Acacia xanthophloea (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
|Carbohydrate / starch|
|Essential oil / exudate|
|Forage / feed|
Acacia xanthophloea Benth.
- Protologue: Trans. Linn. Soc. London 30(3): 511 (1875).
- Family: Mimosaceae (Leguminosae - Mimosoideae)
- Chromosome number: 2n = 52
- Fever tree, sulphur bark, African thorn acacia (En).
- Arbre à fièvre (Fr).
- Árvore da febre, camba (Po).
- Mgunga, mukonge (Sw).
Origin and geographic distribution
Acacia xanthophloea occurs from Somalia south to northern South Africa and Swaziland. It is occasionally planted as an ornamental tree outside its natural distribution area, e.g. in the United States and Taiwan.
Acacia xanthophloea produces a general-purpose timber, which is used in construction and for carpentry, boat building, boxwood, furniture, mortars, domestic utensils, troughs and fence poles. The wood is used as firewood, although the gum leaves a black tar-like deposit when burnt. It makes good quality charcoal. Acacia xanthophloea is a popular garden tree and has been grown in hedges in Tanzania. In Kenya a bark decoction is used in traditional medicine to treat indigestion, and in Tanzania to treat sickle cell anaemia. In South Africa Zulu people use powdered bark as an emetic to treat malaria, and also against eye complaints. The gum, which occurs in large quantities on the trunk, is reportedly edible and is eaten by monkeys. Leafy branches are given as fodder to livestock. Trees produce good bee forage.
Production and international trade
Acacia xanthophloea bark is traded on local markets in South Africa, with an estimated trade volume of 7500 kg in 1988. It is also imported from Mozambique.
The heartwood is pale brown with a reddish tinge and distinctly demarcated from the wide, paler sapwood. It is fairly heavy, with a density of about 900 kg/m³ at 12% moisture content. It should be dried with care because it is prone to splitting and cracking. It finishes smoothly. The wood is susceptible to borer attack.
The bark contains tannin in considerable quantity, but it does not produce good leather. Bark extracts showed weak in-vitro antimalarial activity and considerable antimicrobial activity. Leaf extracts showed antibacterial activity against Staphylococcus aureus and Escherichia coli. The gum is water soluble and contains galactose, arabinose, rhamnose, glucuronic acid and 4-O-methyl-a- D-glucuronic acid.
- Medium-sized tree up to 25 m tall; bole straight, up to 60 cm in diameter; bark smooth and powdery, lemon yellow to greenish yellow; crown open, with spreading branches; young branches first purple then yellowish, with paired, straight stipular spines up to 7(–8.5) cm long.
- Leaves alternate, bipinnately compound, with 3–6(–8) pairs of pinnae; rachis (2.5–)3–7 cm long, almost glabrous; leaflets in 8–19 pairs per pinna, obliquely oblong, 2.5–6.5 mm × 1–2 mm, glabrous.
- Inflorescence an axillary globose head c. 12 mm in diameter, usually in clusters on older twigs; peduncle 1.5–2.5 cm long, with a pair of bracts c. 3 mm long in basal half.
- Flowers bisexual, regular, usually 5-merous, small, creamy white to pinkish or purplish or yellow, fragrant, sessile; calyx cup-shaped, 1–1.5 mm long; corolla shortly lobed, glabrous; stamens numerous, free, up to 5 mm long; ovary superior, 1-celled, style slender.
- Fruit a linear-oblong pod 4–13.5 cm × 0.5–1.5 cm, straight or slightly curved, pale brown, constricted between the seeds, indehiscent but breaking up into segments when fallen, (3–)5–10-seeded.
- Seeds orbicular to ellipsoid, compressed, 4.5–5.5 mm × 3.5–4 mm, smooth, brown.
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 xanthophloea belongs to subgenus Acacia, which comprises all African Acacia species with straight spinescent stipules. Acacia xanthophloea is very characteristic with its smooth, yellow bark and indehiscent pods.
Wood-anatomical description (IAWA hardwood codes):
- Growth rings: 2: growth ring boundaries indistinct or absent.
- Vessels: 5: wood diffuse-porous; 13: simple perforation plates; 22: intervessel pits alternate; (23: shape of alternate pits polygonal); 25: intervessel pits small (4–7 µm); 26: intervessel pits medium (7–10 µm); 29: vestured pits; 30: vessel-ray pits with distinct borders; similar to intervessel pits in size and shape throughout the ray cell; 42: mean tangential diameter of vessel lumina 100–200 µm; 47: 5–20 vessels per square millimetre.
- Tracheids and fibres: 61: fibres with simple to minutely bordered pits; 66: non-septate fibres present; 70: fibres very thick-walled.
- Axial parenchyma: 78: axial parenchyma scanty paratracheal; 79: axial parenchyma vasicentric; (80: axial parenchyma aliform); (81: axial parenchyma lozenge-aliform); 83: axial parenchyma confluent; 90: fusiform parenchyma cells; 91: two cells per parenchyma strand.
- Rays: 98: larger rays commonly 4- to 10-seriate; 104: all ray cells procumbent; 115: 4–12 rays per mm.
