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Aucoumea klaineana Pierre

Protologue
Bull. Mens. Soc. Linn. Paris, n.s. 1: 1241 (1896).
Family
Burseraceae
Chromosome number
2n = 26
Vernacular names
Okoumé, Gaboon mahogany (En). Okoumé (Fr). Ocumé (Po).
Origin and geographic distribution
Aucoumea klaineana occurs naturally in western and central Gabon and in continental Equatorial Guinea, south to Congo, where it is restricted to the Chaillu and Mayombe Massifs. There are small natural stands in southern Cameroon, near the border with Equatorial Guinea. Its natural occurrence in Nigeria, near the border with Cameroon, needs confirmation.
Okoumé has been planted for timber both within its natural range in Gabon and Cameroon, and elsewhere in Cameroon and Côte d’Ivoire. Small-scale planting trials have been made in Congo, DR Congo, Ghana, Madagascar, Indonesia, Malaysia, Suriname and French Guiana.
Uses
Okoumé is one of the best timber species for plywood. It is a major commercial timber in Gabon and Equatorial Guinea, representing more than 60% of timber production, while it is of lesser importance in Congo. It is made into blockboard, particle board and veneer, and is widely used in boat building for decorative interior panelling and for exterior applications. The wood is also suitable for light interior construction, carpentry, furniture, sports equipment, cigar boxes and packing cases. Logs are traditionally used to make canoes. The wood can be used as firewood and is suitable for the production of pulp for papermaking.
Bark resin is used for torches and oil lamps in Gabon and Equatorial Guinea, and also for cosmetic applications in skin care products and nail polish. It is also applied to treat superficial wounds and abscesses, and as a water disinfectant. The astringent bark is used to treat diarrhoea.
Production and international trade
Okoumé timber is mainly exported as logs to Asia (69% in 2002) and Europe (24% in 2002). In the 1960s, Gabon exported 1.1–1.4 million m³ of okoumé per year, Congo about 250,000 m³/year and Equatorial Guinea 60,000–120,000 m³/year. According to ATIBT (Association Technique Internationale des Bois Tropicaux), exports of logs, sawn timber, veneer and plywood from Gabon in 2002 were 1.0 million m³, 60,000 m³, 108,000 m³ and 30,000 m³ respectively. The average export prices of okoumé sawn timber, veneer and plywood in 2001 were US$ 176 per m³, US$ 360 and US$ 233 respectively. China is the largest importer of okoumé with 820,000 m³ in 2002, followed by France with 230,000 m³.
Small quantities of resin are exported from Gabon to France for cosmetic applications.
Properties
The heartwood is salmon pink to pale pinkish brown or reddish brown, darkening upon exposure to light to a mahogany-like colour, and is distinctly or indistinctly demarcated from the white to pale grey sapwood. The grain is usually slightly interlocked, texture medium to moderately fine. Quarter-sawn surfaces with a striped or mottled figure. Growth rings distinct due to alternating paler and darker layers.
Okoumé is a lightweight hardwood. At 12% moisture content, the density is (320–)350–450(–580) kg/m³. Okoumé wood dries rapidly, with little degrade and few defects in either air or kiln drying. The shrinkage rates from green to oven dry are 3.4–7.0% radial and 5.3–11.3% tangential.
The wood is comparatively soft. At 12% moisture content, the modulus of rupture is 51–109 N/mm², modulus of elasticity 5500–10,800 N/mm², compression parallel to grain 25–44 N/mm², shear 4–8 N/mm², cleavage (6–)9–22 N/mm tangential, Janka side hardness 1690–2510 N and Janka end hardness 2790–3560 N.
The wood is easy to work with both hand- and machine tools, but worked surfaces tend to be woolly, so cutting edges must be kept sharp. It contains up to 0.3% silica and sawing requires tungsten-carbide-tipped cutters as saw teeth blunt rather quickly. When planing, an angle of 20° is necessary to prevent tearing. The wood responds well to sanding, can be nailed without pre-boring, and glues and stains well. It can be polished to a lustrous surface. Dimensional stability is rated as fair, and seasoned wood is reported to move moderately after manufacture. Okoumé is often rotary-cut for the production of plywood; figured wood is sliced into highly decorative veneers for panelling and cabinet making.
Okoumé has little natural resistance to decay. Logs are susceptible to forest longhorn beetle attack. While the sapwood is readily attacked by Lyctus beetles, the heartwood is resistant, but susceptible to termites, marine borers and fungal attack. The heartwood is impermeable to preservatives. Glues with fungicides are used for plywood which may come into contact with moisture. The energy value of the wood is 29,970 kJ/kg.
