Khaya grandifoliola (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
Khaya grandifoliola C.DC.
- Protologue: Bull. Soc. Bot. France 54, mém. 8: 10 (1907).
- Family: Meliaceae
- Chromosome number: 2n = 50
- Broad-leaved mahogany, big-leaved mahogany, dry-zone mahogany, Benin mahogany (En).
- Acajou à grandes feuilles, acajou du Bénin (Fr).
- Mogno de Benim (Po).
Origin and geographic distribution
Khaya grandifoliola occurs from Guinea east to Sudan and Uganda. It is occasionally grown in plantations within its natural area of distribution, e.g. in Côte d’Ivoire and Ghana, and trial plantations have been established in Indonesia.
The wood (trade names: African mahogany, acajou d’Afrique) is valued for carpentry, joinery, furniture, cabinet work and decorative veneer. It is suitable for light construction, light flooring, interior trim, ship building, musical instruments, toys, novelties, carving, turnery and pulpwood. Traditionally, the wood is used for furniture, household implements and dug-out canoes. It is also used as fuelwood and for charcoal production.
The bitter-tasting bark is used in traditional medicine. It is widely used against fever caused by malaria. Bark decoctions are taken to treat stomach complaints including gastric ulcers, pain after childbirth, and skin diseases. In Sudan a bark infusion is used to treat diarrhoea caused by intestinal parasites. A decoction of the root bark is drunk to treat gonorrhoea and pulverized root bark is applied externally to skin diseases. In Uganda the bark is used as a fish poison, and in DR Congo the bark is used for washing cloth. Khaya grandifoliola is planted as a roadside tree and ornamental shade tree. In Uganda it is valued for stabilization of river banks.
Production and international trade
The first Khaya logs were shipped to the British timber market around 1833 from Côte d’Ivoire, and export from Ghana started in 1888. Until the 1950s Khaya timber formed up to 70% of the total export from Ghana, with an annual volume of approximately 100,000 m³, but since then its export has steadily declined. Khaya grandifoliola wood is exported from West African countries (e.g. Ghana) in mixed consignments with other Khaya spp., particularly Khaya anthotheca (Welw.) C.DC. and Khaya ivorensis A.Chev. 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 grandifoliola in these amounts is obscure, but is probably much smaller than for the other species. Côte d’Ivoire exported 41,000 m³ of sawn Khaya wood in 2003, at an average price of US$ 397/m³, and 34,000 m³ in 2005, at an average price of US$ 439/m³. Cameroon exported 11,000 m³ of sawn Khaya wood in 2003, and 8600 m³ in 2004 and 2006. In recent years, the United States market has dominated the international trade in Khaya timber, especially as a substitute for American mahogany (from Swietenia) of which the availability has declined considerably.
The heartwood is pinkish brown, darkening to reddish brown upon exposure. It is usually distinctly demarcated from the pale brown to pinkish brown, up to 5 cm wide sapwood, at least in dried wood. The grain is usually interlocked, sometimes straight, texture rather coarse. Wood from savanna trees is reported to be darker than that of forest trees.
The wood is heavier than that of Khaya anthotheca and Khaya ivorensis. It is medium-weight to moderately heavy, with a density of (560–)640–730(–770) kg/m³ at 12% moisture content. It generally air dries rather slowly, but with little degrade. The rates of shrinkage are moderate. Once dry, the wood is stable in service.
At 12% moisture content, the modulus of rupture is 92–119 N/mm², modulus of elasticity 10,600–11,400 N/mm², compression parallel to grain 53–74 N/mm², shear 15–17 N/mm², cleavage 14 N/mm radial and 18 N/mm tangential, and Janka side hardness 6090 N.
The wood is fairly easy to saw and work, with moderate blunting effect on cutting edges. It can be finished to a smooth surface and takes an excellent polish. The wood holds nails and screws well, but it has some tendency to split. It has good gluing properties. The peeling properties are poor because of the higher density in comparison with Khaya anthotheca and Khaya ivorensis and the interlocked grain, but slicing gives decorative veneer. The wood is moderately durable, being resistant to termites but susceptible to Lyctus. The heartwood is strongly resistant to impregnation, the sapwood moderately resistant.
Bark extracts exhibited antimalarial activity in mice infected with Plasmodium berghei. Several limonoids isolated from bark and seeds showed distinct in-vitro antimalarial activity against chloroquine-resistant strains of Plasmodium falciparum, particularly gedunin, 7-deacetylkhivorin, methylangolensate and 6-acetylswietenolide. The most active limonoid, gedunin, exhibited an additive effect when combined with chloroquine. In tests with rats, bark extracts showed hypoglycaemic, hypoproteinaemic and hypocholesterolaemic effects, and also showed good results for the treatment of gastric ulcers. They had positive effects on the production of red blood cells.
