Milicia excelsa (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
|Dye / tannin|
Milicia excelsa (Welw.) C.C.Berg
- Protologue: Bull. Jard. Bot. Etat 52: 227 (1982).
- Family: Moraceae
- Maclura excelsa (Welw.) Bureau (1873),
- Chlorophora excelsa (Welw.) Benth. (1880).
- Iroko, rock elm, African teak, African oak (En).
- Iroko, chêne d’Afrique, teck d’Afrique, teck kambala (Fr).
- Câmbala, amoreira, moreira, teca africana, tumbiro, magundo (Po).
- Mvule (Sw).
Origin and geographic distribution
Milicia excelsa is distributed from Guinea Bissau eastward to Ethiopia and southward to Angola, Zimbabwe and Mozambique. It has been introduced into India and the United States.
The wood of Milicia excelsa and the closely related Milicia regia (A.Chev.) C.C.Berg are not distinguished in the timber trade, and are traded under the trade name ‘iroko’, or ‘odum’ in Ghana. Other frequently used trade names for Milicia excelsa are ‘kambala’ and ‘mvule’. Iroko is a highly valued commercial timber in Africa, for which demand is large. It is used for construction work, shipbuilding and marine carpentry, sleepers, sluice gates, framework, trucks, draining boards, outdoor and indoor joinery, stairs, doors, frames, garden furniture, cabinet work, panelling, flooring and profile boards for decorative and structural uses. It is also used for carving, domestic utensils, musical instruments and toys. As it is resistant to acids and bases, it is used for tanks and barrels for food and chemical products and for laboratory benches. It is used as sliced veneer but only rarely as rotary veneer. The wood is also used as firewood and for making charcoal.
The ripe fruits and cooked young leaves of Milicia excelsa are edible. The fruit juice is used for flavouring in India. Mature leaves have been used as sandpaper. The bark is used for dyeing leather and cloth and for roofs of houses. Formerly the bark of young trees was used for making loincloths. The tree is used for soil conservation and mulch production, and as an ornamental and shade tree. In Nigeria it is planted to mark boundaries between farms or villages. It has also been planted to attract edible caterpillars. The wood is being tested as a substrate for the cultivation of the mushroom Lentinus subnudus, popular e.g. in Nigeria.
Milicia excelsa is widely used in African traditional medicine. A root decoction is taken to treat female sterility. A decoction of the root and stem bark is taken as an aphrodisiac. Preparations from the bark are taken to treat cough, asthma, heart trouble, lumbago, spleen pain, stomach pain, abdominal pain, oedema, ascites, dysmenorrhoea, gonorrhoea, general fatigue, rheumatism, sprains, and as a galactagogue, aphrodisiac, tonic and purgative. Bark preparations are externally applied to treat scabies, wounds, loss of hair, fever, venereal diseases and sprains. They are applied as an enema to cure piles, diarrhoea and dysentery. The latex is applied on burns, wounds, sores and against eczema and other skin problems. It is also taken against stomach problems, hypertension and as a galactagogue, and it is used for treatment of tumours and obstructions of the throat. Leaves are eaten to treat insanity; a leaf maceration is drunk as a galactagogue. A decoction of the leaves is taken for treatment of gallstones. Leaf preparations are externally applied to treat snakebites and fever and as eye drops to treat filariasis.
Milicia excelsa is often considered a sacred tree and is frequently protected near houses and in cultivated fields. In Benin the foliage is used as a fetish, and in Nigeria the tree has a special place in the folklore and traditions of the people.
