Acacia mearnsii (PROSEA)
Acacia mearnsii De Wild.
- Protologue: Pl. Bequaert. 3: 61 (1925).
- Family: Leguminosae
- Chromosome number: 2n= 26
- Acacia decurrens (Wendl.) Willd. var. mollis Lindley (1819),
- Acacia decurrens auct., non Willd. (1806),
- Acacia mollissima auct., non Willd. (1809).
- Black wattle, late black wattle, tan wattle (En)
- Acacie noir (Fr)
- Indonesia: akasia (general).
Origin and geographic distribution
Black wattle is native to south-eastern Australia (New South Wales, Queensland, Victoria and Tasmania), but it has been introduced throughout the tropics and often naturalizing. Large plantations are found in southern and East Africa, Brazil and India. In Indonesia the first trials with the species started at the end of the 18th Century, but better results were obtained with trials started in 1911. In 1933 the Forest Service started planting the species on commercial scale in mountainous areas of Java and by 1941 an area of around 12 000 ha had been planted. Additional plantations were established in southern Sulawesi, around Lake Toba on Sumatra and on Bali. On a smaller scale, plantations have been established in Peninsular Malaysia and in the Philippines.
Black wattle is primarily cultivated for tannin and wood production. The species is the principal source of the world's tanbark; the bark contains up to 40% of excellent tannin especially fitted for use in the manufacture of heavy leather goods. In addition, the powdered bark extract is used to prepare tannin formaldehyde adhesives for exterior grade plywood, particle board and laminated timber. The wood of the tree is widely used as fuelwood for domestic use and village industries (e.g. tobacco curing in central Java), or for charcoal production (e.g. in Kenya and Brazil). The wood may also be used for local construction material, mine props, wooden tools, joinery, flooring and hardboard. It is also used for rayon and paper pulp. The species has also been planted for erosion control and soil improvement, as shelterbelt or firebelt, as a shade tree in tea plantations, and as an ornamental. The leaves are sometimes used for fodder, but are relatively unpalatable and can best be mixed with other feeds.
Production and international trade
The maximum area of black wattle plantations was reached around the 1960s. Since then, a fall in demand for tannin has led to a considerable reduction in area, e.g. from 325 000 ha to 160 000 ha in South Africa and from 27 000 ha to 14 000 ha in Zimbabwe. Around 1980 the estimated plantation area was about 350 000 ha, of which 160 000 ha were in South Africa, 125 000 ha in Brazil, 30 000 ha in East Africa (Zimbabwe, Kenya, Tanzania, Rwanda, Burundi), and 20 000 ha in India. In Indonesia the plantation area is estimated at 15 000 ha (mostly Forest Service plantations); in addition, locally (Dieng Plateau and the slopes of Merapi volcano in central Java, southern Sulawesi) the tree is also grown by farmers in rotation with agricultural crops. In several countries tannin industries based on the species have been developed; the main exporting countries are South Africa (150 000 t/year of bark), Kenya (25 000 t/year) and Tanzania, and the main importing countries are Great Britain, Australia and the United States.
Black wattle is currently the world's major source of vegetable tannin, closely followed by quebracho (Schinopsis spp.).
The bark of black wattle contains 30-40% high-quality tannin on dry weight basis. The tannin belongs to the proanthocyanidins, and is a complex mixture of some 40 components; among the main constituents are (+)catechin, (-)robinetinidol and (+)gallocatechin. The tannin penetrates the hide fast, and gives a firm and durable leather with a light colour, unlike other proanthocyanidin tanning materials (e.g. mangrove extracts) which give a reddish colour. It does not precipitate in acid solution, which influences positively the quality of the leather. It is especially suited for the manufacture of sole leather.
The tannin content varies with bark thickness, age of the tree and average annual rainfall, and decreases from the base of the trunk upwards, the bark of the branches having a low tannin content.
Black wattle extract contains 60-65% tannin. Extracts, usually called "mimosa extract", are commercially available in several forms, each giving different qualities to leather. Some firms in Europe specialize in treating the pure extract for this purpose. Usually the extract is mixed with syntans for use in the leather industry.
