Intsia (PROSEA)

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Plant Resources of South-East Asia
Introduction

List of species

Intsia Thouars

Protologue: Gen. Nov. Madag.: 22 (1806).

Family: Leguminosae

Chromosome number: x= unknown; I. bijuga: 2n= 24

Trade groups

Merbau: heavy hardwood, e.g. Intsia bijuga (Colebr.) O. Kuntze, I. palembanica Miq.

Vernacular names

Merbau:

Malacca teak, mirabow, Moluccan ironwood (En)
Indonesia: ipil (general), kayu besi (Moluccas, Irian Jaya)
Malaysia: merbau ipil (general)
Papua New Guinea: kwila, bendora
Philippines: ipil, ipil laut, malaipil
Burma: tat-takun
Cambodia: krâkâs prêk
Thailand: lumpho (peninsular), lumpho-thale (Surat Thani)
Vietnam: gõ nước (general).

Origin and geographic distribution

Intsia consists of about 8 species and occurs from East Africa and Madagascar towards Melanesia, Micronesia and northern Australia. Within Malesia 3 species are recorded. The most widespread species is I. bijuga.

Uses

Merbau is a very good general-purpose timber. It is suitable for a wide range of purposes because of its favourable physical and mechanical properties, combined with a high natural durability and an attractive appearance. Merbau is used in construction work in house building, especially for high-class exterior joinery such as windows, solid panel doors, framing, and weatherboarding. It is also an excellent timber for high-grade flooring for both light and heavy pedestrian traffic. Other uses include furniture making, panelling, stairs, handrails, shopfittings, truck bodies, turnery, poles, fence posts, musical instruments and carving. Merbau is also particularly useful for waterwork construction such as bridges, wharves, sluices and sheet piles, and it is generally free of ship worm. The timber is suitable for making decorative veneer but is generally too hard for plywood manufacture.

The seeds of merbau are eaten locally after being soaked in salt water for 3-4 days and then boiled. A brown and yellow dye is obtained from an oily substance present in the wood and bark. Bark and leaves are used medicinally against rheumatism, dysentery, diarrhoea and urinary diseases.

Production and international trade

The production and export of merbau is most important in Malaysia, particularly Peninsular Malaysia where log production fluctuated from 181 000 m³in 1982 to 246 000 m³ in 1989 (maximum annual production) and 228 000 m³ in 1990. The export of sawn merbau timber from Peninsular Malaysia was 73 000 m³ (with a value of US$ 15.2 million) in 1981, reached a maximum of 92 000 m³(with a value of US$ 26 million) in 1988, and was 63 000 m³ (with a value of US$ 23 million) in 1992. The average price of sawn merbau on the export market was 409 US$/m³ in 1991. In 1987 the export of merbau logs from Sabah was 7200 m³ (worth US$ 723 000), and in 1992 the export was 32 000 m³of logs and 3500 m³ of sawn timber with a total value of US$ 7.2 million. Most of the Malaysian export of merbau is to the Netherlands, where the principal end uses are for windows (45%) and doors (35%), and to Germany where it is used, for instance, for parquet flooring.

Merbau was much less important in Indonesia until recently. In 1990 the export of sawn merbau timber was 1700 m³ with a value of US$ 825 000. In 1991 the export was much larger, amounting to 17 000 m³ with a value of US$ 7.8 million. The total production of merbau in 1992 in Indonesia was about 137 000 m³. The main production areas are Aceh and the Moluccas (each about 8000 m³/year), and particularly Irian Jaya (about 121 000 m³/year).

In Papua New Guinea Intsia timber is grouped in MEP (Minimum Export Price) group 1, and fetched a minimum export price of US$ 130/m³ for saw logs in 1992. Japan imports comparatively small amounts of merbau from Sabah, Sarawak and especially from Papua New Guinea.

Because of the demand for this timber, it is becoming increasingly difficult to obtain in many areas outside Peninsular Malaysia, and supplies will be limited from these areas until the next forest rotation.

Properties

Merbau is heavy and hard without any characteristic figure. Heartwood light brown, orange-brown, to dark red-brown, darkening on exposure, sharply differentiated from the lighter sapwood, usually without lustre, but light-coloured wood sometimes slightly lustrous and with stripe figure on radial surface and zigzag design on tangential surface. The density is (500-)690-955(-1000) kg/m³at 12% moisture content. The grain is straight, interlocked or wavy, texture moderately coarse to coarse but even.

