Acacia nilotica (L.) Willd. ex Del.

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Leguminosae

Synonyms

  • Acacia arabica (Lam.) Willd.
  • Acacia subalata Vatke.
Author: Le Houérou

Common names

Prickly acacia (Australia), mgunga (East Africa; ol'erbat in Masai), babul (Jhansi, India).

Description

Tree 5-20 m high with a dense spheric crown, stems and branchlets usually dark to black coloured, fissured bark, grey-pinkish slash, exuding a reddish low quality gum. Thin, straight, light, grey spines in axillary pairs, usually in 3 to 12 pairs, 5 to 7.5 cm long in young trees, mature trees commonly without thorns. Leaves bipinnate, with 3-6 pairs of pinnulae and 10-30 pairs of leaflets each; leaflets 4-5 mm long and + / - tomentose, rachis with a gland at the bottom of the last pair of pinnulae. Flowers in globulous heads 1.2-1.5 cm in diameter of a bright golden-yellow color, set up either axillary or whorly on peduncles 2-3 cm long located at the end of the branches. Pods grey, thick, softly tomentose, straight or slightly curved, 5 to 15 cm long on a pedicel, 0.5 to 1.2 cm wide, with constrictions between the seeds giving a necklace appearance, fleshy when young, becoming black and hard at maturity (Andrews, 1952). Deeply constricted in a necklace fashion in subsp. tomentosa, lightly so in subsp. adstringens and glabrous but moniliform (in "pearl string") in subsp. nilotica.

Subsp. nilotica is characterized by glabrous, or nearly so, pods and twigs, while subsp. tomentosa has strongly constricted white-grey hairy pods and subsp. adstringens, also known as var. adansonii (Guill. & Perrott.) Kuntze, exhibits hardly-constricted or non-constricted, densely and persistently tomentose pods, and twigs.

Habitat

  • Subsp. tomentosa is closely tied to regularly flooded areas (3-4 months per annum in the lower Senegal river valley, 4-5 months along the Nile), with required fine textured alluvial clay soils, such as the food plains of the Senegal and Niger rivers and the central Chad Basin.
  • Subsp. adstringens occurs in the terraces above the flood plains with loamy to silty soils and infrequent floodings or no flooding at all, but with a deep water table.
  • Subsp. nilotica occurs in well drained situations along water courses such as on the river banks of the White and Blue Niles and Atbara.

Temperature

The species as a whole is quite tolerant to heat (> 50° C) and air dryness. It is frost sensitive, albeit subsp.subalata has been reported from elevations of 2,000 m in Kenya and therefore subject to light freezing (Dougall & Bogdan, 1958, cited by Baumer, 1983).

Soil

The species as a whole, and particularly subsp. tomentosa, is tolerant to water logging and submersion, to alkalinity ( pH > 9 ; ESP > 60 %) and fairly tolerant to salinity, as shown in India (Abrol, 1986 ; Singh, 1998 ; Singh & al., 1993). It requires permanent or sub-permanent soil moisture, for that reason it is found in wet soils even under climatically desert conditions of the South Sahara, South Arabian Peninsula and East Africa.

Distribution

Widespread in Africa and Asia, and occurs in Australia. In Kenya, it is mainly found at 900 to 2 000 m (Dougall and Bogdan, 1958). It is found in well watered Sahelian and Sudanian savannas to the southern Arabian Peninsula, East Africa and in the Gambia, the Sudan, Togo, Ghana, Nigeria, and on lateritic soil in the Himalayan foothills in India (Whyte, 1964).

Two subspecies are fairly common to the Sahel : subsp. tomentosa and subsp. adstringens, while subsp nilotica is restricted to the Nile valley and tributaries and to the SE Sahel in the Republic of Sudan.

Crop management

The beautiful native groves of the Senegal and Niger river valleys have been destroyed to a large extent over the past 3 decades by over-exploitation and other mis-management, or consecutive to the lowering or exhaustion of the water table or via well-wishing curbed floodings by diversion for irrigation ; similarly the formerly extensive stands along the Nile, and its tributaries have been cut for fuel and railway sleepers and have not had a chance to recover, according to Wickens, in Litteris.

In Maharashtra State, areas of A. nilotica are closed by law for regeneration in five-year rotations (Whyte, 1964). The removal of leaves from this species should be restricted to prevent damage to flower development and a consequent loss of pods.

In Queensland, cattle, sheep, and goats are fond of the plant, but because of the spiny nature of the young trees the species has been declared noxious throughout the state (Everist) 1969). Ivens (1970) has had some success in its control by hand digging, basal bark spraying with 2,4-D and with foliar spraying with a picloram/2,4-D mixture, but a number of treatments may be necessary.

Propagation

A. nilotica has been successfully planted in adequate sites in various Sahel countries, either through direct seeding of treated seeds (as for most acacias) or through nursery-grown seedlings in polyethylene bags.

