Agroforestree Database

Local names:
 Amharic (mimosa), English (green wattle,tan wattle,Australian acacia,black wattle,late black wattle), French (acacie noir), German (gerber- Akazie), Indonesian (akasia), Italian (Mimosa di Mearns), Japanese (wattor-ju), Spanish (acacia negra,Aromo negro)

Acacia mearnsii is a small to large, evergreen, single-stemmed or multi-branched tree, 6-25 m high, with a straight trunk, growing to 50 cm in diameter; crown low, spreading, rounded; spines absent; bark brownish-black, hard and fissured; twigs angled, grey, densely hairy, tinged with yellow when young. Trees in their natural habitat have a spreading, rounded crown, but are erect and slender when crowded in plantations.

Leaves dark green, alternate, feathery, with very soft hair, binnately compound, 8-15 cm long, with 8-21 pairs of pinnae 2-5 cm long, a round gland at base of each pair on upper surface; leaflets very numerous; 20-70 pairs of pinnae on each axis; small, crowded, narrowly oblong, 3 mm long, blunt, with dense soft hairs, yellowish when young becoming dark green, 1.5-4 x 0.5-0.75 mm; petiole 1.5-2.5 cm long, often with a gland above; rachis usually 4-12 cm long, with numerous raised glands all along its upper side both at and between insertions of pinnae pairs. The tree exhibits a superficial root system.

Flower clusters (racemes) along axis at leaf base or terminal, composed of many (20-30) stalked, pale yellow balls (heads) 7-8 mm in diameter; flowers many, tiny, very sweet scented, composed of narrow 5-lobed calyx; corolla of 5 petals; stamens many, threadlike, pale yellow; pistil with long, slender style 2-6 mm long.

Pods (legumes) narrowly oblong, flat, rough, blackish, with fine hair, fairly straight, 5-15 cm long by 4-9 mm wide but often constricted between the seeds, almost moniliform (in Australia pods less moniliform and almost glabrous are found), dehiscing along 1 margin; seeds about 1-14, longitudinal in the pod, beanlike, elliptical, flattened, blackish, 4 mm long; caruncle conspicuous; areole 3.5 x 2 mm.

The generic name ‘acacia’ comes from the Greek word ‘akis’, meaning a point or a barb.

The specific name is after A.R Mearns (1856-1916), an American army surgeon who collected the type specimen form a cultivated tree near Thika in Kenya. The name was first published in Pl. Bequaert in 1925.

Ecology

In Hawaii, A. mearnsii is a noxious weed and spreads prolifically between 600 and 1200 m in the 1000-1200 mm rainfall zones. A. mearnsii prefers a moderate climate, exhibiting great intolerance to extreme heat or cold. Its lower altitudinal range is decided by the fact that trees cannot stand high summer temperatures, and the upper altitudinal limit is based on the fact that the tree does not tolerate temperatures below 0 deg. C. Winter frosts and cold winds during the early part of the rainy season affect growth and survival rate, but older trees can withstand mild frost. Localities experiencing severe hailstorms and snowfall are unsuitable. Adequate soil moisture is a prerequisite for satisfactory growth. Trees cannot withstand drought because of their superficial root system and high rate of transpiration.

Native range
Australia

Tree management

The trees have strong light requirements and respond to thinning in the early years. Growth rate is comparatively slow for the 1st 1 or 2 years. Thereafter, both height and diameter increments are rapid up to the age of 6-7 years, after which they fall off gradually. A. mearnsii has low coppicing power, discouraging people from propagating through coppicing. Protection of trees from fire is necessary, as fire may scorch the bark and reduce its value. A. mearnsii is easily killed by frost during the 1st 2-5 years of its life, although stands fertilized with super-phosphate are more resistant.

Harvesting 10-year-old trees results in the removal of large quantities of soil nutrients. Therefore, before planting, NPK fertilizers are added; application of lime has also been found to considerably increase yield. Taproot development largely depends upon the depth of the soil, but it has the general tendency to develop a superficial lateral root system. Because of this, trees are liable to being uprooted by strong gales during the monsoon season. Therefore windbelts of Eucalyptus globulus (blue gum) and Pinus patula could be planted at the periphery -- 5 rows of E. globulus alternating with 50 rows of A. mearnsii. Eucalyptus globulus may be planted 3 years before A. mearnsii. As the natural regeneration in clear-felled areas is very dense, thinning is done in the 3rd year. Trees are planted at spacing of 2.5-3.3 x 2.5-3.3. The optimum economic densities for the ages of 7, 9 and 11 years would be 1977-2224, 1483-1730 and 1236-1483 trees/ha, respectively.

Seed storage behaviour is orthodox. Viability is maintained for more than 2 years in hermitic storage at room temperature with 13 + or - 2% mc; 63% germination after 17 years open storage; no loss in viability following 17 years in bottle storage at room temperature; a few seeds survived 23 years in a herbarium; viability can be maintained for several years at 10 deg. C with 4.5-9% mc. There are about 66 000-88 000 seeds/kg.

