Acacia (PROSEA Timbers)

Plant Resources of South-East Asia Introduction

List of species

Acacia Miller

Protologue: Gard. Dict. abr. ed. 4 (1754).

Family: Leguminosae

Chromosome number: x= 13; 2n= 26 for the majority of species,A. auriculiformis, A. catechu: 2n= 26,A. leucophloea: 2n= 26, 52

Trade groups

Wattle: medium-weight hardwood, e.g. Acacia auriculiformis A. Cunn. ex Benth., A. catechu (L.f.) Willd., A. mangium Willd.

Vernacular names

• Wattle, brown salwood, acacia (En). Acacie (Fr)
• Indonesia: akasia (general).

Origin and geographic distribution

Acacia is a large genus with over 1300 species, which is widely distributed in the tropics and subtropics. Most of the species are found on the Southern Hemisphere and the main centre of diversity is located in Australia and the Pacific. Within the Malesian region 29 species occur native or naturalized. Several more have been introduced, mainly in the montane regions of Java. Most of the timber-producing species are found in New Guinea.

Uses

The timber of Acacia species is used for furniture and cabinet making, light to heavy construction, door and window frames, mouldings, light flooring, poles, posts, panelling, mine timber, boat building, carts, wheels, joinery, turnery, oil crushers, tool handles, agricultural implements, matchboxes and splints, particle board, hardboard, veneer and plywood, pulp and paper. The wood is tough and resilient and particularly suitable for axe handles and sports equipment. The pulp is suitable for the manufacture of liner boards, bags, wrapping papers and multiwall sacks. The wood makes a good fuelwood and good charcoal, as it has a high energy value. The sawdust provides a good medium for the production of shiitake mushrooms.

The trees are also planted in fire-breaks and wind-breaks, for shade, soil protection, and as ornamentals. The leaves and pods of some species are used for animal fodder. The germinated seeds can be cooked and eaten as a vegetable. Several species are important tannin-producing trees and a dye can be extracted from the bark of one species ( A. mearnsii ). An extract of the heartwood is used medicinally and is sometimes chewed with betel ( Areca catechu L.). A gum produced by the stem or the roots is also used in local medicine.

Production and international trade

Significant areas of plantations, mainly of A. mangium and A. auriculiformis , have been or are being established in Indonesia, Malaysia and Papua New Guinea, and also in India, Sri Lanka and Thailand. The wood from these plantations is mainly used as pulp, but no statistics are available on production and trade. The international trade in wattle timber is relatively small. Wood chips of plantation-grown A. mangium are exported to Japan from Papua New Guinea, and small quantities of A. mangium timber are exported from Peninsular Malaysia and Sabah, for instance to Taiwan.

Properties

Wattle is a medium-weight hardwood. The heartwood is pale olive-brown, grey-brown to pink, darkening to reddish-brown or dark red, and often attractively streaked. The sapwood is yellowish-white, cream or straw-coloured and distinctly demarcated from the heartwood. Heartwood formation varies significantly with provenance. Like the wood of other fast-growing tree species, the wood from wattle plantations has the inherent potential disadvantage of small diameter, knottiness, low density, littl strength, large proportion of reaction wood, greater incidence of spiral growth, greater growth stress and greater proportion of juvenile wood. The density is (490-)560-1000 kg/m³at 15% moisture content; the density of plantation-grown wood of A. mangium can be as little as 450 kg/m³at 15% moisture content. The grain is straight to shallowly interlocked, texture fine to medium and even.

The mechanical properties of A. leucophloea wood from Indonesia have been tested at 14% moisture content, with the following results: the modulus of rupture 85-86 N/mm², modulus of elasticity 10 340-10 780 N/mm², compression parallel to grain 51.5-53.5 N/mm²and shear 8-10.5 N/mm². Wood of A. mangium tested in Australia at 11% moisture content showed a modulus of rupture of 106 N/mm², modulus of elasticity of 11 600 N/mm²and compression parallel to grain of 60 N/mm².

The rates of shrinkage are fairly low to moderate: from green to 12% moisture content 1.0-1.4% radial and 2.3-4.2% tangential. When seasoned with care, end-splitting and surface checking are not significant during drying. Boards 25 mm thick take about 3 months to air dry. The timber kiln dries rapidly but marked collapse may occur in early stages of seasoning; this can be remedied by reconditioning.

