Dipterocarpus (PROSEA Timbers)
Dipterocarpus Gaertner f.
- Protologue: Fruct. 3: 50 (1805).
- Family: Dipterocarpaceae
- Chromosome number: x= 10, 11; D. costatus, D. elongatus, D. kunstleri, D. obtusifolius: 2n= 20, D. baudii, D. cornutus, D. oblongifolius: 2n= 22, D. alatus: 2n= 20, 22
Keruing: medium-weight to heavy hardwood, e.g. Dipterocarpus baudii Korth., D. cornutus Dyer, D. costulatus v. Slooten, D. crinitus Dyer, D. verrucosus Foxw. ex v. Slooten.
Because of the comparatively wide variation in density of the wood a subdivision into lightweight keruing, medium-weight keruing and heavy keruing is sometimes made.
- Indonesian gurjun (En).
- Keruing, kruen (Fr)
- Indonesia: lagan (Sumatra), tempudau, kerup (Kalimantan)
- Philippines: apitong, panau
- Burma: kanyin, eng, in
- Cambodia: thbaèng, khlông, chhe: ti:ël
- Laos: nha:ng
- Thailand: yang-na (general), yang-khon (Chanthaburi), yang-pai (northern)
- Vietnam: dầu, lông, dầu rái, chò.
Origin and geographic distribution
Dipterocarpus consists of some 70 species and is distributed from Sri Lanka, India and Burma, through Indo-China, southern China and Thailand towards western Malesia. Within Malesia the genus is found in Peninsular Malaysia, Sumatra, Java, Bali, Sumbawa, Borneo, the Philippines and intervening islands. Hence it does not cross Wallace's line (between Borneo and Sulawesi), which is regarded as an important biogeographical barrier, except between Bali and Sumbawa. The oldest fossil records are from the Miocene.
Keruing is an important source of general construction timber, for medium and heavy construction. Because it is not very durable when in contact with the ground its main applications are for interior purposes such as beams, joists, staircases, stringers, door and window frames, vehicle and wagon bodies, heavy-duty furniture and flooring (except for the heaviest pedestrian traffic). A special application is in laboratories where a high impermeability for chemicals is required. When treated with a preservative the timber can be used outdoors, for example for telegraph and powerline poles, foundation piles, fences, railway sleepers, and for ship building and wharves. Other purposes are heavy-duty pallets, general-duty crates and boxes, agricultural tools and toys. Because of the resinous nature of several keruing species, their wood is generally less suitable for flooring and woodwork exposed to the sun, or to be painted. The wood is widely used for the manufacture of veneer and plywood, especially for the structural grade. Oily keruing is not suitable for the production of low-density particle board. The surfaces of such boards are rough, probably because of the stiffness of the flakes. Keruing flakes can, however, be used as core material in the manufacture of 3-layered particle board. Hardboard of good quality can be obtained from keruing, and good-quality paper can be produced from keruing pulp. Locally the wood is made into good-quality charcoal.
An oleo-resin, known as "minyak keruing" (Malaysia) or "apitong oil" (the Philippines), is produced by all species of keruing, but only a few produce sufficient amounts to make collecting on a commercial scale feasible. This oleo-resin is used locally as a coat for waterproofing paper, for caulking baskets and boats, as a varnish for walls and furniture, in the preparation of lithographic ink or, sometimes mixed with bark of Melaleuca sp., for torches. It has also been tried as a substitute for linseed oil. Medicinally the oil is applied to ulcers as well as sores and foot diseases of cattle. Contradictory results have been obtained when this oil was applied in the treatment of gonorrhoea. Destructive distillation of the wood yields a tar rich in creosote oils. In the Philippines, substituting apitong oil for diesel fuel gave positive results but needs further research.
The bark is considered to be a tonic, and a hot decoction of it is used to cure rheumatism and liver complaints. Locally it is used for house walls. Bark extracts yield a tannin-formaldehyde adhesive. Locally the nuts are boiled and eaten as a vegetable or chewed as chew-nuts (e.g. in Thailand).
