Prota 7(1): Timbers/Bois d’œuvre 1
Veg. Ut. Afr. Trop. Franç. 5: 207 (1909).
2n = 50
Red mahogany, Lagos mahogany (En). Acajou rouge, acajou du Gabon, acajou Bassam, acajou à peau rugueuse (Fr). Acaju de Bassan (Po).
Origin and geographic distribution
Khaya ivorensis is distributed from Côte d’Ivoire east to Cameroon and south to Cabinda (Angola); it possibly also occurs in Guinea, Liberia, the Central African Republic and Congo. It is fairly widely grown in plantations within its natural area of distribution, but also in tropical Asia and tropical America.
The wood (trade names: African mahogany, acajou d’Afrique) is highly valued for furniture, cabinet work, decorative boxes and cases, and veneer, and is also commonly used for window frames, panelling, doors and staircases. It is suitable for light construction, light flooring, ship building, vehicle bodies, handles, ladders, sporting goods, musical instruments, toys, novelties, precision equipment, carving, turnery and pulpwood. The wood is in demand for making backs or sides of acoustic guitars as it is considered to have good acoustical characteristics. Traditionally, the wood is used for dugout canoes. It is also used as fuelwood and for charcoal production.
The bitter-tasting bark is widely used in traditional medicine. Bark decoctions are taken to treat cough, fever and anaemia, and are applied externally to wounds, sores, ulcers and tumours, and as an anodyne to treat rheumatic pains and lumbago. Root pulp is applied as an enema to treat dysentery. Ground young shoots and leaves are applied externally as an anodyne. The seeds are used in soap production. In Nigeria Khaya ivorensis trees are locally retained in cocoa plantations to serve as shade trees and ultimately for timber production.
Production and international trade
Khaya ivorensis wood is exported from West African countries in mixed consignments with other Khaya spp., particularly Khaya anthotheca (Welw.) C.DC. Côte d’Ivoire has long been the main exporter of Khaya wood: as early as in 1925 it exported around 10,000 m³ per year, and during in 1965–1974 it exported more than 1.5 million m³ Khaya logs and about 115,000 m³ of sawn wood, more than all other African countries together. Exports from Côte d’Ivoire have decreased since then, but 41,000 m³ of sawn Khaya wood was exported in 2004, at an average price of US$ 397/m³, and 34,000 m³ in 2005, at an average price of US$ 439/m³. Ghana exported 11,000 m³ of sawn Khaya wood in 2003, at an average price of US$ 714/m³, 14,000 m³ in 2004, at an average price of US$ 527/m³, and 17,000 m³ in 2005, at an average price of US$ 755/m³. The export of Khaya veneer from Ghana was 4000 m³ in 2003, at an average price of US$ 443/m³, 6000 m³ in 2004, at an average price of US$ 1677/m³, and 5000 m³ in 2005, at an average price of US$ 1938/m³. The proportion of Khaya ivorensis in these amounts is obscure. Cameroon exported 11,000 m³ of sawn Khaya wood in 2003, and 8600 m³ in 2004 and 2006. Exports of logs from Gabon amounted to 21,300 m³ in 2000, 18,700 m³ in 2001, 14,300 m³ in 2002, 17,800 m³ in 2003, and 18,600 m³ in 2004. In recent years, the United States market has dominated the international trade in Khaya timber, especially as a substitute for American mahogany (from Swietenia), the availability of which has declined considerably.
The heartwood is pale pinkish brown to pale red, darkening to deep brown with a golden lustre upon exposure. It is more or less distinctly demarcated from the creamy white, up to 5 cm wide sapwood. The grain is straight or interlocked, texture rather coarse.
The wood is medium-weight, with a density of (420–)460–570 kg/m³ at 12% moisture content. It generally air dries and kiln dries easily with little degrade, but some warping may occur due to the presence of interlocked grain. The rates of shrinkage are moderate, from green to oven dry 2.2–4.1(–5.0)% radial and 5.0–6.9(–8.4)% tangential. Once dry, the wood is fairly stable in service.
At 12% moisture content, the modulus of rupture is 71–126 N/mm², modulus of elasticity 8700–10,800 N/mm², compression parallel to grain 37–48 N/mm², compression perpendicular to grain 4–8 N/mm², shear 8–12 N/mm², cleavage 10–17 N/mm, Janka side hardness 3210–3700 N and Janka end hardness 4810 N.
