Prota 7(1): Timbers/Bois d’œuvre 1
Bull. Soc. Bot. France 58, Mém. 8: 138 (1912).
Sterculiaceae (APG: Malvaceae)
2n = 48
Mansonia, African black walnut, African walnut (En). Bété, mansonia (Fr).
Origin and geographic distribution
Mansonia altissima occurs from Guinea and Côte d’Ivoire east to the Central African Republic and northern Congo.
The wood (trade names: mansonia, bété, African black walnut, pruno) is used for general and high-class joinery, cabinet work, furniture, turnery, decorative veneer and handicrafts. It is also used in construction for doors and windows, in railway coaches and shop fittings, and for boxes and crates. Well-coloured wood resembles American black walnut and is commonly used as a substitute, e.g. for gun stocks and grips, musical instruments and loudspeaker enclosures.
Wood waste can be used as a substrate for the edible fungus Pleurotus tuber-regium. Preliminary observations indicate that the fermented substrate has some value as cattle feed.
The bark is very poisonous and in parts of south-western Côte d’Ivoire it is the main component of a very effective arrow-poison, also used in spear traps for large game. In other parts of its range the bark is a component of arrow poisons too. In Nigeria and Ghana products from the bark have been used in the treatment of leprosy. A bark extract is drunk or an infusion of the root is applied as enema as an aphrodisiac. A root decoction is given as enema against leprosy. A decoction of the twig bark is applied as a bath against yaws, scabies and syphilis.
Production and international trade
The wood of Mansonia altissima was first exported from Nigeria in the 1930s as a substitute for walnut; at that time the sustainable annual yield was estimated to 4800 m³ per year. Between 1959 and 1970, however, Nigeria exported 15,500 m³ of logs and 800 m³ of sawn wood per year. Côte d’Ivoire exported about 131,400 m³ of logs and 2700 m³ of sawn wood per year in 1967–1972. In 1994 Côte d’Ivoire exported 314 m³ of veneer. Ghana exported 2700 m³ of logs in 1998 and only 300 m³ of logs in 2000 and in 2001. In 2004 Ghana exported 1000 m³ of plywood at an average price of US$ 367/m³. Cameroon exported 1900 m³ of sawn wood in 2003. Europe and the United States are the main importers.
The heartwood is yellowish brown to dark grey-brown or even dark brown, often with purple, reddish or greyish green streaks, often in alternating light and dark bands. It fades on exposure to a somewhat dull brown. It is distinctly demarcated from the 2–4(–6) cm wide, white to pinkish sapwood. The grain is usually straight, texture fine. The wood is moderately lustrous.
The wood is of medium weight with a density at 12% moisture content of 590–720 kg/m³. It dries fairly rapidly with little distortion or degrade. In Congo it takes about 15 weeks to air dry boards 5.5–6 cm thick from 75% to 23% moisture content. Shrinkage from green to oven dry is (2.6–)4.1–5.7(–7.0)% radial and (5.2–)7.1–9.7% tangential. Movement in service is medium.
At 12% moisture content the modulus of rupture is (61–)114–177(–183) N/mm², modulus of elasticity 9320–12,800 N/mm², compression parallel to grain 43–68(–96) N/mm², shear 6–15 N/mm², cleavage 9–23 N/mm, Janka side hardness 5690–7470 N and Janka end hardness 5740–7470 N.
The wood is easy to work with hand and machine tools with little or only moderate blunting effect on cutting edges and no charring. The surface can be worked to a good finish. The wood holds nails and screws fairly well, but iron tends to corrode in contact with the wood. Gluing properties are good. The wood takes stain and polish well. The wood peels well, but requires softening beforehand; veneer is rather fragile. Steam-bending is easy unless knots are present.
The heartwood is very durable, being resistant to fungi, borers and termites; in logs the sapwood is susceptible to pinhole borer and longhorn beetle attacks. The heartwood is extremely resistant to impregnation with preservatives, sapwood is permeable.
In processing the wood causes high levels of dust, and it may cause serious health problems to workers. The dust may cause dermatitis, nose-bleeding, throat irritation, asthma and eczema, the reactions becoming stronger on repeated exposure. Heart troubles have also been reported. Pigs given the sawdust as litter developed severe contact dermatitis and piglets even died. The toxins in the wood, even in the form of thin veneer, are resistant to oxidation and are heat-stable; they remain effective for many years.