Production and international trade
Keruing is one of the most important export timbers of South-East Asia, second only to meranti. In Indonesia keruing is exported together with kapur ( Dryobalanops spp.) in a combined export group. In 1987 the export of sawn kapur/keruing timber from Indonesia was 213 000 m3(with a value of US$ 39 million), increasing to 463 000 m3(with a value of US$ 99 million) in 1989. Keruing accounts for about 82% of this combined timber group. Moreover, keruing is much used in plywood production in Indonesia. In Peninsular Malaysia the export of sawn keruing timber was 475 000 m3(worth US$ 55 million) in 1981, decreasing gradually to 192 000 m3(worth US$ 19 million) in 1984. Thereafter, exports increased again, and by 1989 had almost reached the level of 1981, with 428 000 m3(worth US$ 65 million); exports in 1990 fell back to 363 000 m3(with a value of US$ 59 million) and in 1992 to 190 000 m3(with a value of US$ 37 million). Sabah and Sarawak export keruing in even larger amounts. In Sabah the export of round logs in 1987 was 1.1 million m3with a value of US$ 85 million; exports in 1992 were 234 000 m3of logs and 260 000 m3of sawn timber with a total value of US$ 87 million.
The annual export of keruing from the Philippines was in 1980 and 1981 over 100 000 m3(worth US$ 7-8 million); more than 90% of the exported timber volume was unprocessed. Exports fell to 6000 m3(worth US$ 850 000) in 1987 (entirely sawn timber), and to only 22 m3(worth US$ 3000) in 1990. In Thailand, keruing timber is mostly for local use. In 1986 the production was 706 000 m3, but it fell dramatically to 203 000 m3in 1989 and 48 000 m3in 1990, illustrating the decline of the natural forest.
The annual production of keruing wood-oil in Thailand peaked in 1984 with 1.7 million l, but fell to 640 000 l in 1989 and 293 000 l in 1990.
Keruing is a moderately heavy to heavy hardwood. The sapwood is yellowish to greyish-brown and usually distinctly demarcated from the heartwood, which is greyish-brown to red-brown, usually not distinctly lustrous on planed surfaces. The density is (500-)600-980(-1070) kg/m3at 15% moisture content. The grain is straight, sometimes interlocked, texture moderately coarse to coarse but even. The wood has a distinctive smell of resin and the surface is often sticky.
At 15% moisture content, the modulus of rupture is 76-133 N/mm2, modulus of elasticity 12 900-22 300 N/mm2, compression parallel to grain 43-68 N/mm2, compression perpendicular to grain 5-9 N/mm2, shear 6-12.5 N/mm2, cleavage 53-76 N/mm radial and 56-93 N/mm tangential, Janka side hardness 3575-7300 N and Janka end hardness 4250-6135 N.
Keruing is a timber with high shrinkage. The rate of shrinkage from green to 15% moisture content is (1.6-)2.0-4.7% radial and (3.5-)4.0-7.0(-7.4)% tangential, from green to oven dry (3.5-)5.0-7.0% radial and (8.5-)9.6-13.0(-13.5)% tangential. The wood is rather difficult to dry because of this high shrinkage and the great difference between radial and tangential shrinkage; it is prone to cupping, bowing, springing and end checking. Usually there is little splitting, surface checking, staining and insect attack during drying. Boards of 15 mm, 20 mm, 40 mm and 50 mm thick take about 3, 4, 5 and 8 months, respectively, to air dry. For kiln drying of 25 mm thick boards, kiln schedule D is recommended in Malaysia. The kiln-drying characteristics differ between species. Wood of D. grandiflorus is very resinous and 25 mm thick boards may take up to 15 days to kiln dry from 50% to 10% moisture content and are prone to cupping and bowing. D. cornutus boards can be dried in about 7 days and show considerably less warping. Hence, when wood of various species of keruing is to be kiln dried as a mixed charge, the wood should be sticker-stacked at 450 mm spacing and a longer equalization treatment of 2-3 days is recommended. Resin exudation can be reduced, if desired, by using moderately low dry-bulb temperatures.
