Gmelina (PROSEA Timbers)

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Plant Resources of South-East Asia
Introduction
List of species


Gmelina L.

Protologue: Sp. pl.: 626 (1753), Gen. pl. ed. 5: 274 (1754).
Family: Verbenaceae
Chromosome number: x= unknown; G. arborea: 2n= 36, 38

Trade groups

  • Yemane: lightweight hardwood, Gmelina arborea Roxb.
  • White beech: lightweight hardwood, e.g. G. moluccana (Blume) Backer ex K. Heyne.

Vernacular names

Yemane

  • gmelina, gumhar, Malay beechwood (En)
  • Burma: yemani, mai saw
  • Thailand: so (northern), so-maeo (Narathiwat). Vietnam: lõi thọ, nghiến dất.

White beech:

  • grey teak, northern white beech (En).

Origin and geographic distribution

Gmelina consists of about 33 species of trees and shrubs and is distributed from Pakistan and India, Sri Lanka and southern China through the Malesian Archipelago towards northern and western Australia, Fiji, New Zealand and New Caledonia. About 12 species occur in Malesia. G. arborea is the best known species of the genus. It has frequently been planted as a fast-growing plantation tree in South-East Asia as well as in India, tropical Africa and Brazil.

Uses

Yemane wood is suitable for general utility purposes, especially light construction and structural work, general carpentry, packaging, carvings, utility furniture and decorative veneers with excellent woodworking properties. Additionally, the wood has been used in light flooring, for musical instruments, matches, particle board, as a mine timber, in vehicle bodies and ships. The usually poor form and tapering of the bole limit its use for sawn timber.

The wood of yemane produces good quality pulp. Unmixed semi-chemical pulp is only suitable for carton board or low grade writing paper, but kraft pulp of yemane wood is suitable for higher grades of writing paper.

White beech is used for a very wide variety of purposes where light structural timber is needed. It is suitable for all purposes mentioned for yemane and, additionally, it is used for canoe-making.

Roots, bark, leaves, fruits and seeds of yemane are used in Hindu medicine. Both the fruit and bark have medicinal properties against bilious fever. Yemane is sometimes planted as avenue tree and is valuable in coffee and cocoa plantations to protect young trees and to suppress noxious grasses. The leaves are widely used as cattle fodder. Yemane is recommended for silkworm culture. Both wood ash and fruit yield a very persistent yellow dye. Outside South-East Asia the wood of yemane is used as firewood and for charcoal. Flowers of yemane produce abundant nectar from which a high-quality honey is produced.

Production and international trade

The majority of yemane timber is consumed locally in South-East Asia, mainly for construction purposes and ship building. Until 1990 it was not exported much. Elsewhere other comparable timbers are usually available. This fast-growing and comparatively cheap timber can supply local markets when no special demands with regard to the wood are required.

The largest single plantation of yemane was at Jarilandia, Brazil.

Properties

Yemane is a lightweight hardwood. The colour is uniformly cream to light yellowish-brown, turning reddish-brown with age. The wood has a high and silky lustre, and the odour and taste are not distinct. The density is 400-580 kg/m3 at 15% moisture content. It has been found to increase gradually from the pith outwards in planted trees of 8 years old in Nigeria, and also upwards in the bole. Studies in Nigeria have shown that there is a high correlation between density and age. The grain is straight to interlocked, texture coarse.

At 15% moisture content, the modulus of rupture is 61-75(-82) N/mm2, modulus of elasticity 8900-9600 N/mm2, compression parallel to grain 28-39 N/mm2, compression perpendicular to grain 2.5-3 N/mm2, shear 7.5-9(-10.5) N/mm2, cleavage 49 N/mm radial and 53 N/mm tangential, and Janka side hardness 2580-3440 N. Tests carried out with wood harvested in Malaysia, Burma and India have shown differences in strength properties. The modulus of elasticity, impact bending and hardness of the introduced trees in Malaysia are higher than those of trees tested in Burma and India.

