Grevillea (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Grevillea R. Br. ex Knight

Protologue: Cult. Prot.: 120 (1809).

Family: Proteaceae

Chromosome number: x= 10; G. banksii,G. robusta: 2n= 20

Vernacular names

• Silky oak (En, trade name), grevillea (En). Lacewood (Am).

Origin and geographic distribution

Grevillea comprises about 270 species ocurring mainly in Australia, 9 occur in New Caledonia and 4 are native to the Malesian region. The latter are found in Sulawesi, the Moluccas and New Guinea. A few Australian species are cultivated within Malesia.

Uses

The wood of Grevillea is used for light construction, boat building, interior trim, fine finish, furniture, cabinet work, parquet flooring, panelling, turnery, boxes, toys and novelties. It is also applied as decorative veneer and for the production of blockboard, particle board and hardboard, and is suitable as a pulpwood. It also yields a high-quality firewood, particularly in semi-arid regions.

G. robusta is a well-known auxiliary plant used widely throughout Asia and Africa in agroforestry systems (e.g. as a shade tree). It is also widely planted as an ornamental because of its attractive fern-like foliage and bright orange inflorescences. G. banksii is also planted as wind-break, for firewood, to provide fodder for goats, and as an ornamental.

Production and international trade

In 1996 only 58 m³ of "silky oak" logs were exported from Papua New Guinea at an average free-on-board (FOB) price of US$ 102/m³.

Properties

Grevillea yields a medium-weight hardwood with a density of 540-720 kg/m³ at 15% moisture content. Heartwood pale pink-brown turning to yellow-brown or red-brown upon exposure, moderately clearly differentiated from the cream-coloured to pale pink, up to 4 cm wide sapwood; grain straight to wavy; texture medium to coarse and uneven; wood lustrous; prominent silver grain on radial surface. Growth rings indistinct; vessels moderately small to moderately large, solitary and in short radial multiples, in tangential arrangement, also in clusters, vessels with white, infrequent yellowish or pinkish to dark brown substances present; parenchyma apotracheal in narrow regularly spaced bands looping from ray to ray, paratracheal vasicentric, aliform or confluent; rays of 2 sizes, moderately fine and moderately broad to extremely broad, prominent on all surfaces; ripple marks absent; axial canals occasionally present.

Shrinkage upon seasoning is low to moderate; seasoning properties are rated from good to poor; wood air dries slowly. It has a tendency to warp and check and thick material should be air dried slowly followed by a mild kiln schedule to avoid honeycombing. When used as a firewood it needs only a few days drying. The wood is hard, of low strength, but elastic. It is easy to work with hand and machine tools but a cutting angle of 10is required to obtain a good finish on quarter-sawn faces. Some fair-skinned people are allergic to the sawdust which can cause skin irritation. The wood can be peeled and sliced satisfactorily. It is moderately durable to non-durable and the heartwood of G. robusta shows an absorption of creosote of 128 kg/m³ and 321 kg/m³, respectively, when treated by the open tank method and pressure treatment. The wood is susceptible to marine borer, pinhole borer and termite attack. The sapwood is susceptible to Lyctus.

The gross energy value of G. robusta wood is about 20 400 kJ/kg. The average fibre length of G. robusta is 1.50-1.75 mm and it is suitable as a raw material to produce chemical pulp.

See also the tables on microscopic wood anatomy and wood properties.

Botany

• Small to medium-sized shrubs or trees up to 25(-40) m tall; bole straight, branchless for up to 15 m, up to 80(-120) cm in diameter, usually without buttresses; bark surface fissured, sometimes pustulate, dark grey to dark brown, inner bark reddish-brown; crown conical, dense, with branches projecting upwards.
• Leaves arranged spirally to alternate, simple to pinnate with linear to pinnatifid segments, often dimorphous even within a single tree, exstipulate.
• Flowers in a terminal or axillary, simple or branched raceme, often in pairs, protandrous; tepals 4, united into a tube that is mostly recurved under the broadened apex (limb), eventually splitting open; stamens 4, sessile in the concave limb segments; disk annular or semi-annular, sometimes bi-lobed; ovary superior, 1-locular with 2 ovules, style curved and protruding from a slit in the perianth tube before the apex is free from the limb, eventually straight, persistent.
• Fruit a coriaceous to woody follicle, usually oblique and opening along the ventral margin.
• Seeds 1-2, flat, generally winged all around.
• Seedling with epigeal germination; cotyledons emergent; hypocotyl elongated.