The wood is easy to work with all tools, but boards of A. auriculiformis tend to split when sawn. It is recommended to saw the comparatively heavy wood of A. catechu when green. Wattle wood planes easily to a smooth, lustrous surface using cutting angles of 15-25and finishes well with sharp tools. It drills quite easily, provided the base is supported to prevent end-chipping, and it turns well under low to moderate pressure. The nailing and screwing properties are satisfactory. The wood takes a good polish.

Wattle wood is usually durable when exposed to the weather, but is not durable in contact with theground. It is mostly resistant to termite attack, but A. auriculiformis wood can be attacked by the root fungus Ganoderma lucidum and is liable to marine borer attack. The heartwood is moderately resistant to preservative treatment, but the sapwood is permeable.

The pulping properties are excellent and comparable to commercial eucalypts. In tests in Australia using the sulphate process, wood chips of A. mangium from a 9-year-old plantation required only moderate amounts of alkali to yield in excess of 50% of screened pulp with excellent paper-making properties. Pulp yields were even higher (up to 75%) with the neutral sulphite semichemical process, and the pulp was readily bleached to brightness levels acceptable for use in fine papers. The hybrid of A. mangium and A. auriculiformis has a yield of over 55% in sulphate pulping and the quality of the pulp is generally better than that of A. mangium or A. auriculiformis .

A. auriculiformis wood contains 66% holocellulose, 35%α-cellulose, 31% lignin, 16% pentosan and 1.5% ash; the solubility is 9.7% in alcohol-benzene, 10.6% in hot water and 24.0% in alkali. Wood of A. mangium contains 78% holocellulose, 46.5%α-cellulose, 27% lignin, 14% pentosan and 0.2% ash; the solubility is 3.8% in alcohol-benzene, 3.3% in hot water and 13.4% in alkali. Wood of the hybrid between these species from Sabah contains 79% holocellulose, 47%α-cellulose, 26.5% lignin, 13.5% pentosan and 0.6% ash; the solubility is 3.8% in alcohol-benzene, 2.5% in hot water and 13.9% in alkali. The energy value of A. mangium wood is 20 100-20 500 kJ/kg and of A. mearnsii wood is about 19 700 kJ/kg.

The bark and wood contain abundant tannins, e.g. up to 40% on dry weight basis in the bark of A. mearnsii , making wattles commercially important for tanning sole leather.

Description

Armed or unarmed lianas, shrubs or small to fairly large trees up to 35(-39) m tall; bole branchless for up to 21 m, up to 100 cm in diameter, not buttressed; bark surface (of timber trees) dark grey or brown, deeply longitudinally fissured, inner bark pale brown or red to pink. Leaves arranged spirally, bipinnate and consisting of many opposite, sessile or short-stalked leaflets, or a phyllode made up of a flattened petiole and the proximal part of the rachis; extrafloral nectaries usually present on petiole and rachis; stipules present, spinescent or not. Inflorescences consisting of pedunculate glomerules or spikes borne in axillary clusters or aggregated into terminal panicles. Flowers bisexual, or male and bisexual, actinomorphic, 4-5-merous, white or pale greenish to yellow; calyx and corolla connate, valvate; stamens many, free or united only at base; ovary solitary, superior, 1-celled, style filiform, stigma small. Fruit a dehiscent or indehiscent pod, very variable in shape, texture and indumentum. Seeds in 1 row, usually elliptical to oblong, more or less flattened; testa hard; funicle usually without an aril. Seedling with epigeal germination; cotyledons borne above the soil level, petiolate, ear-shaped with flabellate venation; basic foliage sequence from pinnate to bipinnate to a phyllode.

Wood anatomy

• Macroscopic characters:

Heartwood pale pinkish-brown to dark brown, sometimes olive-brown to grey-brown, clearly demarcated from the pale yellow to straw-coloured sapwood which is up to 60 mm wide in A. auriculiformis , narrower in other species. Grain usually straight, sometimes interlocked. Texture fine to medium, even; streaky figure sometimes evident due to darker coloured streaks, wood lustrous. Growth rings indistinct to absent, but reportedly visible in wood from plantations of A. mangium in Thailand; vessels intermediate to large and distinct to the naked eye, evenly distributed; parenchyma not abundant, around pores, very occasionally in irregular bands; rays small, invisible to barely visible to the naked eye as individual rays, more conspicuous on radial surface particularly when extraneous materials abundant; ripple marks absent.