At 12% moisture content the modulus of rupture is 105-147 N/mm², modulus of elasticity 14 000-18 000 N/mm², compression parallel to grain 60-80 N/mm², compression perpendicular to grain 10-27 N/mm², shear 13-18 N/mm², cleavage 70-75 N/mm radial and c. 83 N/mm tangential, Janka side hardness 6700-8500 N and Janka end hardness 7750-8550 N.

The rates of shrinkage of merbau are very low, from green to 15% moisture content 0.6-0.9% radial and 0.7-1.6% tangential, from green to 12% moisture content 1.1-1.3% radial and 1.9-2.6% tangential. Generally merbau dries rather slowly by air seasoning and kiln drying. Test specimens of 30 cm × 20 cm × 2.5 cm took 53 days to air dry from 60% to 20% moisture content, and 20 days in the dehumidifying chamber. Extremely mild kiln schedules are required to obtain good results when drying thicker boards. When the timber is dried very slowly, degrade is almost negligible.

Working properties may vary considerably with the density and the direction of the grain of the wood, but generally merbau cuts cleanly in most operations. The surface of green timber may be marred by an oily exudate; during sawing the sawteeth may become covered with a gummy substance. Sawing of dried wood has a moderately blunting effect on cutting edges. A reduction of the cutting angle of 20° is advantageous in planing quarter-sawn material with interlocked grain and to prevent picking-up of grain. Turning and boring give no problems and generally a smooth finish. Pre-boring for nailing and screwing is advisable since the wood is liable to split. The wood stains and polishes satisfactorily, but requires considerable filling. Merbau is difficult to cut or slice into veneer, even after pre-steaming, mainly due to its high density. However, tests in Indonesia showed that 1.5 mm thick veneer can be made from logs of I. palembanica from the Moluccas at a peeling angle of 92° without pretreatment, while logs from South Kalimantan produced inferior veneer, even after boiling for 24 hours. When in contact with water, water-soluble extractives create blackish-lined dark-coloured spots on the surface which are very difficult to remove. Manufactured exterior joinery products have to be protected from water or rain on building sites until the finishing operation is completed. Merbau does not corrode iron, but it does stain black in the presence of iron and moisture. It glues satisfactorily with most modern types of glue.

The heartwood is durable. Stake tests show an average service life in contact with the ground of 6-11.5 years under tropical conditions and about 20 years under temperate conditions. Merbau is highly resistant to dry wood termites as well as subterranean termites; it is also resistant to decay caused by Polystyctus versicolor. When used for construction under cover (e.g. as roofing timber), it may be expected to last 30-40 years in the humid tropics. It is, however, reported to be susceptible to powder-post beetle attack. Merbau may have considerable resistance to marine borers, but no average lifetime can safely be predicted from the information available. The heartwood is very difficult to treat with preservatives.

Wood of I. bijuga contains 47% cellulose, 23% lignin, 17% pentosan, 0.9% ash and 0.2% silica. The solubility is 5.6% in alcohol-benzene and 8.2% in hot water.

Description

Medium-sized or large, evergreen or deciduous trees up to 50 m tall; bole sometimes of poor shape, branchless for up to 20 m and with a diameter of up to 160(-250) cm; buttresses usually present; bark 0.5-1 cm thick, smooth to dimpled or scaly or rarely with boat-shaped fissures (like Calophyllum), grey to brown-red, flaking off in small pieces; inner bark very firm, fibrous, sometimes with an obscure granular layer at the surface, orange to pink outwards, pale red or pale fawn inwards; crown rather compact and dark-coloured.
Leaves alternate, paripinnate with 2-4(-5) pairs of leaflets; stipules connate at base, forming a persistent scale; leaflets opposite or subopposite, often with a gland at the base on one or both sides, otherwise glabrous except for the midrib.
Flowers arranged spirally in simple racemes or branched terminal or lateral panicles; bracts and bracteoles deciduous; sepals 4, with a distinct tube, imbricate, unequal, pulverulent; petals 1, large, clawed; stamens 3, very long, connate at base with the 4-7 filiform staminodes; ovary stipitate, connate to the dorsal side of the calyx tube, with many uniseriate ovules, style with a small dark stigma.
Fruit a stipitate pod, compressed, dehiscent with more or less leathery valves, with several seeds.
Seed large, hard, without an aril, dark brown at maturity.
Seedling with epigeal germination.

Wood anatomy

Macroscopic characters

Heartwood light brown with some darker brown streaks, distinctly demarcated from the whitish to pale yellowish sapwood.
Grain straight. Texture moderately coarse; wood more or less lustrous.
Growth rings usually distinct, because generally vessels are less numerous in the outer part of a growth ring; vessels visible to the naked eye, in heartwood often filled with red-brown gum or yellow contents; aliform to confluent parenchyma distinct, rays only visible with a lens; ripple marks absent.

