5663

5(3): Timber trees; Lesser-known timbers

Khaya A. Juss.

Mém. Mus. Hist. Nat. Paris 19: 249 (1830).

MELIACEAE

x = 25; K. anthotheca, K. grandifoliola, K. ivorensis, K. senegalensis: 2n = 50

African mahogany (En). Acajou d'Afrique (Fr). Indonesia: kaya (general).

Khaya comprises 6 species, 4 in tropical Africa and 2 in Madagascar and the Comores. Several species have been introduced widely in other parts of both the wet and dry tropics for their timber. K. ivorensis is planted in trials in Peninsular Malaysia, whereas another 3 species have been planted in many trials in Indonesia.

The wood of African sources of Khaya is used for furniture and cabinet work, high quality decorative interior finishing, high quality staircases, panelling, flooring, boat planking and cabins, banisters, handrails, carving, turnery and occasionally for construction, canoe building and boat decking. The wood is highly valued for the production of decorative veneer and plywood, which is also its main application in South-East Asia.
K. ivorensis and K. senegalensis have been used for enrichment planting (especially the former) and afforestation and the latter has also been planted in parks and along roads, e.g. in Jakarta and Singapore. K. anthotheca has been used in a taungya system. The wood produces a good quality charcoal. Seeds of K. senegalensis and K. grandifoliola yield an oil used in West Africa for cooking. The bark is used medicinally against fever.

In South-East Asia Khaya wood is not exported yet, all wood being sold on the domestic market. In trade K. anthotheca and K. ivorensis are considered as the lighter weight African mahogany species, K. grandifoliola and K. senegalensis as the heavier and somewhat darker coloured ones.

No description of the wood properties of Khaya grown in South-East Asia exists; the following is entirely based upon African material. Khaya yields a medium-weight hardwood with a density of 460-800(-950) kg/m³ at 12% moisture content with K. senegalensis and K. grandifoliola from relatively dry areas yielding the heavier timber; growing sites significantly influence the structure and density of the wood. Heartwood pink when freshly sawn, turning pale pink-brown or dark reddish-brown upon exposure, not well defined, or well defined in K. grandifoliola and K. ivorensis, from the yellowish-brown, up to 6 cm wide sapwood; grain interlocked, sometimes straight; texture moderately coarse; wood usually with attractive figure and lustre, sometimes with irregular streaks. Growth rings indistinct to distinct, boundaries indicated by denser fibres; vessels moderately small to moderately large, solitary and in radial multiples of 2-8 and in small clusters, vessel lines usually conspicuous and with reddish streaks, occasionally with white deposits; parenchyma sparse to moderately abundant, paratracheal vasicentric, occasionally apotracheal in marginal or seemingly marginal bands; rays of 2 distinct sizes, moderately fine and medium-sized, distinct to the naked eye; ripple marks mostly absent or when present, sometimes irregular, but not marked; traumatic gum ducts occasionally present in tangential rows.
Shrinkage upon seasoning is moderate, the lighter-weight stock dries fairly rapidly, the heavier seasons rather slowly, generally with little degrade but care is needed to prevent warp in larger sizes and splitting in heavier material. Kiln schedule F is recommended. The wood is moderately hard and of medium to fairly low strength but tough. It works fairly well to excellently (K. grandifoliola), although interlocked grain on quarter-sawn surfaces may give rise to woolly surface in planing, making it necessary to have sharp tools and to reduce the cutting angle to 15-20°. It can be peeled and sliced well to produce face veneer (except for K. grandifoliola), polishes and finishes well; the machining properties are medium to fairly good and slightly better for K. senegalensis than for K. grandifoliola. The wood is moderately durable. The heartwood is extremely resistant and the sapwood is moderately resistant to impregnation. Logs are susceptible to pinhole borer and longhorn beetle attacks, the wood of the lighter-weight species also to termite attack. The sapwood is susceptible to Lyctus. The sawdust may cause skin irritations to those people allergic to it.
The seeds of K. senegalensis have an oil content of 67% and are rich in oleic acid (66%). The gross energy value of the wood of K. senegalensis is about 19 990 kJ/kg.
See also the tables on microscopic wood anatomy and wood properties.