The rates of shrinkage of yemane are low, from green to 15% moisture content only 0.5-0.6% radial and 1.1% tangential, from green to 12% moisture content 1.2-1.5% radial and 2.4-3.5% tangential, from green to oven dry 2.4% radial and 4.9% tangential. Seasoning is reported as either good and fairly rapid or slow with some warping. Air seasoning may take about 3.5 months for boards of 12.5 mm and about 11 months for boards of 38 mm thick. Slow drying is probably due to numerous knots in the wood. Kiln drying of the wood is satisfactory; the recommended temperature is 71°C for boards up to 38 mm. Higher temperatures are required for thicker boards and this may cause some darkening on the surface. It takes about 2 weeks to kiln dry boards of 25 mm thickness from green to 12% moisture content.

Yemane timber saws easily and has only slight blunting effects on tools. It planes to a smooth finish and polishes well. When knots are present in the wood, cutting angles should be reduced. The wood is too soft for satisfactory turning. Nailing is fairly easy, but pre-boring is recommended for screwing as the wood tends to split. Rotary peeling is easy even without pretreatment, and the veneers are easy to handle without a tendency to tear; they remain flat after drying. The gluing properties are reported as good. Yemane is in general very stable in service. Pulp of yemane is usually rather short-fibred, but the fibres are comparatively flexible. The quality of paper made from it can be improved by the addition of small amounts of long fibres.

The timber is non-durable, having an average life of 1.3 years when buried in the ground under tropical conditions, but the more dense heartwood is moderately durable. The resistance to termite and marine borer attack is variable, but the wood is usually classified as susceptible. The heartwood is difficult to treat with preservatives, probably due to the presence of numerous tyloses. Yemane was found to absorb only 32 kg/m3 of creosote into the heartwood using the full-cell process, while the sapwood absorbed 112 kg/m3 in the same test.

Results from various analyses of the chemical composition of the wood of yemane are fairly uniform. The lignin content is 27%, ash content 1% and extractives content 5%. The content of holocellulose is generally high and varies between 67% and 81%.

Yemane is not very suitable for firewood. The energy value is 20 150-20 750 kJ/kg. It burns quickly, and the charcoal burns well and without smoke, but leaves much ash.

White beech wood (from G. moluccana) has a density of about 465 kg/m3 at 12% moisture content. At the same moisture content, the modulus of rupture is 61 N/mm2, modulus of elasticity 8830 N/mm2, compression parallel to grain 36 N/mm2, shear 7-8 N/mm2, cleavage 45 N/mm tangential, and Janka side hardness 2000 N. Shrinkage and drying properties are comparable with yemane, but a large variation in moisture content of the fresh wood has been observed, being reflected in drying times. Within G. moluccana two forms are reported: white and red. The red wood often cracks during the drying process and is therefore used less often.

The working properties and durability of white beech are also comparable to yemane. However, sawdust may clog up sawteeth, and coated nails should be used because the wood has corrosive properties. The gluing properties are moderate due to a greasy surface. The heartwood is extremely difficult to treat with preservatives.

Description

  • Small to medium-sized trees or shrubs, up to 30 m, rarely 40 m tall with cylindrical bole having a diameter of up to 100(-250) cm, without buttresses but sometimes flanged; bark smooth or scaly, pale brown to grey; twigs glabrous or pubescent, spinous or unarmed.
  • Leaves opposite, simple, entire, toothed or lobed, often strongly varying in shape within the same plant, reticulately veined, lacking stipules.
  • Inflorescence a terminal or axillary cymose raceme or panicle.
  • Flowers zygomorphic, bisexual, with small bracts; calyx tubular or somewhat campanulate, with 4 or 5 teeth or subentire, generally with large glands; corolla with (4-)5 fused petals, tube slender below, ventricose upwards, more or less 2-lipped, hairy outside, yellow to orange or purplish; stamens 4, didynamous, alternate with the corolla-lobes, inserted in the lower part of the corolla-tube, filaments flat, filiform, often sparsely glandular, anthers 2-celled, dorsifixed, elliptical to oblong; ovary 4-locular, with one ovule in each cell, ovule attached to an axile placenta at or above the middle, style filiform, with two unequal stigmatic lobes.
  • Fruit a succulent drupe, with hard and stony endocarp, usually 4-celled.
  • Seed oblong.
  • Seedling with epigeal germination; cotyledons somewhat fleshy.

Wood anatomy

Macroscopic characters

  • Demarcation between heartwood and sapwood indistinct, heartwood light brown to yellowish-brown, sometimes with a pinkish tinge, sapwood whitish, sometimes with a greenish or yellowish tinge.
  • Grain straight to interlocked.
  • Texture coarse. Growth rings generally not distinct.

Microscopic characters

  • Growth rings indistinct, if present, marked by smaller size of latewood pores.
  • Vessels diffuse, 3-6(-12)/mm2, solitary and in radial multiples of 2-4, occasionally with narrow vessels between the larger ones, 130-240μm in tangential diameter; perforations simple; intervessel pits alternate, c. 8μm; vessel-ray and vessel-parenchyma pits enlarged, reticulate or palisade-like, simple to half-bordered; tyloses abundant.
  • Fibres 0.7-1.5 mm long, septate, with up to 5 septa per fibre, very thin-walled (walls in G. arborea 2-3.5μm thick), with minutely bordered pits mainly confined to the radial walls.
  • Parenchyma paratracheal, usually vasicentric and partly aliform, rarely confluent, and apotracheally diffuse.
  • Rays 5-7/mm in G. arborea, 10-13/mm in G. dalrympleana, usually (1-)2-4 seriate, maximum height 420-550μm, homocellular, often seemingly heterocellular in tangential section due to the presence of one row of taller marginal, procumbent cells.
  • Crystals present in ray cells, small, acicular.
  • Silica inclusions in fibres and/or axial parenchyma cells of G. moluccana only.

Species studied: G. arborea, G. dalrympleana, G. moluccana.

Growth and development

Germination of yemane is epigeal with the stony endocarp opening by lateral valves. The radicle then appears, followed by the cotyledons. The primary root is long and slender at the initial stage but then thickens, producing a moderate number of lateral roots. The depth of the root system varies. Growth of yemane is very rapid during the first 6 years, but decreases sharply from the 7th year onwards. Yemane is a short-lived species which reaches an age of 30(-50) years. Under favourable conditions it can reach a height of 30 m and a diameter of 50 cm in 20 years. Growth is strongly site-dependent. Trees of 10 years old can vary in height from 5-31 m.

Trees of yemane are deciduous, shedding their leaves around January or February in all areas where it is planted. New leaves are produced in March to April. Flowering occurs when new leaves have just begun to develop, but intensity of flowering varies. Trees 3-4 years old are able to flower and fruit regularly. The fruit matures within 2-3 months, during which it changes colour from green to yellow and falls to the ground directly when ripe. Fruiting tends to be annual.

White beech has been found to flower and fruit in almost every month of the year. The mechanisms inducing flowering are still not known.

Other botanical information

The genus Gmelina is closely related to the large genus Vitex , from which it is distinguished by its large and usually broadly bell-shaped corolla-tubes, and the well-shaped bole (in Vitex the corolla-tube is short and cylindrical, and the bole is usually poorly shaped).

Several varieties within G. arborea are sometimes distinguished. At least the typical variety (var. arborea) and a form with a glabrous lower leaf surface (var. glaucescens C.B. Clarke) are known to be cultivated in South-East Asia.

Ecology

Yemane is rather common in its natural distribution area where it occurs in habitats varying from rain forest to drier deciduous forest. It reaches its maximum size in the more humid forests of Burma, especially in humid fertile valleys. It can grow up to 1300 m altitude but is then usually stunted. It thrives in climates with mean annual temperature of 21-28°C, with mean maximum temperature of the hottest month 24-35°C, and mean minimum temperature of the coldest month 18-24°C. In its natural range the annual rainfall varies from 750-5000 mm. Its optimum lies at an annual rainfall of 1800-2300 mm in areas with a dry period of 3-5 months and a relative humidity of at least 40%.

Although yemane can be found on a variety of soils it prefers deep moist soils with an ample supply of nutrients. When established under poor conditions the trees suffer dieback after 15 years and are also very sensitive to weed competition. Growth of yemane on leached acid soils is poor. In plantations it requires a well-drained fertile soil.

Yemane is an opportunist species in the rain forest and has been classified as a long-lived pioneer. It has a high light requirement. White beech is frequently found in primary or secondary, more open forests, along streams and on ridges. It often occurs in drier areas as compared to yemane, sometimes even in grasslands. It usually prefers well-drained, moist soils and occurs from sea-level to 1200 m altitude. It is a pioneer species, just like yemane, and may regenerate gregariously.

Propagation and planting

The species is normally propagated by seed. The weight of 1000 seeds is approximately 400 g. Freshly collected seeds (stones) of yemane yield the best germination results. The germination rate is 65-80%. The fruit wall should still be yellow-green to yellow. Any dark coloured fruits should be discarded. Small amounts of fruits can be depulped by stepping on them to extract the stones. For large quantities, a modified coffee depulper can be used which may process 50 kg of fruits per minute. About 400 kg of fresh fruits are needed for 50 kg of wet depulped seeds. Further washing is required to remove any remnants of the pulp which will otherwise cause rapid fungal infection during storage. After depulping the seed must be dried immediately, to prevent any loss in viability. Considerable loss in viability occurs within one year of storage at room temperature, often even within 6 months. Storage at 4°C reduces the loss but seeds stored for 3 years tend to be no longer viable. Stored seeds should be soaked in cold water for 1-2 days before sowing. Fresh seeds do not need soaking.

In the nursery the seeds are sown on germination beds, preferably in a mixture of sand and loam. The blunt end of the seed should be approximately level with the soil surface and the acute end should point downward. Spacing should be 2 cm × 5 cm and the seeds should be covered by a thin layer of sand. Germination occurs after 2-3 weeks. The seedlings are potted in polyethylene bags after the first pair of leaves has appeared. They can be planted in the field when they reach a height of 23-30 cm, which is usually after about 6 months. Balled seedlings are preferred to bare-rooted ones.

Stumps have also been used in artificial regeneration. Mainly because of the high mortality rate (about 50% has been reported), stump planting is no longer practised in more recent plantation projects with yemane. Instead, direct sowing after extensive site preparation is used, for example at the JARI project in Brazil. The usual spacing ranges from 2.5 m × 2.5 m to 3.5 m × 3.5 m.

Silviculture and management

It is important to balance the spacing with the development of good stem forms. Narrow spacing combined with early and regular thinnings improves the stem form and reduces heavy branching and forking. Pruning is essential to produce long clear boles. Cutting off all leaves of the sapling except for the upper 2-3 pairs has been recommended in order to get a straight bole. As yemane has a high light requirement and is sensitive to competition, good site preparation and clearing by weeding or fire is required to ensure good growth and establishment.

Rotations of 6 years are used for pulpwood purposes while for sawnwood the usual rotation is usually 10 years. The second rotation is produced by coppicing. Seedling or stump planting is employed for a third rotation. Weeding is carried out 3-4 times during the first 2 years. With 10-year rotations, the stands are thinned to 50% after 5 years and again after 7 years.

Studies have shown that extensive addition of fertilizer is required to maintain sufficient growth of yemane during the second cycle.

In dense plantations trees shade out weeds and it might be necessary to take measures on sloping ground before planting, to avoid erosion. Natural regeneration is usually not abundant in plantations as the seedling demands light.