Bark extracts showed nematicidal activity against Pratylenchus brachyurus, a nematode injurious to many crops worldwide. They inhibited egg-hatch in sugarcane cyst nematode (Heterodera sacchari). Bark and seed extracts showed toxic effects on Aedes aegypti mosquitoes. Seed extracts resulted in high mortality in the cotton stainer (Dysdercus sp.). The mexicanolide-type limonoids isolated from the seeds showed antifeedant activity against larvae of the Mexican bean beetle (Epilachna varivestis), a notorious pest of beans and other leguminous crops. Bark and seed extracts, as well as the limonoids khivorin isolated from the bark and fissinolide isolated from the seed, have molluscicidal properties.
The gum from the bark is extremely resistant to hydrolysis and highly acidic (pH 3.0–4.0). It contains the sugars rhamnose, arabinose and galactose, as well as glucuronic acid and galacturonic acid. Research showed that the gum can serve as a useful formulating agent in the pharmaceutical industry. It showed good properties for sustained-release tablets for up to 5 hours, whereas a combination with hydroxypropylmethylcellulose could be used to provide a release for longer periods. The gum was effective as a coating to extend the storage life of cassava roots, and it could also be used as ice cream stabilizer.
Adulterations and substitutes
The wood of Khaya grandifoliola resembles true mahogany (from Swietenia spp.) more closely than the woods of Khaya anthotheca and Khaya ivorensis do. However, it is usually exported from West Africa in mixed consignments with these species as ‘African mahogany’ or ‘acajou d’Afrique’. The wood of makore (Tieghemella) is similar, but more durable.
- Usually deciduous, monoecious, medium-sized to large tree up to 40 m tall; bole branchless for up to 23 m, often twisted or leaning near the top, up to 120(–200) cm in diameter, usually with buttresses up to 3 m high; bark surface greyish brown, rough, exfoliating in small circular scales and becoming pitted, inner bark dark pink to reddish, with white streaks, exuding a clear gum; crown large, rounded; twigs glabrous.
- Leaves arranged spirally but clustered near ends of branches, paripinnately compound with 3–5 pairs of leaflets; stipules absent; petiole and rachis together up to 50 cm long; petiolules 0.5–1 cm long; leaflets opposite or nearly so, elliptical to ovate-elliptical or oblong-elliptical, (10–)12–20(–30) cm × 5–10 cm, cuneate to obtuse or rounded and slightly asymmetrical at base, shortly but distinctly acuminate at apex, often with twisted acumen, margins entire or wavy, thickly papery to thinly leathery, glabrous, pinnately veined with 9–15 pairs of lateral veins.
- Inflorescence an axillary panicle up to 40 cm long.
- Flowers unisexual, male and female flowers very similar in appearance, regular, usually 5-merous, whitish, sweet-scented; pedicel 1–2 mm long; calyx lobed almost to the base, with rounded lobes c. 1.5 mm long; petals free, elliptical, c. 5 mm × 2 mm, somewhat hooded; stamens fused into an urn-shaped tube c. 5 mm long, with usually 10 included anthers near apex, alternating with rounded lobes; disk cushion-shaped; ovary superior, globose to conical, 1–2 mm in diameter, usually 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 6–9 cm in diameter, greyish brown, dehiscent by 5 valves, many-seeded.
- Seeds disk-shaped or quadrangular, strongly flattened, c. 2 cm × 3.5 cm, narrowly winged all around the margin, brown.
- Seedling with hypogeal germination, cotyledons remaining enclosed in the seed coat; epicotyl c. 6 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. Khaya grandifoliola is very close to Khaya anthotheca (Welw.) C.DC. and may even be conspecific. The latter differs in its usually smaller and thicker leaflets, and thinner fruit walls. Hybrids between the two species have been recorded.
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); 92: four (3–4) cells per parenchyma strand; 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); (138: prismatic crystals in procumbent ray cells).
Growth and development
In Nigeria saplings reached an average height of 4.2 m and bole diameter of 7 cm 4 years after planting. In Côte d’Ivoire Khaya grandifoliola trees planted in the open in the semi-deciduous forest zone reached an average height of 13.5 m and an average bole diameter of 17 cm after 10 years. However, trees planted in the evergreen forest zone only reached 9 m in height and 11.5 cm in diameter after 8 years. In Nigeria an average height of 21 m after 20 years was recorded. However, in natural forest in Nigeria the average bole diameter for trees 100 years old was estimated at only 60–70 cm.
Trees are usually deciduous in the dry season; young leaves are strikingly reddish and often occur together with flowers. The flowers are pollinated by insects such as bees and moths. In Côte d’Ivoire fruiting is in January–March. Dispersal of the seeds is by wind, but most seeds fall close to the parent tree.