Several terpenoids are present in the bark resin, including α-terpineol and β-phellandrene and several tetracyclic and pentacyclic triterpenes.
Description
Dioecious, medium-sized to large evergreen tree up to 50(–60) m tall; bole cylindrical, often contorted and bent, up to 110(–240) cm in diameter, with buttresses up to 3 m high, and clear of branches up to 21 m; bark 0.5–2 cm thick, greyish to orange-brown, detaching in more or less thick rectangular brown scales revealing orange bark in adult trees, with lenticels, slash strongly resinous, pinkish-red, fibrous; crown rather open structured. Leaves alternate, imparipinnate; stipules absent; rachis up to 40 cm long; leaflets 7–13, with stalks up to 4 cm long, ovate to oblong, 10–30 cm × 4–7 cm, rounded at base, acuminate at apex, entire, leathery. Inflorescence an axillary or terminal panicle up to 20 cm long; male inflorescence comprising up to 5 times more flowers than female one. Flowers unisexual, regular, 5-merous; sepals lanceolate, up to 5 mm long, hairy, greenish; petals spatulate, 5–6 mm long, hairy on both sides, whitish; disk consisting of 2-lobed nectaries; male flowers with 10 stamens and rudimentary pistil; female flowers with 10 staminodes and a superior, 5-celled ovary, style columnar, with headlike stigma. Fruit a capsule up to 5 cm × 3 cm, opening with 5 valves from the base, 5-seeded. Seeds enclosed by endocarp (‘pyrenes’), ovoid, extending into a wing 2–3 cm × 0.5 cm. Seedling with epigeal germination; cotyledons rounded, leafy.
Other botanical information
Aucoumea comprises a single species, and is characterized by its extra-staminal disk and dry, dehiscent fruit (pseudocapsule), which after opening releases 5 seeds covered by a winged endocarp.
Anatomy
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; (26: intervessel pits medium (7–10 μm)); 27: intervessel pits large ( 10 μm); 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); 42: mean tangential diameter of vessel lumina 100–200 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; 65: septate fibres present; 68: fibres very thin-walled. Axial parenchyma: 75: axial parenchyma absent or extremely rare; 78: axial parenchyma scanty paratracheal; 92: four (3–4) cells per parenchyma strand; 93: eight (5–8) cells per parenchyma strand. Rays: 97: ray width 1–3 cells; 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. Mineral inclusions: 159: silica bodies present; 160: silica bodies in ray cells.
(M. Thiam, P. Détienne & E.A. Wheeler)
Growth and development
Okoumé is shallow-rooted, with virtually no taproot. At 6–12 years, populations become organized into ‘biological cells’ through joining root grafts. When 10–25 years old, natural and planted populations segregate into dominant and suppressed trees. Trees reach an average height of about 10 m in the first 4 years; from 10–50 years, the growth rate of dominant trees is about 1 m per year, and eventually they reach a height of about 50 m. Suppressed trees grow very slowly and reach 15 m in 50 years. Growth in bole diameter is variable and depends on age, stand density, site fertility, hierarchical position of the tree and thinning rates. In coastal Gabon, bole diameters of 30–80 cm after 50–60 years for dominant trees and 10–40 cm for suppressed trees have been recorded. The mean annual diameter increment for dominant trees gradually decreased from 1.4 cm (7–12 years old) to 0.7 cm (50–60 years old) and for suppressed trees from 0.6 cm (5–20 years old) to 0.1 cm (10–40 years old). Other studies at different sites in the interior of Gabon recorded mean bole diameter growth rates of 0.5–0.7 cm/year, without taking into account the presence of dominant and suppressed individuals.
New leaves appear from September to December and are bright red for one week. Trees start to flower when they are about 10 years old, but fruiting only begins after 15 years. Flowering starts in August and lasts for 1–2 months depending on weather conditions. Okoumé is dioecious; male and female flowers are found on different trees. Individual flowers last for a few days and are pollinated by bees and flies. Fruiting starts in September; fruits are fully grown in about 40 days, but mature after about 80 days. Fruiting is annual, but large quantities of seeds are produced only every 2–3 years. A healthy, dominant mature tree can produce up to 20,000 seeds. Seeds are winddispersed up to 80 m from the parent tree.