• Microscopic characters:

Growth rings indistinct or absent, sometimes poorly defined growth zones evident. Vessels diffuse, 4-6(-9)/mm², solitary (c. 40%) and in radial multiples of 2-3(-4), round to mostly oval, average tangential diameter (90-)120-160(-270)μm; perforations simple; intervessel pits alternate, vestured, polygonal and often crowded, 6-9μm in diameter; vessel-ray pits similar to intervessel pits but half-bordered; helical thickenings absent; tyloses absent. Fibres (0.9-)1.1-1.2(-1.3) mm long, non-septate, thin-walled to moderately thick-walled, with inconspicuous and simple to minutely bordered pits; tension-wood fibres common. Parenchyma sparse to moderately abundant paratracheal, vasicentric, usually in prominent sheaths, 2-4 cells wide around the pores, tending to aliform particularly around the smaller pores, in 2-4-celled strands. Rays 4-6(-8)/mm, 1-2(-3)-seriate, 0.2-0.4 mm (10-40 cells) high, homocellular. Prismatic crystals in chambered parenchyma strands. Silica absent. Wood showing fluorescence in UV light.

Species studied: A. aulacocarpa , A. auriculiformis , A. crassicarpa , A. decurrens (Wendl.) Willd., A. mangium , A. mearnsii .

The heartwood of plantation-grown material tends to be paler. Sometimes the wood of Albizia may superficially resemble pale-coloured wattle, but it can easily be differentiated from wattle by more abundant parenchyma and, in some species, septate fibres; additionally, the density is lower.

Growth and development

Most Acacia species grow fast. The considerable amount of growth data on A. mangium confirms that it can achieve a mean annual diameter increment of upto 5 cm and a height increment of up to 5 m in the first 4 or 5 years. A. mangium is reported to grow 3 m tall in the first year in Sabah and Sumatra, and in the Philippines it reached an average height of 8.3 m and diameter of 9.4 cm after 2 years. However, growth declines rapidly after 7 or 8 years and except under ideal conditions or over long periods (more than 20 years), the tree will probably not exceed 35 cm in diameter and 35 m in height. In Sabah, 14-year-old A. mangium trees were 30 m tall and 40 cm in diameter. Provenances from Papua New Guinea consistently show better growth in height and diameter, and the form is also superior.

Early growth of A. auriculiformis , A. crassicarpa and A. leptocarpa is fast during the first 6 months. A. leptocarpa trees reached a mean height of 3.0 m in 6 months. A. auriculiformis in Papua New Guinea reached 6 m height and 5 cm diameter in 2 years, in Malaysia it reached 9-12 m height after 3 years on clay soils and 6 m height on nutrient-poor sandy soils; in Sabah the growth rate is comparable to A. mangium . However, early growth of A. leucophloea is slow, and the mean annual diameter increment of A. catechu in Thailand is only 0.8-1.3 cm.

In the first 2 years both the diameter growth and height growth of A. mangium trees are significantly greater at a spacing of 2 m × 2 m and 2.5 m × 2.5 m than at 3 m × 3 m. Height growth is almost halved on sites dominated by the grass Imperata cylindrica (L.) Raeuschel

A. mangium trees form a symbiosis with soil bacteria of the genus Rhizobium , leading to nodules, in which the bacteria transform free nitrogen into organic and inorganic compounds containing nitrogen. Some Rhizobium strains are more effective in promoting growth than others. Optimal growth is achieved most effectively if vesicular-arbuscular mycorrhizal (VAM) fungi such as Glomus fasciculatus and Gigaspora margarita are present in combination with Rhizobium . Uninoculated seedlings died after 2 years in degraded grasslands. Technologies for the commercial production of rhizobial and VAM inoculants are now available in South-East Asia. The ecto-mycorrhizal fungus Thelephora ramaroides has been identified in Sabah in association with A. mangium .

A. mangium and A. auriculiformis flower more or less continuously. A. mearnsii can be expected to flower and fruit profusely every year. A. mangium flowers precociously and viable seed can be harvested 24 months after planting. The flowering to fruiting period is 6-7 months. The fragrant flowers are pollinated by insects such as bees.

