Tree of economic importance in South and East Africa, Rhodesia, India, and Rio Grande do Sul area of South America etc. for tanning of soft-leather. Ranging from 30–54 percent tannin in dried bark. Wood furnishes badly needed fuel and building material in some areas. Trees not only provide tannin and fuel, but also add nitrogen and organic material to improve the soil. Bark is used for wood adhesives and flotation agents (Duke, 1981a). The pulp is suitable for wrapping paper and hardboard. Some regard it as an attractive ornamental. Sometimes used for erosion control on poor sloping soils unsuitable for agriculture. Densely packed plantations are effective in preventing further erosion on 50° slopes. Some farmers claim that tobacco and vegetable yields are doubled in rotating with the black wattle. In places it is regarded as a “green cancer”, spreading vigorously as a weed (NAS, 1980; Little, 1983).
Products are often used in folk medicine as styptics or astringents (Duke, 1981).
Black wattle bark contains (-)-robinetinidol and (+)-catechin; the biflavonoids (-)-fisetinidol-(+)-catechin (2 diastereoisomers), (-)-robinetinidol-(+)-catechin and (-)-robinetinidol-(+)-gallocatechin; triflavonoids and condensed tannins. The heartwood is rich in (+)-leucofisetinidin (mollisacacidin) together with (-)-fisetinidol, (+)-fustin, butin, fisetin, butein, and biflavonoid condensates (tannins) (Duke, 1981).
Tree 6 to 20 m tall, 10 to 60 cm in diameter; crown conical or rounded; all parts except flowers usually pubescent or puberulous; stems without spines or prickles; leaves bipinnate, on petioles 1.5–2.5 cm long, with a gland above; rachis 4–12 cm long with numerous raised glands all along its upper side; pinnae in 8–30 pairs, pinnules in 16–70 pairs, linear-oblong, 1.5–4 mm long, 0.5–0.75 min wide; flowers in globose heads 5–8 mm in diameter, borne in panicles or racemes, on peduncles 2–6 mm long; pale yellow and fragrant; pods gray-puberulous, or sometimes glabrous, almost moniliform, dehiscing, usually 3–10 cm long, 0.5–0.8 cm wide, with 3–14 joints; seeds black, smooth, elliptic or compressed ovoid, 3–5 mm long, 2–3.5 mm wide; caruncle conspicuous; areole 3.5 mm long, 2 mm wide. Seeds 66,000 to 110,000/kg (Duke, 1981a).
Can be crossed with Acacia decurrens, hybrids show more sterility than parents. Meiosis is regular, with no gross cytological abnormalities, and sterility may be due to gene differentiation between species. There is little geographic overlap in the native Australian ranges of the species, and there are differences in phenology (flowering; seedset). Most of the characters that vary among the species are quantitative. The development of black wattle strains or of hybrids with enhanced vigor, better quality bark, outstanding stem form, or resistance to insect pests and disease would benefit the wattle industry. Assigned to the Australian Center of Diversity, black wattle or cvs thereof is reported to exhibit tolerance to drought, laterite, and poor soil (Duke, 1981). For an Acacia, it is relatively tolerant to frost, and its growth is slowed by high temperatures. (2n = 26.)
Native to Southeast Australia (Victoria to New South Wales and southern Queensland) and Tasmania. Introduced and cultivated widely for afforestations. See Sherry (1971) for details.
In Kenya grows on or near Equator at altitudes of 2,000–2,800 m, is well adjusted to the climate of East Africa. Grows well at 30°S Lat. in South America on rolling terrain at altitudes of 50–70 m. Thrives on poor, dry soils but favors deeper, moister, more fertile soils. In Australia, black wattle may occur on soils derived from shales, mudstones, sandstones, conglomerates, and alluvial deposits. In Kenya on podsols, krasnozems, sandy hills, lava flows or on mixtures of lava and contemporaneous volcanic tuffs and breccias. In South America, grown on red clay or sandy soils that have suffered from severe erosion and soil depletion (ferruginous clay loams with little or no free silica). In East Africa grows where annual rainfall is 1,041–1,321 mm, (about 75% between April and September). On the equator where black wattle is grown in South America, the rain pattern is nearly opposite, mean annual temperature range is 17–23°C; there is little seasonal variation, but considerable diurnal variation. At higher altitudes in South America, frost is a risk and heavy snows may break tree limbs. Tannin content varies inversely with precipitaton. Ranging from Warm Temperate Dry through Tropical Thorn to Tropical Moist Forest Life Zones, black wattle is reported to tolerate annual precipitation of 6.6–22.8 dm (mean of 6 cases=12.6), annual mean temperature of 14.7–27.8°C (mean of 6 cases=2.6°C), and pH of 5.0–7.2 (mean of 5 cases = 0.5).