Deciduous, monoecious, small to large or very large trees up to 50(-60) m tall; bole straight and cylindrical to slightly sinuous, branchless for up to 25(-30) m, up to 150(-400) cm in diameter, larger trees with prominent buttresses up to 7 m high; bark surface smooth but becoming scaly, the circular scales leaving shallow pits, grey and brown mottled. Leaves arranged spirally, in tufts at the end of twigs, paripinnate, exstipulate; leaflets entire. Flowers in an axillary large, much-branched thyrse, unisexual, 4-5-merous; calyx lobed almost to the base; petals free. Male flower with urceolate to cup-shaped staminal tube, bearing 8-10 anthers; disk cushion-shaped. Female flowers with a superior, 4-5-locular ovary with 12-16(-18) ovules in each cell, style-head discoid; disk indistinct. Fruit an erect, globose, woody capsule; columella sharply ridged. Seeds 8-18 per cell, in single rows, flattened and narrowly winged all around. Seedling with epigeal germination; cotyledons not emergent; hypocotyl slightly developed or absent; first 2 leaves opposite, simple, subsequent ones arranged spirally and trifoliate or imparipinnate, later paripinnate.
Early growth can be very rapid, namely 13 m in height in 4 years (K. ivorensis), but in enrichment planting trials and in taungya systems annual height increment in the first 2 years was only 0.3-0.5 m (K. anthotheca, K. ivorensis). Extensive trials with K. anthotheca, K. grandifoliola and K. senegalensis in Indonesia showed a mean annual increment of 0.8-2.4 cm in diameter and 0.5-1.5 m in height. In 26-year-old plantations of K. ivorensis in Peninsular Malaysia a mean annual increment of 1.7-1.9 cm in diameter and 1 m in height was achieved. In Nigeria growth of K. grandifoliola seedlings was promoted by inoculation with Endogone spores and they developed vesicular-arbuscular mycorrhizas. K. anthotheca and K. ivorensis are clearly the larger trees, while K. grandifoliola and K. senegalensis are much smaller. K. ivorensis develops according to Rauh's architectural tree model, characterized by a monopodial trunk which grows rhythmically and so develops tiers of branches. In K. ivorensis the monoaxial state may persist to a height of 10 m. In West Africa K. anthotheca flowers from January to February and again from September to October. Fruits develop in 2-3 months. Seeds are probably dispersed by wind.

K. senegalensis is a species of seasonal climates where it is found in a wide range of habitats, from closed deciduous forest to savanna, up to 1500 m altitude, with an annual rainfall 650-1300 mm and a dry season of 4-7 months. K. anthotheca and K. ivorensis occur in wet evergreen to semi-deciduous or deciduous forest. K. anthotheca is generally found scattered or rarely in small groups, often in the vicinity of water, in areas with an annual rainfall of 1200-1800 mm and a dry season of 2-4 months, up to 500 m altitude. Annual rainfall in areas where K. ivorensis occurs is 1600-2500 mm, and the dry season lasts 2-3 months. K. grandifoliola is found in dry evergreen forest, gallery forest and savanna woodlands at up to 1300 m altitude in areas with an annual rainfall of 1200-1800 mm and a dry season of 3-5 months.

Khaya can be propagated by seed and by cuttings; tissue culture on an experimental scale is successful. In Indonesia direct sowing of K. anthotheca in the field failed. Seed counts range from 3200-8600 per kg, but counts from specimen in the Bogor Botanical Gardens average 2700-3450 seeds/kg. Seed viability drops sharply after only two weeks, but the germination rate of seed of K. senegalensis stored at 5°C and -15°C in airtight receptacles for 2 years was still 30%. Pretreatment is not necessary before sowing and the germination rate of fresh seed is high, being (55-)75-90% in 10-30 days. Young seedlings require light shading. Seedlings are left to grow fairly tall, 0.6-1.8 m, and stay for up to three years in the nursery before they are made into stumps or striplings. The latter perform better under South-East Asian conditions. Vegetative propagation of K. anthotheca by means of cuttings applying 50 mg of Rootone F/cutting showed a rooting of about 75%. Generally, Khaya is planted under light shade, but full overhead light is required for good growth. K. anthotheca requires fertile deep soils and plenty of water; K. senegalensis is very resistant to flooding and can be considered for planting on swampy soils. The stem form of K. ivorensis is variable and strong lateral competition is needed to obtain straight and upright trunks. When natural regeneration has become established under the canopy the old trees are harvested with various cutting and regeneration systems. Attacks of the shoot borer Hypsipyla robusta can be overcome by close spacing, admixing with other species which are planted 2-3 years later, or planting in the shade of a nurse crop. In the latter case, the nurse crop is removed when the desired clear bole length is achieved, after which Khaya can grow fast. There is some evidence that Hypsipyla attack is reduced by planting under shade, possibly because this suppresses lateral shoot growth which provide the best conditions for the multiplication of the borer, or because predators prefer shaded conditions. K. ivorensis proved more resistant to shoot borer than Swietenia spp. In Malaysia a final density of 80 trees/ha and a rotation of about 30 years for K. ivorensis in mixed plantations is recommended. In Malaysia the mean annual increment of 26-28-year-old plantations is 7.4-7.7 m³/ha. K. senegalensis proved to be successful in enrichment planting of woodland in Vietnam.

Provenances of Khaya species, but mainly of K. anthotheca and K. ivorensis, are kept in various tropical countries. In Sabah selection of K. ivorensis is focussing on resistance to Hypsipyla and involves 9 provenances and 21 progenies.

In Malaysia K. ivorensis in particular is considered one of the most important species for enrichment planting or plantations. The other Khaya species also have the potential to become important for these purposes. When Hypsipyla-resistant trees can be bred, the importance of Khaya for plantations will increase, given its favourable growth rate and high quality timber.

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S.I. Wiselius

Khaya anthotheca
Khaya grandifoliola
Khaya ivorensis
Khaya senegalensis

Wiselius, S.I., 1998. Khaya A. Juss.. In: Sosef, M.S.M., Hong, L.T. and Prawirohatmodjo, S. (Editors): Plant Resources of South-East Asia No 5(3): Timber trees; Lesser-known timbers. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea

The following species in this genus are important in this commodity group and are treated separatedly in this database:
Khaya anthotheca
Khaya grandifoliola
Khaya ivorensis
Khaya senegalensis