The wood is yellowish to light red. It is fairly light, 550-850 kg/m3 (in Indonesia 600-700 kg/m3), depending on site conditions, moderately hard to hard, durable, and fairly tough and strong. It has an energy value of about 19 700 kJ/kg and ash content of ca. 1.5%. The energy value of charcoal is about 32 000 kJ/kg.
Seed weight is low; 1 kg contains 50 000-80 000 seeds.
- A small to medium-sized evergreen tree, 6-25 m tall, with straight trunk to 50 cm in diameter and a spreading rounded crown of feathery, dark green foliage (if cultivated in plantations erect and slender); bark brownish-black, fissured, but in younger stems grey-brown and smooth; twigs unarmed, angled, grey, densely hairy, tinged with golden yellow when young.
- Leaves alternate, bipinnately compound, 8-15 cm long, with 8-20 pairs of pinnae 2-5 cm long; rachis with glands at base of each pair of pinnae on upper surface; leaflets very numerous, 20-70 pairs crowded on each pinna, narrowly oblong and small, 1.5-4 mm × 0.5-0.8 mm, blunt, with dense soft hairs, dark olive green.
- Flowers minute and numerous in pale yellow globose heads, 5-8 mm in diameter, arranged in axillary racemes or panicles, very sweet-scented, 5-merous, with numerous stamens and a pistil with long slender style.
- Fruit a narrowly oblong or linear, flat pod, (3-)5-15 cm × 0.5-1 cm, constricted between the seeds, pubescent, dark brown to blackish when ripe, dehiscent along one suture, 3-14-seeded.
- Seeds ovoid, 3-5 mm × 2-3.5 mm, smooth and black.
Growth and development
Seeds stay viable for many years, especially when stored in air-tight containers. Seeds can remain in the ground for a long time and start to germinate after a forest fire, but seedlings are susceptible to fire. After germination the radicle grows vertically downwards to form a taproot of about 1 m. After a few weeks a relatively extensive network of lateral roots starts to develop; at a later stage these lateral roots may develop sinkers. Nodules of nitrogen-fixing bacteria are readily formed at the tips of the lateral roots. The plumule starts to develop somewhat later than the radicle; it has an erect growth. Black wattle is a light-demanding species with rapid early stem growth. Growth rates of up to 3 m/year are reached after 3-5 years. Trees start to flower when about 2 years old. The flowers are insect (bee) pollinated. Copious fruiting normally occurs after 5-6 years, and fruits mature in 12-14 months. The total lifespan is 15-20 years.
In some areas such as Hawaii and parts of South Africa the species has become a noxious weed due to its aggressive colonization of denuded areas, especially if fires occur.
Other botanical information
There has been considerable confusion about A. mearnsii and some closely allied species: A. decurrens (Wendl.) Willd. (green wattle) and A. dealbata Link (silver wattle). These species have been considered for a long time as conspecific with A. mearnsii, although usually distinguished as varieties. They are now usually thought to represent distinct species. The name A. mollissima has often been used erroneously for A. mearnsii. In fact, A. mollissima Willd. is a synonym for A. pubescens (Vent.) Ait.f. This makes literature on these species very confusing. In Indonesia, the name A. decurrens is still commonly used for A. mearnsii. However, although unlikely, it cannot be completely ruled out that the true A. decurrens, which also has a high tannin content in the bark, has also been introduced in Java.
Black wattle can be crossed with green wattle ( A. decurrens). Hybrids show more sterility than their parents.
In its natural area of distribution (35-44°S latitude), black wattle occurs in the understorey of tall open forests, in fringes of closed forests or in dense thickets on recolonized lands. Its range is from sea-level to 900 m, but its main occurrence is from near sea-level to about 200 m in areas with a warm subhumid to humid climate. The mean maximum temperature of the hottest month is 21-28°C, the mean minimum of the coolest month 1-7°C, with up to 20-40 frost days. Annual rainfall varies between (440-)625-1000(-1600) mm. The species is sensitive to severe drought and to frosts of -4°C or lower.