The working properties vary with the resin and silica content of the wood. In general, keruing is easy to cross cut but slightly difficult to resaw as the resin tends to clog the sawteeth. Wood with a high silica content quickly blunts sawteeth. Occasionally dabbing the saw with kerosene, and frequent sharpening, can overcome the problem. With a few exceptions (e.g. D. baudii ) keruing is easy to plane and gives smooth to moderately smooth surfaces. Ease of boring ranges from easy to slightly difficult with smooth to moderately smooth surfaces. D. cornutus wood has a tendency to split and tear grain during boring. Turning is usually slightly difficult, giving rough surfaces. Keruing is peeled for plywood manufacture, especially for the structural grade. The resin interferes with the gluing properties and the species which are too oily are generally not used. Resin patches are usually seen on the surface of the veneer. Pre-heating is essential for good peeling. Keruing flakes can be used as core material in the manufacture of 3-layered particle board, while a lighter wood can be used for faces. Hardboard made from keruing was found to have similar mechanical properties but better water repellency when compared with hardboards made from Japanese oak and beech wood. In general, keruing is a technically suitable raw material for hardboard production. Heat-tempered hardboard made from D. crinitus wood was found to meet the requirements of the British standards.
Graveyard tests in Malaysia showed an average service-life of stakes (50 mm × 50 mm × 600 mm) in contact with the ground ranging from 0.8 years for D. kerrii to 4.1 years for D. verrucosus. Under temperate conditions, stakes may last 10-15 years. Most keruing wood is classified as moderately durable. The resistance to wood rotting fungi is quite variable, and to dry-wood termites usually poor, except wood of D. elongatus and D. lowii. In general, the sapwood is readily susceptible to fungal, borer and dry-wood termite attack and should be rejected. Keruing is fairly resistant to marine borers (mean service life of 6-7 years). Preservatives are absorbed very readily by most keruing species, with the exception of D. crinitus and D. lowii. An absorption of 100-130 kg/m3of an equal mixture of creosote and diesel fuel can be obtained when using the open tank method, and 300 kg/m3of copper-chromium-arsenic based preservatives, using the full-cell pressure treatment. Treated keruing can be very durable in exposed conditions, more than 20 years in the tropics.
Wood of D. gracilis contains 51% cellulose, 19% lignin, 17% pentosan, 0.9% ash and 0.6% silica. The solubility is 3.9% in alcohol-benzene, 0.3% in cold water, 3.2% in hot water and 11.7% in a 1% NaOH solution. The energy value is 20 425 kJ/kg.
The resin consists of sesquiterpenoids, and has fungicidal and termiticidal properties, as was demonstrated for D. kerrii.
- Medium-sized to large, resinous trees of up to 65 m tall; bole usually branchless for as much as 35 m, straight with little taper, with a diameter often exceeding 150 cm with a maximum of 260 cm and usually with small and concave or sometimes tall and straight stout buttresses; bark surface orange-brown bleached by the sun to greyish, usually scaly and warty-lenticelled, rarely fissured or scaly-fissured, outer bark dark rust-brown, inner bark pale yellow-brown to dark rust-brown, homogeneous; resin produced on freshly cut surfaces; crown usually relatively narrow, even or irregular (not cauliflower-shaped), dome-shaped, frequently rather flat, open, with a few large, strongly ascending, twisted branches; twigs variable in tomentum and appearance, with distinct, usually swollen and pale, amplexicaul stipule scars; buds in dormant stage, prominent and specifically diagnostic, not much broader than the twigs.
- Leaves alternate, simple, leathery, rarely thin, very variable in size and tomentum, pinnately veined, with a sinuate or straight margin, plicate in bud and corrugated on opening; secondary veins prominent beneath, straight, curved only near the margins; petiole geniculate at the joint with the lamina, stout or slender; stipules paired, large, hastate to lorate, obtuse, more or less succulent, caducous, characteristically carpeting the forest floor in the growing season.
- Inflorescence simple or branched, racemose, short, stout, zig-zag, few-flowered; bracts as the stipules but smaller, fugaceous.
