Swietenia (PROSEA)

Plant Resources of South-East Asia
Introduction

List of species

Swietenia Jacq.

Protologue: Enum. syst. pl. Carib. 4, 20 (1760).

Family: Meliaceae

Chromosome number: x= unknown; S. macrophylla: 2n= 46, 48, 54; S. mahagoni: 2n= 46, 48, 54, 56

Trade groups

Mahogany: medium-weight hardwood, Swietenia macrophylla King and S. mahagoni (L.) Jacq.

Vernacular names

Mahogany

baywood (En).
Acajou (Fr)
Indonesia: mahoni (general)
Thailand: mahokkani-baiyai, mahokkani-bailek (Bangkok)
Vietnam: giái ngựa.

Origin and geographic distribution

Swietenia consists of 3 species and is distributed in tropical America between 20° N and 18° S. The area of distribution extends from central Mexico through Central America and the West Indies, including southern Florida, towards Bolivia, Peru and Brazil. At present, mahogany is widely cultivated throughout the tropics including Malaysia, Indonesia and the Philippines.

Uses

Mahogany is regarded as the worlds finest timber for high-class furniture and cabinet work. Its popularity is especially due to its attractive appearance in combination with ease of working, excellent finishing qualities and dimensional stability. Mahogany is also often used for interior trim such as panelling, doors and decorative borders. It is used for boat building, often as a decorative wood for luxury yachts and ocean liners, although it is also used when a medium-weight timber with other good qualities is required. It is sometimes applied in boat building as plywood for planking and deck housing. Its outstanding technical qualities make it particularly suitable for precision woodwork such as models and patterns, instrument cases, clocks, printer's blocks and parts of musical instruments; for these purposes, uniform straight-grained material is used. Other minor uses include burial caskets, wood carvings, novelties, toys and turnery.

An oil can be extracted from the seed kernels which might be of some commercial value. The bark is used for dyeing and tanning leather. A gum is produced for Bombay (India) markets from cuts in the bark, both pure and mixed with other gums. Various medicinal uses of various parts of the tree are reported from Central America. The crushed fruit shells have been used as a potting medium. Mahogany is also used in reforestation projects, and has proved to be suitable in areas not protected from grazing. It is used as a shade tree, for example for young plantations of dipterocarps.

Production and international trade

Mahogany is one of the most important tropical timbers on the world market. Most mahogany traded is from natural stands, although small quantities are available from planted trees. Main exporting countries are Brazil, Bolivia and Peru. The most important importers are the United States (buying mainly from Brazil) and Great Britain (in 1989 85 000 m³ of sawn timber). The trade of plantation-grown mahogany in South-East Asia is very limited. Malaysia, Indonesia and the Philippines export only small amounts of sawn mahogany.

Properties

Mahogany is a medium-weight timber which is rather soft. The heartwood is reddish or pinkish, the colour darkening with age to a deep red or brown, sapwood usually yellowish and up to 40 mm wide. The density is 500-800(-850) kg/m³ at 15% moisture content. The grain of the wood is interlocked, sometimes straight, texture fine to moderately coarse. The surfaces are glossy, and the wood is often nicely figured because of the irregular grain.

Tests in Indonesia showed the following mechanical properties at 15% moisture content: modulus of rupture 55-61 N/mm², modulus of elasticity 9000-9550 N/mm², compression parallel to grain 36-40 N/mm², shear 4-7.5 N/mm², cleavage 51-55 N/mm radial and 58-64 N/mm tangential, Janka side hardness 2650-3840 N and Janka end hardness 3700-3840 N.

The rates of shrinkage are low, from green to 15% moisture content about 0.9% radial and 1.3% tangential, from green to 12% moisture content 1.2-1.4% radial and 1.8-2.2% tangential, and from green to oven dry 3.0-3.3% radial and 4.1-5.7% tangential. Mahogany seasons well and easily without much checking or distortion. Boards of S. macrophylla wood can be air dried in 11 weeks (50 mm thick) and 6 weeks (25 mm thick) from 40% moisture content to 15%. The wood kiln dries satisfactorily when moderate schedules are used (temperatures of 43-76° C and corresponding relative humidity of 75-33%). Boards of 50 mm thick can be kiln dried in approximately 8 days, and boards of 25 mm in 4 days. After drying, the wood is stable in service.