The presence of endotrophic mycorrhizal fungi in nurseries is important; inoculation with Endogone spores markedly improved the growth of seedlings.
Khaya grandifoliola occurs in semi-deciduous forest, especially in drier types and in savanna, but in the latter case usually along watercourses, in areas with 1200–1800 mm annual rainfall and a dry season of 3–5 months. It occurs up to 1400 m altitude. Sometimes it can be found in rocky and hilly parts of moist semi-deciduous forest, where Khaya anthotheca also occurs. In Sudan and Uganda it occurs in lowland forest, particularly in gallery forest. It prefers moist but well-drained soils, and is locally common on alluvial soils in valleys.
Seeds can germinate in full sun as well as in the shade, but natural regeneration may be very sparse in the forest. In Nigeria it was found that although seedlings could become established in closed forest, they showed very poor growth and rarely survived for long. In Sudan Khaya grandifoliola reportedly does not regenerate under a closed forest canopy. Natural regeneration can be abundant in savanna which is close to the forest and protected from fire. In gallery forest in Nigeria regeneration was most abundant in the boundary zone with the savanna. In Nigeria it has been noted that more seeds and seedlings are produced in years with abundant rainfall.
Propagation and planting
Khaya grandifoliola is propagated by seed. The 1000-seed weight is 200–300 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 seeds can be stored in a cool place; in a test in Benin the germination rate was still 76% after 4 months. It is recommended to add ash during storage to reduce insect attacks. The seeds are very liable to desiccation. It is not necessary to treat seeds before sowing. They can best be sown in seed beds in the nursery or in pots. Upon sowing seeds should be covered with only a thin layer of soil, or left partially uncovered. Fresh healthy seeds have a high germination rate, about 90%, but this decreases rapidly to nearly zero after 2 months under natural circumstances. Germination takes 10–35 days. It has been recommended to provide light shade to young seedlings until they are 1–2 months old, but in Nigeria it was found that seedlings grew best with full light, watering at least every second day and weekly supplementation with NK solution. Seedlings can be left in the nursery for about one year until they are 0.5–1 m tall, after which the root system is pruned to a length of about 30 cm and most leaves stripped off before planting into the field. Stumps can also be planted out, leaving 2–3 cm of stem and 25–30 cm of root. In Uganda seedlings of 2.5 m tall with a stem diameter of 5–8 cm at the base have been used for planting into the field. Normal spacing is 2–4 m × 2–4 m. Wildlings are sometimes collected for planting.
Enrichment planting in natural forest has been applied in Uganda, but failed possibly due to poor silvicultural management operations. In young plantations weeding is necessary; the young trees are susceptible to suppression by weeds, and also to fire. In Côte d’Ivoire Khaya grandifoliola 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 grandifoliola trees. The first thinning of 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 at 25 m tall and 25 cm diameter to 125–150 stems/ha and the fourth at 30 cm diameter to 75–100 stems/ha. In tropical Africa Khaya grandifoliola has been planted successfully in mixed plantations, e.g. with Milicia excelsa (Welw.) C.C.Berg, Triplochiton scleroxylon K.Schum., Gmelina arborea Roxb. and Margaritaria discoidea (Baill.) Webster.
Realistic rotation cycles in natural forest are probably in the range of 80–100 years, but in plantations a rotation of 40–60 years is feasible.
Diseases and pests
In plantations Khaya grandifoliola 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. Seeds are commonly attacked by seed-boring beetles and eaten by small rodents.
The minimum bole diameter for harvesting of Khaya grandifoliola trees in natural forest is 60 cm in Côte d’Ivoire, 80 cm in Cameroon, Central African Republic and DR Congo, and 110 cm in Ghana.
Handling after harvest
Logs 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 grandifoliola is included in the IUCN Red list as a vulnerable species because of habitat loss and degradation, and selective felling. Like other Khaya spp., populations have been depleted in many regions through centuries of commercial exploitation.
More research is needed on appropriate management systems in natural forest to ensure a sustainable exploitation of Khaya grandifoliola. Its fair growth rate makes more extensive establishment of plantations an option, 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. Research priority should be given to range-wide selection of genotypes which are resistant to stem-borer attack, fast growing and have acceptable wood quality. The establishment of appropriate methods of vegetative propagation including tissue culture is urgently needed.
The bark demonstrated several interesting pharmacological activities, especially against malaria and gastric ulcers. This deserves more research attention for possible development into new drugs. The insecticidal, molluscicidal and nematicidal activities of bark and seeds are also noteworthy. The gum has potential as compression coating for drugs targeting the colon, and as a stabilizing and protective agent.
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Correct citation of this article
Opuni-Frimpong, E., 2008. Khaya grandifoliola C.DC. 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 12 April 2019.
- See the Prota4U database.