The wood is usually fairly easy to saw and work, although the presence of interlocked grain may cause some difficulties. Saws should therefore be kept sharp to prevent a woolly finish and a cutting angle of 15–20° is recommended. The wood can be finished to a smooth surface, but the use of a filler is required in staining and varnishing. The wood holds nails and screws well and glues satisfactorily. The bending properties are poor. The wood peels and slices well, producing an excellent quality of veneer. It turns fairly well. The wood dust may cause irritation to the skin.
The wood is moderately durable and can be susceptible to termite and pinhole borer attacks. The heartwood is strongly resistant to impregnation, the sapwood moderately resistant. The wood is suitable for paper production, and even peeler cores, often regarded as waste, are suitable for pulp production.
Limonoids have been isolated from the bark and seeds. Some of these showed significant antifeedant activity in insects, and some antifungal and antibacterial activities. Bark extracts showed weak antitrypanosomal and antiplasmodial activities in tests with mice. Tests in rats showed that the bark has dose-dependent anti-inflammatory activity and that it is toxic only at high doses. The bark showed anticonvulsant activity in mice. The seeds contain 17–27% oil, with palmitic acid, oleic acid and linoleic acid as the dominant fatty acids. Another analysis indicated an oil content of about 48% at 7% moisture content, and a fatty acid composition of: palmitic acid 7%, stearic acid 32%, oleic acid 15% and linoleic acid 45%. The oil can be used as an additive in liquid soaps, and may act as an antibacterial and antifungal agent because of the presence of limonoids such as methylangolensate. In Nigeria the bark was found to contain 27% extractable tannins. Smoke from the wood showed good results in tests of smoking fish, protecting the fish effectively against fungi.
Adulterations and substitutes
The wood of Khaya anthotheca is very similar to that of Khaya ivorensis. The wood of both species and that of Khaya grandifoliola C.DC. is exported from West Africa in mixed consignments as ‘African mahogany’ or ‘acajou d’Afrique’. The wood of makore (Tieghemella) is similar, but more durable.
Evergreen or deciduous, monoecious, large to very large tree up to 60 m tall; bole branchless for up to 30 m, usually straight and cylindrical, up to 160(–210) cm in diameter, with large buttresses up to 2(–4) m high, sometimes extending into prominent surface roots; bark surface brown and slightly rough, exfoliating in small circular scales leaving a pock-marked, mottled greyish brown and orange brown surface, inner bark pink to reddish; crown massive, rounded; twigs glabrous. Leaves arranged spirally but clustered near ends of branches, paripinnately compound with (3–)4–7 pairs of leaflets; stipules absent; petiole 1–4 cm long, rachis 6–20 cm long; petiolules 0.5–1 cm long; leaflets opposite, oblong to oblong-elliptical, 5–14 cm × 2–6 cm, cuneate to obtuse and slightly asymmetrical at base, distinctly acuminate at apex, margins entire, leathery, glabrous, pinnately veined with 5–10 pairs of lateral veins. Inflorescence an axillary panicle up to 20 cm long. Flowers unisexual, male and female flowers very similar in appearance, regular, (4–)5-merous, whitish, sweet-scented; pedicel 1–3 mm long; calyx lobed almost to the base, with rounded lobes c. 1 mm long; petals free, elliptical, c. 4 mm × 2 mm, somewhat hooded; stamens fused into an urn-shaped tube c. 5 mm long, with (8–)10 included anthers near apex, alternating with rounded lobes; disk cushion-shaped; ovary superior, globose to conical, 1–2 mm in diameter, 5-celled, style up to 1 mm long, stigma disk-shaped; male flowers with rudimentary ovary, female flowers with smaller, non-dehiscing anthers. Fruit an erect, nearly globose, woody capsule 5–7 cm in diameter, greyish brown, dehiscent by 5 valves, many-seeded. Seeds disk-shaped or quadrangular, strongly flattened, 2–2.5 cm × 2.5–3.5 cm, narrowly winged all around the margin, brown. Seedling with hypogeal germination, cotyledons remaining enclosed in the seed coat; epicotyl 5–10 cm long; first 2 leaves opposite, simple.