- Flowers large, actinomorphic, bisexual, scented, nodding; calyx persistent, 5-merous, united round the ovary into a tube, but not fused to it, with valvate lobes, two of them long, oblong to spatulate, more or less distinctly 3-veined, and 3 short, or rarely all 5 short; petals large, oblong to narrowly oblong, strongly contorted, loosely cohering at base on falling, cream-white with a prominent pink, red or purple stripe down the centre; stamens 15-40, persistent at first in a ring round the ovary after the petals fall, filaments of variable length, broad, compressed, connate at base, tapering apically, connective prolonged into a short, sharp or blunt point or a long awn; ovary 3-celled with 2(-3) ovules in each locule, the base enclosed in the calyx tube, the apex ovoid to conical, shortly tomentose, stylopodium present, shortly tomentose, narrowed gradually into a filiform glabrous style, stigma small, simple.
- Fruit a nut, surrounded by the calyx, comparatively large; fruit calyx tube woody, becoming more or less distinctly constricted into a distal neck as the nut expands, smooth, pustulate, tubercled, ridged, winged or plicate, fruiting calyx lobes developed into 2 large wings and 3 ear-shaped lobes or rarely vestigial; nut ovoid, with a woody pericarp, tomentose, with a short acute apical style remnant.
- Seedling with epigeal (cryptocotylar) germination; first two leaves opposite, subsequent leaves arranged spirally.
- Heartwood varying from greyish-brown, pink-brown to red-brown, sometimes with a purple tinge, darkening on exposure, sapwood pale with grey tinge, 50-75 mm wide, often but not always clearly demarcated from the heartwood, some species with characteristic white blotches.
- Grain straight, interlocked grain rare, fissile.
- Texture medium to coarse, even. Growth rings indistinct; vessels medium-sized to moderately large, mostly visible to the naked eye as individual pores, vessel lines long and conspicuous on longitudinal surfaces, varying amounts of tyloses visible; parenchyma absent or sparse.
- Rays of two sizes, fine and larger sized ones individually distinct to the naked eye; ripple marks absent.
- Axial intercellular canals in short tangential arcs spanning several rays, empty or filled with chalky white or black resin often producing resinous exudation on end surfaces.
- Growth rings indistinct.
- Vessels diffuse, 3-10/mm2, predominantly solitary (over 95%), a few in pairs, uniformly distributed, tending to oblique arrangement, distinctly oval, average tangential diameter (120-)180-250(-280)μm; perforation plates simple, horizontal and rounded; intervessel pits sparse, loosely alternate, rounded, tending to be horizontally elongated, vestured, pit border diameter approximately 5-7μm; vessel-ray pits simple, rounded, with large apertures of c. 20μm; tyloses varying from sparse to abundant.
- Vasicentric tracheids few to common. Fibres 1.7-2.0 mm long, non-septate, walls moderately thick to thick, pits small, distinctly bordered.
- Parenchyma paratracheal, partially surrounding pores, to aliform with short wings, diffuse, surrounding axial intercellular canals, in 4-5-celled strands, free from extraneous materials. Rays 6-8/mm, uniseriate and multiseriate, the latter 3-6(-8) cells wide, up to 2.0 mm high, heterocellular with 1-3 rows of square to upright marginal cells (Kribs type heterogeneous III and II, occasionally I), uniseriate rays few, short; sheath cells present.
- Silica bodies abundant in ray cells of all species; crystals absent; dark staining material usually abundant.
- Horizontal intercellular canals absent; axial gum canals in short tangential series of 2-7 individual canals, (50-)80-120(-170)μm, smaller, as large as, or larger than the vessels, occasionally forming continuous tangential series in D. confertus and D. elongatus.
Species studied: D. acutangulus, D. borneensis, D. caudatus, D. caudiferus, D. confertus, D. costatus, D. costulatus, D. crinitus, D. dyeri, D. elongatus, D. eurynchus, D. geniculatus, D. globosus, D. gracilis, D. grandiflorus, D. kunstleri, D. lowii, D. oblongifolius, D. palembanicus, D. rigidus, D. verrucosus.
The axial gum canals arranged in typically short tangential series of 2-7 canals distinguish Dipterocarpus from most other Dipterocarpaceae. Anisoptera differs from Dipterocarpus by the wood lacking pink tints and by the lack of oblique arrangement of the vessels.