Variability

Acacia nilotica is a complex species with 9 subspecies, of which 6 are native to the African tropics, 3 of them to the Sahel ; 3 others are native to the Indian sub-continent, of which 1 has been introduced to East Africa from India (subsp. indica). This complex taxonomy has been cleared by Brenan, 1983. The Brenan taxonomy is as shown below in a simplified fashion : Subspecies distribution, according to Brenan, (1983), for the most part.

  • nilotica Nile Valley, SW Sahel and Sudano-Guinean ecozone of W Africa
  • adstringens (Schum. & Thonn.) Roberty South Sahara, Sahel and Soudano-Guinean ecozones, E Africa.
  • indica (Benth.) Brenan India (Rajasthan), Yemen, Oman, Pakistan, Manyanmar (= Burma) ; introduced to E Africa (Somalia).
  • cupressiformis (J.L. Stewart) Ali & Faruqi NW India, Rajasthan
  • hemispherica Ali & Faruqi W Pakistan, Sind.
  • kraussiana (Benth.) Brenan E Africa (Kenya, Ethiopia,Somalia)
  • leiocarpa Brenan S & SE Africa (Zambia, Zimbabwe, Malawi, Mozambique, Botswana, Namibia, S Africa
  • subalata (Vatke) Brenan E & SE Africa (Uganda, Kenya, Tanzania, Somalia, Ethiopia)
  • tomentosa (Benth.) Brenan Flooding areas in the Sahel, Sudanian and Guinean ecozones N of the Equator to NW Uganda.

Note that subsp. subalata (meaning "almost winged"), is sometimes incorrectly written subulata (meaning "acuminate"), referring to the tip of the pods.

Polyploidy (Fagg, 1992, Fagg & Stewart, 1994, Wickens et al., 1995) :

  • subsp. tomentosa hexadecaploid, 16 n = 208, riverine
  • subsp. nilotica, octoploid, 8 n = 104, riverine
  • subsp.adstringens, tetraploid, 4 n = 52, sub-riverine savanna.

The other subsp. are tetra ploid, 4 n = 52, dry savannas and woodlands.

The complex taxonomy of this species is not considered further, it is, however, important to realize for the sake of comprehension of this complexity, that this taxonomy, by Brenan, is based on the combination of a few simple traits :

  • Habit of the trees canopy (e.g. umbrella, erect, spreading, hemispherical)
  • Pilosity of the twigs (glabrous, glabrescent, puberulous, pubescent, tomentose)
  • Shape of the pods (neck-lace, crenate, constricted, straight)
  • Margins of the pods irregularly shaped or regularly shaped
  • Pubescence of the pods (glabrous, glabrescent, puberulous, tomentose).

Products & uses

Leaves used for feeding sheep and goats in the Hissar district in India. In Kenya, the fleshy pods are readily eaten by goats, sheep and cattle, but some tribes believe they cause bloat. They are occasionally browsed by goats (Dougall and Bogdan, 1958). The wood is hard and heavy, difficult to work as it blunts tools for its high content in silica ; it is regarded, however, as excellent quality timber and service wood, poles, carpentry, boat and house construction, it is also considered a very good fire-wood and produces an excellent charcoal. Bark and pods are used in the tanning industry, leaves are readily browsed by stock, they have an average 12 % crude protein content. Young pods and seeds are eaten roasted by humans. Subsp. indica is regarded as an excellent forage tree producing large amounts of nutritious pods with little tannin. Bark and leaves are used to treat haemorrhage, colds, diarrhoea, scurvy, dysentry and ophtalmia etc.

Nutritional Quality and Animal Production

The leaves contain up to 12 percent crude protein and 21.35 percent crude fibre. In all subsp. pods are more or less rich in tannins (up to 32 % on the DM in subsp tomentosa), except in subsp. indica, and used for tanning (particularly subsp. tomentosa whose fresh pods are not palatable) and so is the bark. Subsp. indica, however, produces great quantities of pods with a low tannin content and for that reason has been introduced to North Somalia (Hargeisa, Burao, Las Anod) as a forage tree. Such introductions could also be done in the Sahel countries for the same purpose, in areas which are not fit for Faidherbia albida or A. tortilis.

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References

Brenan 1957a ; Brenan 1959 ; Brenan 1983 ; Catinot 1967 ; El Amin 1973 ; El Amin 1990 ; Kerharo & Adam 1974 ; Giffard 1974a ; Giffard 1975 ; Berhaut 1975 ; Delwaulle 1978 ; Delwaulle  1979 ; Ross 1979 ; Fagg & Greaves 1990 ; Geerling 1982/88 ; Baumer 1983 ; Von Maydell 1983/86 ; Fagg 1991 ; Fagg 1992 ; Fagg & Stewart 1994 ; Burkill 1995 ; Wickens et al. 1995 ; Vassal 1972 ; Vassal 1998 ; Dommergues et al. 1999.