Other botanical information
Khaya comprises 4 species in mainland Africa and 1 or 2 endemic to the Comoros and Madagascar. It belongs to subfamily Swietenoideae and seems most closely related to Carapa and Swietenia. Khaya species strongly resemble each other in flowers and fruits, and differences are most prominent in their leaflets. There appears to be a more or less gradual transition in species according to ecological gradients, from the moist evergreen forest zone through semi-deciduous forest to the savanna zone. Studies on anatomical, chemical and physical properties of the wood confirmed the close relationship of Khaya species, with main differences in fibre lumen/wall ratio, percentage multiseriate rays and wood density, which could also be explained, at least partly, by ecological conditions. Khaya anthotheca (Welw.) C.DC. is very close to Khaya ivorensis, but differs, apart from its ecological requirements, in its more ovate-elliptical, short-acuminate leaflets and light-coloured, more smooth bark.
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; 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; 42: mean tangential diameter of vessel lumina 100–200 μm; (43: mean tangential diameter of vessel lumina ≥ 200 μm); 46: ≤ 5 vessels per square millimetre; 47: 5–20 vessels per square millimetre; 58: gums and other deposits in heartwood vessels. Tracheids and fibres: 61: fibres with simple to minutely bordered pits; (65: septate fibres present); 66: non-septate fibres present; 69: fibres thin- to thick-walled. Axial parenchyma: 78: axial parenchyma scanty paratracheal; 79: axial parenchyma vasicentric; (89: axial parenchyma in marginal or in seemingly marginal bands); 93: eight (5–8) cells per parenchyma strand. Rays: 98: larger rays commonly 4- to 10-seriate; (103: rays of two distinct sizes); 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. Secretory elements and cambial variants: 131: intercellular canals of traumatic origin. Mineral inclusions: 136: prismatic crystals present; 137: prismatic crystals in upright and/or square ray cells; (140: prismatic crystals in chambered upright and/or square ray cells).
(N.P. Mollel, P. Détienne & E.A. Wheeler)
Growth and development
In nurseries in Côte d’Ivoire 1-year-old Khaya ivorensis seedlings were 90–100 cm tall, 2-year-old saplings 2.5–3 m, 4-year-old saplings 4–6 m, and 6-year-old saplings were 12–15 m tall and 10–18 cm in diameter. Trees planted in the open in the evergreen forest zone reached an average height of 12 m and an average bole diameter of 15 cm after 8 years. In mixed plantations the average bole diameter was 39 cm at 27 years after planting, with dominant trees being 28 m tall and 47 cm in diameter. At 34 years after planting dominant trees were 76 cm in diameter, but the bole was branchless for only 12 m. In Nigeria the average height of saplings was 4.5 m after 4 years, with an average stem diameter of 8 cm. In 26-year-old plantations in Malaysia, a mean annual increment of 1.8 cm in diameter and 1 m in height was achieved. In a 40-year-old plantation in Malaysia, trees had an average height of 23.5 m and average bole diameter of 29.5 cm, with dominant trees 30 m and 47 cm, respectively. Young trees have a slender stem and a small crown. Extensive lateral growth starts when the upper canopy of the forest has been reached. The tree develops according to Rauh’s architectural tree model, characterized by a monopodial trunk which grows rhythmically and so develops tiers of branches. The monoaxial state may persist to a height of 10 m.
Trees are sometimes leafless for a short period at the beginning of the dry season. In moist evergreen forest Khaya ivorensis can be found flowering and fruiting throughout the year and bearing flowers and fruits at the same time; usually flowering is seasonal, in West Africa being most abundant in June–October. Fruits mature in about 6 months. In Côte d’Ivoire there are generally 2 fruiting periods: February–April and July–August; in Ghana the main fruiting period is February–May. Trees of 30 years old may produce fruits and seeds abundantly. Mass production of seeds is reported to occur every 3–4 years. Dispersal of the seeds is by wind, but most seeds fall close to the parent tree.
Khaya ivorensis is most abundant in evergreen forest, but can also be found in moist semi-deciduous forest, in areas with 1600–2500 mm annual rainfall and a dry season of 2–3 months, up to 700 m altitude. In moist semi-deciduous forest it may occur together with Khaya anthotheca. Khaya ivorensis often occurs along watercourses. It prefers alluvial soils which are moist but well-drained, but it can also be found on slopes on lateritic soils. Seeds can germinate in full sun as well as in the shade, but natural regeneration is apparently sparse in large gaps. Seedlings can survive in dense shade, but for good growth opening of the forest canopy is needed. Regeneration of Khaya ivorensis is not promoted by large disturbances in the forest, but it benefits from small gaps.