Ecology
Okoumé occurs naturally from sea-level to about 600 m and occasionally to 1400 m altitude in areas with a mean annual rainfall of 1200–3000 mm and a marked dry season (less than 50 mm/month) from June to August/September, during which evaporation is reduced by the relatively low temperatures and high relative humidity. Mean annual temperatures range from 23–26°C, with mean maximum temperatures of the hottest month 32–36°C, and mean minimum temperatures in the coldest month 10–15°C. The natural distribution of Aucoumea klaineana is directly related to rainfall. In the north-east of Gabon, a second dry season (during the fruiting period) accounts for the absence of the species there, whereas in the south of Gabon the annual rainfall of less than 1200 mm is the limiting factor.
Okoumé is a long-lived pioneer of, in particular, large forest clearings and fire-protected savanna edges, where it often becomes mono-dominant. It requires full sun to grow well. Seedlings and saplings can, however, survive in shade for some years. Huge individuals occur in what seems to be virgin forest, but is in fact old secondary forest.
Okoumé grows well on a wide range of acid soils (such as ferralitic arenosols, ferralitic soils, podzoluvisols) developed on various substrates. It can grow on infertile sandy soils but prefers fertile, deep sandyclayloams. It can tolerate a certain degree of impeded drainage, but not long periods of waterlogging.
Propagation and planting
Seeds lose viability within 1 month in the field, but can be stored at 4°C in airtight containers for up to 3 years after being dried to 8% moisture content. The mean 1000-seed weight is 98 g. Propagation by seed is the preferred method. Seeds do not require pretreatment and can be sown directly in polythene bags (20–30 cm high, 10–15 cm in diameter) filled with a mixture of sand and clay. The addition of NPK fertilizer is recommended. Two seeds are placed per bag; one seedling is selected after 3–4 weeks. After 2.5 months, the seedlings are 20–25 cm tall, have 5–7 simple leaves and are ready for transplanting. Propagation by grafting and cuttings is possible.
Management
Good regeneration requires a sufficient number of seed trees, large canopy openings or clearings (> 2500 m²) and clean soil during the fruiting season. These conditions are found in shifting cultivation or in logged areas (e.g. log yards and wide logging tracks). Selective logging or natural tree fall events do not produce sufficiently large canopy openings. If site conditions are favourable, okoumé dominates regrowth.
Two main site preparation methods are used for establishing plantations. The first is mechanized clear felling, in which existing woody vegetation is cleared using bulldozers and placed into windrows and burned. Seedlings are planted between the windrows. The second consists of cutting the vegetation to 50 cm above the ground to allow resprouting and regrowth. Seedlings are planted in lines cut through the regrowth. In both methods, existing large trees are killed. Recommended spacings for seedlings are from 625–950 trees/ha. It is necessary to eliminate climbers, especially Mikania species, and trees such as Musanga cecropioides R.Br. ex Tedlie, which compete for light and space, for up to 5 years after planting.
Thinning in both natural and artificial stands is advisable, but should be conducted carefully to avoid increased sensitivity to black canker attack resulting from lateral illumination of the stem. In both mixed and almost pure stands, thinning is beneficial for diameter growth. Suppressed trees are much more responsive to thinning than dominant ones. In almost pure stands, thinning should be restricted to those young stands of less than 15 years, because in older stands thinning will remove potentially commercial trees.
In plantations, a thinning regime to reduce stem density to 350 stems/ha after 5 years, 200–250 stems/ha after 10 years and 150 stems/ha after 15 years is recommended. If resources only allow 2 thinning operations, stand density can be reduced to 250–300 stems/ha after 5 years and 150 stems/ha after 13 years. If thinning is not by cutting but by killing standing trees, it involves girdling okoumé trees, or girdling and poisoning other species, taking care to avoid intense bole illumination of lower strata trees. Okoumé trees should not be poisoned because of the risk of affecting adjacent trees via the connected root systems.
Diseases and pests