Production and international trade
Milicia excelsa and Milicia regia are both traded as iroko and the share of Milicia excelsa in that commerce is unknown. Iroko is a major timber in international trade; during the 1960s Côte d’Ivoire exported about 55,000 m³ of iroko logs and 6000 m³ of iroko sawnwood per year, and Ghana 28,000 m³ of sawnwood. In 1973 Côte d’Ivoire exported as much as 136,500 m³ of logs and 16,000 m³ of sawnwood. In 1994 Cameroon exported 65,000 m³ of logs and 12,000 m³ of sawnwood, Congo 10,000 m³ of logs, and Ghana at least 47,000 m³ of sawnwood. In the season 1998–99, an estimated 133,400 m³ of iroko timber was cut in Cameroon. From 1998 to 2003 Gabon exported about 28,500 m³ of logs per year. Exportation of iroko logs is now forbidden in Côte d’Ivoire, Ghana and Cameroon, but export of sawnwood is allowed.
In 2003 Cameroon exported 33,000 m³ of iroko sawnwood (US$ 948/m³), Congo 28,000 m³ of iroko logs (US$ 221/m³) and 1000 m³ of iroko veneer (US$ 343/m³), the Central African Republic 2000 m³ of logs (US$ 412/m³) and 5000 m³ of sawnwood (US$ 465/m³), Ghana 8000 m³ of sawnwood (US$ 754/m³) and Togo 1000 m³ of sawnwood (US$ 260/m³).
In the past Tanzania and Uganda were major suppliers of iroko, and small volumes are still exported from East Africa.
The heartwood is pale yellow to yellow, darkening on exposure to yellowish or greenish brown or sometimes to chocolate brown; it is clearly demarcated from the 5–7.5 cm wide yellowish white sapwood. The grain is interlocked, texture medium to coarse, figure mottled. The wood is somewhat greasy and is odourless. Logs from closed forest are usually straight and cylindrical, but savanna trees are often damaged by fire and their wood is affected by eccentric growth, ingrown bark or calcium carbonate plugs.
The wood has a density of 550–750 kg/m³ at 12% moisture content. Shrinkage rates from green to oven dry are 1.7–4.1(–5.6)% radial and 2.4–6.3(–9.8)% tangential. The wood dries well in open air and kilns, with little degrade. Boards of 4.2 cm thick can be kiln dried from 60% to 15% moisture content in 8 days at a drying temperature of 50–80°C, and a corresponding relative humidity of 85% to 40%. Once the wood is dry, movement in service is small.
At 12% moisture content, the modulus of rupture is 75–156 N/mm², modulus of elasticity 8300–13,300 N/mm², compression parallel to grain 42–65 N/mm², shear 5.4–14.1 N/mm², cleavage 10.3–20.9 N/mm, Janka side hardness 4400–5610 N, and Janka end hardness 5360–6640 N.
Working properties for hand and machine tools are generally good but variable; the interlocked grain may hamper sawing and planing. The wood is rather abrasive due to the presence of hard deposits (‘iroko stones’, mainly consisting of calcium carbonate), which can blunt cutting edges. Tearing in planing can be avoided by using cutting angles of 15° or less. The wood has good nailing, screwing, mortising and gluing properties and turns easily. It finishes well, but filler is needed. The wood contains the stilbene derivative chlorophorin, which prevents oil-based paints from drying, and which corrodes metal in contact with it. Steam-bending properties of the wood are moderate.
Although the heartwood is generally considered very durable, reports on its resistance to marine borers and termites vary. It is resistant to fungal attack. The sapwood is liable to attack by Lyctus beetles. The heartwood is impermeable to preservatives, but sapwood is permeable.
The wood and sawdust may cause dermatitis, irritation to nose and throat, and asthmatic reactions, due to the presence of chlorophorin. Chlorophorin has shown inhibitory effects on melanin biosynthesis; its more stable derivative hexahydrochlorophorin may have potential for use in skin-whitening agents and for treating disturbances in pigmentation. Two phenolic compounds (chlorophorin and iroko) have shown in-vitro anti-amoebic activity. A methanol extract of the stem bark has shown in-vivo anti-inflammatory properties in mice and rats.