In tropical countries plantations occur under hotter and wetter conditions than in the natural area of distribution. These plantations are found in the highlands (1500-2500 m) with mean annual temperature 12-20°C, coolest month minimum temperature 2-8°C, hottest month maximum temperature 18-24°C and mean annual precipitation 700-2000 mm. In Indonesia the species has been planted at 1000-2000 m altitude with annual rainfall of 1000 mm or more. In South Africa (Natal) the species is cultivated in areas at 300-1000 m altitude where ecological conditions are intermediate between the tree's native conditions and tropical conditions.
Black wattle can grow on a variety of soils. The best soils are moist but well-drained, relatively deep and light-textured with pH 5-6.5. The species tolerates moderately heavy or shallow soils with moderate to poor fertility. It does not grow on poorly-drained, calcareous or very infertile sites.
Propagation and planting
Black wattle is usually propagated by seeds, which are either directly sown in the field, or in containers when raised as nursery stock. In Java direct seeding at 3 m × 1 m distance with 5 seeds per hole is most common. Germination is rapid if seeds have been pre-treated with very hot water (90°C). Sometimes scarification is used. Seeds retain their viability for several years. Vegetative propagation is not very successful. Normally no inoculation with Rhizobium is needed.
Standard nursery practices can be used to raise seedlings in the nursery. Plantation sites should be well prepared by ploughing or soil ripping. Normally black wattle is not mixed with other species because its rapid growth hinders the development of other species. In Indonesia and other countries, young plantations are sometimes temporarily intercropped with food crops. In such cases in Indonesia, Leucaena leucocephala (Lamk) De Wit or Cestrum spp. are interplanted along the contours to control erosion.
During the first year plantations should be weeded. To maintain vigorous growth, thinning should start as early as the 2nd or 3rd year, and should be repeated regularly. The degree of thinning depends on the management objectives; severe thinning favours stem diameter growth and the related bark production; denser stands are needed for good timber production. In Indonesia optimal bark production is obtained by reducing the early tree density of around 3000 trees/ha to 200-380 trees/ha at 8 years of age, depending on the quality of the sites. Such thinning allows the most productive trees to be selected and trees with gummosis to be removed. Normally no pruning is needed.
Because of the short rotations and high biomass harvest it is important to replace nutrients by fertilization. Proper care should be taken to control erosion, especially when plantations are burnt (e.g. to promote natural regeneration). But if properly managed, black wattle may help to enrich soil nitrogen as a result of rhizobial nitrogen fixation, and rehabilitate degraded lands. Good results are obtained in the Wonosobo region (Central Java) where local farmers have cultivated the species in rotation with vegetable crops and tobacco for many years.
Diseases and pests
In its native range the species is not cultivated because of serious damage by indigenous insects including the fireblight beetle Pyrgo orphana; sometimes severe damage may occur in Brazil, too. But in most tropical countries, disease and pest attacks are generally not serious, although attacks by various insects including defoliators (e.g. wattle bagworm, Acanthopsyche junode), stem-borers (e.g. Platypus solidus) and caterpillars (e.g. wattle looper caterpillar, Achaea lienardi) may occur. Sometimes a physiological disorder, called "gummosis" occurs whereby gum is exuded in the absence of any obvious injury.
In Indonesia, most damage occurs from fungal attacks of Armillaria, Corticium, Fomes and Phytophtora spp. under humid conditions with more than 3000 mm annual precipitation.
Plantations for tannin bark are usually harvested after (7-)8-10(-12) years, when trees are more than 18 m tall and have a diameter of at least 15 cm. The bark is harvested by ripping the bark at several points near the base of the stem with a hatchet or short iron bar flattened at the end; the loosened strips of bark are pulled from the stem. After stripping the bark is cut to bundle length. Trees are cut according to normal practices.
