3420

5(1): Timber trees; Major commercial timbers

Cotylelobium Pierre

Fl. forest. Cochinch. fasc. 15, text accomp. pl. 235 (1890).

DIPTEROCARPACEAE

x = unknown

Trade groups Resak: heavy hardwood, e.g. Cotylelobium burckii (Heim) Heim, Cotylelobium lanceolatum Craib and Cotylelobium melanoxylon (Hook.f.) Pierre ex Heim. Resak often includes timber of the genus Vatica and sometimes also that of Upuna. Timber of Cotylelobium spp. is traded in Indonesia under the name 'giam', which may cause confusion because this is the trade name for the heavy wood of Hopea spp.

Resak. Brunei: resak batu. Indonesia: giam, resak bukit. Thailand: khiam (general).

Cotylelobium comprises 5 species and occurs in Sri Lanka (2 species), peninsular Thailand (2 species), Peninsular Malaysia (2 species), Sumatra (1 species) and Borneo (3 species). Within Malesia 3 species are recognized.

Resak is used for piling (also in contact with the ground or water), construction of houses (beams, posts, rafters, boards, door and window frames), bridges and ships (keels and ribs), for heavy-duty flooring, turnery, cabinet work and railway sleepers. Less heavy resak timber is also used for furniture, package, pallets and as a substitute for keruing (Dipterocarpus spp.), e.g. in Japan. Resak timber of Cotylelobium spp. has a high silica content (timber of Vatica spp. lacks any silica) which makes it especially suitable for saltwater piling and shipbuilding.
The resin may occasionally be used for caulking boats and for illumination but is of no commercial importance. An extract from the bark is used locally to prevent frothing during the boiling of sap of the sugar palm for sugar manufacture, and to arrest fermentation of toddy and local wine.

The timber is traded together with timber of Vatica spp. as resak, particularly on local markets. As Cotylelobium trees are often larger, they are considered more valuable for their timber than Vatica trees. The export of resak logs from Sabah was 13 000 m³ in 1992, and of sawn timber 5700 m³; the total value was US$ 2.4 million.

Resak is a heavy and hard timber. The heartwood is yellowish-brown to reddish-brown when fresh, darkening to dark brown on exposure. When the wood is fresh, the sapwood is clearly differentiated from the heartwood and is light yellowish-brown; when dry it can be less sharply defined. The density is 810-1160 kg/m³ at 15% moisture content. The grain is straight or only slightly interlocked, texture fine and even.
At 15% moisture content, the modulus of rupture for Cotylelobium melanoxylon from Indonesia is 133-135 N/mm², modulus of elasticity 17 700-18 630 N/mm², compression parallel to grain 68-69 N/mm², compression perpendicular to grain c. 6 N/mm², shear 7-8 N/mm², cleavage c. 77 N/mm radial and 82 N/mm tangential, Janka side hardness 8860 N and Janka end hardness 10 200 N.
The rates of shrinkage are high, for Cotylelobium melanoxylon from green to oven dry 6.1% radial and 10.4% tangential. The wood is moderately slow in drying, and there is a slight risk of checking and deformation. Boards of 25 mm thick take about 2.5 months to dry from 35% to 15% moisture content.
Working qualities are rated as moderately easy to moderately difficult. Sawing is usually rather easy, although silica is present. Planing and boring is easy and has a smooth finish; turning is slightly difficult.
The wood is very durable and is resistant to termites and teredos. It is very difficult to treat with preservatives, but for most applications treatment is not required.
Wood of Cotylelobium burckii contains 52% cellulose, 31.5% lignin, 7% pentosan, 0.3% ash and 0.1-0.2% silica; wood of Cotylelobium melanoxylon contains up to 0.4% silica. The solubility of Cotylelobium burckii wood is 3.4% in alcohol-benzene, 2.8% in cold water and 3.7% in hot water. Charcoal made from Cotylelobium wood has an energy value of over 29 000 kJ/kg.

Small, medium-sized or large trees, up to 50 m tall, bole frequently twisted, sometimes cylindrical and straight, branchless for up to 30 m but often forked at lower height and up to 160(-200) cm in diameter, buttresses small and rounded; outer bark thin, brittle, smooth at first and hoop-marked, greyish, becoming irregularly flaky, leaving a dippled or scroll-marked surface, inner bark about 1.5 cm thick, often separated from the outer by a red line, pinkish to pale yellow, with prominent bast fibres, resinous; crown hemispherical, rather small; twigs often densely stellate hairy or with short tufted hairs. Leaves alternate, simple and entire, leathery, margin revolute, lower surface scaly; midrib sunken or not prominent above, slightly prominent beneath, secondary veins parallel, bifurcating and anastomosing towards the margin to form a looped intramarginal vein; tertiary venation reticulate; petiole comparatively short, straight; stipules often early caducous. Inflorescence axillary or terminal, racemose, usually short. Flowers bisexual, actinomorphic, 5-merous, lanceolate in bud, sweet smelling; sepals imbricate, free, subequal, densely hairy; petals free, elliptical-oblong, cream to pink; stamens 15, subequal, in 3 whorls, filaments short, more or less triangular, anthers 4-celled, the inner cells smaller than the outer, oblong, setose along the margins, with a short, subulate appendage; ovary superior, free from the calyx, globose, without a stylopodium, densely tomentose, style 1, slender, more than 3 times as long as the ovary, tomentose towards the base, stigma small, trifid. Fruit a 1-seeded, globose nut; fruit calyx lobes free almost to the base, all distinctly enlarged, 2 of them obtuse and larger than the other 3 acute ones. Seedlings with the first pair of leaves opposite, succeeding leaves arranged spirally.