Okoumé is attacked by various pathogens, which are only serious in pure stands. The most important disease is black canker, a complex infection which begins with a primary infestation of scale insects (mainly Asterolecanium pustulans) spread by ants (Crematogaster, Oecophylla). Subsequent bark injuries are then infested by a secondary fungal pathogen, Botryodiplodia theobromae, resulting in an external proliferation of smut which blackens the bark and causes abnormal resin secretion. Very dense or severely thinned plantings and regrowth on abandoned human settlements are particularly prone to black canker. The problem can be avoided by good silvicultural practices, such as selecting a suitable planting site, adequate spacing and careful thinning. Seedlings and young plants are attacked by psyllids (Pseudophacopteron spp.) and centipedes, although serious infestations only occur in badly managed nurseries and only affect young plantings. During December and January, the foliage of both young and adult trees is often attacked by caterpillars of the moth Pleuroptya balteata, sometimes resulting in total defoliation of pure stands, but trees only suffer a reduction in growth. Locally, elephants cause serious destruction in young and pure stands.
Harvesting
Only 1 or 2 okoumé trees per ha are usually extracted in natural forest. Logs float and are commonly transported by river. During seasonally low river levels or where there are no rivers, logs are transported by road or rail.
Yield
In established plantations, the estimated potential yield varies from 7 m³/ha/year at 43 years on a moderately fertile site without thinning, to 11 m³/ha/year at 32 years on a fertile site with thinning. However, actual production is lower because 30% of trees above minimum felling diameter (70 cm) are usually not suitable for timber due to their poor shape. The yield potential in pure natural stands can be 5.5–7.5 m³/ha/year, but commercial yields are 1–2 m³/ha/year due to selective logging.
Handling after harvest
80% of okoumé timber is peeled for plywood production. It is considered one of the best woods for this purpose. Peeling can be performed with or without controlled steaming or seasoning, although the latter increases the overall quality of end products.
Sawn timber is becoming important as both Gabon and Congo are attempting to develop their woodprocessing capacities. While good-quality logs are exported, or locally processed for plywood, lower grades are now used for sawn timber.
Genetic resources
In order to safeguard the genetic diversity and sustained production of okoumé, stands should be identified and protected throughout the natural range of the species. Special attention should be given to natural stands not having a long history of exploitation. Field experiments indicate a poor performance of provenances from sites having a long history of exploitation.
Breeding
There are currently no major selection and breeding programmes as all planting has ceased, the current trend favouring natural regeneration. However, the few studies on provenance and progeny testing revealed large variability. This diversity at the provenance/progeny level is confirmed by recent genetic studies, indicating that there are possibilities for selecting better planting material. On the other hand, a study of chloroplast DNA variation showed a relatively low level of differentiation between okoumé populations in Gabon.
Prospects
Okoumé will remain a major commercial timber for Gabon and Equatorial Guinea, but its climatic requirements restrict its importance elsewhere. Breeding and selection programmes are needed.
Major references
• Brunck, F., Grison, F. & Maître, H.F., 1990. L’okoumé (Aucoumea klaineana Pierre). Monographie. Centre Technique Forestier Tropical (CIRAD-Forêt), Montpellier, France. 112 pp.
• CTFT (Centre Technique Forestier Tropical), 1988. Fiche technique: Okoumé. Bois et Forêts des Tropiques 215: 27–35.
• de Kam, M., Verkaar, H.J.P.A., Evers, P.W., van Dam, B.C. & Breteler, F.J., 1996. Biologie de l’okoumé. Rapport final de l’appui technique. IBN Research Report No 96-8. 185 pp.
• Fuhr, M., Nasi, R. & Minkoué, J.M., 1998. Les peuplements d’okoumés éclaircis au Gabon. Bois et Forêts des Tropiques 256: 5–20.
• Fuhr, M., Nasi, R. & Delegue, M.-A., 2001. Vegetation structure, floristic composition and growth characteristics of Aucoumea klaineana Pierre stands as influenced by stand age and thinning. Forest Ecology and Management 140: 117–132.
• Koumba Zaou, P., Mapaga, D., Nze Nguema, S. & Deleporte, P., 1998. Croissance de 13 essences de bois d’oeuvre plantées en forêt Gabonaise. Bois et Forêts des Tropiques 256(2): 21–32.
