5998

5(3): Timber trees; Lesser-known timbers

Ochroma Sw.

Prodr.: 97 (1788).

BOMBACACEAE

x = unknown; O. pyramidale: 2n = 72, 78, 88, 90

Balsa (trade name).

Ochroma is a monotypic genus occurring in tropical Central and South America from southern Mexico to Bolivia. The only species is O. pyramidale (Cav. ex Lamk) Urban (synonyms: O. bicolor Rowlee, O. grandiflora Rowlee, O. lagopus Sw.) which is planted in many tropical countries; in the Malesian region in the Philippines, Malaysia, Java and Papua New Guinea where it has locally become naturalized.

The extremely lightweight wood of Ochroma is used for buoys, life-jackets and life-belts, surf boards, aircraft construction, toys, model making, entomology mounting boards, core stock in sandwich construction, surgical splints, packaging of fragile articles and as insulation for heat, vibration, sound and formerly also for electricity. The heavier wood of Ochroma is suitable for matches, popsicle sticks, toothpicks, etc. and for the production of pulp and paper.
The kapok from the fruits is suitable for filling pillows and mattresses. Ropes have been made from the fibrous bark. The tree is sometimes planted as an ornamental or to provide shade expediently.

Ecuador is the main exporting country of Ochroma wood supplying 80-90% of the volume traded on the world market.

Ochroma is an extremely lightweight hardwood with a density of (40-)90-310 kg/m³ at 12% moisture content, but with a very large difference between the outer most wood and that from the centre, the new wood being on average 2.2 times heavier than the old wood as determined for wood samples from Costa Rica. Most of the commercial stock is sapwood. Heartwood white to grey-white, sometimes with a pinkish tinge near the heart in older trees, not clearly demarcated from sapwood; grain straight; texture coarse and even; wood with silky lustre. Growth rings indistinct; vessels moderately small to moderately large, solitary and in radial multiples of 2-3, rarely more than 4, open, vessels lines conspicuous appearing as coarse brown scratches; parenchyma paratracheal mostly vasicentric, and apotracheal diffuse-in-aggregates, indistinct; rays medium-sized to moderately broad, conspicuous on all surfaces producing a silver grain on the radial surface; ripple marks absent.
Shrinkage upon seasoning is low to moderate, the wood seasons easily and rapidly (air drying from green to 15% moisture content takes 1-3 weeks) with very little degrade, but is highly prone to blue stain. It should be converted rapidly after felling to prevent extensive splitting and staining. Kiln drying is preferable to air drying, to minimize splitting and warping; kiln schedule H is recommended. The wood is very soft and very weak but with a good strength to weight ratio. The wood from old trees tends to be brittle. The wood is very easy to work with hand and machine tools, but sharp tools are essential to prevent crumbling. It has good insulating properties and high buoyancy. The wood is non-durable and prone to attack by Anobium, Lyctus, termites and longhorn beetles. Both sapwood and heartwood are permeable to impregnation, the retention of preservative by the pressure heating method is about 560 kg/m³ for sapwood and about 334 kg/m³ for heartwood.
The fibres are very long and yield a high-quality chemical pulp with a recovery ratio of 45-50%, whereas the raw pulp can be easily bleached.
See also the tables on microscopic wood anatomy and wood properties.

Deciduous or evergreen, medium-sized or rarely large trees up to 30(-50) m tall; bole straight, usually short, cylindrical, up to 100(-180) cm in diameter, with short buttresses in older trees; bark surface smooth, grey-white mottled. Leaves arranged spirally, simple, angular-lobate, palmately veined; stipules present. Flowers axillary, solitary; calyx tubular, with 5 unequal lobes; petals 5, whitish; stamens united into a briefly 5-lobed staminal column bearing sessile wavy anthers from the middle to the apex; ovary superior, 5-locular with many ovules in each cell, style club-shaped, stigma spiralled. Fruit an oblong, ribbed, 5-valved dehiscent capsule, densely lanate inside.
Growth of O. pyramidale trees can be extremely rapid and is according to Koriba's architectural tree model, characterized by orthotropic axes which branch to produce initially equivalent modules. Subsequently one of these modules becomes dominant, constituting one unit of the sympodial trunk. In Peninsular Malaysia growth is too slow to produce the very low-density wood which is sought after for many of its uses. In trials in Indonesia the mean annual increment was 2.0-6.6 cm in diameter and 1.2-3.3 m in height. In Papua New Guinea trees with a diameter of about 20 cm displayed a mean annual diameter increment of 1.7-4.3 cm. In South America mean annual diameter increment is even up to 10 cm and after 10-12 years, when growth stabilizes, trees are generally 20-25 m tall and about 100 cm in diameter. After 12-15 years growth slows down and the trees deteriorate rapidly. Only under specific circumstances may trees reach 50 m in height. In Java and Peninsular Malaysia O. pyramidale flowers throughout the year and is pollinated by bats. Trees start producing viable seed after (2-)3-4 years and dispersal is by wind.
O. pyramidale is highly variable, which is probably why the genus was formerly thought to comprise at least 11 species.