- Macroscopic characters:
Heartwood red-brown, darkening on exposure to darker brown, clearly demarcated from the pale yellow-brown sapwood. Grain straight to shallowly interlocked, producing a ribbon or silver figure. Texture fine and even. Growth rings indistinct; vessels just visible to the naked eye, vessel lines conspicuous on longitudinal surfaces; parenchyma moderately abundant, rather indistinct with a hand lens; rays of two sizes, fine- and larger-sized ones, individually distinct to the naked eye on the end grain, not conspicuous on longitudinal surfaces; ripple marks absent. Axial intercellular canals approximately half the size of larger vessels, scattered singly or in pairs, rarely in tangential series, empty or filled with chalky white deposits of dammar.
- Microscopic characters:
Growth rings indistinct. Vessels diffuse, 8-10/mm² (but c. 20/mm² in Cotylelobium burckii), predominantly solitary (95-98%), pairs rare, uniformly distributed and moderately numerous, mostly round, average tangential diameter 120-140 µm; perforation plates simple; intervessel pits rare, loosely alternate when present, vestured, 4-5 µm; vessel-ray pits simple, rounded, with large apertures of c. 20 µm; tyloses abundant. Fibres 1-1.5 mm long, non-septate, thick- to very thick-walled, bordered pits moderately conspicuous, most prominent in Cotylelobium melanoxylon. Parenchyma partially surrounding pores, to aliform with short wings, diffuse, diffuse-in-aggregates, occasionally forming short lines spanning several cells, also surrounding axial intercellular canals. Rays 6-8/mm, uniseriate and multiseriate 2-6(-8) cells wide, up to 1(-2) mm high, weakly heterocellular with 1-3 rows of square to upright marginal cells (Kribs type heterogeneous III and II, type heterogeneous I present in Cotylelobium burckii), uniseriates few, short, sheath cells present, prominent in Cotylelobium melanoxylon. Silica grains in ray cells of all species, prismatic crystals in addition to silica in upright cells of Cotylelobium melanoxylon. Horizontal intercellular canals absent. Axial gum canals diffuse, sometimes in pairs, rarely in short tangential arcs, average diameter 50-60 µm, commonly occluded with chalky white deposits.
Species studied: Cotylelobium burckii, Cotylelobium lanceolatum, Cotylelobium melanoxylon.
The diffuse axial gum canals distinguish resak (Cotylelobium and Vatica) from Dipterocarpus, Dryobalanops, Hopea and Shorea. Anisoptera can be distinguished from Cotylelobium by larger pores (and by the lower density and distinctive yellowish colour of the wood). Cotylelobium is readily separated from Upuna and Vatica by the presence of silica in ray cells.

Resak trees tend to flower periodically and synchronously over wide areas. In seasonal climates, they flower generally in the dry season, e.g. from January to March in Thailand. Fruiting is often prolific and fruits mature about 2 months after flowering. In Thailand mature fruits, which are brown, are present at the beginning of the rainy season (March - June). There, germination of the seeds usually starts with the first rains, either on the ground or already on the mother trees. During the first 3-5 years, the seedlings are stunted and hardly growing in height; they actually increase the girth of the stem. They normally produce shoots during the wet season, but these often die back in the dry season. After about 5 years (or more), when the stem and root system are strong enough, the seedlings will start to grow in height. The trees grow slowly, and it takes a long time to achieve commercial size. In Sarawak Cotylelobium melanoxylon showed an annual diameter increment of 0.2 cm.

The genus Cotylelobium is very closely related to Vatica especially to its section Sunaptea (Griffith) Burck. This close relationship is expressed in characters of the perianth, fruit and wood anatomy. The two taxa differ in their gynoecium, androecium and leaf venation. In Cotylelobium the secondary veins of the leaf anastomose to form a distinct intramarginal vein (which is absent in Vatica), the anthers are narrower and more hairy, and the style is longer than in Vatica. It has been proposed to divide the genus Vatica into three different genera and at the same time merge Cotylelobium with one of these (Sunaptea). Cotylelobium belongs to the tribe Dipterocarpeae and, although no chromosome counts have been performed, the expected basal number (x) is 11. The Malesian species of Cotylelobium differ only in the indumentum, shape and venation of the leaves. They are sometimes difficult to identify because of the occurrence of more or less intermediate specimens.