An ethanolic extract of the wood showed hepatotoxic and haematotoxic effects when administered orally to rats. Ethanol and chloroform extracts of the bark have shown a very strong toxicity against several mammals. Extracts of the bark also inhibit the growth of Mycobacterium tuberculosis. Because of its antibacterial properties, addition of small amounts of bark to the diets of chicken, as a substitute of commercial antibiotics, improved feed conversion rates.
The bark contains the highly toxic compound mansonine, related to cardenolides from Digitalis and ouabain from Strophanthus. Its aglycon was identified as strophanthidin. In addition some 30 toxic glycosides have been identified.
From the wood similar glycosides, an azulene derivative, and a range of quinones, called mansonones A–I and L, have been isolated. Mansonone A is the main causal agent of the allergic reaction to the sawdust. The seeds too contain a large number of cardenolide glycosides, including glycosides of strophanthidin and nigrescigenin. The glycosides strophothevoside and strophalloside are about as toxic to mammals as ouabain.
Evergreen medium-sized to fairly large tree up to 45 m tall; bole branchless for up to 30 m, up to 100(–150) cm in diameter, generally straight, cylindrical, sometimes with narrow buttresses; bark surface fissured lengthwise, clear brown, inner bark yellowish; crown small, ovoid, dense, with branches almost horizontal, later drooping; branchlets hairy or glabrous. Leaves alternate, simple; stipules present, early falling; petiole 2–5 cm long, hairy; blade obovate to orbicular, 15–30 cm × 8–15 cm, cordate at base, rounded and sometimes short-acuminate at apex, margin slightly wavy or toothed, papery, densely hairy below, with 6–7 basal veins and 4–5 pairs of lateral veins. Inflorescence a large, stalked, terminal cyme 12–15 cm long, densely short-hairy, many-flowered. Flowers bisexual, fragrant; pedicel c. 7 mm long; calyx c. 1 cm long, split unilaterally, folded back, stellate hairy; petals 5, obovate, c. 12 mm × 6 mm, glabrous, white, twisted in bud; androgynophore well-developed, c. 18 mm long; stamens 10 in a single circle, nearly sessile; staminodes 5, scale-like; ovary superior, consisting of 5 free carpels, velvety hairy, each carpel with a threadlike, flexuous style with minute stigma. Fruit consisting of 1–2 (–3) ovoid nuts c. 0.5 cm in diameter, surface reticulate, with a large papery wing 5–6 cm × c. 2 cm. Seedling with epigeal germination.
Other botanical information
Mansonia comprises 4 or 5 species, 2 in Africa, 1 in India and 1 in Myanmar and Thailand. In Mansonia altissima 2 varieties are recognized: var. altissima occurring from Liberia to Nigeria, and var. kamerunica Jacq.-Fél. which occurs from Nigeria east to the Central African Republic. The latter is characterized by glabrous branchlets. Mansonia nymphaeifolia Mildbr., which is endemic to Cameroon, is doubtfully different from Mansonia altissima var. kamerunica. Mansonia diatomanthera Brenan is a large tree known only from 2 or 3 locations in Tanzania. The use of its wood has not been recorded, but a bark infusion is used as a bath to treat scabies. Mansonia diatomanthera is critically endangered.
Wood-anatomical description (IAWA hardwood codes):
Growth rings: (1: growth ring boundaries distinct); (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); 24: intervessel pits minute (≤ 4 μm; 30: vessel-ray pits with distinct borders; similar to intervessel pits in size and shape throughout the ray cell; 41: mean tangential diameter of vessel lumina 50–100 μm; 42: mean tangential diameter of vessel lumina 100–200 μm; 47: 5–20 vessels per square millimetre; (48: 20–40 vessels per square millimetre). Tracheids and fibres: 61: fibres with simple to minutely bordered pits; 66: non-septate fibres present; 69: fibres thin- to thick-walled. Axial parenchyma: 76: axial parenchyma diffuse; 77: axial parenchyma diffuse-in-aggregates; 78: axial parenchyma scanty paratracheal; (91: two cells per parenchyma strand); 92: four (3–4) cells per parenchyma strand. Rays: 97: ray width 1–3 cells; 106: body ray cells procumbent with one row of upright and/or square marginal cells; 107: body ray cells procumbent with mostly 2–4 rows of upright and/or square marginal cells; 115: 4–12 rays per mm. Storied structure: 118: all rays storied; 120: axial parenchyma and/or vessel elements storied; 121: fibres storied. Mineral inclusions: 136: prismatic crystals present; 137: prismatic crystals in upright and/or square ray cells; (138: prismatic crystals in procumbent ray cells); (140: prismatic crystals in chambered upright and/or square ray cells); (141: prismatic crystals in non-chambered axial parenchyma cells).