Diseases and pests

Serious fungal infestation has been observed in various locations. Armillaria mellea, Ceratocystis fimbriata, Ganoderma colossum, Gnomonia sp. and Poria rhizomorpha are some of the fungi found to cause serious damage to planted trees of yemane. Additionally, a parasitic plant, Loranthus scurrula L. can also cause damage. The latter is controlled by spraying herbicides.

Plantation trees of yemane tend to be attacked by the stem-boring beetle Xylotrupes gideon. Termites which damage the heartwood near the ground have been observed in Peninsular Malaysia but do not cause any serious problems in other regions. Plantations in India have been reported to be defoliated by the yemane leaf beetle (Calopepla leayana) which damages the leaves, buds and twigs, and the defoliator Ozola minor (Lepidoptera, Geometridae) occurs in the Philippines.

Yield

Under favourable conditions yemane is capable of reaching an annual increment of 20-25 m3/ha with impressive exceptions of over 30 m3/ha with a maximum of 38 m3/ha. On poor sandy soils a yield of only 84 m3/ha after 12 years was reported, whereas on very favourable soils a production of 304 m3/ha after 10 years can be reached.

Genetic resources and breeding

The various species of Gmelina are usually widely distributed, and although they are not abundant (except on favourable sites), none of them seems to be threatened with extinction.

Owing to the introduction of yemane into plantation forestry, extensive breeding programmes have been carried out in the South-East Asian region, particularly Malaysia and Indonesia. An international provenance trial has been established in the region by the Danish International Development Agency (DANIDA). A major project was started on Kolombangara (the Solomon Islands).

Prospects

Although yemane is extensively planted in industrial plantations, its success is still doubtful because of the problems encountered at the JARI plantation in Brazil. The main problem is that the species performs very differently on different soil types. Moreover, the need for additional nutrients makes the establishment of plantations expensive. Breeding might overcome this problem, as well as bringing about an improvement of the shape of the bole and tree.

Literature

  • Ani Sulaiman & Lim, S.C., 1989. Some timber characteristics of Gmelina arborea grown in a plantation in Peninsular Malaysia. Journal of Tropical Forest Science 2: 135-141.
  • Boulet-Gercourt, M., 1977. Monographie du Gmelina arborea. Revue Bois et Forêts des Tropiques 172: 3-23.
  • Charomaini, M., 1989. Pre-sowing treatment of Gmelina arborea Roxb. seeds to accelerate and improve germination. Buletin Penelitan Hutan 515: 29-39.
  • Christine, J.H., 1988. The ecology of a Gmelina arborea plantation established after shifting cultivation in the Niah Forest Reserve. Forest Research Report, Forest Ecology 3: 1-62.
  • Cortes, E.V., 1979. Wood quality and utilization of Yemane (Gmelina arborea). Forpride Digest 8: 24-32.
  • Freezaillah Che Yeom & Sandrasegaran, K., 1966. Growth and yield of yemane (Gmelina arborea Roxb.). Malayan Forester 29: 140-151.
  • Henderson, C.P. & Hancock, I.R., 1988. A guide to the useful plants of the Solomon Islands. Research Department Ministry of Agriculture and Lands, Honiara. pp. 218-219.
  • Moldenke, H.N., 1984. Additional notes on the genus Gmelina IV. Phytologia 56: 102-126.
  • Munir, A.A., 1984. A taxonomic revision of the genus Gmelina L. (Verbenaceae) in Australia. Journal of the Adelaide Botanical Garden 7: 91-116.
  • Wong, W.C. & Khoo, K.C., 1980. Gmelina arborea - A literature review. Report 14. Forest Research Institute Malaysia, Kepong.

Selection of species

Authors

  • S.K. Yap (general part, properties),
  • M.S.M. Sosef (general part, selection of species),
  • S. Sudo (wood anatomy)