Mahogany saws, planes and moulds easily in both green and dry condition. In general it finishes to a smooth surface, but a woolly surface may occur on bands of reaction wood or interlocked grain. Finishing is easy and the wood takes an excellent polish. Gluing and nailing properties are good, but discoloration in contact with iron, copper and brass may occur under humid conditions. Mahogany slices and rotary cuts into fine and decorative veneer, without preliminary treatment, at a peeling angle of 92°. The veneer can be glued with casein extended with 30% lime, to produce plywood of satisfactory quality. Satisfactory results are obtained with pulping (kraft pulp yield of 49.5%).

The heartwood of trees from natural stands can be reasonably durable, but plantation-grown wood is not considered suitable for applications in contact with the ground. Graveyard tests in Indonesia showed an average service life in contact with the ground of 2.7 years for S. macrophylla and 3.3 years for S. mahagoni. The wood is resistant to wood-rotting fungi. The sapwood is susceptible to Lyctus borer attack and the heartwood may also be attacked by pinhole borers and termites; the wood has little resistance to marine borers. Mahogany is resistant to impregnation with preservatives by pressure methods, but plantation-grown wood can be amenable to boron diffusion techniques.

Wood of S. macrophylla contains 47% cellulose, 27% lignin, 16% pentosan, 0.6% ash and 0.1% silica. The solubility is 2.4% in alcohol-benzene, 0.4% in cold water, 4.5% in hot water 4.5% and 19% in a 1% NaOH solution. The energy value of the wood is about 19 600 kJ/kg. The essential oils in the wood are rich in sesquiterpenes.

Description

Small to large, monoecious but often functionally dioecious trees up to 40(-60) m tall with a straight cylindrical bole branchless for up to 18(-25) m, up to 150(-200) cm in diameter and with broad buttresses; outer bark of older trees scaly, shaggy, deeply longitudinally furrowed and brownish-grey to reddish-brown, inner bark red-brown or pinkish-red; crown dome-shaped.
Leaves alternate, usually paripinnate; stipules absent; leaflets usually opposite, entire, glabrous. Inflorescence consisting of small cymes united into axillary panicles.
Flowers unisexual, but with well-developed vestiges of the opposite sex; calyx 5-lobed, lobes rounded or obtuse, imbricate; petals (4-)5, contorted, strongly reflexed at anthesis; stamens united into a tube constricted at the apex and bearing 8-10 anthers; disk present; ovary (4-)5(-6)-locular with 9-16 ovules per locule, style with a discoid head blocking the entrance to the staminal tube, with (4-)5 stigmatic lobes.
Fruit erect, capsular, woody, ovoid to obovoid, opening by usually 5 valves consisting of a thick outer and thin inner layer, the 5-winged receptacle visible after the valves and seeds have been shed.
Seeds winged, flat, imbricate, two rows per locule, hanging down and attached with the wing to the receptacle; cotyledons thin.
Seedling with hypogeal, cryptocotylar germination; first two leaves opposite, simple just like the alternate third and fourth leaves, later ones trifoliolate or imparipinnate.

Wood anatomy

Macroscopic characters

Heartwood reddish, pinkish, salmon-coloured or yellowish when fresh, darkening to deep red or brown with age, distinct from the yellowish or whitish sapwood.
Grain interlocked or sometimes straight, with some irregularities producing an attractive stripe figure on quarter-sawn surfaces such as mottled, fiddle-back, raindrop, wavy or curly figures.
Texture fine, moderately fine to rather coarse; wood with golden lustre.
Growth rings present but not always distinct, wood rarely ring-porous; vessels mostly diffuse, large to very small and readily visible to the naked eye, with dark, sometimes white gum deposits; parenchyma surrounding pores and pore groups very distinct, paler than background; rays fine and variable in distinctness; ripple marks present or absent, when present uniform or irregular, visible without lens, 18-22 tiers per cm; gum ducts occasionally present in compact peripheral rows and filled with dark red deposits, very distinct on tangential sections.

