- Protologue: Mus. Bot. Ludg.-Bat. 1: 231 (1850).
- Family: Anacardiaceae
- Chromosome number: x= unknown;D. dao:n= 18
Dao: lightweight to medium-weight hardwood, e.g. Dracontomelon dao (Blanco) Merr. & Rolfe.
- Dao: New Guinea walnut, Papua New Guinea walnut, Pacific walnut (En, Am)
- Indonesia: dahu (general), sengkuang (Kalimantan), basuong (Irian Jaya)
- Malaysia: sengkuang (Peninsular, Sabah), unkawang (Sarawak)
- Papua New Guinea: mon (Pidgin), laup (Tolai)
- Philippines: dao (general). Burma (Myanmar): nga-bauk
- Thailand: phrachao ha phra ong (Chiang Mai)
- Vietnam: sấu.
Origin and geographic distribution
Dracontomelon consists of about 8 species which are distributed in India, Burma (Myanmar), Indo-China, China, Thailand, the Malesian area (3 species), towards the Solomon Islands, New Caledonia, and Fiji. D. dao has the largest area of distribution, from India to the Solomon Islands.
The usually colourful timber is extensively used for furniture and interior finish, and also for joinery, cabinet work, shop fittings, decorative (sliced) veneers, plywood, panelling, moulding, flooring, light construction, fence posts, house posts, rafters, boat-building, shop fittings, cladding, lining, boxes, matches, turnery, carving and artifacts.
The fruits of most species are edible but sour; the kernel of the seeds is also edible. Locally, flowers and leaves are cooked and eaten as vegetable, and they may also be used as food flavouring, or medicinally. The bark is occasionally used in traditional medicine. Furthermore, the tree is planted as an ornamental in roadside plantings and used for firewood.
Production and international trade
The export of dao from the Philippines varied considerably in the 1980s, but was never significant; in 1982 about 75 m3of processed timber was exported with a value of US$ 15 000 (US$ 200/m3), and in 1987 only 5 m3with a value of US$ 900 (US$ 180/m3). In Papua New Guinea, dao timber is ranked in MEP (Minimum Export Price) group 1, which fetches comparatively high prices; in 1992 the minimum export price for saw logs was US$ 80/m3and the best quality logs fetched US$ 140/m3. Its importance has declined from a major export timber to contributing less than 1% of the total volume exported from Papua New Guinea in 1993. Japan imports it mainly from Papua New Guinea.
Dao is a lightweight to medium-weight hardwood. The heartwood is greyish, greenish-yellow to walnut brown, often with irregular dark brown to nearly black bands or finestreaks; it is more or less clearly defined from the paler sapwood (pale yellow with pinkish or greyish tinge), which is up to 10 cm wide. The density is (330-)370-790 kg/m3at 15% moisture content. The grain is straight or interlocked, texture medium to coarse; a stripe or ribbon figure with black pencil-like streaks is usually present on quarter-sawn surfaces.
At 12% moisture content, the modulus of rupture is 81-100 N/mm2, modulus of elasticity 11 000-12 600 N/mm2, compression parallel to grain 46-49.5 N/mm2, compression perpendicular to grain c. 7.5 N/mm2, shear 10.5-11 N/mm2, cleavage c. 48 N/mm radial and 67 N/mm tangential, Janka side hardness 3650-5030 N and Janka end hardness 4050-5580 N.
The rates of shrinkage are moderate: from green to 15% moisture content 0.7-1.4% radial and 2.4-3.7% tangential, from green to 12% moisture content 1.2-2.1% radial and 3.0-4.6% tangential, and from green to oven dry 2.5-4.1% radial and 5.7-8.7% tangential. Air drying under cover occasionally results in some surface checking, twisting and cupping in back-sawn boards. Weighting of stacks is recommended. It takes about 2 months to air dry boards 25 mm thick from green to 15% moisture content, and 9-10 days to kiln dry boards 25 mm thick from green to 12% moisture content. Deformation on cross section (collapse) may be severe during kiln drying. The wood is stable in service once dry.
The wood is readily converted and easy to work with hand and machine tools, but tension wood is sometimes present, giving rise to a slight woolly surface on sawn material. It is easy to saw, as it is non-siliceous, and generally it can be planed to a smooth finish. Dao produces decorative veneer and plywood with satisfactory gluing properties and a good finish and polish. It is very easy to peel without pretreatment at a peeling angle of 91for 1.5 mm thick veneer. The wood is moderately suited for particle board, fibreboard and pulping and not suited for cement board.