Growth and development
Viable fruits start to germinate a few days to a few weeks after they have fallen on the ground. The radicle extrudes from the apex of the nut, causing the pericarp to split irregularly. The folded cotyledons remain in the nut close to the soil surface, but the plumule frees itself by elongating the petioles of the cotyledons. In some species with small nuts, the nuts may be raised from the ground. The plumule grows from between the petioles of the cotyledons, first producing a pair of opposite leaves, but with subsequent leaves arranged spirally; the leaves have large stipules. Twigs and petioles of seedlings are usually hairier than those of the mature trees, leaves are considerably larger, frequently more prominently acuminate and have proportionately shorter petioles.
Keruing seedlings need shade for optimal growth. For D. oblongifolius maximum height increment after one year is obtained at about 30% of full daylight, but root weight (more important for transplanting) is maximum at about 60% of full daylight. Seedlings of D. hasseltii show optimal growth at 50% shading. The average annual height growth and annual diameter growth differ between species. In experiments in Java, D. retusus had an average height growth of 50 cm/year and an average diameter increment of 0.7 cm/year; for D. grandiflorus and D. tempehes the corresponding figures were 58 cm/year and 0.9 cm/year, and 83 cm/year and 0.9 cm/year, respectively. A comparatively large average annual height growth of 160 cm/year was reported for D. gracilis on Java. Several species may reach large bole diameters in a comparatively short time. The following bole diameters of 40-year-old planted trees have been reached or estimated in Malaysia: D. oblongifolius 79 cm, D. kerrii 74 cm, D. costulatus 73 cm, D. baudii 65 cm, D. cornutus 61 cm, D. grandiflorus 58 cm, D. hasseltii 52 cm, D. chartaceus 51 cm, and D. crinitus 48 cm. Other species appear to grow slowly, e.g. D. kunstleri and D. palembanicus, which reportedly reach maximum bole diameters after 40 years of only 24 cm and 20 cm, respectively, and D. caudiferus which needs over 150 years to reach a diameter of 75 cm (estimated in Sabah).
Some keruing species are reported to flower and fruit annually. Examination of flowering shoots of D. oblongifolius in Malaysia revealed alternating periods of flowering and vegetative growth; the tree investigated flowered and fruited annually in April - August and sometimes again in October - January. The apical reiteration of primary branches (i.e. starting new copies of its growth model) has been described for D. kunstleri.
Keruing trees probably need mycorrhizae for optimal growth, but data are lacking. Ectomycorrhizae have been recorded for D. cornutus and D. obtusifolius, but the identity of the fungal symbionts is unknown. Self-compatability has been demonstrated for D. oblongifolius in Malaysia.
Other botanical information
The genus Dipterocarpus is characterized by its bark with warty lenticels, its generally amplexicaul stipule scars, its prominent resting buds, the leathery, plicate, corrugated leaves with swollen petioles at their base, and its type of venation. It belongs to the tribe Dipterocarpeae which is characterized by the fruit calyx lobes being valvate at the base, and a basic chromosome number of 11. Within the tribe Dipterocarpeae the genus is most closely related to Anisoptera. Dipterocarpus has been divided into 5 sections on the basis of characters of the fruit calyx tube, but these do not correlate with other characters and they vary within a species. Natural hybrids between species of the genus have been observed and are mentioned in the subsequent species treatments. The hybrids are generally found in small patches together with the parent trees. D. intricatus Dyer, D. tuberculatus Roxb., and D. turbinatus Gaertner f. supply much of the timber used in Indo-China and Thailand, but they do not occur in Malesia.
Most species grow scattered, but some, such as D. elongatus, D. gracilis and D. obtusifolius, frequently occur gregariously. This may be due to their fire-resistance, their high germination rate or to peculiar chemical properties of the soil. Keruing species occur in evergreen forest, semi-evergreen forest or savanna woodland up to 1000(-1400) m altitude. In Thailand D. obtusifolius (and some other species) is frequently found in association with pines, forming the Pine-Dipterocarp association.
