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Acacia nigrescens Oliv.

Fl. trop. Afr. 2: 340 (1871).
Mimosaceae (Leguminosae - Mimosoideae)
Chromosome number
2n = 26
Vernacular names
Knobthorn (En).
Origin and geographic distribution
Acacia nigrescens occurs from Tanzania southward to north-eastern Namibia, Botswana and north-eastern South Africa.
The wood is used for parquet flooring, carving, turnery, fence posts, railway sleepers and mine props. It is occasionally made into furniture, although it is usually considered too heavy for this purpose. It is also used as firewood and for charcoal production. The foliage is browsed by livestock. Traditional healers of the Shona people in Zimbabwe make an ointment from burnt roots to treat convulsions. In Tanzania a root decoction is used as an aphrodisiac. The bark is used for tanning.
The heartwood is dark brown, with light and darker streaks, and distinctly demarcated from the narrow, whitish yellow sapwood. The grain is often irregular, texture moderately coarse, even. The wood is very heavy, with a density of 1000–1200 kg/m³ at 12% moisture content. The wood seasons slowly and care is needed to avoid surface checking. It is very hard. At 12% moisture content, the modulus of rupture is 126 N/mm², modulus of elasticity 14,810 N/mm², compression parallel to grain 73 N/mm², shear 17.5 N/mm², Janka side hardness 19,080 N and Janka end hardness 19,080 N. It is difficult to saw, even when green, and dulls tool edges. A good finish can be obtained with waxes and oil. The wood has very good natural durability, being extremely resistant to fungal, borer and termite attack. The heartwood is extremely resistant to preservative treatment.
Promelacacinidins and a variety of tetrahydroxyflavonoids have been isolated from the heartwood.
Medium-sized deciduous tree up to 20(–30) m tall; bole usually straight, up to 75(–90) cm in diameter, usually beset with prickles on large knobs, but these often absent in old trees; bark rough, fissured, dark brown to blackish; slash pale to deep pink or red; crown conical in young trees and rounded in mature trees; branchlets usually glabrous, with pairs of hooked, blackish prickles up to 7 mm long just below the nodes. Leaves alternate, bipinnately compound, with 2–4 pairs of pinnae; stipules small, caducous; petiole with or without gland, rachis sometimes with gland between top 1–2 pairs of pinnae; leaflets in 1–2 pairs per pinna, obliquely obovate-orbicular to obovate-elliptical, (7–)10–35(–50) mm Χ 7–30(–50) mm, apex rounded to emarginate, usually glabrous. Inflorescence an axillary spike 1–10(–12) cm long, often in fascicles at the base of young shoots; peduncle 0.5–2.5 cm long. Flowers bisexual, regular, 5-merous, small, creamy white but reddish brown in bud, fragrant, sessile; calyx cup-shaped, 1.5–2 mm long; corolla 2–2.5 mm long, with short lobes; stamens numerous, free, 3.5–6 mm long; ovary superior, very shortly stalked, style slender. Fruit an oblong pod 6–18 cm Χ 1.5–2.5 cm, straight, glabrous, dark brown, longitudinally dehiscent, 3–7-seeded. Seeds flattened orbicular to lens-shaped, 12–13 mm in diameter.
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 nigrescens belongs to subgenus Aculeiferum, which comprises all African Acacia species with non-spinescent stipules and hooked prickles. Acacia nigrescens is characterized by prickles on large knobs on the trunk and the large and few leaflets.
Growth is slow, with maximum annual increment of 60 cm in height, usually much less. Acacia nigrescens is completely deciduous and bare for several months. It frequently flowers when leafless. The flowers are an important food resource to giraffe in the late dry season, and it has been suggested that giraffe could be a pollen vector. Nodulation and nitrogen fixation have been confirmed.
Acacia nigrescens occurs in woodland and bushland, commonly near rivers, up to 1200(–1600) m altitude. It usually grows on shallow soils on rocky hillsides and on alluvial soils in the valleys. It is often common and locally dominant on loamy soils. The tree is resistant to fire.
Acacia nigrescens has proven easy to produce in the nursery. The seeds are usually sown into seedbeds, but sowing directly into pots is also possible because of a high germination rate. Seedlings suitable for transplanting have been produced in less than 9 months.
Genetic resources and breeding
Locally Acacia nigrescens has been subjected to heavy cutting, but in general it is still fairly common.
Acacia nigrescens will remain a source of durable timber for local use, e.g. for fence poles and flooring. It does not seem to be a promising plantation tree because of its slow growth.
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.
• Ross, J.H., 1968. Acacia nigrescens Oliv. in Africa, with particular reference to Natal. Boletim da Sociedada Broteriana, 2a sιrie, 42: 181–205.
• Timberlake, J., Fagg, C. & Barnes, R., 1999. Field guide to the Acacias of Zimbabwe. CBC Publishing, Harare, Zimbabwe. 160 pp.
Other references
• Bleys, J.A., Mazibuko, W.K.M. & Allen, J.A., 1982. The silviculture of indigenous and exotic trees other than pines and eucalypts in Swaziland. South African Forestry Journal 121: 24–27.
• Bolza, E. & Keating, W.G., 1972. African timbers: the properties, uses and characteristics of 700 species. Division of Building Research, CSIRO, Melbourne, Australia. 710 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.
• Gelfand, M., Mavi, S., Drummond, R.B. & Ndemera, B., 1985. The traditional medical practitioner in Zimbabwe: his principles of practice and pharmacopoeia. Mambo Press, Gweru, Zimbabwe. 411 pp.
• Greenway, P.J., 1941. Dyeing and tanning plants in East Africa. Bulletin of the Imperial Institute 39: 222–245.
• Howell, H., Malan, E., Steenkamp, J.A., Brandt, E.V. & Brand, J., 2002. Identification of two novel promelacacinidin dimers from Acacia nigrescens. Journal of Natural Products 65: 769–771.
• Neuwinger, H.D., 2000. African traditional medicine: a dictionary of plant use and applications. Medpharm Scientific, Stuttgart, Germany. 589 pp.
• Takahashi, A., 1978. Compilation of data on the mechanical properties of foreign woods (part 3) Africa. Shimane University, Matsue, Japan, 248 pp.
• Williamson, J., 1955. Useful plants of Nyasaland. The Government Printer, Zomba, Nyasaland. 168 pp.
• R.H.M.J. Lemmens
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

• D. Louppe
CIRAD, Dιpartement Environnements et Sociιtιs, Cirad es-dir, Campus international de Baillarguet, TA C-DIR / B (Bβt. C, Bur. 113), 34398 Montpellier Cedex 5, France
• A.A. Oteng-Amoako
Forestry Research Institute of Ghana (FORIG), University P.O. Box 63, KNUST, Kumasi, Ghana
• M. Brink
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
General editors
• R.H.M.J. Lemmens
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
• L.P.A. Oyen
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
• J.R. Cobbinah
Forestry Research Institute of Ghana (FORIG), University P.O. Box 63, KNUST, Kumasi, Ghana

Correct citation of this article:
Lemmens, R.H.M.J., 2006. Acacia nigrescens Oliv. In: Louppe, D., Oteng-Amoako, A.A. & Brink, M. (Editors). Prota 7(1): Timbers/Bois d’œuvre 1. [CD-Rom]. PROTA, Wageningen, Netherlands.