Propagation by seed is easy. Seeds retain their viability for several years. For germination seed are covered with boiling water and allowed to stand until cool. This cracks the hard outer coat and facilitates germination. Seeds may be broadcast or sown in rows on any barren site. Usually they are sown about 5 cm apart in seedbeds, and are transplanted after 3–6 months. In South America, fields are usually plowed and harrowed in April or May. Seedlings are set out May–November, but usually in winter, June–August, after a rain. Plants are spaced 2 m each way, at rate of 2,500/ha. Propagation by cuttings is almost impossible without mist. Air layering is more promising. Two types of farmers grow acacia: the tanner or business man plants 200 ha or so entirely to black wattle, usually one section at a time so that he can plant and harvest within the same year and continue year after year; the farmer plants half or less of his land to black wattle and the rest to crops such as corn, beans, maniac, sugarcane, other vegetables, or pasture. He plants 2–6 hectares of acacian each year and thus evenly distributes work and production. Oxen may be useful for plowing, but most work is by hand. Usually only plows and hoes are used in Cultivation. Intercrops may be grown the first year during which trees grow about 4–5 m in height, and about 2.5 cm in diameter (Duke, 1981).
Trees provide bark 5–10 years after seeding (avg 7). Bark is stripped from lower part of tree, then tree is felled, the remaining bark removed, and tree and bark are cut into 1 m lengths. Thoroughly dried bark is arranged in bales of 75 to 80 kg when ready for transportation. Tanning power improves by 10–15% in bark carefully stored for a season. Percent tannin does not differ between barks harvested in dry and wet seasons. However, the amount of bark on trees may be less on poor than on rich soils. Tannin runs about 25–35% per kilo of dried bark, on either poor or rich soil. Acacia bark may be sold as baled bark, or bark powder. Dried bark may go first to commercial bark processors where it is ground or shredded in a hammermill, then sold in 40-kg sacks. Bark powder is sold in 60-kg sacks. Liquid extract is sold in 300-kg wooden barrels. In Rio Grande do Sul an estimated 5,000 MT of liquid extract is produced annually (Duke, 1981a).
Except for some mangrove species, black wattle in pure stand produces more tannin per hectare than most tanniniferous plants. In South Africa well-managed have produced the equivalent of 3 MT/ha tannin, about twice the average, when grown in rotations in excess of 12 years. One 7-yr-old tree produces 3–5 kg of dried bark. Twelve trees produce 1 cu m of firewood. The wood of debarked trees is dried and used for mine timbers, pulpwood, and fuel. Moisture l
oss is rapid in first 4 weeks after felling, then much slower. Wood weighs 708.7 kg/cu m. One tree can produce up to 10 cwt of bark or about 5 cwt stripped. One ton of black wattle bark is sufficient to tan 2,530 hides, best adapted for sole leather and other heavy goods; the leather is fully as durable as that tanned with oak bark. One ton of bark yields 4 cwt of extract tar. Destructive distillation of the wood yields 33.2% charcoal, 9.5% lime acetate, and 0.81 methyl alcohol. As a source of vegetable tannin, black wattle shares with quebracho and chestnut a large portion of the world market for vegetable tannins. According to Sherry (1971), plantation grown wattle in South Africa, Rhodesia, Tanzania, Kenya, and Brazil supplied about 38% of world demand for tannin. South Africa was the largest producer, with annual output of 72,000 MT of ca 120,000 MT on the world market. Eucalyptus grandis produces more wood than wattle, but it is inferior for fuel and charcoal. At one time in South Africa, 56% of the proceeds from wattle was from bark, the balance from timber (Duke, 1981a).
An efficient N-fixer, it is reported to annually yield 21–28 MT/ha wet leaves containing 245–285 kg N. If we put the information in our cultivation paragraph and our yields paragraph, we find the improbable 2,500 plants per hectare, with 12 producing 1 m3 firewood, suggesting a potential of more than 200 m3/ha for 7 year old trees, suggesting annual yields of ca 30 m3/ha. NAS (1980a) reports annual thickwood production of 10–25 m°3/ha and bark production of 0.8–4.0 MT. The dense wood (sp. grav. = 0.7–0.85) 3,500–4,000 kcal/kg (oven-dry Indonesian specimens 4,650 kcal/kg), its ash content ca 1.5%. The charcoal (sp. grav. = 0.3–0.5) has a calorific value of 6,600 kcal/kg, with an ash content of 0.4%.
The most serious disease is disback, caused by Phoma herbarum. Other fungi attacking black wattle include: Chaetomium cochliodes, Daldinia sp., and Trichoderma viride. In Rio Grande do Sul, disease and insects cause about 20% loss of trees. Principal insects attacking Brazilian wattle are Molippa sabina, Achryson surinamum, Placosternus cyclene, Eburodacrys dubitata, Neoclytus pusillus, Oncideres impluviata, Oncideres saga, and Trachyderes thoracica. Ants, termites, and borers are the most damaging. The sauva ant which attacks the leaves is fought constantly with arsenicals and carbon disulfide. Nematodes reported on this species include Meloidogyne arenaria, M. incognita acrita, and M. javanica (Golden, pers. commun. 1984).
- Duke, J.A. 1981a. Handbook of legumes of world economic importance. Plenum Press. NewYork.
- Little, E.L. Jr. 1983. Common fuelwood crops: a handbook for their identification. McClain Printing Co., Parsons, WV.
- NAS, 1980.
- N.A.S. 1980a. Firewood crops. Shrub and tree species for energy production. National Academy of Sciences, Washington, DC.
- Sherry, S.P. 1971. The black wattle (Acacia mearnsii de Wild.). University of Natal Press. Pietermatitzburg.