Acacia robusta (PROTA)

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tree habit (Virtual Field Herbarium)
tree habit (Virtual Field Herbarium)
fruiting branch (Virtual Field Herbarium)

Acacia robusta Burch.


Protologue: Trav. S. Africa 2: 442 (1824).
Family: Mimosaceae (Leguminosae - Mimosoideae)
Chromosome number: 2n = 26, 52

Vernacular names

  • Splendid thorn, splendid acacia, ankle thorn (En).
  • Mungu manzi, egamosema (Po).
  • Mgunga (Sw).

Origin and geographic distribution

Acacia robusta occurs from Ethiopia and Somalia south to Namibia and northern and eastern South Africa. It has been introduced elsewhere, e.g. in South Asia.

Uses

The wood is occasionally used for furniture, shelves and yokes, although its use is limited because of considerable warping. It is also used as firewood and among the Mijikenda people of Kenya it is even the preferred type of firewood. The foliage and pods are browsed by livestock. In traditional medicine, root powder is applied to swellings, a root decoction is used to treat dysmenorrhoea, female sterility and schistosomiasis, a stem bark decoction to treat gonorrhoea, abdominal pains and skin ailments, and leaves to treat snakebites. Some medicinal applications of unknown plant parts have been recorded: to treat malaria and bubonic plague. Acacia robusta is occasionally grown as a bonsai plant.

Properties

The heartwood is pinkish brown to reddish brown and distinctly demarcated from the wide whitish sapwood. The texture is moderately coarse to coarse and even. The wood is moderately heavy, with a density of about 850 kg/m³ at 12% moisture content. Warping is often considerable during drying. The wood is brittle and moderately durable, being moderately susceptible to borer and termite attack; the sapwood is susceptible to sap stain. The pulping properties of the wood have been rated as good. The energy value of the wood is 18,100 kJ/kg.

The crude protein content of the seeds is 12.5%. The gum from the bark contains 18% of protein bound to an arabinogalactan. In screening tests root bark of Acacia robusta showed strong antibacterial activity against Staphylococcus aureus and moderate activity against Escherichia coli, but these activities were not confirmed in a later test.

Description

  • Shrub or small to medium-sized tree up to 20(–25) m tall; root system moderately deep, with strongly spreading lateral roots; bole up to 70 cm in diameter; bark smooth to fissured, grey-brown to dark brown or black; crown spreading; young branches smooth, with paired, straight stipular spines up to 6(–11) cm long.
  • Leaves alternate, bipinnately compound, with 2–6(–10) pairs of pinnae; rachis 2.5–7 cm long, pubescent or glabrous; leaflets in 9–27 pairs per pinna, obliquely oblong, (2–)3.5–13(–16) mm × 1–5(–7) mm, glabrous or ciliate on the margins.
  • Inflorescence an axillary globose head 9–12 mm in diameter, usually in clusters; peduncle 2–2.5 cm long, with a pair of small bracts in basal half.
  • Flowers bisexual, regular, usually 5-merous, small, creamy white, fragrant, sessile; calyx cup-shaped, with short lobes; corolla shortly lobed, glabrous; stamens numerous, free, up to 5 mm long; ovary superior, 1-celled, style slender.
  • Fruit a linear pod 7–19 cm × 0.5–3 cm, straight or curved, more or less woody, glabrous, brown, longitudinally veined, dehiscent, up to 15-seeded.
  • Seeds quadrangular to ellipsoid, compressed, 8–15 mm × 5–9 mm, smooth.

Other botanical information

Acacia is a large pantropical genus, comprising more than 1300 species; most of them distributed in Australia (more than 900), more than 200 in America, and about 130 in Africa. Acacia robusta belongs to subgenus Acacia , which comprises all African Acacia species with straight spinescent stipules. Acacia robusta is variable, and is subdivided into 3 subspecies:

  • subsp. clavigera (E.Mey.) Brenan (synonym: Acacia clavigera E.Mey.), which may develop into a medium-sized tree and is characterized by its pubescent leaf rachis and curved, comparatively thin pods;
  • subsp. robusta, which remains a small tree (up to 8 m tall) and is characterized by its glabrous leaf rachis and straight, comparatively wide pods;
  • and subsp. usambarensis (Taub.) Brenan (synonym: Acacia usambarensis Taub.), which is characterized by its glabrous leaf rachis and very narrow pods. Intermediates between the subspecies are rather common.