Microscopic characters

Growth ring boundaries marked by marginal parenchyma bands and/or differences in vessel frequency.
Vessels diffuse, 2-3(-5)/mm², solitary and in radial multiples of 2-3(-5), usually completely surrounded by parenchyma, round to oval, mutually flattened in multiples, average tangential diameter 185-280μm, of vessels in multiples 60-230μm; perforations simple; intervessel pits alternate, distinctly vestured, usually polygonal, 6μm, regularly with coalescent apertures; vessel-ray and vessel-parenchyma pits similar but less crowded and half-bordered.
Fibres 560-790μm long, non-septate, thick-walled, with simple pits confined to the radial walls.
Axial parenchyma rather abundant, mainly paratracheal, but also marginal or seemingly marginal banded; paratracheal parenchyma lozenge- to winged-aliform, sometimes confluent surrounding 2-5 vessels, at the growth ring boundary lateral extensions forming long marginal bands 2-3 cells wide; parenchyma strands (2-)4-celled.
Rays 5-8/mm, 1-3-seriate (often 2-seriate), up to 4-25 cells high, homocellular, entirely composed of procumbent cells, one row of marginal cells usually shorter in radial direction and higher longitudinally.
Crystals prismatic, only in chambered axial parenchyma cells of both inner and outer sides of paratracheal and banded parenchyma.
Deposits red-brown to sulphur-yellow, granular, often present in vessels; brown to yellow-brown contents often present in libriform fibres, axial and ray parenchyma cells.
All elements non-storied.

Species studied: I. bijuga.

Growth and development

Experiments showed that the cotyledons contribute to seedling development up to the stage of the first pair of leaves. Initial growth of the seedling is fast; seedlings reach an average height of 40-55 cm after 3 months. Then growth will slow down, and in the period from 3-10 months after germination it averages only 5-6 cm. Merbau seedlings need a high light intensity for optimal growth, and in the open growth is much faster than under closed canopy conditions. However, experiments in Indonesia showed that growth of seedlings under shelter or under shade trees was faster than in full sunlight. At Bogor, Indonesia, an 8-year-old stand of I. bijuga had an average height of 10.7 m and an average diameter of 15 cm. The maximum diameter which may be reached in 40 years is 43 cm for I. palembanica.

Other botanical information

The genus Intsia is closely related to Afzelia (formerly called Pahudia). It is impossible to assign sterile material to either genus. Intsia differs from Afzelia by its three fertile stamens, its flat seeds lacking an aril, and its leathery pods. The differences between the South-East Asian species are small.

Ecology

I. bijuga occurs most frequently in coastal areas, often in a zone behind the mangrove and is, therefore, often treated as a member of the Indo-Pacific strand flora. I. palembanica is found inland up to 1000 m altitude. Merbau prefers a rainfall of more than 2000 mm a year and grows in primary or old secondary forests on a wide variety of soils but usually not on peat. It is sometimes found growing gregariously, and occasionally dominantly. In New Guinea merbau is an especially common component of the lowland ridge flora in association with Anisoptera and Hopea.

Propagation and planting

Merbau seeds are protected by a hard seed-coat which is impermeable to water. Mature seeds have a moisture content of less than 10% and hence can survive for more than 3 years without any specific treatment. Germination may take up to nearly 2 years. To promote rapid and simultaneous germination, scarification followed by soaking in water is necessary. The most effective scarification technique is to use a file to scrape off the small protrusion of the seed-coat (the strophiole) located at the opposite side of the hilum. Treatment with fungicides (e.g. benlate, daconil) may protect scarified seeds from damage by fungi. Immersion in concentrated sulphuric acid for one hour is also effective in rupturing the seed-coat. Seeds must be planted vertically with the hilum downward, so that the seed-coat is shed as the hypocotyl emerges from the soil. Seed may also be sown direct into the field. Almost 100% of scarified seeds of I. palembanica germinate after about 11 days, and the germination rate is the same in full sun and shade. Seeds of I. bijuga germinate better in full sun. Seedlings of I. palembanica grow best in soil mixed with 25% sand. Appropriate drainage and high air humidity are important. Seedlings can be transplanted into the field at about 3 months after sowing at a spacing of 3 m × 4 m or 5 m × 5 m.

Vegetative propagation of merbau by means of 60 cm long cuttings was tested in the Philippines. Six weeks after planting in a sandy clay-loam medium the mortality rate was 62%.