Microscopic characters

Growth rings demarcated by concentric lines or bands of parenchyma, and sometimes also by differences in vessel diameter.
Wood usually diffuse-porous, but sometimes semi-ring- to ring-porous.
Vessels 5-12/mm², solitary and in radial multiples of 2 to several pores each, average tangential diameter 130-230 μm; perforations simple; intervessel pits alternate, 2-4(-6) μm; vessel-ray and vessel-parenchyma pits similar but half-bordered; vessel walls without spiral thickenings; gum-like deposits present; tyloses absent.
Fibres 1140-1460μm long, both septate and non-septate fibres occurring together (S. macrophylla), thin-walled, with simple to minutely bordered pits mainly confined to the radial walls.
Parenchyma predominantly paratracheal, vasicentric and in terminal lines or bands and apotracheal diffuse, in 2-4(-6)-celled strands.
Rays 5-9/mm, (1-)3-4-seriate, maximum ray height 0.69 mm (mostly Kribs type heterogeneous III). Prismatic crystals present in ray and parenchyma cells.
Traumatic axial canals sometimes present.
Vessel elements and rays storied.

Species studied: S. macrophylla, S. mahagoni.

Growth and development

Healthy seeds start germinating about 15 days after sowing. Young trees have straight and slender stems, and branches are formed 2-3 m above the ground with a position oblique to the main stem. Initial growth is fast, depending on site conditions. Under optimal conditions, seedlings of S. macrophylla may reach 3 m in one year and 6 m in two years. In Indonesia the average height of 10-year-old trees in plantations is 10 m (average diameter 9 cm), and the average height of 40-year-old trees is 25 m (average diameter 35 cm). In Peninsular Malaysia the maximum diameter of trees after 40 years is reported to be 45 cm. However, trees planted in the open field under optimal conditions may occasionally reach 50 cm in diameter in 25 years.

Flowering mahogany trees have male and female flowers (about ten times as many male as female flowers; often only the central flower of a cyme is female), but the flowers of both sexes are very similar. Trees are sometimes functionally dioecious. In mixed inflorescences, male flowers open first, but self-pollination may occur. Fruits may be produced once a year, and trees start to produce fruits regularly when about 15 years old. The seeds are provided with a thin tail-like wing that makes them rotate when they fall, and are thus dispersed by wind as far as 500 m from the mother tree.

Other botanical information

The name mahogany is applied to several other timbers. African mahogany (species of the genus Khaya) is indeed similar to genuine mahogany but is of a lesser quality. The name Philippine mahogany is truly misleading since the timber traded under this name (several species of the dipterocarp genera Shorea and Parashorea) only superficially resembles true mahogany.

Swietenia belongs to the subfamily Swietenioideae, and within this subfamily to the tribe Swietenieae which contains 9 genera. It is obviously closely related to Khaya but a proposed union of the two does not seem justified, the main differences being the shape of the capsule and that of the seed wing.

Ecology

Under natural conditions mahogany thrives in both deciduous and evergreen rain forest and occurs scattered or in small groups, but more than 4-8 trees/ha are rarely encountered. The optimum annual rainfall is 1400-2500(-3500) mm with a dry period of 0-4 months. Mahogany grows from sea-level to 1500 m altitude, in areas with a mean annual temperature of 20-28° C, the range of the coldest and warmest month being 11-22° C and 22-30° C, respectively. Mahogany is largely unspecific as to soil requirements. Within its natural range it has been found on alluvial soils, volcanic soils, heavy clays, lateritic soils, soils derived from limestone, granite and other sedimentary, igneous or metamorphic rock formations and even on shallow rendzinas. In plantations in Java it grows on very poor soils but performs best on deep, fertile, well-drained soils with a pH of 6.5-7.5. It does not tolerate waterlogging. In tropical America mahogany is among the pioneer species reoccupying degraded agricultural land. It has been shown that teak is outcompeted by mahogany in a mixed stand. S. macrophylla is reported to be very wind firm (resistant to cyclones) in the Philippines.