Acacia robusta grows comparatively fast. Trees are usually deciduous for a short period. Inflorescences appear together with or after the new leaves. Dispersal of the seeds is probably by browsing animals. Acacia robusta is nodulated by nitrogen-fixing Rhizobium bacteria. In Australia it is considered invasive.

Ecology

Acacia robusta occurs in woodland and wooded grassland, often near rivers, where large specimens can be found, up to 1800 m altitude. The tree is drought and frost resistant.

Management

The germination rate of untreated seeds can be very low, often only about 3%; it is higher when the seeds have been ingested by herbivores. Mechanical scarification may improve germination to over 90%, and treatment with boiling water or sulphuric acid may enhance germination to over 80%. Bruchid species may attack the seeds, especially when these are on the ground. Therefore, seeds for propagation should be collected from the canopy and the period of storage of untreated seeds should be minimized. Trees resprout well after coppicing.

Genetic resources

Acacia robusta is widespread and locally common and not in danger of genetic erosion.

Prospects

The wood of Acacia robusta is considered of rather poor quality and of limited use only. Some types are spectacular when flowering and may be good garden trees, even in frost-prone regions.

Major references

  • Brenan, J.P.M., 1970. Leguminosae (Mimosoideae). In: Brenan, J.P.M. (Editor). Flora Zambesiaca. Volume 3, part 1. Crown Agents for Oversea Governments and Administrations, London, United Kingdom. 153 pp.
  • Coates Palgrave, K., 1983. Trees of southern Africa. 2nd Edition. Struik Publishers, Cape Town, South Africa. 959 pp.
  • Palmer, E. & Pitman, N., 1972–1974. Trees of southern Africa, covering all known indigenous species in the Republic of South Africa, South-West Africa, Botswana, Lesotho and Swaziland. 3 volumes. Balkema, Cape Town, South Africa. 2235 pp.
  • Timberlake, J., Fagg, C. & Barnes, R., 1999. Field guide to the Acacias of Zimbabwe. CBC Publishing, Harare, Zimbabwe. 160 pp.

Other references

  • Beentje, H.J., 1994. Kenya trees, shrubs and lianas. National Museums of Kenya, Nairobi, Kenya. 722 pp.
  • Brenan, J.P.M., 1959. Leguminosae subfamily Mimosoideae. In: Hubbard, C.E. & Milne-Redhead, E. (Editors). Flora of Tropical East Africa. Crown Agents for Oversea Governments and Administrations, London, United Kingdom. 173 pp.
  • Churms, S.C. & Stephen, A.M., 1984. Structural studies of an arabinogalactan-protein from the gum exudate of Acacia robusta ssp. clavigera. Carbohydrate Research 133(1): 105–124.
  • Khan, M.R., 2001. Antibacterial activity of some Tanzanian medicinal plants. Pharmaceutical Biology 39(3): 206–212.
  • Khan, M.R., Ndaalio, G., Nkunya, M.H.H., Wevers, H. & Sawhney, A.N., 1980. Studies on African medicinal plants. Part I. Preliminary screening of medicinal plants for antibacterial activity. Planta Medica Suppl. 1980: 91–97.
  • Kokwaro, J.O., 1993. Medicinal plants of East Africa. 2nd Edition. Kenya Literature Bureau, Nairobi, Kenya. 401 pp.
  • Luoga, E.J., Witkowski, E.T.F. & Balkwill, K., 2004. Regeneration by coppicing (resprouting) of miombo (African savanna) trees in relation to land use. Forest Ecology and Management 189: 23–35.
  • Neuwinger, H.D., 2000. African traditional medicine: a dictionary of plant use and applications. Medpharm Scientific, Stuttgart, Germany. 589 pp.
  • Pakia, M. & Cooke, J.A., 2003. The ethnobotany of the Midzichenda tribes of the coastal forest areas in Kenya: 1. General perspective and non-medicinal plant uses. South African Journal of Botany 69(3): 370–381.
  • Robbertse, P.J., 1974. Germination of Acacia seed. Journal of South African Botany 40(4): 269 273.

Author(s)

  • R.H.M.J. Lemmens, PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Correct citation of this article

Lemmens, R.H.M.J., 2006. Acacia robusta Burch. In: Louppe, D., Oteng-Amoako, A.A. & Brink, M. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. Accessed 27 November 2017.