Propagation and planting
Khaya ivorensis is propagated by seed. The 1000-seed weight is 130–310 g. The seeds are often already attacked by insects while they are still on the tree, and undamaged seeds should therefore be selected before sowing. The optimal temperature and moisture content for storage of seeds were found to be 3°C and 6%, respectively. The seeds are best sown in seed beds in the nursery. Germination is rather slow, taking 11–40 days. The germination rate of fresh healthy seed is high, nearly 100%, but it decreases rapidly and after 3 months viability of the seeds is only 5%. In the nursery, light shade is advantageous for seedlings up to 2 years old; this reduces attacks by Hypsipyla shoot borers and the development of leaf galls. The application of 0.5 g of inorganic fertilizer to seedlings has been recommended, promoting height and collar diameter growth. Seedlings are planted out when 60–90 cm tall, usually as stumps or striplings.
In experiments vegetative propagation by means of cuttings from seedlings less than 2 years old was successful with the application of auxin (IBA) at a concentration of 200 μg per cutting. Cuttings from basal nodes rooted better than those from apical nodes, longer (4 cm long) cuttings better than shorter (2 cm long) ones, whereas trimming the leaf area to 10 cm² also promoted rooting.
In Côte d’Ivoire the first plantation of Khaya ivorensis was established in 1927, at a density of 2500 seedlings/ha, but after 30 years only 72 trees/ha had an annual diameter growth of over 1 cm. Later it was planted in lines as enrichment of degraded forests, mixed with other species, at a distance of 7–25 m between lines and 3–7 m within the line, and some trees showed an annual diameter growth of over 2.5 cm after 14 years. Between 1969 and 1995 about 1730 ha of pure plantations have been established with trees planted at a spacing of 3 m × 3 m. More recently it has been planted successfully in 3 rows to mark the boundary of forestry reserves.
Khaya ivorensis occurs scattered or in small groups in the forest, usually in low densities. In southern Cameroon on average 0.02–0.08 boles of over 60 cm diameter per ha and 0.17–0.64 m³ of wood per ha have been recorded, and in Gabon an average of 0.27 m³ of wood per ha. In Côte d’Ivoire an average density of less than 1 exploitable tree per 10 ha has been recorded, but locally 1 exploitable tree per 2 ha has been found.
Enrichment planting in natural forest is locally applied in Côte d’Ivoire. In Gabon 4-month-old seedlings have been planted after clear-cutting of the forest, and in other sites after removal of the forest undergrowth and thinning of the upper canopy. After 6 years, the seedlings showed 92% survival in the clear-cut localities and nearly 100% in localities where the forest was cleared from undergrowth and the canopy thinned. Average heights were 10.7 m and 11.7 m, respectively, and average bole diameters 12.9 cm and 9.3 cm. After 11 years average heights were 16.1 m and 17.1 m, respectively, and average bole diameters 43.1 cm and 36.5 cm.
In Côte d’Ivoire Khaya ivorensis trees have been planted under the shade of 2-year-old Leucaena leucocephala (Lam.) de Wit, which suppresses weeds and fixes nitrogen into the soil. Regular thinning of the shade trees in the first years is needed for good growth of the Khaya ivorensis trees. The first thinning of Khaya ivorensis trees in a plantation of 1000 stems/ha is done when trees have reached 15 m in height and 15 cm in bole diameter, to a density of 400–500 stems/ha. The second thinning can be done when trees are 20 m tall and 20 cm in diameter to 200–250 stems/ha, the third one at 25 m tall and 25 cm diameter to 125–150 stems/ha and the fourth one at 30 cm diameter to 75–100 stems/ha. In tropical Africa Khaya ivorensis has been planted successfully in mixed plantations, e.g. with Heritiera utilis (Sprague) Sprague, Terminalia ivorensis A.Chev., Tieghemella heckelii (A.Chev.) Roberty and Triplochiton scleroxylon K.Schum. In mixed plantations with Khaya ivorensis in Malaysia, a final density of 80 trees/ha and a rotation of 30 years is recommended. Realistic rotation cycles in natural forest are probably in the range of 60–80 years.