In its natural range G. robusta is semi-deciduous, shedding most of its leaves in the dry season. Trials in Java have confirmed that under suitable circumstances G. robusta may show an annual height and diameter increment of about 2 m and 2 cm, respectively, during the first 10 years. On the most favourable sites height increments of 3 m have been observed. In Thailand the average height and diameter 1 year after planting of G. baileyana are 0.5-1.6 m and 0.8-1.7 cm, respectively. It first flowers when about 6 years old; in October-November in its natural environment but sporadically throughout the year in equatorial latitudes. In Java G. robusta has mature fruits in September-January. The main flowering period of G. banksii in Australia is from August to October, whereas in Java and Peninsular Malaysia it flowers throughout the year. In Papua New Guinea Grevillea flowers mainly in April-September particularly in the monsoonal areas. It may produce viable seeds already when 3-4 years old. Flowers contain nectar and pollination is presumably by birds and/or arboreal marsupials (Phalangeridae). Seed dispersal is by wind. Grevillea is known to form vesicular-arbuscular mycorrhizae.

Grevillea belongs to the tribe Grevilleeae and is closely related to Finschia which differs mainly by the indehiscent fruits.

Ecology

Malesian Grevillea species generally occur in dry savanna forest, up to 1800 m altitude. They are found on well-drained sites on slopes and ridges, but also along rivers, sometimes associated with Melaleuca or Eucalyptus . G. baileyana also occurs in monsoon forest.

Silviculture

Grevillea can be propagated by seed and by vegetative techniques (cuttings, air layering). Wildlings of G. robusta are used by farmers as planting stock. G. robusta has 24 000-110 000 seeds/kg. Seeds can be stored for at least 2 years when dried to below 8% moisture content and stored cool (20 °C or less). Seeds do not need a pretreatment before sowing and have 60-80% germination starting approximately 2 weeks after sowing. Seedlings are pricked out when their second leaf pair starts to develop. The fast-growing taproot needs to be pruned several times in the nursery. G. robusta is usually planted at 2.5-3 m × 3-4 m. The survival rate of G. baileyana 1 year after planting in a trial in Thailand was 56-73%. For optimal development G. robusta needs deep and loose soils and is not very demanding with regard to soil fertility. The relative open canopy of G. robusta plantations make it less suitable for areas with erosion hazard. G. robusta coppices poorly, but can be pruned and pollarded with good regrowth. It also easily regenerates naturally, especially in agricultural fields. Older trees are fairly resistant to frost and wind. For firewood production rotations of 10-20 years are applied and annual volume increments of 5-15 m³/ha may be expected. A growth reduction after 20 years is reported for G. robusta. G. robusta is known to produce substances toxic to its own seedlings.

Genetic resources and breeding

The Australian Tree Seed Centre of the Commonwealth Scientific and Industrial Research Organization (CSIRO) and the Queensland Forest Service have distributed seed from 27 natural provenances of G. robusta, covering the altitudinal and geographical range of sites where it occurs, for evaluation in other countries. G. banksii cv. Fosteri is a dense shrub planted in fences, whereas several hybrids of G. banksii are planted as an ornamental.

Prospects

The conspicuous silver grain of Grevillea wood makes it attractive for decorative purposes. G. robusta has potential as a multipurpose tree in the highlands of South-East Asia.

Literature

124, 163, 193, 209, 238, 265, 300, 301, 304, 333, 341, 343, 348, 364, 403, 406, 418, 419, 436, 464, 488, 493, 536, 658, 660, 697, 736, 768, 817, 861, 896, 1177, 1199.