The durability of the heartwood is variable depending on area of origin, but in general it is regarded as non-durable and not resistant to termite and marine borer attack. The sapwood is also susceptible to powder-post beetle attack. The wood is susceptible to staining. The penetration of preservatives in both sapwood and heartwood is unsatisfactory. The retention of heartwood by the pressure treating method is only 22 kg/m3, but that of the sapwood is much better: 370 kg/m3.
The wood contains 42-46% cellulose, 25-34% lignin, 9-10.5% pentosan and 0.8-2.2% ash. The solubility is 2.2-3.4% in alcohol-benzene, 2.4-4.5% in cold water, 4.7-7.5% in hot water and 14.0-18.1% in a 1% NaOH solution. The energy value is about 16 600 kJ/kg. Unlike many other species of Anacardiaceae , dao contains no irritant sap.
Medium-sized to large evergreen or sometimes deciduous trees up to 45(-55) m tall; bole straight, branchless for up to 20(-25) m, up to 100(-150) cm in diameter, with distinct, thin buttresses up to 5 m high; bark surface smooth except for greenish-brown or grey-brown scales irregularly peeling off and forming depressions, inner bark soft, straw-brown or bright yellow to pinkish, exuding some watery, pale pink sap; crown rounded, spreading, heavily branched, branchlets with large leaf scars. Leaves arranged spirally, crowded towards the end of twigs, large, imparipinnate; leaflets opposite to alternate, slightly asymmetrical, ovate to oblong, entire, usually with hairy or glabrous domatia below. Inflorescence axillary or terminal, paniculate; bracts and bracteoles caducous. Flowers bisexual, actinomorphic, 5-merous, slightly fragrant, white to greenish-white; calyx lobed; petals valvate but imbricate at the apical part, puberulous outside or on both surfaces, or glabrous; stamens 10, in 2 whorls, those opposite the calyx lobes longer than those alternating with them, filaments glabrous, anthers dorsifixed; disk intrastaminal, puberulous but glabrescent, or glabrous; pistil composed of 5 carpels which are free but connate at base and apically, ovary superior, 5-celled, with a single ovule in each cell, styles 5, stigma capitate with the stigmatic tissue lateral. Fruit a drupe, 5-celled, or seemingly 1-celled by abortion, each cell with a distinct operculum, endocarp woody and hard. Seed pendulous from an apical, axial placenta; testa free from the endocarp. Seedling with epigeal germination; cotyledons free, plano-convex; first 2 leaves trifoliolate and opposite, subsequent leaves arranged spirally, with increasing numbers of leaflets.
- Macroscopic characters:
Heartwood greyish or greenish-yellow to walnut grey-brown or reddish-brown, with irregular dark brown to black streaks, often clearly demarcated from the wide pinkish or greyish-yellow sapwood. Grain straight or interlocked. Texture moderately coarse to coarse and even, lustrous, wavy grain sometimes producing a coarse fiddle-back figure. Growth rings not evident, but occasionally darker bands of tissue present due to streaks; vessels large and distinct to the naked eye, evenly distributed; parenchyma abundant, paratracheal, vasicentric to aliform with some confluence, with occasional wide bands of parenchyma; rays moderately sized, visible to the naked eye as individual rays; ripple marks absent.
- Microscopic characters:
Growth rings inconspicuous, occasionally with narrow zones of thicker-walled fibres. Vessels diffuse, 1-3(-5)/mm2, solitary and in radial multiples of 2-3(-6), mostly oval, tangential diameter 100-300μm; perforations simple; intervessel pits alternate, coarse, rounded to polygonal,10-14(-18)μm in diameter; vessel-ray pits large, with strongly reduced borders or simple, mostly irregular, sometimes elongated; helical thickenings absent; tyloses abundant. Fibres 1.2-2.0 mm long, mostly septate, moderately thin-walled, with inconspicuous simple to minutely bordered pits confined to the radial walls. Parenchyma paratracheal, vasicentric, aliform to confluent particularly in smaller pores, with occasional wide bands, in 2-4-celled strands. Rays 5-7/mm, (1-)2-3(-5)-seriate, 400-600μm (15-30 cells) high, heterocellular with 1-2 rows of square or upright cells (Kribs type heterogeneous II). Prismatic crystals abundant in procumbent and marginal ray cells. Silica absent. Horizontal intercellular canals absent.
Species studied: D. dao .
Superficially Mangifera wood with dark streaks may resemble dao wood, but it can be easily distinguished by the absence of septate fibres and consistent apotracheal bands of parenchyma.