In South Africa typical yields of fertilized plantations are 15-25 m3/ha per year of wood and 1.5-2 t/ha of dry bark. In other tropical regions yields range between 25-35 m3/ha per year of wood and 0.9-2 t/ha of dry bark. In Indonesia mean wood yields are 14-21 m3/ha per year and 11-16 m3/ha per year in plantations of 8 and 12 years old, respectively. Dry bark production is 1.2-2 t/ha per year and 0.9-1.5 t/ha per year in plantations of 8 and 12 years old, respectively. At the best sites 60-65% of the yield consists of first grade bark from stems of at least 15 cm diameter, on poorer sites this is only 40-50%.
Handling after harvest
The harvested bark may be transported immediately or first dried locally. Drying should be done in partial shade; the inner bark darkens if exposed to direct sunlight. The bark discolours if it is re-wetted after drying. To obtain good bark quality kiln drying is practised sometimes. In Indonesia and elsewhere trials have been done on portable charcoal-burning drying kilns in which the bark can be completely dried in about 60 hours. During drying the bark curls inwards; these "sticks" are bundled for transport. At processing plants the bark may either be extracted or prepared for marketing as dry bark. Fresh bark is preferred for extraction. Dry bark is graded according to thickness, maturity, lightness of colour, absence of corkiness and freedom from mould. It is marketed as chopped bark, ground bark or sometimes as dust, in pressed bales or in bags.
In small tanneries in Indonesia, chopped bark is used directly for tanning hides in pits in the ground. The bark is cut into chips about 5 cm long, which are soaked in water for 10 days. Cow hides are immersed in the solution for a period of about 6 weeks.
It is thought that the seed used for black wattle plantations outside Australia derived from a limited part of the natural range and further provenance testing should be carried out. Germplasm collections exist at the CSIRO Division of Forest Research (Canberra, Australia) and at the Wattle Research Institute (Pietermaritzburg, South Africa).
Major breeding objectives are trees with enhanced vigour, better bark quality and stem form, and resistance to pests and diseases. Trials in South Africa to hybridize black and green wattle have not given promising results.
Due to the substitution of leather by plastics and the subsequent decline in the importance of tannin since the 1960s, black wattle cultivation has decreased in importance. In Indonesia several wattle plantations have been transformed into timber or clove plantations, and in central Java the cultivation of black wattle in rotation with agricultural crops has also diminished with the advance of artificial fertilizer and commercial vegetable production. Nonetheless, the species deserves more attention because of its multipurpose functions and its adaptability to a wide range of ecological conditions including degraded sites. It is a potential substitute for the synthetic tannins, which are now so widely used in the tanning industry and which cause problems for the environment. Special consideration should be given to using black wattle for combined production of bark and fuel or local construction material, and for soil rehabilitation in local land use systems.
- Berenschot, L.M., Filius, B.M. & Hardjosoediro, S., 1988. Factors determining the occurrence of the agroforestry system with Acacia mearnsii in Central Java. Agroforestry Systems 6(2): 119-135.
- Booth, T.H. & Jovanovic, T., 1988. Climatology of Acacia mearnsii. 1. Characteristics of natural sites and exotic plantations. New Forests 2: 17-30.
- Coster, C., 1939. De betekenis van de cultures van Acacia decurrens in Nederlandsch Indi√´ [The importance of Acacia decurrens in the Dutch Indies]. Tectona 32: 368-388 (in Dutch).
- Ferguson, J.H.A., 1948. Opbrengttafels voor Acacia decurrens Willd. var. mollis Lindl., bewerkt naar nagelaten tabellen van Dr. H.E. Wolff von Wülfing, 1945 [Yield tables for Acacia decurrens Willd. var. mollis Lindl., revised after posthumus tables from Dr. H.E. Wolff von W√ºlfing, 1945]. Tectona 38: 283-290.
- Sherry, S.P., 1971. The black wattle (Acacia mearnsii De Wild.). University of Natal Press, Pietermaritzburg, South Africa. 402 pp.
- Turnbull, J.W., 1986. Acacia mearnsii. In: J.W. Turnbull (Editor): Multipurpose Australian trees and shrubs; lesser-known species for fuelwood and agroforestry. Australian Centre for International Agricultural Research, Canberra. pp. 164-167.
155, 162, 369, 474, 560, 649, 650, 672, 687. Timbers
- K.F. Wiersum
- F. Arentz