Propagation and planting
Viability of the seeds is short. In the nursery, seeds should be sown immediately (no later than 7 days after collecting the nuts), preferably under a cover of dry hay, and kept continuously wet. Seeds of D. humeratus can be successfully stored for up to 8 weeks at half of their initial moisture content and at 15°C in sealed polyethylene bags filled with nitrogen gas. Seeds of D. oblongifolius survived at 4°C for at least 2 months. The germination rate, however, is often low. Tests in the Philippines showed a germination rate for D. grandiflorus of 56% and a survival percentage of seedlings of 22%; the comparable figures for D. gracilis are 16% and 6%, and for D. hasseltii are 14% and 3%, respectively. Fruits of D. crinitus seem to be particularly prone to insect attack and viable seeds are difficult to obtain. The weight of a nut of D. crinitus is approximately 1.5 g, for D. gracilis about 3 g. Germinated seeds are often immediately put into plastic bags and kept under shade. After one year the seedlings have reached 50 cm in height on average, and can be planted out in the field. It has been recommended to plant Paraserianthes falcataria (L.) Nielsen as a source of mycorrhiza and as a shade tree, before transplanting the keruing seedlings.
Experiments with vegetative propagation have had varying degrees of success, but were not successful at commercial levels. In Malaysia, stem cuttings of D. chartaceus showed a rooting success of 60-80%, and in the Philippines the air layering of D. grandiflorus resulted in only 10% of the branches developing roots. Wildlings of D. retusus survive much better than cuttings and are recommended as planting stock for enrichment planting.
In plantations spacing is 3 m × 3 m, 4 m × 4 m or 2 m × 4 m, to attain straight boles. In strip planting, spacing is 2-3 m in the strip, and 6-10 m between strips.
Silviculture and management Keruing seedlings and saplings can persist in the forest for years under heavy shade. In the first 2 years, major openings in the canopy are not tolerated, but after the seedlings have been well established (about 120 cm tall) the canopy can be opened up, to speed up growth. Many species regenerate well only in primary forest, and for these species enrichment planting after selective cutting is recommended. In plantations, weeding is necessary during the first 3 years. Thinnings should be carried out after 5, 10, 15 and 25 years. In the first 2-3 years, shade trees are used such as Acacia auriculiformis Cunn. ex Benth. and Paraserianthes falcataria.
Diseases and pests
Diseases reported for D. grandiflorus in the Philippines are "wildling blight" caused by Botryodiplodia theobromae and "apitong wilt" for which the most frequently associated organism is a Polyporus sp. In Peninsular Malaysia the fungus Cylindrocladium scoparium is pathogenic to seedlings of D. grandiflorus.
Insects may damage seeds, e.g. Alcidodes crassus (Coleoptera), Alcidodes dipterocarpi, Nanophyes shoreae (Coleoptera) and Cydia pulverula (Lepidoptera).
Keruing timber is usually obtained from natural forests using selective cutting systems, just as for most other dipterocarp timbers. Fresh logs usually sink in water and cannot be transported by river, but the logs of species with lighter wood may float.
For firewood production, coppicing is an adequate method in Thailand to harvest trees of D. obtusifolius (a savanna species) with a diameter of less than 20 cm.
D. kerrii appears to be by far the most important species for tapping keruing oil in Malaysia. Other species tapped are D. chartaceus, D. cornutus and D. gracilis, and in Thailand also D. baudii and D. costatus. Usually the tap consists of an axe-cut hole in the stem, 90-120 cm from the base of the trunk, sloping down to the centre of the bole. The lower part of the notch is often hollowed out to catch the wood oil. The flow of wood oil is stimulated by firing which is spasmodic, depending on the flow of the oil. There have been no studies to test whether tapping does or does not affect timber production, but in Thailand tapping is usually done within 18 months before felling of the tree. It is reported that the maximum quantity of wood oil can be obtained when the bole is tapped on the side which bears the largest branch.