Adulterations and substitutes
The wood properties of iroko are similar to those of teak (Tectona grandis L.f.). The wood of Guibourtia arnoldiana (De Wild. & T.Durand) J.Léonard (trade name: mutenye) has uses similar to iroko and teak. As iroko is locally overexploited, it is recommended that for some specific uses it is replaced by species such as Piptadeniastrum africanum (Hook.f.) Brenan (trade name: dabéma), Lophira alata Banks ex P.Gaertn. (trade name: azobé), Nauclea diderrichii (De Wild. & T.Durand) Merr. (trade name: bilinga) or Afzelia spp. (trade name: doussié). Logs of Morus mesozygia Stapf ex A.Chev. (trade name: difou) have sometimes been sold mixed with iroko logs.
- Large, dioecious tree up to 50 m tall; bole straight, cylindrical, branchless for up to 25(–30) m, up to 2.5(–3) m in diameter, buttresses absent or small; surface roots often long and prominent, red-brown with yellow lenticels; outer bark grey to dark brown or black, lenticelled, becoming scaly, inner bark thick, fibrous, cream-coloured speckled with orange-brown spots, exuding a white or yellowish latex; crown spreading; branches obliquely ascending.
- Leaves distichously alternate, simple; stipules free, slightly clasping the stem, up to 5 cm long, caducous; petiole 1–6 cm long; blade oblong to elliptical, 6–20(–33) cm × 3.5–10(–15) cm, base cordate to obtuse, often very unequal, apex acuminate, margin almost entire to wavy but toothed in juvenile specimens, papery to leathery, above glabrous or slightly hairy on the main veins, below densely short-hairy between the finest veins, pinnately veined with 10–22 pairs of lateral veins.
- Inflorescence a catkin, usually solitary in leaf axils or on leafless nodes at the base of twigs, white hairy, flowers numerous in longitudinal rows alternating with rows of bracts; male inflorescence 8–20(–32) cm × 0.5–1 cm, hanging, peduncle 0.5–2.5 cm long; female inflorescence 2–4 cm × 0.5–2 cm, peduncle 0.5–2 cm long.
- Flowers unisexual, 4-merous, sessile; male flowers c. 1.5 mm long, white, tepals 4, basally fused, stamens 4 and inflexed in bud, rudimentary pistil present; female flowers 2–3 mm long, with 4 basally fused tepals, greenish, ovary superior, c. 1 mm long, 1-celled, stigmas 2, one 3–7 mm long, the other up to 1 mm long.
- Fruit an ellipsoid, compressed achene 2.5–3 mm long, arranged in infructescences up to 5 cm × 1.5 cm.
- Seed c. 2 mm long.
- Seedling with epigeal germination.
Other botanical information
Milicia comprises 2 species, Milicia excelsa and Milicia regia, both in tropical Africa. The 2 species mainly differ in the venation and hairiness of the leaves. Milicia excelsa has 10–22 pairs of lateral veins and is short-hairy below between the finest veins, Milicia regia has 6–11 pairs of lateral veins and is glabrous below between the finest veins. Juvenile plants of the 2 species can be distinguished by differences in leaf hairiness and in the colour of the midrib beneath (yellow in Milicia excelsa, red in Milicia regia), and by growth habit and leaf arrangement (in Milicia excelsa sympodial, leaves in 2 rows; in Milicia regia monopodial, with spirally arranged leaves).
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; 27: intervessel pits large (≥ 10 μm); (30: vessel-ray pits with distinct borders; similar to intervessel pits in size and shape throughout the ray cell); 31: vessel-ray pits with much reduced borders to apparently simple: pits rounded or angular; 32: vessel-ray pits with much reduced borders to apparently simple: pits horizontal (scalariform, gash-like) to vertical (palisade); 43: mean tangential diameter of vessel lumina ≥ 200 μm; 46: ≤ 5 vessels per square millimetre; (47: 5–20 vessels per square millimetre); 56: tyloses common.