- Mineral inclusions: 136: prismatic crystals present; 142: prismatic crystals in chambered axial parenchyma cells; (156: crystals in enlarged cells).
Growth and development
The growth rate of seedlings is fast, to a maximum of 7 m tall in 3 years. A growth rate of 1.5 m/year and 2 cm in diameter are common in young trees. The tree is deciduous. The flowers are pollinated by insects such as bees and butterflies. In southern Africa flowering occurs in September–November, fruiting in January–April. Acacia xanthophloea suffers stripping of bark, browsing and breakage by elephants, but exhibits high resilience to disturbance. The pods are a favourite food of vervet monkeys, which may play a role in seed dispersal. The light pod segments are probably also dispersed by wind and water.
Acacia xanthophloea occurs in swampy localities and riverine forest, often on flood plains or in depressions, up to 2100 m altitude. It often grows on alluvial black clay soils. This preferred habitat, which offers ideal breeding possibilities for malaria mosquitoes, is the reason that Acacia xanthophloea was associated with malaria; hence the name ‘fever tree’. It often grows gregariously and is often even dominant. The tree can tolerate moderate frost. It is nodulated by nitrogen-fixing Rhizobium bacteria.
Propagation and planting
Seed production is often poor as a result of predation by animals. There are 24, 000–30,000 seeds per kg. Germination is generally fair, reaching about 70% after 2 weeks. Seed should be soaked in water for 24 hours, in hot water overnight, or be mechanically scarified. Seeds can be stored for a long time in a dry place, but they are susceptible to insect damage. When seedlings have reached the 2-leaf stage 6–8 weeks after sowing, they should be transplanted from seedling trays into nursery bags. Care should be taken not to damage the long taproot. Acacia xanthophloea can be propagated by cuttings.
Acacia xanthophloea often occurs in apparently even-aged stands of large trees with few young plants, probably as a result of prolific regeneration after flooding in the past. Natural stands of Acacia xanthophloea in South Africa comprise about 85 stems of all size classes per ha.
The bark is removed with a knife and bark collectors usually focus on larger-sized trees. The tolerance of Acacia xanthophloea to damage is high, and usually trees recover from bark removal as well as from elephant damage. However, excessive destructive harvesting is locally common.
In many riverine localities Acacia xanthophloea occurs gregariously. However, in many regions riverine woodlands have declined as a result of clearing for agriculture, changes in climatic conditions and soil salinity, and because of tree damage by growing elephants populations and overgrazing by livestock. Locally, large Acacia xanthophloea trees have been felled for timber. In South Africa there is considerable pressure on Acacia xanthophloea populations because of excessive bark harvesting in a non-sustainable way.
Acacia xanthophloea is a multipurpose tree that is important for people as a source of timber, firewood, medicine and forage. It is a characteristic element of riverine habitats in eastern and southern Africa, a popular tree amongst birds for nest building and an important food source for animals such as vervet monkey and elephant, and as such it plays an important ecological role. Monitoring of natural populations is needed, and cultivation of this fast-growing species should be encouraged. In regions where bark is harvested for medicinal purposes on a large scale, methods of sustainable harvesting should be developed and promoted. After harvesting, bark should be given sufficient time to grow back. With its open crown Acacia xanthophloea is a promising plantation tree in agroforestry systems, and its popularity as an ornamental tree is also likely to increase.
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- Chhabra, S.C. & Uiso, F.C., 1991. Antibacterial activity of some Tanzanian plants used in traditional medicine. Fitoterapia 62(6): 499–503.
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- Katerere, D.R. & Eloff, J.N., 2004. Variation in chemical composition, antibacterial and antioxidant activity of fresh and dried acacia leaf extracts. South African Journal of Botany 70(2): 303–305.
- Mugedo, J.Z.A. & Waterman, P.G., 1992. Sources of tannin: alternatives to wattle (Acacia mearnsii) among indigenous Kenyan species. Economic Botany 46(1): 55–63.
- Neuwinger, H.D., 2000. African traditional medicine: a dictionary of plant use and applications. Medpharm Scientific, Stuttgart, Germany. 589 pp.
- Odee, D.W., Haukka K., McInroy, S.G., Sprent J.I., Sutherland, J.M. & Young, J.P.W., 2002. Genetic and symbiotic characterization of rhizobia isolated from tree and herbaceous legumes grown in soils from ecologically diverse sites in Kenya. Soil Biology and Biochemistry 34(6): 801 811.
- Otieno, D.O., Schmidt, N.W.T., J.I. Kinyamario J.I. & J. Tenhunen, J., 2005. Responses of Acacia tortilis and Acacia xanthophloea to seasonal changes in soil water availability in the savanna region of Kenya. Journal of Arid Environments 62(3): 377–400.
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Sources of illustration
- Timberlake, J., Fagg, C. & Barnes, R., 1999. Field guide to the Acacias of Zimbabwe. CBC Publishing, Harare, Zimbabwe. 160 pp.
- 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 xanthophloea Benth. 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 26 November 2017.
- See the Prota4U database.