Other botanical information

Acacia is easily distinguishable from other genera of the subfamily Mimosoideae by its many stamens which are free or united only at the base. The genus is subdivided into 3 subgenera. Pollen morphological characters support this division. The mainly African and American subgenus Acacia is characterized by its spinescent stipules. The pantropical subgenus Aculeiferum Vassal has non-spinescent stipules but the internodes are armed with prickles. The large and mainly Australian subgenus Phyllodineae (DC.) Seringe (synonym: subgenus Heterophyllum Vassal) usually bears non-spinescent stipules, whereas the leaves are generally reduced to phyllodes. It has been suggested to treat the latter subgenera as distinct genera ( Senegalia for subgenus Aculeiferum and Racosperma for subgenus Phyllodineae ), but no consensus has been reached on this yet.

Ecology

The species of Acacia mainly occur in savanna ecosystems, having a greater tendency to exploit arid or semi-arid regions rather than wetter forested regions, and may constitute a characteristic element of the vegetation there. The exceptions are several tropical species (including A. auriculiformis and A. mangium ) found in areas of high rainfall in northern Australia, New Guinea and adjacent islands. The prevailing climate in these areas is usually strongly seasonal, with rainfall of less than 50 mm/month in June to October. Average annual rainfall is 1450-1900 mm in southern New Guinea, and 2100 mm in northern Queensland. A. mangium appears to have a preference for slightly higher and drier sites than other Acacia species found in the same area, whereas A. auriculiformis prefers moister soils.

In their natural habitat the species are found in a wide variety of vegetation types, ranging from grassland, swamp grassland, savanna, savanna woodland, to dry evergreen monsoon forest. The timber-producing species native in South-East Asia and northern Australia occur at low altitudes, on well-drained sandy, stony, or limestone soils, or on poorly drained floodplains and on the margins of swamps and mangroves. Acacia species are often found associated with Melaleuca , Eucalyptus , Tristania , Alstonia , Dillenia , Xanthostemon , Grevillea , Planchonia and Syzygium spp.

In general, Acacia can grow on a variety of soils, including very infertile, clayey, acidic, or saline soils with impeded drainage. A. mangium has been successfully planted on abandoned areas of shifting cultivation colonized by Imperata cylindrica grass, but it does not tolerate waterlogging and soils derived from ultrabasic rocks. The optimum soil pH range is 4-6. A. auriculiformis performs well on extremely infertile sand tailings and on heath soils.

Propagation and planting

Acacia can be propagated from seed (direct sowing or in the nursery), and by air layering, cuttings, grafting and tissue culture.

For the production of seedlings, the pods should be processed as soon as possible after harvesting. Pods and seeds should not be left to dry in the sun for too long, as temperatures exceeding 43C reduce viability. It is difficult to extract the seed, but pods can be broken open by being tumbled ina cement mixer with heavy wooden blocks for 10 minutes or by beating in a commercial thresher. Threshing produces highly irritating dust and causes respiratory problems for some people; operators should wear protective gear. One kg of A. mangium pods yields (16-)56-86 g of seed.

The number of seeds/kg is 40 000-80 000 for A. aulacocarpa , 30 000-62 000 for A. auriculiformis , 15 000-40 000 for A. catechu , 35 000-50 000 for A. crassicarpa , 60 000-120 000 for A. leptocarpa , 32 500-37 500 for A. leucophloea , 63 000-189 000 for A. mangium and 66 000-80 000 for A. mearnsii . Seed can retain its viability for many years if stored cool (0-5C) in airtight containers. The seed of all species except A. catechu needs to be pretreated before sowing. A good method is to pour seed into 5-10 times their volume of water at 100C and stir for 30 seconds (2 minutes for A. auriculiformis ). The hot water is then drained off, cold water is added, and the seed is left to imbibe for 24 hours. Manual scarification is another pretreatment used for Acacia species. Pretreated seed can be sown, or may be dried immediately after the hot water treatment and then stored and transported.