- Tracheids and fibres: 61: fibres with simple to minutely bordered pits; 66: non-septate fibres present; 69: fibres thin- to thick-walled.
- Axial parenchyma: 80: axial parenchyma aliform; (81: axial parenchyma lozenge-aliform); (82: axial parenchyma winged-aliform); 83: axial parenchyma confluent; (84: axial parenchyma unilateral paratracheal); (85: axial parenchyma bands more than three cells wide); (86: axial parenchyma in narrow bands or lines up to three cells wide); 91: two cells per parenchyma strand; 92: four (3–4) cells per parenchyma strand.
- Rays: 98: larger rays commonly 4- to 10-seriate; 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: 132: laticifers or tanniferous tubes.
- Mineral inclusions: 136: prismatic crystals present; 137: prismatic crystals in upright and/or square ray cells; 141: prismatic crystals in non-chambered axial parenchyma cells; (154: more than one crystal of about the same size per cell or chamber); (155: two distinct sizes of crystals per cell or chamber).
Growth and development
Seeds normally germinate 2–4 weeks after sowing. Young Milicia excelsa trees grow continuously, but growth of adult trees is periodical. In the dry season Milicia excelsa is deciduous for a short period. From West Africa to Sudan it flowers from December to March. It takes 5–6 weeks from fertilization to fruit maturation. Seed dispersal is mostly by birds, bats and squirrels.
In a 6-year-old plantation in northern Côte d’Ivoire some trees were over 6 m tall, but the variability was large. In Ghana the annual increment in diameter is 0.33–0.59 cm. In Cameroon a mean diameter growth of over 1 cm per year has been recorded for trees about 25 years old. In the south-western Central African Republic (annual rainfall 1500 mm; dry period 2 months) the average annual increment in diameter of Milicia excelsa trees is 0.57 cm; it decreases with age from 0.93 cm for trees with a diameter of less than 10 cm to 0.45 cm for trees with 110–120 cm diameter. On average it takes 130 years here for a tree to reach a diameter of 80 cm. In Uganda 32-year-old trees planted at a spacing of 4 m × 4 m were 18 m tall and well shaped.
Milicia excelsa occurs in deciduous, semi-deciduous or evergreen, primary or secondary forest, with an apparent preference for drier forest types. It often occurs in gallery forest and in forest islands or as lone trees in savanna regions, and is sometimes left as a lone tree in old cultivated areas. It is usually found up to 1200(–1500) m altitude, although it has been found at 4500 m altitude on Mount Kilimanjaro in Tanzania. In West Africa Milicia excelsa occurs in regions with an average annual temperature of 25–35°C and an average annual rainfall of 1150–1900 mm. It is considered a pioneer species, demanding intense light and unable to stand deep shade. In young secondary forest, for example, it cannot compete with climbers and shrubs.
Although Milicia excelsa grows on a large variety of soils, it is reported to be rather demanding with respect to soil fertility, especially the presence of K and P. It is considered to be an indicator of fertile soil suitable for cultivation. It prefers well-drained soils and does not tolerate impeded drainage.
Propagation and planting
Milicia excelsa is mostly propagated by seed. The 1000-seed weight is 1–4 g. About 40 kg of fruits is needed for 1 kg of seeds. As the colour of the infructescences does not change during ripening, maturity has to be determined by cutting the infructescence to see if the pulp has softened. If unripe infructescences are picked from the tree, it is necessary to leave them in the shade for some days to ripen. It is easier to collect them from the ground, but seeds should be extracted before the infructescences have begun to ferment. Seeds can be separated by crushing the infructescences after immersing them in water for about one day. Viable seeds sink in water and can be easily separated from floating unviable seeds. Fresh seeds normally germinate well; the germination rate may be more than 90% within 4 weeks. Seeds are best sown within 3 months after collection, because viability decreases rapidly. Seeds dried to 8% moisture content can be stored at 0–5°C for at least a year.