Although keruing is common over large areas, and is often outnumbered only by meranti (Shorea sp.), the trees usually occur scattered (e.g. in Thailand with an approximate density of 8 trees/ha). Indiscriminate logging of trees belonging to a genus with so many species may endanger the less common ones. The establishment of reserves of sufficient extent, located in areas with optimal species richness, seems to be the best way to protect genetic diversity.
The exploitation of keruing in Indonesia and Malaysia has gained importance towards the end of the 1980s. Keruing is now one of the valuable timbers in the development of the forest industry, particularly for the production of veneer and plywood (e.g. in Kalimantan). Keruing is only planted on a very small scale, and research on silvicultural aspects is urgently needed to enable large-scale planting to ensure supplies for the future.
- Ashton, P.S., 1982. Dipterocarpaceae. In: van Steenis, C.G.G.J. (Editor): Flora Malesiana. Ser. 1, Vol. 9. Martinus Nijhoff/Dr W. Junk Publishers, The Hague, Boston, London. pp. 291-326.
- Browne, F.G., 1955. Forest trees of Sarawak and Brunei and their products. Government Printing Office, Kuching. pp. 102-111.
- Burgess, P.F., 1966. Timbers of Sabah. Sabah Forest Records No 6. Forest Department, Sabah, Sandakan. pp. 99-117.
- Choo, K.T. & Sim, H.C., 1981. Malaysian timbers - keruing. Malaysian Forest Service Trade Leaflet No 48. Malaysian Timber Industry Board, Kuala Lumpur. 18 pp.
- Fyfe, A.J., 1950. Tapping of keruing for oil. Malayan Forester 13: 227-229.
- Martawijaya, A., Kartasujana, I., Kadir, K. & Prawira, S.A., 1986. Indonesian wood atlas. Vol. 1. Forest Products Research and Development Centre, Bogor. pp. 54-59.
- Masano, Alrasjid, H. & Hamzah, Z., 1987. Planting trials of dipterocarp species outside their natural distributional range in the Haurbentes experimental forest, West Java. In: Kostermans, A.J.G.H. (Editor): Proceedings of the third round table conference on dipterocarps, Samarinda. UNESCO, Jakarta. pp. 19-37.
- Maury-Lechon, G., Hassan, A.M. & Bravo, D.R., 1981. Seed storage of Shorea parvifolia and Dipterocarpus humeratus. Malaysian Forester 44: 267-280.
- Quiniones, S.S., 1980. Notes on the diseases of forest trees in the Philippines. Sylvatrop 5(4): 263-271.
- Smitinand, T., Santisuk, T. & Phengklai, C., 1980. The manual of Dipterocarpaceae of mainland South-East Asia. Thai Forest Bulletin (Botany) 12: 1-110.
Selection of species
- Dipterocarpus acutangulus
- Dipterocarpus alatus
- Dipterocarpus applanatus
- Dipterocarpus baudii
- Dipterocarpus borneensis
- Dipterocarpus caudatus
- Dipterocarpus caudiferus
- Dipterocarpus chartaceus
- Dipterocarpus confertus
- Dipterocarpus conformis
- Dipterocarpus cornutus
- Dipterocarpus costatus
- Dipterocarpus costulatus
- Dipterocarpus crinitus
- Dipterocarpus dyeri
- Dipterocarpus elongatus
- Dipterocarpus eurynchus
- Dipterocarpus geniculatus
- Dipterocarpus globosus
- Dipterocarpus gracilis
- Dipterocarpus grandiflorus
- Dipterocarpus hasseltii
- Dipterocarpus humeratus
- Dipterocarpus kerrii
- Dipterocarpus kunstleri
- Dipterocarpus lowii
- Dipterocarpus mundus
- Dipterocarpus oblongifolius
- Dipterocarpus obtusifolius
- Dipterocarpus palembanicus
- Dipterocarpus retusus
- Dipterocarpus rigidus
- Dipterocarpus sublamellatus
- Dipterocarpus tempehes
- Dipterocarpus validus
- Dipterocarpus verrucosus
- T. Smitinand (general part, selection of species),
- C. Phengklai (general part, selection of species),
- W.C. Wong (properties),
- J. Ilic (wood anatomy),
- L.E. Groen (selection of species)