• Koumba Zaou, P., Mapaga, D. & Verkaar, H.J., 1998. Effect of shade on young Aucoumea klaineana Pierre trees of various provenances under field conditions. Forest Ecology and Management 106: 107–118.
• Leroy-Deval, J., 1976. Biologie et sylviculture de l’Okoumé (Aucoumea klaineana Pierre). Tome 1: La sylviculture de l’Okoumé. 355 pp. Tome 2: Maladies et défauts de l’Okoumé. Centre Technique Forestier Tropical, Nogent-sur-Marne, France. 76 pp.
• Muloko-Ntoutoume, N., Petit, R.J., White, L. & Abernathy, K., 2000. Chloroplast DNA variation in a rainforest tree (Aucoumea klaineana, Burseraceae) in Gabon. Molecular Ecology 9(3): 359–363.
• Nasi, R., 1997. Les peuplements d’okoumés au Gabon. Leur dynamique et croissance en zone côtière. Bois et Forêts des Tropiques 251: 5–27.
Other references
• Aubréville, A., 1962. Burséracées. Flore du Gabon. Volume 3. Muséum National d’Histoire Naturelle, Paris, France. pp. 53–95.
• Biraud, J., 1959. Reconstitution naturelle et amélioration des peuplements d’okoumé du Gabon. Bois et Forêts des Tropiques 66: 3–28.
• Biraud, J. & Catinot, R., 1960. Les plantations artificielles d’okoumé au Gabon. Bois et Forêts des Tropiques 73: 3–23.
• Caballé, G., 1978. Essai sur la géographie forestière du Gabon. Adansonia, séries 2, 17: 425–440.
• de Saint-Aubin, G., 1963. La forêt du Gabon. Publication No 21 du Centre Technique Forestier Tropical, Nogent-sur-Marne, France. 208 pp.
• Grison, F., 1978. Amélioration génétique de l’okoumé. Bois et Forêts des Tropiques 178: 3–15.
• Grison, F., 1978. Note sur les fleurs de l’Okoumé (Aucoumea klaineana Pierre, Burseraceae). Adansonia, séries 2, 17(3): 335–342.
• Grison, F., 1978. Où en est l’amélioration génétique de l’okoumé au Gabon? Revue Forestière Française 31: 504–511.
• InsideWood, undated. [Internet] http://insidewood.lib.ncsu.edu/search/. Accessed May 2007.
• Lebacq, L. & Deschamps, R., 1964. Essais d’identification anatomique des bois de l’afrique centrale. Koninklijk museum voor midden Afrika / Musee royal de l’afrique central, Tervuren, Belgium. 101 pp.
• Leroy-Deval, J., 1973. Les liaisons et anastomoses racinaires. Bois et Forêts des Tropiques 152: 37–49.
• Leroy-Deval, J., 1974. Structure dynamique de la rhizosphere de l’okoumé dans ses raports avec la sylviculture. Centre Technique Forestier Tropical, Nogent-sur-Marne, France. 113 pp.
• Leroy-Deval, J., 1975. Les possibilités du traitement de l’okoumé en taillis pour la production de bois papetier. Bois et Forêts des Tropiques 161: 23–34.
• Raponda-Walker, A. & Sillans, R., 1961. Les plantes utiles du Gabon. Paul Lechevalier, Paris, France. 614 pp.
• Reitsma, J.M., 1988. Végétation forestière du Gabon – Forest vegetation of Gabon. Tropenbos Technical Series 1. Tropenbos Foundation, Ede, Netherlands. 142 pp.
• Richter, H.G. & Dallwitz, M.J., 2000. Commercial timbers: descriptions, illustrations, identification, and information retrieval. [Internet]. Version 18th October 2002. http://delta-intkey.com/wood/index.htm. Accessed January 2005.
• Tessier, A.M., Delaveau, P. & Piffault, N., 1982. Oleo-résine d’Aucoumea klaineana. Planta Medica 44: 215–217.
• Touzard, J., 1964. Etude sur la conservation des graines d’okoumé. Bois et Forêts des Tropiques 98: 37–40.
• van Valkenburg, J.L.C.H., Ketner, P. & Wilks, C.M., 1998. A floristic inventory and preliminary vegetation classification of the mixed semi-evergreen rain forest in the Minkébé region, North East Gabon. Adansonia, séries 3, 20(1): 139–162.
• White, L. & Abernethy, K., 1997. A guide to the vegetation of the Lopé Reserve, Gabon. 2nd edition. Wildlife Conservation Society, New York, United States. 224 pp.
• Wilks, C. & Issembé, Y., 2000. Les arbres de la Guinée Equatoriale: Guide pratique d’identification: région continentale. Projet CUREF, Bata, Guinée Equatoriale. 546 pp.
Sources of illustration
• Aubréville, A., 1962. Burséracées. Flore du Gabon. Volume 3. Muséum National d’Histoire Naturelle, Paris, France. pp. 53–95.
• Grison, F., 1978. Note sur les fleurs de l’Okoumé (Aucoumea klaineana Pierre, Burseraceae). Adansonia, séries 2, 17(3): 335–342.
• Wilks, C. & Issembé, Y., 2000. Les arbres de la Guinée Equatoriale: Guide pratique d’identification: région continentale. Projet CUREF, Bata, Guinée Equatoriale. 546 pp.
• Tailfer, Y., 1989. La forêt dense d’Afrique centrale. Identification pratique des principaux arbres. Tome 2. CTA, Wageningen, Pays Bas. pp. 465–1271.
Author(s)
J.L.C.H. van Valkenburg
Biosystematics Group, Wageningen University, Generaal Foulkesweg 37, 6703 BL Wageningen, Netherlands