Propagation and planting

Ripe fruits must be collected to achieve a good germination rate. These open after 2 days of storage and the seeds are very viable. The germination rate of fresh seeds is 60-90%. They can be stored up to 2 months, or longer (up to one year) if kept cool (2-5° C) in sealed containers at about 45% relative humidity. The weight of 1000 seeds of S. macrophylla is 400-500 g.

Seeds are sown in the nursery in drills of 2-4 cm deep, or they are pushed into flat beds, leaving part of the wing exposed. The seed-bed should be well manured and shaded, and seedlings should be kept under light shade for 3-6 months. Mahogany seedlings may have a 70% survival rate, even if planted during the dry season, provided they are partially shaded and watered whenever soil moisture drops below 30%. For field planting, bare-root stock, balled seedlings, stumps (stem length 20 cm, root length 20-40 cm, diameter of root collar 0.5-2.5 cm) or striplings are used. The average production of S. macrophylla seedlings in the Philippines in the period 1979-1982 was 15 million seedlings/year. Spacing in the field is usually 2.5 m × 2.5 m or 3 m × 3 m.

Silviculture and management

Although natural regeneration in S. macrophylla stands may be plentiful, it is usual to plant seedlings raised in nurseries. One-year-old mahogany seedlings attain optimal height and diameter growth when fertilized with 3.6 g N, 2.4 g P₂O₅ and 3.6 g K₂O. Phosphorus appears to be the most limiting element for mahogany seedling growth.

Monoculture plantations of mahogany are susceptible to pests, and for that reason mixed plantations with other fast-growing species are often preferred. Leucaena leucocephala (Lamk) de Wit and Paraserianthes falcataria (L.) Nielsen are used as shade trees in young mahogany plantations.

Thinning usually starts 6 years after planting, and progressively reduces the number of trees to 220-400 trees/ha in plantations of 20 years old, and to 120-150 trees/ha in 35-year-old plantations. The rotation is usually 40-60 years.

S. macrophylla is used for reforestation and afforestation. In the Philippines mahogany is recommended for the revegetation of scrubland and denuded areas, and in the Philippines and Indonesia it is used in agroforestry systems, e.g. in Java with maize, upland rice and cassava.

Diseases and pests

In the Philippines stem rot of mahogany trees is caused by Botryodiplodia theobromae ; it also infests the seeds. In South-East Asia the most destructive pest is the mahogany shoot-borer moth Hypsipyla robusta Moore. The attack is commonly noticed on saplings and pole-size trees when terminal shoots show symptoms of dieback, finally resulting in malformed trees. Often, multiple leaders are formed. Options suggested for the Philippines for control of the pest are close spacing to reduce the development of vigorous lateral branches, the planting of Paraserianthes falcataria a few years ahead, to provide mild shade for the mahogany planted later (this minimizes lush growth of terminals and laterals), and mixed planting with Leucaena leucocephala in alternate rows to provide mild shade. In Java, trees 3-6 years old and 2-8 m tall are most severely damaged by the shoot-borer. S. mahagoni is more susceptible to pests and diseases than S. macrophylla.

Harvesting

Mahogany plantations are clear-cut when the rotation age has been reached (40-60 years) and subsequently replanted with nursery-raised seedlings.

Mahogany logs float in water and can be transported by river. The sapwood is susceptible to staining; an anti-sapstain dip should be used during drying.

Yield

In rotations of 50-60 years, average annual volume increments of 15-20 m³/ha can be achieved for plantations of S. macrophylla; on poor sites 7-11 m³/ha. S. mahagoni grows slower, but the quality of the timber is slightly better.