Growth and development
Young trees of D. dao reach a height of 3-4 m after 2 years and 6.5 m after 5.5 years. Larger branches develop in tiers.
In Peninsular Malaysia, Sarawak and Brunei D. dao is deciduous, shedding its leaves for only a short while after marked periods of dry weather. In Papua New Guinea the genus is deciduous to semi-deciduous and leaves are often shed just before the rainy season. Inflorescences are produced at the base of new shoots and the tree flowers just before all old leaves have fallen and new bronze-coloured leaves appear. However, D. dao is also reported to bear flowers almost throughout the year.
Dao is regarded as having bat-adapted fruits with colours duller than those of bird-dispersed fruits and a strong musty odour. The fruits ripen on the tree and at some distance from the foliage, to facilitate visits by bats.
Other botanical information
The genus Dracontomelon belongs to the tribe Spondieae , together with e.g. the genera Koordersiodendron and Spondias . This tribe is characterized by the presence of compound leaves, united carpels with a pendulous ovule and number of stamens being twice the number of petals. Unlike many Anacardiaceae , most of the species in this tribe are not allergenic.
There is some confusion about the number of species within Dracontomelon , mainly because of the great variability within D. dao , especially in leaf hairiness. Many of the formerly recognized species have proven to be conspecific with D. dao , resulting in many synonyms. In Papua New Guinea, many of the former species are referred to as varieties of D. dao .
In the literature the generic name is often misspelled as Dracontomelum .
Dracontomelon species occur scattered in primary or secondary, evergreen or semi-deciduous (monsoon) forest at low altitude, rarely at 500-1000 m altitude. Dao occurs particularly in areas of high rainfall. In South Kalimantan, it is usually found on organosols, gley humus soils or red-yellow podzolic soils where annual rainfall is 1800-2900 mm. The species are found on well-drained to poorly drained soils, mainly on alluvial flats and in swampy areas.
Propagation and planting
Propagation is usually by seed. Seed should be extracted immediately after the fleshy fruits have been collected, to avoid fermentation and heating. Pulp and seed can be separated by maceration. For D. dao , one kg contains 520-620 seeds or about 70 fresh fruits. Seed does not retain its viability for a long time: the germination rate is approximately 33% when sown fresh, 11% when stored for one month, 7% when stored for two months, and 0.5% when stored for 4 months, whereas no germination was observed after 6 months of storage under ambient conditions for D. dao in Java. In Malaysia, 85-95% of fresh seed of D. dao is reported to germinate and germination takes 28-67 days. Seedlings can be planted out whithout problems. In trial plantations in Java where direct sowing had been practised, trees were present in 70% of the sown spots after 5 years. A trial plantation of D. dao in Java was established through direct sowing with a spacing of 1 m × 3 m. When clearing land for shifting cultivation, trees may be retained for their fruit production.
Silviculture and management
In the Philippines stands of natural forest have been encountered with 8-10 trees per ha over 20 m tall. In the Bismarck Archipelago (Papua New Guinea), dao may contribute up to 7% of the total volume of commercial standing timber (trees over 50 cm in diameter) with an average of 30 m3/ha.
Dao regenerates easily in abandoned agricultural plots. The canopy of a plantation of D. dao planted at 1 m × 3 m closes after 8 years. In Papua New Guinea, dao plantations have been established under the taungya system. D. dao tolerates shade. Since natural pruning is good, artificial pruning is seldom necessary, although big wounds heal very easily.
In a trial with experimental felling of D. dao in Papua New Guinea a very high percentage of the trees appeared to be defective having hollow stems due to over-maturity. This phenomenon has also been recorded in Irian Jaya and Java. Younger trees do not show this defect.
A 15-year-old plantation of D. dao in Java had a mean annual increment of 5.4 m3/ha. Even after 15 years, planted trees had hardly formed any heartwood.
Handling after harvest
Logs are debarked when still fresh and should be sprayed immediately with insecticides to prevent attack of Platypus spp., Xyleborus spp. and Heterobostrycus aequalis . Dao logs sink in water.
D. dao is considered a vanishing timber in the Philippines. Elsewhere, e.g. in Papua New Guinea, the importance of Dracontomelon is also decreasing as resources are depleted. Since the area of distribution of D. dao is extensive, it is expected that its genetic variation may be considerable.
Streaked dao wood is particularly sought after and is highly valued. Provenance trials might allow the selection of superior trees with desirable wood characteristics (e.g. the presence of nicely figured wood) and growth rates for the establishment of timber plantations. More research should be conducted on the silviculture of dao and the economic viability of commercial plantations.
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