The germination rate is high, generally 75-90%, and germination is rapid, usually within one month (2-10(-35) days for A. mangium ). Seed may be sown in seed beds and pricked out 6-10 days after sowing; however, the recovery rate for A. mangium is only about 37%. Sowing in germination trays ("wet-towel method"), and pricking out the seedlings 6-10 days after sowing when the radicle emerges, gives over 85% recovery. Another option is direct sowing in containers (polythene bags, open-ended hanging pots called "root trainers" or other permanent pots) followed by pricking out to maintain one seedling per container. There are no specific requirements for the type of substrate; mixtures of topsoil, peat, old sawdust, rice husks, sand and vermiculite are used. Even pure peat with a pH of 3.1 presented no problems. A mixture of peat (70-80%) and rice husks (30-20%) has been used successfully for A. mangium in Sumatra. NPK fertilization is generally applied in the nursery, but fertilization is stopped when "hardening off" the plants by reducing watering and exposing them to full sunlight. The appropriate height for planting is 25-40 cm, when seedlings have been in the nursery for 9-16 weeks. A direct seeding trial with A. mangium in Sabah gave 66% survival after 3 months and 30% after 6 months.

A. mangium can be propagated vegetatively through single-node stem cuttings 4-5 cm long and 0.5-1.5 cm in diameter, leaving 0.5-1 phyllodes. The application of 500-1000 ppm indolebutyric acid (IBA) or rooting powder enables 65-75% rooting to be achieved. However, rooting is reported to be slow. Air layering trials in Thailand gave a success rate of 80% in A. aulacocarpa and A. auriculiformis ; promising results were also obtained for A. crassicarpa and A. mangium . The explants for tissue culture are 2-3 mm lengths of aseptically-germinated one-month-old seedlings and the optimum induction of multiple shoots is achieved in a Murashige and Skoog basal medium supplemented with 0.5 mg/l of benzylamino purine (BAP). Excised shoots longer than 0.5 cm root easily in a humidified rooting chamber. Plants in the nursery do not need to be inoculated with Rhizobium , because nodulation is prolific; however, the seedlings should be checked for the presence of active nitrogen-fixing root nodules prior to planting. There is little experience with bare-root planting stock, but in the Philippines plantations have been succesfully established using this technique. In Malaysia, plants are hardened off by wrenching them every 2-4 weeks and watering only once every 6 days. The spacing applied varies according to country and to the objective of the plantation, from 2 m × 2 m to 4 m × 4 m. Dense planting for the production of saw logs reduces the incidence of large branches and the inherent risk of infections.

Silviculture and management

Acacia species are pioneers and demand full light for good development; in shade A. mangium grows stunted and spindly. Acacia trees are renowned for their robustness and adaptability, which makes them good plantation species. Survival after planting out is high: 60% for A. mangium planted in a windbreak in Imperata grassland, and over 90% when planted on more favourable sites. In A. mangium plantations canopy closure occurs after 9 months to 3 years, depending on soil fertility, weediness and initial spacing (e.g. in Sabah in a plantation with an initial spacing of 3 m × 3 m canopy closure was achieved in one year).

In the first year, the plantation should be protected from livestock as they browse the trees, and it should be weeded, taking particular care to remove climbers, creepers and vines. Imperata cylindrica is a strong competitor on relatively wet sites with heavier soils. A. mangium has been found to be very sensitive to herbicides.

As A. mangium has a strong tendency to produce multiple leaders from the base, "singling" is carried out at 4-6 months after planting. Persistent branches are pruned out only in plantations where the objective is to produce quality saw or veneer logs. Usually, pruning is done twice; the second time, branches are pruned out further up the trunk, often to a height of 6 m. Pruning out branches with a diameter of 2 cm or more makes the trees susceptible to infections, especially heart rot.

A. mangium is very responsive to extra growing space. The thinning caried out in plantations for pulpwood production is aimed at achieving a final stock of 600-700 stems/ha from the 1250 trees/ha planted. It is executed after 18 months. These plantations are clear felled after 6-8 years. Thinnings in plantations for the production of quality saw logs generally reduce the initial numberof trees from 900/ha to 100-200/ha in two or three thinnings. The first thinning is done when trees are about 9 m tall, i.e. before 2 years of age. The rotation here is 15-20 years. However, no definitive pruning and thinning schedules have yet been established for A. mangium and other schedules are also applied. In Papua New Guinea, plantations grown on a 7-8-year rotation for pulpwood are not thinned.

