Acacia mearnsii, also known as black wattle, late black wattle, or green wattle, is a species of flowering plant in the family Fabaceae.
Black Wattle
It is usually an erect tree with smooth bark, bipinnate leaves and spherical heads of fragrant pale yellow or cream-coloured flowers followed by black to reddish brown pods. Acacia mearnsii is a spreading shrub or erect tree that typically grows to a height of 10 m (33 ft) and has smooth bark, sometimes corrugated at the base of old specimens.
The current total planted black wattle area is 110,000 ha in South Africa and about 170,000 ha in Brazil. Because of the commerce of wattle woodchips in the international market, it is a profitable crop in South Africa, with timber providing 85% of revenue and bark the other 15%.
The species is named after American naturalist Edgar Alexander Mearns, who collected the type from a cultivated specimen in East Africa. Acacia mearnsii De Wild. was first described by Belgian naturalist Emile Auguste Joseph De Wildeman in 1925.
Acacia mearnsii flowers
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Ecological Role and Distribution
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 and 1,700 metres (2,000 and 5,600 ft). 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 66 and 228 cm (26 and 90 in), an annual mean temperature of 14.7 to 27.8 °C (58.5 to 82.0 °F), and a pH of 5.0-7.2.
A. mearnsii plays an important role in the native ecosystem of Australia. As a pioneer plant it quickly binds the erosion-prone soil following the bushfires that are common in its Australian habitats. Like other leguminous plants, it fixes atmospheric nitrogen in the soil.
The cracks and crevices in the wattle's bark are home for many insects and invertebrates. The rare Tasmanian hairstreak butterfly lays her eggs in these cracks, which hatch to produce caterpillar larva attended by ants (Iridomyrmex sp.) that feed off the sweet exudates from the larva.
Invasive Potential
In some parts of the world, A. mearnsii is considered to be an invasive species. In South Africa it is listed as a Category 2 invader in the National Environmental Biodiversity Management Act. While it is an important plantation species for tannin production and woodchip exports (Brazil and South Africa), in many countries it is an invasive species partly due to its ability to produce large amounts of long-live seeds.
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Seed may remain viable for up to 50 years. It may replace grass communities, increase rainfall interception and transpiration, causing soil to dessicate more quickly. It can destabilize stream banks and support a lower diversity of species. The black wattle population has now expanded over 2.5 million hectares of land. It also damages South Africa’s biodiversity.
Distribution Map of Acacia mearnsii
Traditional and Medicinal Uses
African wattle is commonly used in African traditional medicine, especially the bark and the roots. They are taken internally to treat a range of digestive disorders and as general tonics, whilst externally they are used to treat wounds and sores.
In South Africa, used for treatment of microbial infections. Bark, ground and mixed with water, drunk as decoction for treatment of stomachache and diarrhea. The aboriginal Ngunnawal people of the Australian Capital Territory use a bark infusion to treat indigestion.
Tannin-rich bark can be used as astringent for treatment of diarrhea and dysentery., or applied externally for treatment of wounds, hemorrhoids, or used as mouthwash.
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Study evaluated crude acetone extracts for antimicrobial activity using both agar diffusion and macrobroth dilution methods and cytotoxicity effect with brine shrimp lethality assay. Results showed inhibition of both bacterial and fungal isolates, with bacteria showing more susceptibility than fungi, and was more bactericidal (75%) than bacteriostatic (25%) and more fungicidal (66.67%) than fungistatic (33.33%). On brine shrimp lethality assay, the extract was nontoxic with LC50 >100 µg/mL.
Other Applications
- Agri: Primary source of tannins. Use as soil conditioner and stabilizer and for restoring native woodlands.
- Timber: Used as building material; charcoal as fuel; pulp and wood chips used to produce paper. Significant secondary products are charcoal and firewood.
- Dietary supplement: Following discovery of its strong antioxidant activity, a wattle tannin dietary supplement has been developed.
Here's a summary of the primary uses of the African Wattle tree:
| Use | Description |
|---|---|
| Tannin Production | Wattle bark is a major source of tannins used in leather production and adhesives. |
| Wood and Fuel | The timber is used for building materials, and the wood is a valuable source of fuel and charcoal. |
| Soil Stabilization | The tree's extensive root system helps prevent soil erosion, especially on steep slopes. |
| Medicinal Purposes | Bark and root extracts are used in traditional medicine to treat digestive disorders, wounds, and microbial infections. |
| Agroforestry | Used as a soil conditioner, stabilizer, and for restoring native woodlands. |