Silviculture and management

Extensive stands of merbau occur in northern Papua New Guinea on sites destroyed by fire during natural droughts. The stands in the Gogol River valley were probably established after the 1918-1920 droughts. After logging, dormant seeds in the soil can germinate abundantly in gaps around the stumps. Merbau is apparently a successful secondary forest tree.

In Papua New Guinea a 50-year rotation for natural stands has been reported as suitable, but in Malaysia it has been estimated that a rotation of some 120 years would be required for merbau in plantations.

Diseases and pests

The roots of seedlings of I. bijuga may be infected by nematodes; in the Philippines Rotylenchulus sp. and Helicotylenchus sp. have been found in nurseries. In Kalimantan young plantations suffered from grazing by deer and mouse deer, while rats are also considered a serious pest.

Harvesting

Fresh logs sink in water and must be transported over land.

Yield

Forest inventories in Peninsular Malaysia (1970-72 and 1981-82) showed that there is still sufficient merbau timber in the natural forests. Annual log production figures have been more or less stable from 1981-1991. The standing stock in Peninsular Malaysia is estimated at 2.8 trees/ha (gross volume 11.7 m³/ha) over 15 cm diameter and 1.9 trees/ha (gross volume 10.8 m³/ha) over 45 cm diameter. The yield is still good enough to make merbau one of the most important export timbers of Peninsular Malaysia.

Genetic resources

Merbau was proposed for inclusion in Appendix II of CITES in 1992. It includes species which, although not necessarily threatened with extinction now, may become so unless trade in specimens of such species is strictly regulated in order to avoid over-utilization. Export of products from these species requires an export permit. Stands of merbau have been exploited for many years, because it is a popular hardwood for both domestic and international markets, and the Intsia species are considered to merit conservation. Malaysia successfully objected to the inclusion of merbau in CITES Appendix II based on the forest inventories and log production figures of Peninsular Malaysia, which apparently show no sign of stands being threatened. However, merbau seems to be seriously threatened with extinction in several areas outside Peninsular Malaysia, e.g. in parts of Borneo, the Philippines and Thailand.

Prospects

The prospects for merbau seem to depend on whether the exploitation of the remaining stands is properly controlled. It is evident that cutting cycles should be long for these rather slow-growing trees; at least 60 years, though 80 years is probably more realistic. This means that a decrease in the amount of timber harvested is necessary in most areas. Nevertheless, supplies of merbau may be fair in the future, as merbau usually regenerates well in logged-over areas, and enrichment planting with nursery-raised seedlings seems promising. This very good and useful timber justifies more research and effort to ensure better and more sustainable management of natural stands.

Literature

Bolza, E. & Kloot, N.H., 1966. The mechanical properties of 81 New Guinea timbers. Technological Paper No 41. Division of Forest Products, CSIRO, Melbourne. pp. 24-27.
Boonnab, C., 1975. Intsia palembanica Miq. Vanasarn (Thailand) 33(3): 261-263.
de Guzman, E.D., Umali, R.M. & Sotalbo, E.D., 1986. Guide to Philippine flora and fauna. Vol. 3: Dipterocarps, non-dipterocarps. Natural Resources Management Center, Ministry of Natural Resources and University of the Philippines, Manila. pp. 203-204.
Hidyat, S. & Karnasudirdja, S., 1987. Sifat pengeringan alami dan dehumidifikasi beberapa jenis kayu Indonesia [Air drying and dehumidifying properties of some Indonesian timber species]. Jurnal Penelitian Hutan 4(3): 41-44.
Malaysian Timber Industry Board, 1986. 100 Malaysian timbers. Kuala Lumpur. pp. 26-27.
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. 91-96.
Sasaki, S., 1980. Storage and germination of some Malaysian legume seeds. Malaysian Forester 43: 161-165.
Sasaki, S. &. Ng, F.S.P., 1981. Physiological studies on germination and seedling development in Intsia palembanica. Malaysian Forester 44: 43-59.
Ser, C.S., 1982. Malaysian timbers - merbau. Malaysian Forest Service Trade Leaflet No 65. Malaysian Timber Industry Board, Kuala Lumpur. 7 pp.
van Alphen de Veer, E.J. & Verduyn Lunel, F.A., 1950. Kweekproeven met Intsia palembanica Miq. en Intsia bijuga O. Ktze [Germination tests with Intsia palembamica Miq. and Intsia bijuga O. Ktze]. Tectona 40: 336-345.

Selection of species

Authors

R.J. Johns (general part),
P.B. Laming (properties),
R.W. den Outer (wood anatomy),
M.S.M. Sosef (selection of species)