Editors
D. Louppe
CIRAD, Département Environnements et Sociétés, Cirad es-dir, Campus international de Baillarguet, TA C-DIR / B (Bât. C, Bur. 113), 34398 Montpellier Cedex 5, France
A.A. Oteng-Amoako
Forestry Research Institute of Ghana (FORIG), University P.O. Box 63, KNUST, Kumasi, Ghana
M. Brink
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
General editors
R.H.M.J. Lemmens
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
L.P.A. Oyen
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
J.R. Cobbinah
Forestry Research Institute of Ghana (FORIG), University P.O. Box 63, KNUST, Kumasi, Ghana
Photo editor
E. Boer
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Correct citation of this article:
van Valkenburg, J.L.C.H., 2005. Aucoumea klaineana Pierre. In: Louppe, D., Oteng-Amoako, A.A. & Brink, M. (Editors). Prota 7(1): Timbers/Bois d’œuvre 1. [CD-Rom]. PROTA, Wageningen, Netherlands.
Distribution Map wild


1, base of bole; 2, leaf; 3, female flower, front sepals and petals removed; 4,male flower, front sepals and petals removed; 5, fruit with opening valves; 6, fruits after dropping of valves
Redrawn and adapted by M.M. Spitteler



plantation of about 50 years old


tree habit


base of bole


bark and slash


bole and crown


female flowers


fruits


seedling


forest exploitation


wood in transverse section


wood in tangential section


wood in radial section