In Hawaii, A. mearnsii is a noxious weed and spreads prolifically between 600 and 1200 m in the 1000-1200 mm rainfall zones. A. mearnsii prefers a moderate climate, exhibiting great intolerance to extreme heat or cold. Its lower altitudinal range is decided by the fact that trees cannot stand high summer temperatures, and the upper altitudinal limit is based on the fact that the tree does not tolerate temperatures below 0 deg. C. Winter frosts and cold winds during the early part of the rainy season affect growth and survival rate, but older trees can withstand mild frost. Localities experiencing severe hailstorms and snowfall are unsuitable. Adequate soil moisture is a prerequisite for satisfactory growth. Trees cannot withstand drought because of their superficial root system and high rate of transpiration.

A. mearnsii regenerates naturally from seed after burning in clear-felled plantations. Seed may lie dormant in the soil for up to 6 years without loss of viability. Profuse seed production and prolonged viability of seed are the features particularly suitable for the silvicultural system of clear felling with natural regeneration. However, to obtain a crop by this method, controlling grass and frost damage is necessary. In practice, after the extraction of bark and timber, the remaining slash is spread along the contours and is lightly burnt. Controlled burning not only serves to keep the grass under control but also scorches the hard testa of A. mearnsii seed, thus creating conditions favourable for profuse regeneration. Apart from introducing a light fire, it is desirable to improve seedbed conditions by working the soil lightly in well-spaced, narrow strips. Also, soon before the onset of dry and hot weather, weeding and cleaning to conserve moisture and eliminate fire are essential. There has been a great deal of controversy on the benefits of burning, as it destroys humus and microorganisms in the soil, and it increases soil erosion and run-off. Profuse regeneration also produces intense root competition and creates problems for thinning.

Up to 1956, artificial regeneration involved using the direct sowing method. Subsequently the technique was changed to planting seedlings, which gives uniform stocking and better initial growth. Recently the technique is mainly being replaced by planting seedlings in polythene bags. Seed pretreatment involves mechanical scarification, but seed can also be immersed in very hot water (90 deg. C) for 30-60 seconds followed by rinsing in cold water. Alternatively, seeds may be immersed in boiling water for 5 minutes, which is as good as soaking them for 24 hours. A germination rate of 75% occurs in 7-15 days. Standard nursery beds are 10 x 1 m. About 300 g of seed is sown per a standard bed, giving about 2500 seedlings for planting out. The sown seeds are covered with bracken fronds to protect the beds from excessive evaporation and runoff during watering. The bracken cover is removed after germination. Seedlings are pricked out when they are 5-6 cm tall with 3 or 4 pairs of leaves, planted into polythene bags 13 x 8 cm, and watered daily until out-planting. It is essential to shift the seedlings frequently to prevent them from striking roots in the ground. The seedlings should be about 9 months old at the time of planting. Plantations should be closed to grazing for the 1st 3 years of the trees’ lives. Vegetative propagation by rooted cuttings is difficult. Seedlings are sensitive to competition from grasses and weeds, and they should be protected from frost, cold winds and fires. If the seedlings grow under these adverse conditions, the actual yield may drop to about 25% of the original yield.

Erosion control: Wattles grow well at high elevations even on slopes with shallow or poor acid soils that are unstable and will not support agricultural crops. They can therefore be very effective in preventing soil erosion. Densely packed plantations have proved effective in preventing further erosion, even on hillsides of up to 50 degrees slope.

The leaves have a high protein content (about 15%). Palatability trials with sheep showed milled leaves to be unpalatable on their own and were acceptable only when mixed with other feedstock. In Hawaii, A. mearnsii has been fed to cattle during drought periods.

Apiculture:  The extra floral nectaries of A. mearnsii (containing about 20% pollen protein and 40% sugar) and its late flowering makes the tree a suitable bee forage.

Originally distributed as a source of tannin, black wattle is now recognized as a valuable fuel wood. Wood is moderately dense with specific gravity about 0.75, splits easily and burns well with a calorific value of 3500-4600 kcal/kg. The charcoal is extensively used in Brazil and Kenya, and in Indonesia the tree is extensively used as a domestic fuel and for curing tobacco.

Fibre:  The pulp productivity of A. mearnsii is about 320 kg/cubic m. Testing has shown that A. mearnsii yields a pulp with good strength characteristics and is suitable for wrapping paper and hardboard. It is also used for rayon.

Timber:  The wood is moderately hard to hard, light yellowish to light red, heavy, durable, fairly tough and strong, with a specific gravity of 0.7-0.85; it is moderately easy to work and polishes well. It is used for house poles, mine props, tool handles, cabinet work, joinery, flooring, construction timber and matchwood.

Shade or shelter:  The species has been planted as a shelterbelt, a firebelt and as a shade tree in plantations.

Tannin or dyestuff: Wattle bark is the most widely used tannin material in the world. It contains 30-45% (dry basis) high-quality tannins that are used in tanning many classes of skins and hides for many different classes of leather. Such tannins are particularly effective on hard leathers for shoes and saddles. They give better color to leather than other tannins, do not precipitate in acid solution, and penetrate hides faster. A powdered bark extract is also used to prepare tannin formaldehyde adhesives for exterior grade plywood, particleboard and laminated timber.

Nitrogen fixing:  It is an effective nitrogen fixer and has an annual yield of wet leaves of 21-25 t/ha, containing 240-285 kg of nitrogen.

Ornamental:  A. mearnsii is an attractive tree, flowering profusely in late winter.

Soil improver: An efficient nitrogen-fixer and good source of green manure, it thus can restore and regenerate soils. 

Intercropping:  In India, A. mearnsii is sometimes raised in taungya systems in conjunction with potato crops.