It is not possible to regenerate from coppice for the second generation, as the coppice shoots do not develop to tree size. Only A. aulacocarpa coppices well and suckers from its roots. A. auriculiformis coppices when cut more than 50 cm from ground level. A. mangium regenerates abundantly in clear-felled areas or where a light fire has occurred. However, there are no reports of experience in tending, pruning and thinning a crop grown from natural regeneration. Natural regeneration of A. auriculiformis is also profuse and rapid after the mature stand has been felled. For the production of tannin from A. mearnsii in Java the initial 3300 trees/ha are thinned heavily to the final stocking of 275 trees/ha at the age of 8 years, when the plantation is harvested. For A. mearnsii up to 8 short rotations have been harvested from the same site without apparent growth decline. Only A. crassicarpa and A. leucophloea are resistant to fire. Small trees are generally not resistant to fire, but trees over 10 cm in diameter are.

About 50 000 ha of A. mangium plantations have been established in Sabah, and about 42 000 ha in Peninsular Malaysia. In North Sumatra, one enterprise plants 16 000 ha annually and in South Sumatra 300 000 ha of forest land is planned to be planted, predominantly with A. mangium .

Diseases and pests

Damping-off is the most serious disease in the nursery. It is caused by a wide variety of fungi, but can be overcome with the use of fungicide. Other common diseases in nurseries are also found on young plants of A. mangium .

Heart rot is the most serious disease of A. mangium in plantations. It invades through branch wounds (e.g. caused by pruning) and is also known as white rot, as the affected wood becomes whitish, spongy or fibrous and is surrounded by a dark stain. Heart rot is much less common in Sabah than in Peninsular Malaysia. Dead or broken branches, wounds, and cankers indicate its presence. Only Phellinus noxius has been positively identified as causal organism. In Peninsular Malaysia, the Forestry Department recently suspended the planting of A. mangium , pending an evaluation of the impact of heart rot. However, this suspension has now been lifted. Root rot is caused by Phellinus spp. ( P. noxius in the Philippines) and by Ganoderma spp., causing 29% mortality in Papua New Guinea after 5 years. In Sarawak, "pink disease" caused by Corticium salmonicolor is locally important and causes the crown to die.

About 48 groups of insects attack A. mangium . Only the pests of major economic importance are mentioned below. Coptotermes curvignathus (a termite found in Sumatra, Malaysia and Thailand) feeds on young seedlings' roots or stems near ground level and penetrates to the heartwood. Attack on trees is primary, regardless of wound or decay, and damage is greatest in dry plantation sites after the old forest has been cleared, and on low-lying moist sites. All affected wood at the site should be destroyed before replanting. The beetle Sinoxylon anale (a branch and twig borer) is found on A. mangium , A. auriculiformis , A. catechu and other Acacia species in Thailand. It primarily bores into sapwood of cut logs or into diseased and weak poles, but occasionally it tunnels into shoots and young stems to feed. Larvae of Sternocera aequisignata (the green-leg flat-headed borer) bore at root collars and can kill trees in the nursery; this pest is especially destructive during the first 2 years after planting. It attacks A. mangium and A. auriculiformis in Thailand. The larvae of Zeuzera coffea (the red coffee borer) tunnel in young twigs and stems and are found on A. mangium and A. auriculiformis . They are especially injurious to one-year-old seedlings or small saplings in nurseries and plantations. Many other pests may become locally important and deserve attention, including control measures.

Harvesting

A. mangium plantations are felled for pulpwood 6-8 years after planting; for sawn timber the rotation is 15-20 years. In old trees and in A. aulacocarpa and A. crassicarpa the lower part of the bole is often fluted. A. mearnsii trees are harvested when 8 years old, with the main objective of collecting the bark for tannin production, whereas A. auriculiformis is harvested after 10-12 years and A. leucophloea after 12 years.

Yield

The productivity of A. mangium in Kalimantan has been found to be closely related to "total" soil potassium (K) levels (The latter accounted for 50% of the variation in the data). However, in Malaysia phosphorus (P) appears to be the most important nutrient.

Measurements of the diameter at breast height provide sufficiently accurate and reliable yield estimates in A. mangium plantations. Untended stands of 9-year-old A. mangium in Sabah had an annual increment of 46 m³/ha. Even on poor sites a mean annual increment of 20 m³/ha is often achieved. The performance of A. mangium in plantations in Malaysia, however, is variable and is below expectations. In Java, the mean annual increment of A. auriculiformis on relatively fertile soils is 15-20 m³/ha and on less fertile soils it is 8-12 m³/ha. The mean annual increment of an 8-year-old plantation of A. mearnsii in Java is 11 m³/ha, and an additional 7 m³/ha from thinnings. The final yield of undried bark in this plantation was 15 400 kg/ha, and an additional 8800 kg/hawas obtained from thinnings. In general, a mean annual increment of 10-25 m³/ha can be expected for this species. The mean annual increment over the 12-year rotation period of A. leucophloea is 9 m³/ha of stemwood (bole) and 11 m³/ha for wood over 7 cm diameter.

