The Giant Forest Tree Called IROKO

Iroko Tree

In the forest there is a giant tree called by the Yorubas the “Iroko,” with scientific name Milicia excelsa. Seeing the thick branches and mighty trunk of the Iroko, woodcutters are often scared to cut the tree down. 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.

Iroko is a large hardwood tree from the west coast of tropical Africa. It is one of the woods sometimes referred to as African Teak, although it is unrelated to the teak family. The wood colour is initially yellow but darkens to a richer brown over time. It is yielded mostly (probably) by Milicia excelsa. In much of the literature on this timber the names of the trees that yields it are given as Chlorophora excelsa and Chlorophora regia.

The tree is feared in some cultures where it originates and hence is shunned or revered with offerings. Yoruba people believe that the tree is inhabited by a spirit, and anybody who sees the Iroko-man face to face becomes insane and speedily dies. According to the Yoruba, any man who cuts down any Iroko tree causes devastating misfortune on himself and all of his family, also claim that the spirit of the Iroko can be heard in houses which use Iroko wood, as the spirit of the Iroko is trapped in the wood. In Nigeria similar claims are made about other trees.

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.

The wood is used for a variety of purposes including boat-building, domestic flooring and furniture. From the late 1990s, it was used as part of the txalaparta, a Basque musical instrument constructed of wooden boards, due to its lively sound.

Iroko is one of the traditional djembe woods. It is a very durable wood; iroko does not require regular treatment with oil or varnish when used outdoors. Although it is almost as durable as teak, it does not have the same stability.  In the UK there are no trade restrictions on the machining of this timber. The only reported adverse effects known to be caused by the dust from Iroko are asthma, dermatitis and nettle rash

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.

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.

Post Credit: Wikipedia, database.prota.org