Genetic resources and breeding

Populations of mahogany have been depleted through centuries of commercial exploitation in Central and South America. S. mahagoni is mentioned as one of the most striking examples of tropical trees which have suffered genetic erosion, and the comparatively few individuals of good genetic stock should be protected. S. macrophylla is also becoming rare or even extinct in parts of its natural area of distribution. Both species were proposed but rejected for inclusion in Appendix II of CITES in early 1992. Included species are subject to strict regulations concerning export and trade. The third species of the genus (S. humilis Zucc.), which is not planted, is already included in CITES Appendix II.

There is no restriction for trade and export of plantation-grown mahogany. The genetic resources of planted mahogany are reasonably comprehensive because trees have been being planted for many decades and over large areas and are largely still pristine.

Selection work has been carried out recently on an experimental scale in Java (Indonesia), especially towards improving growth and germination rates. The two main mahogany species hybridize freely, and hybrids often show promising features, combining the fast growth of S. macrophylla and the good-quality wood of S. mahagoni, and greater resistance to diseases and pests.

Prospects

Mahogany has very good prospects for large-scale timber production in plantations and for reafforestation. A major problem in plantations is the susceptibility to Hypsipyla attack. Research priority should be given to the selection of resistant trees which are fast-growing and have an acceptable wood quality. The establishment of optimal methods of vegetative propagation is urgently needed.

Literature

Albay, J.L., 1983. Plantation establishment methods and techniques in the Philippines. In: Te Aho, T. & Hosking, M.R. (Editors): Workshop on nursery and plantation practices in the ASEAN, Jakarta, Indonesia. ASEAN-New Zealand Afforestation Project. New Zealand Forest Service, Welington. pp. 259-278.
Haslett, A.N., 1986. Properties and uses of the timbers of western Samoa. Plantation-grown exotic hardwoods. Ministry of Foreign Affairs, Wellington. pp. 20-21.
IUCN Species Survival Commission Trade Specialist Group, 1992. Analyses of proposals to amend the CITES appendices. IUCN - the World Conservation Union. pp. 195-198.
Lamb, F.B., 1966. Mahogany of tropical America, its ecology and management. The University of Michigan Press, Michigan. 220 pp.
Martawijaya, A., Kartasujana, I., Kadir, K. & Prawira, S.A., 1986. Indonesian wood atlas. Vol. 1. Forest Products Research and Development Centre, Bogor. pp. 59-63.
Styles, B.T., 1972. The flower biology of the Meliaceae and its bearing on tree breeding. Silvae Genetica 21: 175-182.
Suratmo, F.G., 1977. Infestation of the leading shoots of mahogany (Swietenia macrophylla King) by Hypsipyla robusta Moore in West Java, Indonesia. BIOTROP Special Publication 2. BIOTROP-SEAMEO, Bogor. pp. 121-132.
Tesoro, F.O., 1978. Wood quality and utilization of Philippine plantation species: Mahogany (Swietenia spp.). University of the Philippines, Los Baños. 4 pp. (also in: IUFRO Conference on Wood Quality and Utilization of Tropical Species, Los Baños, Laguna (Philippines), 30 Oct. - 3 Nov. 1978. Forpridecom, Laguna).
Webb, D.B., Wood, P.J., Smith, J.P. & Henman, G.S., 1984. A guide to species selection for tropical and sub-tropical plantations. Tropical Forestry Papers No 15. 2nd ed. Unit of Tropical Silviculture, Commonwealth Forestry Institute, University of Oxford. p. 245.
Yao, C.E., 1981. Survival and growth of mahogany (Swietenia macrophylla King) seedlings under fertilized grassland condition. Sylvatrop 6(4): 203-217.

Selection of species

Authors

S. Prawirohatmadjo (general part),
J. Suranto (general part),
W.G. Keating (properties),
Ani Sulaiman (wood anatomy),
M.S.M. Sosef (selection of species)