Handling after harvest

The observed A. mangium trees in Peninsular Malaysia have problems especially regarding early forking and damage by fungi and insects. Only a small portion of the total amount could be used as saw or veneer logs. The bulk, about 60%, is only fit for pulpwood.

Genetic resources

Extensive seed collections have been made by CSIRO (Australia) from a range of Acacia species in Indonesia (Moluccas, Irian Jaya), Papua New Guinea and in northern Queensland. The natural stands are accessible but not threatened by logging. Moreover, several species are planted on a large scale.

Breeding

A large international provenance trial has been set up for A. mangium , involving 24 provenances tested in 19 sites in 8 countries. Provenances from Papua New Guinea consistently show better growth in height and diameter, and stem form is also superior. In most countries in South-East Asia provenance trials for other species have been set up and preliminary results are available. A. mearnsii and A. auriculiformis are generally outcrossing, and have estimated outcrossing rates of 67-89% and 93%, respectively. A. mangium has a stronger tendency to selfing. Natural hybrids have been found between A. auriculiformis and A. leptocarpa and between A. mangium and A. auriculiformis (8% hybrids in an A. mangium research plot in Peninsular Malaysia). The tree form of the latter hybrid is satisfactory where it inherits the better stem straightness of A. mangium and the self-pruning ability and better stem roundness of A. auriculiformis . The hybrid's height and diameter increments are significantly better. Moreover, it has intermediate physical and mechanical wood properties (better than A. mangium ) and it also appears to be more resistant to heart rot. Natural hybrids in Sabah, however, tend to inherit the poor stem form of A. auriculiformis . Hybridization techniques have been developed and the production of hybrid plants could be accelerated through tissue culture. A. leptocarpa is possibly very variable genetically, and selection of good provenances may easily raise the productivity in plantations. Straight-stemmed A. auriculiformis trees have been found in Papua New Guinea and Sabah, however, the trunks of most trees of this species are crooked. Selection and breeding of A. auriculiformis may considerably enhance its utilization in plantations. In Thailand, a programme for selection and breeding of A. auriculiformis started in 1983 with the selection of plus trees and the identification of plantations which can be transformed into seed stands. Many countries in South-East Asia have started research on breeding on a number of Acacia species.

Prospects

The future for the increased utilization of A. mangium wood for the production of particle board and medium-density fibreboard is promising, and the quality of wood chips for pulp and paper is satisfactory. The wood quality observed in Peninsular Malaysia is less promising for general utility timber. Silvicultural schedules, especially those regarding the spacing, pruning and thinning and management of subsequent rotations, are not well known yet or at least are not well publicised. Present problems with heart rot may be overcome by carefully matching species to site, by selection and breeding, and by hybridization. The high incidence of heart rot in Peninsular Malaysia might be the result of the absence of high seasonality in rainfall. In Thailand, farmers are now planting A. mangium and selling the produce to industry. This interesting example of small-scale plantations of A. mangium deserves to be copied in other countries. Most Acacia species are fast growing and suitable for planting on Themeda and Imperata cylindrica grasslands (although the growth is not optimal under this condition) and sites degraded by logging. The hybrid A. mangium x A. auriculiformis appears to be very promising, as its characters and growth are superior to both parents. In Thailand, preliminary results from research indicate that A. crassicarpa may prove to be a very valuable species for industrial plantations.

Literature

• Awang, K. & Taylor, D.A. (Editors), 1993. Tropical Acacias in East Asia and the Pacific. Proceedings of a first meeting of the Consultative Group for Research and Development of Acacias (COGREDA) held in Phuket, Thailand, June 1-3, 1992. Winrock International Institute for Agricultural Research, Bangkok. 106 pp.
• Awang, K. & Taylor, D.A. (Editors), 1993. Acacias for rural, industrial, and environmental development. Proceedings of the second meeting of the Consultative Group for Research and Development of Acacias (COGREDA) held in Udorn Thani, Thailand, February 15-18, 1993. Winrock International and FAO, Bangkok. 258 pp.
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