Cotylelobium species belong to the main and often dominant canopy trees of vegetations on dry acid soils with a pH of 3.7-4.5, especially on coastal hills up to 400 m altitude. They usually occur scattered but are sometimes semi-gregarious in primary dipterocarp forest. They grow on sandy loam soils, giant or regular podzols, peaty soils overlying limestone or sandstone ridges up to 1500 m altitude. In peninsular Thailand they occur in an area with a mean annual rainfall of about 2400 mm, 175 rainy days and a very short or no dry period. The average minimum temperature is 22.2°C, the average maximum temperature 33.7°C.

Mature fruits should be collected from mother trees. Fruits lying on the ground are mostly damaged by insects. They should be treated with fungicides, bactericides and insecticides. The nuts are soaked in water for one day before being sown in nursery beds. Using this method, a germination rate of 80-95% may be reached. When seeds are kept for more than 3 weeks, the germination rate decreases to 30-40% or less. The nursery beds should be shaded to about 50% of full sunlight. After about 2 weeks seeds will start to germinate. The seedlings are transferred to containers filled with a mixture of 25% black ash and 75% sandy loam and some organic fertilizer when they have reached a height of about 10 cm. Like all dipterocarps the seedlings need mycorrhizal infection for optimal growth. They are ready for planting into the field when they are 1.5-2 years old and have attained a height of about 50 cm and a stem diameter at soil level of about 30 mm. Spacing is 3 m 3 m or 4 m 4 m.

Management systems applied to lowland dipterocarp forest used to have a 30-year rotation. At present the 'selection cum improvement felling system' is applied in peninsular Thailand; the minimum girth for which cutting is allowed under this system is 200 cm at breast height. Since Cotylelobium trees are slow growers, cutting cycles should be long. In the Bako forest, Sarawak, on average 83 saplings and 623 seedlings of Cotylelobium melanoxylon were counted per ha.
Resak trees should be planted under quick growing nurse trees to protect them from strong sunlight and to achieve better growth.

Cotylelobium species usually occur scattered in the forest, and although regeneration may be plentiful, it takes a very long time for trees to reach a commercially interesting size. When large-scale logging operations and comparatively short cutting cycles are employed, resak may easily become endangered, except when forest reserves of sufficient size are maintained.

Cotylelobium timber is most valuable, but the very slow growth of the trees and their scattered occurrence in natural forest hampers commercial management and means that the survival of the species is at risk under indiscriminate exploitation without good management. Further research on silvicultural aspects is urgently needed.

Ashton, P.S., 1964. Manual of the dipterocarp trees of Brunei State. Oxford University Press, London. pp. 56-60.
Ashton, P.S., 1982. Dipterocarpaceae. In: van Steenis, C.G.G.J. (Editor): Flora Malesiana. Ser. 1, Vol. 9. Martinus Nijhof/Dr. W. Junk Publishers, The Hague, Boston, London. pp. 237-552.
Browne, F.G., 1955. Forest trees of Sarawak and Brunei and their products. Government Printing Office, Kuching. pp. 94-96.
Burgess, P.F., 1966. Timbers of Sabah. Sabah Forest Records No 6. Forest Department Sabah, Sandakan. pp. 226-235.
Lim, S.C., 1982. Malaysian timbers - resak. Malaysian Forest Service Trade Leaflet No 62. Malaysian Timber Industry Board, Kuala Lumpur. 9 pp.
Martawijaya, A., Kartasujana, I., Mandang, Y.I., Prawira, S.A. & Kadir, K., 1989. Atlas kayu Indonesia [Indonesian wood atlas]. Vol. 2. Forest Products Research and Development Centre, Bogor. pp. 50-55.
Phengklai, C., 1972. Economic timbers of Thailand. Dipterocarpaceae. Royal Forest Department, Bangkok. pp. 117-210.
Smitinand, T., Santisuk, T. & Phengklai, C., 1980. The manual of Dipterocarpaceae of mainland South-East Asia. Thai Forestry Bulletin 12: 1-110.
Symington, C.F., 1941. Foresters' manual of dipterocarps. Malayan Forest Records No 16. Forest Department, Kuala Lumpur. pp. 232-236.
van Slooten, D.F., 1929. The Dipterocarpaceae of the Dutch East Indies V. The genus Cotylelobium. Bulletin du Jardin Botanique de Buitenzorg, sér. 3, 10: 393-406.

C. Niyomdham (general part, selection of species), W.C. Wong (properties), J. Ilic (wood anatomy)

Cotylelobium burckii
Cotylelobium lanceolatum
Cotylelobium melanoxylon

Niyomdham, C., Wong, W.C. & Ilic, J., 1993. Cotylelobium Pierre. In: Soerianegara, I. and Lemmens, R.H.M.J. (Editors): Plant Resources of South-East Asia No 5(1): Timber trees; Major commercial 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:
Cotylelobium burckii
Cotylelobium lanceolatum
Cotylelobium melanoxylon