Diseases and pests
In plantations Khaya ivorensis may suffer seriously from Hypsipyla robusta shoot borers that kill the main stem of young trees, causing excessive branching and contributing to mortality. Silvicultural techniques such as overhead shading of saplings, mixed planting and removal of lateral shoots can reduce damage by shoot borers. In Nigeria in the 1960s, for instance, shoot borers were found to attack 90–100% of Khaya ivorensis trees in pure stands, 0–30% in stands mixed with other Meliaceae, 0–50% in stands mixed with non-Meliaceae and 0% when planted in lines in natural forest. In Brazil Khaya ivorensis is used for reforestation because of its resistance to Hypsipyla grandella, the major pest of Brazilian mahogany. However, since 1999 a high incidence of leaf spot caused by the fungus Thanatephorus cucumeris (teleomorph of Rhizoctonia solani) has been observed, causing numerous lesions on leaves of larger trees and 100% leaf fall in seedlings.
Seeds are commonly attacked by seed-boring beetles and eaten by small rodents. Attacks of living trees by wood borers (Apate spp.) have been observed. The bark of saplings is sometimes eaten by porcupines and squirrels, which can kill the plants. In nurseries in Côte d’Ivoire seedlings are frequently attacked by psyllids (Phacosema spp.), bugs and scale insects, after which they are infested by secondary fungal pathogens, resulting in a smut blackening the leaves.
The minimum bole diameter for exploitation is 60 cm in Côte d’Ivoire and Gabon, 80 cm in Cameroon and 110 cm in Ghana. The boles of Khaya ivorensis trees are occasionally so large that they cannot be sawn with normal equipment. The high buttresses at the base of the bole often necessitate the construction of a platform before felling can take place, or the removal of the buttresses before felling to recover more timber.
A natural forest tree with a bole diameter of 80 cm yields on average 6.6 m³ of timber, a tree 120 cm in diameter 15.5 m³, and a tree 160 cm in diameter 17.9 m³.
For plantations at an age of 30 years in tropical Africa, the annual wood production is 2–4 m³/ha. On good soil in Côte d’Ivoire a 31-year-old stand with 70 trees/ha (on average 37–40 m tall and 57 cm in diameter) produced 8 m³/ha/year. In 26–28-year-old plantations in Malaysia, mean annual increments of 7.4–7.7 m³/ha have been recorded.
Handling after harvest
Logs may have a spongy or brittle heart, and care is needed in felling and sawing operations. They are susceptible to attack by longhorn beetles and should be processed not too long after felling. The sapwood is often removed soon after felling to prevent attacks by ambrosia beetles. The boles float in water and thus can be transported by river.
Khaya ivorensis is included in the IUCN Red list as a vulnerable species because of habitat loss and degradation, and selective felling. It has been proposed for inclusion in CITES appendix I or II, but it has not been listed due to insufficient information on regeneration, extent of plantations and sustainability under current management regimes.
Provenance trials in Ghana showed a fairly high heritability for height growth, with the growth of the highest-ranking progenies being nearly twice that of the lowest-ranking ones. This may be partly due to some genetic resistance against Hypsipyla robusta attack. Provenance trials have also been planted in Côte d’Ivoire.
Natural regeneration of Khaya ivorensis after logging is often poor due to the often low density of mature trees in the forest and low regeneration rates in heavily disturbed forest. It has been suggested that the addition of seeds at favourable sites is a realistic option to obtain sufficient regeneration after logging. More research is needed on appropriate management systems in natural forest to ensure a sustainable exploitation. Management under the tropical shelterwood system seems most appropriate.
Khaya ivorensis is considered one of the most important timber species for plantations, combining fast growth and good timber quality. More extensive establishment of plantations of Khaya ivorensis is certainly desirable in tropical Africa, but Hypsipyla attack is a serious drawback. The combined effects of selection of provenances with genetic resistance and appropriate silvicultural practices could have a substantial positive impact on the damage caused by Hypsipyla robusta stem borers. The integration of Khaya ivorensis in agroforestry systems, as is already the case in cocoa based systems in Nigeria, can be considered economically and technically feasible and an ecologically sound strategy.
Extensive biosystematic studies on Khaya are recommended, covering the whole range of the genus and also considering the ecological requirements.
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Correct citation of this article:
Lemmens, R.H.M.J., 2008. Khaya ivorensis 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, base of bole; 2, leaf and inflorescence; 3, dehisced fruit; 4, seed.
Redrawn and adapted by Iskak Syamsudin
67-year-old tree, Gabon
11-year-old plantation, Gabon
base of bole
wood in transverse section
wood in tangential section
wood in radial section
transverse surface of wood