O. pyramidale is a typical pioneer, colonizing clearings. It grows gregariously with a preference for alluvial flats, on rich, well-drained or volcanic soils up to 1000 m altitude. In natural conditions it prevails under an annual precipitation of 1250-3000 mm and a mean annual temperature of 22-27°C; it can tolerate a dry season of up to 4 months, but only if the relative humidity does not normally drop below 75%.

O. pyramidale can be propagated by seed. The very fine seeds (112 000-150 000 dry seeds/kg) should be collected from standing trees and can be stored for several years in jute bags or in closed receptacles. In Papua New Guinea seed is collected in June-August. Seeds can be sown directly in the field or in the nursery. Freshly collected seed has only 10% germination. Seeds contain an impervious testa which must be ruptured by heat (boiling water, fire) before they will germinate. Under natural conditions forest clearance exposes the soil to the sun and this triggers germination of Ochroma seeds. In the nursery seeds are sown in lines 3-4 cm apart under slight shade and in sterilized soil to prevent damping-off. Pretreated seeds show 65-75% germination in 6-28 days and seedlings are pricked out and transferred to containers. When they are 3-4 months old and 20-25 cm tall, they can be planted out in the field at 4-5 m x 4-5 m, possibly in a taungya system with a spacing of 2 m x 3 m. In Papua New Guinea a higher initial density of 2268 trees/ha (about 2.1 m x 2.1 m) is applied. As the roots of young plants are extremely sensitive to damage, bare-rooted plants cannot be used and direct seeding is preferred using seed holes at 3 m x 3 m. The soil should be deep, well-aerated and fertile, to allow for rapid growth; slightly inferior sites retard growth and will produce wood with a higher density (over 160 kg/m³), which is not of commercial interest as only the extremely lightweight wood is in demand. Thinning is comparatively heavy and aims at creating enough growing space for the trees to allow for rapid growth. For Papua New Guinea a reduction of the initial 2268 trees/ha to 99 trees/ha before clear felling at the age of five is advocated. A rotation of 8 years is applied in Indonesia with a reduction in stems from 900-1225 to 90-170 per ha, depending on the site. Great care should be taken to avoid damage to the remaining trees, as they heal very poorly or not at all. A mean annual volume increment of 17-30 m³/ha can generally be expected, although 69 m³/ha and 90 m³/ha has been achieved in Indonesia in 8- and 6-year-old plantations, respectively and 50 m³/ha is commonly accepted as realistic. Rotations do not generally exceed 7-8 years, as the rapid growth slows down appreciably after 7-12 years. At this age heartwood development starts with a much higher density and a darker colour rendering it less suitable for the special purposes. After harvest, residues can be burnt, which gives rise to abundant natural regeneration. Zeuzera coffeae, a wood borer, has been observed in a 1.5-year-old plantation in Java seriously damaging the majority of trees which had to be clear felled to avoid attack of other trees.

Differences in wood density in Ochroma may be a starting point for further selection and breeding.

O. pyramidale has lost some importance due to the increased use of synthetic materials, but it will probably remain the best material for some special applications, e.g. model making. Increased use for such special applications and for pulpwood is possible.

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S.I. Wiselius

Wiselius, S.I., 1998. Ochroma Sw.. In: Sosef, M.S.M., Hong, L.T. and Prawirohatmodjo, S. (Editors): Plant Resources of South-East Asia No 5(3): Timber trees; Lesser-known timbers. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea