce surface runoff and decrease water ability, causing an estimated annual economic loss of $US 2.8 million. According to KwaZulu-Natal Wildlife (the governmental agency responsible for managing protected areas in KwaZulu-Natal Province, South Africa) the advance of alien plants (particularly "Chromolaena odorata", "Lantana camara", "Acacia dealbata", and "Acacia mearnsii") is the most significant past and future threat to conservation in these areas.Goodman, P.S. 2003. Assessing Management Effectiveness and Setting Priorities in Protected Areas in KwaZulu-Natal, BioScience53 (9): 843 - 850.]


The species is grown commercially in many areas of the world for a variety of uses. Commercial stands have been established in Africa, South America and Europe. The tannin compounds extracted from the bark of "A. mearnsii" are commonly used in the production of soft leather. A range of other products, such as resins, thinners and adhesives, can also be made from bark extracts. The timber is used for building materials, the charcoal is used for fuel and the pulp and wood chips are used to produce paper. In rural communities in South Africa the trees are important as a source of building material and fuel. "A. mearnsii" has some known medical applications, such as its use as a styptic or astringent. The planting of wattles has also been used as a soil stabiliser to decrease erosion (preferably far from river courses to minimise the water loss caused by the tree's high rate of transpiration). The agroforestry industry promotes the use of the species (among other similar species) as a potential "soil improver".Paiva, J. 1999. Acacia. In Talavera, S. Aedo, C, Castroviejo, S, Romero Zarco, C, Sáez, L, Salgueiro, F.J & Velayos, (ed). Flora Iberica - Plantas Vasculares de la Península Ibérica e Islas Baleares. Vol.VII(I). Leguminosae. Real Jardín Botánico, CSIS. Madrid. ISBN 84-00-06221-3. pp. 11- 25.] Duke, J. A. 1983. "Acacia mearnsii". Handbook of Energy Crops. Unpublished. [ web] ] Franco, J.A. 1971. Nova Flora de Portugal (Continente e Açores). Vol. 1. Franco, J.A. (Ed.). Lisboa.] Tutin, T. G., Heywood, V.H., Burges, N.A., Moore, D.M., Valentine, D.H., Walters, S.M. & Webb, D.A. 1992. Flora Europaea. Vol.2 Rosaceae to Umbelliferae. (reprint). Cambridge University Press. Cambridge. ISBN. 0 521 06662 X pp. 84-85] Young, A. 2002. Effects of Trees on Soils, The Natural Farmer (Special Supplement on AgroForestry Soil Fertility and Land Degradation). [ PDF] ]

"Acacia mearnsii" has been shown to contain less than 0.02% alkaloids. [ [ Chemotaxonomie der Pflanzen By Robert Hegnauer] ]

Geographical range

Native Range: Australia.Known introduced range: North America, South America, Asia, Europe, Pacific, Africa and Europe.

Invasion pathways to new locationsAgriculture: The use of the species as a potential "soil improver" is advocated by some agriculturists.For ornamental purposes: Used as an ornamentalForestry: "A. mearnsii" is a popular source of timber and tannins and is planted globally by the forestry industry. One example of a commercial company that funds research on and establishment of wattle plantations is the South African Wattle Growers Union.Franco, J.A. 1971. Nova Flora de Portugal (Continente e Açores). Vol. 1. Franco, J.A. (Ed.). Lisboa.]

Local dispersal methods

* Consumption and excretion: The seeds are potentially distributed by rodents or birds.Carr, G. D. Acacia mearnsii University of Hawaii, Botany Department. [ web] ]
* For ornamental purposes (local)
* By animals: The dispersal of the seeds of "A . mearnsii" is believed to be aided by cattle and birds.Milton, S.J., Dean W.R.J. and Richardson, D.M. 2003. Economic Incentives for Restoring Natural Capital in Southern African Rangelands, Front Ecol Environ+C26 1 (5): 247 254.]
* By people: Local people collecting branches and logs for firewood may spread seeds.
** Transportation of soil: The seeds may also be spread by the movement of seed-contaminated soil.
* By water: The hard-coated seeds are spread readily down water courses.

Management information

Preventative measures

A Risk Assessment of "Acacia mearnsii" for Hawai'i and other Pacific islands was prepared by Dr. Curtis Daehler (UH Botany) with funding from the Kaulunani Urban Forestry Program and US Forest Service. The alien plant screening system is derived from Pheloung "et al." (1999) with minor modifications for use in Pacific islands (Daehler et al. 2004). The result is a score of 15 and a recommendation of: "Likely to cause significant ecological or economic harm in Hawai'i and on other Pacific Islands as determined by a high WRA score, which is based on published sources describing species biology and behavior in Hawai'i and/or other parts of the world."


Saplings sensitive to foliar applications of triclopyr. Dicamba, glyphosate and picloram applied cut-surface effective, triclopyr probably effective, although applications to drilled holes in larger trees probably necessary. Cut-surface (notching) applications of picloram provided complete control, glyphosate and dicamba caused 80% control, and 2,4-D was inadequate at Kala'e, Molokai. Alton Arakaki (Univ. Hawai'i) and Ed Misaki of the Nature Conservancy (TNC) confirmed the efficacy of picloram but got much better results with glyphosate and dicamba, each resulting in over 90% control at Kamakou Preserve. Basal bark and stump bark treatments with 2,4-D or triclopyr effective. Pat Bily (TNC) reported that basal bark applications with triclopyr ester at 20% in oil was effective, as was cut stump application of triclopyr amine at 50% in water. Hawai'i Volcanoes National Park (HAVO) staff got good control with triclopyramine at 10% in water applied to cut stumps (Chris Zimmer, HAVO). Anecdotes indicate that wattle is sensitive to basal bark treatment with diesel alone and to girdling.


The use of Cecidomyiidae gall midges to inhibit reproduction of Acacia species is being researched for use in South Africa. A Dasineura sp. has been identified as a promising control agent as it prevents fruit formation (and thus reproduction) without affecting vegetative growth (which may be a concern for industries or commerce that rely on the species). "Dasineura" is also known to have a narrow host range. "Melanterius maculatus", a seed-eating weevil (native to Australia), was introduced into South Africa in 1993 and caused reductions in "A. mearnsii" seed numbers in some areas. Larvae feed on developing seeds inside the pods and adults feed on the green pods and pinnules. In South Africa a local stump-colonising fungus has been isolated and added to cut trees to prevent regrowth. Finally, a wasp ("Bruchophagus acaciae") has been described that attacks the seeds of some "Acacia" species but not those of "A. mearnsii" (Adair et al., 2000; Adair, 2002; ARC-PPR, 2003; Hill, Gordon and Neser, 1999).

Integrated management

The Working for Water programme implemented by the South African Government is a collaborative program that aims to ameliorate the problems caused by Acacia species and other invasive plants. The program consists of more than 300 sub-projects in all nine provinces in the country and consists of the clearing of weeds from water courses (by mechanical and chemical methods). Between 1995 and 2000 over $100 million of poverty-relief funds on the program which was labor intensive and provided job opportunities for local communities. After seven years of implementation of the project it became clear that rehabilitation of sites (following the removal of alien plant species) would sometimes be needed in order to prevent or reduce the soil erosion stimulated by the clearing of plants (Van Wilgen "et al.", 2002, Milton, Dean and Richardson, 2003).


"A. mearnsii" produces copious numbers of small seeds that are not dispersed actively. The species may resprout from basal shoots following a fire.PIER, 2003. Pacific Island Ecosystems At Risk [ web] ] It also generates numerous suckers that result in thickets consisting of clones. Seeds may remain viable for up to 50 years.Wessa (2002) Alien Invader Plants. [ web] ]



External links

* [ The IUCN/SSC Invasive Species Specialist Group]

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