(P. Ng’andwe, H. Beeckman & P.E. Gasson)
Growth and development
Seedlings attained a height of 1–2 m in 4 years in natural forest and 6 m in 4 years in logged areas. In Kumasi (Ghana) trees attained a bole diameter of 6.5–16 cm and a height of 8–15 m in 10 years. For plantations in Cameroon faster growth has been recorded, with trees growing 2 m in height per year after 2 years decreasing to 1.3 m per year after 10 years. In Côte d’Ivoire mean annual bole diameter growth of Mansonia altissima trees for all diameter classes was 2.5–7 mm. Mean growth for the diameter class 30–50 cm was 5.1 mm per year. An average tree would reach a bole diameter of 50 cm in 90 years.
In Ghana flowering occurs from May to October with a peak in June and fruits ripen in July–April. The fruits are dispersed by wind at the end of the dry season.
Mansonia altissima is characteristic of the dense semi-deciduous forest in areas with about 1600 mm annual rainfall and a pronounced dry season. In Côte d’Ivoire the southern limit of its distribution area largely corresponds with the transition of semi-deciduous forest to evergreen forest; to the north its distribution extends to patches of dense forest in the savanna. Seedlings are most common on fertile soils on drier sites; they are fairly drought resistant. Seedlings planted in the wet forest zone died within a few years. In Ghana Mansonia altissima is most common in the semi-deciduous forest of the north-western part of the country, where it occurs mainly in exposed conditions. In southern Ghana seedlings up to 1 m tall are most common in disturbed forest, while smaller seedlings are more common in undisturbed forest. Adult trees are more common in logged or burnt forest than in undisturbed forest.
Propagation and planting
The 1000-seed weight is about 330 g. Germination starts after about 10 days and may take one week, but a dormancy period of several months has also been observed. Dormancy caused little loss of seed viability. Germination occurs under both light and dark conditions, but is strongly depressed in large gaps in the forest. During the first 2 years after germination slight shade stimulates growth and the formation of healthy leaves. Leaves are held vertically in exposed conditions, more horizontally in the shade. After 2 years Mansonia altissima requires full sunlight and it has been classified as a non-pioneer light demander.
Mansonia altissima has been tried in plantations and in enrichment plantings, but no results are known. In Cameroon 420 ha have been planted, and in Côte d’Ivoire 78 ha. It is occasionally retained or grown in agroforestry systems.
Diseases and pests
Ambrosia beetles attack the bole of the standing tree. Caterpillars of the moth Godasa sidae may cause complete defoliation in plantations. Wood-boring caterpillars of Eulophonotus spp. may cause damage to the sapwood. Larvae of the polyphagous grasshopper Zonocerus variegatus may seriously attack the foliage of especially older trees. In plantations they may become serious pests.
The minimum felling diameter is 40 cm in the Central African Republic, 50 cm in Côte d’Ivoire, 60 cm in Liberia and Cameroun, and 70 cm in Ghana.
A tree 40 cm in diameter yields about 1.8 m³ of timber, trees 50, 60 and 70 cm in diameter yield 2.8 m³, 4.1 m³ and 5.6 m³, respectively.
Handling after harvest
Freshly cut logs are usually too heavy to be transported by river.
Mansonia altissima is protected by law in Côte d’Ivoire. In Ghana it has been ranked as a pink star species, which means that it is considered common and moderately exploited. In 1984 FAO recommended its in-situ conservation.
The wood of Mansonia altissima is likely to remain valuable for furniture and decoration, for which there is a stable market and growing demand. Because reliable statistics on production are lacking, it is not possible to estimate the amounts of Mansonia altissima timber that can be extracted sustainably. Because of its high value, it is recommended to test and include Mansonia altissima in plantation and enrichment programmes or in agroforestry systems.
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Sources of illustration
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Correct citation of this article:
Ohene-Coffie, F., 2008. Mansonia altissima (A.Chev.) A.Chev. In: Louppe, D., Oteng-Amoako, A.A. & Brink, M. (Editors). Prota 7(1): Timbers/Bois d’œuvre 1. [CD-Rom]. PROTA, Wageningen, Netherlands.
1, flowering twig; 2, fruit.
Redrawn and adapted by Iskak Syamsudin
bark and slash
leaves and fruits
obtained from Arnhemse Fijnhouthandel
wood in transverse section
wood in tangential section
wood in radial section
transverse surface of wood