Seeds are sown in a seedbed and transplanted to pots or nursery beds 3 weeks after germination. Seedlings should be grown under shade to limit attacks by Phytolyma spp. About 4 months after sowing the seedlings are around 30 cm tall and ready for planting out in the field. Young plants transplant well. In Ghana seedlings planted during the long rainy season have shown much better growth than seedlings planted during the short rainy season. The better growth persisted for at least 9 years. Planting in a mixed stand with Terminalia superba Engl. & Diels (in equal proportions at planting) gave better growth than planting in pure stands.
Milicia excelsa can be propagated vegetatively by stem and root cuttings, grafting, layering and in-vitro tissue culture. Successful propagation has been achieved using stem cuttings from 1- and 2-year-old trees, but from mature trees cuttings should be taken from coppice shoots. Stakes and posts made of branches may strike root like cuttings.
The wide sapwood of Milicia excelsa means that thinnings in plantations are of little value, so it is recommended to plant at wide spacings.
Milicia excelsa is mostly extracted from natural forest, as plantations are severely affected by pest problems. It prunes and coppices well.
Diseases and pests
The major constraint on the cultivation of Milicia excelsa and Milicia regia are gall-forming Phytolyma spp. (iroko gall flies). Eggs are laid on buds, shoots or young leaves, and after the emergence of nymphs galls are formed, followed by dieback of foliage down to the woody tissue. This disrupts physiological processes, causes growth reduction, and in many cases kills seedlings. Secondary infection by fungi probably aggravates the damage. Mature leaves are not seriously affected. Efforts to control this pest have had little success, but the development or selection of more resistant Milicia genotypes may offer prospects. Planting in light shade, in mixtures and in low plant densities seems to help to reduce damage by Phytolyma. Elephants eat the bark and may destroy plantations.
In Gabon Milicia excelsa is scattered in all forest types with a commercial bole volume of about 0.1 m³/ha.
Handling after harvest
Immediately after felling, the wood of Milicia excelsa is too heavy to be transported by river, and transport is by trucks. The sapwood is usually removed before shipping. Bark to be used medicinally is sometimes pounded, dried and compressed into balls that are kept in wrappers.
Exploitation is often unsustainable: in the 1980s the extraction rate of iroko timber (from Milicia excelsa and Milicia regia) in Ghana was estimated to be about 173,000 m³ per year, whereas the regeneration rate was estimated to be only about 29,000 m³ per year. Milicia excelsa is classified as ‘lower risk but near threatened’ in the 2006 IUCN Red list of threatened species, the main threats being habitat loss and degradation due to expanding agriculture, overexploitation of the wood, and Phytolyma attacks. Milicia excelsa is considered a priority for in-situ conservation. As genetic diversity within populations is low, but diversity between populations large, it is recommended that different populations are included in in-situ conservation efforts. This may be facilitated by the fact that in some areas Milicia excelsa is conserved on farm, in sacred groves, in public places and in cemeteries. It is protected by legislation in Côte d’Ivoire and Mozambique; in Ghana and Tanzania a permit is needed for its exploitation. It is subject to a log export ban in various countries, such as Côte d’Ivoire, Ghana, Cameroon and Tanzania.
Partial genetic resistance to Phytolyma within natural Milicia populations has been identified, which may lead to the development of resistant lines for vegetative propagation. Selection work is underway in Ghana.
Iroko belongs to the most valuable timbers of Africa, due to its attractive appearance, durability, stability and good working properties. At present its exploitation is not sustainable in most countries. It requires protection and exploitation has to be limited if it is to become sustainable. Plantation is difficult due to a pest problem. The identification of sources of resistance to the iroko gall fly deserves high priority and can possibly be complemented by the development of effective control methods, especially the use of natural parasites or predators of the iroko gall fly.
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Sources of illustration
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Correct citation of this article
Ofori, D.A., 2007. Milicia excelsa (Welw.) C.C.Berg. 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 30 November 2022.
- See the Prota4U database.