Acacia senegal ssp. senegalensis (Houtt.) Roberty

Acacia senegal is a deciduous shrub, growing to 15 m tall and usually branched from the ground. Branches fork repeatedly and in mature trees commonly form a rounded, flat-topped crown. The trunk may vary in diameter up to about 30 cm. The bark is greyish-white, although in old trees growing in the open it may be dark, scaly and thin, showing the bright green cambium layer just below the surface if scratched with a nail. The slash is mottled red. Powerful hooked thorns, 3-5 mm long, with enlarged bases appear at the nodes of the branches, usually in 3s. They are sharp, with some pointing forwards and others backwards. Leaves bipinnate, 3-8 pinnae (glands between uppermost and lowermost pinnae); rachis up to 2.5 cm long; pinnacles are pairs of 8-15, green; 2 stipular spines strongly recurved with a 3rd pseudo-stipular between them. Flowers yellowish-white and fragrant, in cylindrical, axillary pedunculate spikes, 5-10 cm long; calyx of each flower has 5 deep lobes, 5 petals and a mass of short stamens; pistil inconspicuous. The pods are straight, thin, flat, shortly stipitate and oblong (7.5 x 2 cm), green and pubescent when young, maturing to shiny bronze, often with dark patches and bearing prominent veins; seeds 3-6, smooth, flat, rather small, shiny, dark brown. Varietal differences in A. senegal are based on variation in natural distribution as well as differences in morphological characteristics such as the presence or absence of hair on the axis of the flower spike, colour of the axis, shape of pod tips, number of pinnae pairs, occurrence of a distinct trunk and shape of the crown. Four different varieties of Acacia senegal are recognized: var. senegal, var. kerensis Schweinf., var. rostrata Brenan and var. leiorhachis Brenan. The generic name ‘acacia’ comes from the Greek word ‘akis’, meaning a point or a barb.

A. senegal is presumably insect pollinated. Flowering starts from June to July in Sudan, December to January in South Africa, February to March in Pakistan, and August to December in India. Fruits ripen in January in Burkina Faso, July-September in Kenya, August in Pakistan, October in South Africa, and November-December in southern and central Niger. The wind shakes seeds from the dehiscent pods, and sheet wash and grazing animals may extend the seed dispersal range.

Varies from coarse-textured, deep sandy soils to dry, rocky soils, slightly acidic to moderately alkaline. It is intolerant to waterlogging.

Drought-tolerant, A. senegal is the characteristic species in the drier parts of Anglo-Egyptian Sudan and the northern Sahara and is to be found throughout the vast area from Senegal to the Red Sea and to eastern India. It extends southwards to northern Nigeria, Uganda, Kenya, Tanzania and southern Africa. In India it is found chiefly in Sind and Ajmere. In Sudan, the tree both exists in the wild and is cultivated - mainly on sandy hills, but it also grows well in cotton soil. A. senegal is associated with a wide range of vegetation types, from semi-desert grasslands to Anogeissus woodlands. It prefers clay plains and rocky hill slopes. It can easily become a serious weed. South Africa and Australia have policies against further introduction of this noxious weed.

Sudan gum arabic has been an article of commerce for some 2000 years. Originally the gum was sent to Arabian ports, and then to Europe, hence the name ‘gum arabic’. In the Middle Ages, it often went first to Turkish ports and so acquired the name ‘Turkey gum’, which was once commonly used for it in the trade. Today, A. senegal is grown primarily for gum, but plays a secondary role in agricultural systems, restoring soil fertility and providing fuel and fodder.

Fresh seed requires no pretreatment if sown immediately after harvest. Seed collected in previous seasons, however, requires pretreatment to break seed dormancy. Soaking seed in water for 12-24 hours gives good results and is simple to apply. Seeds can also be nicked. More mature seeds can be immersed in concentrated sulphuric acid for 3-15 minutes or dropped in boiling water for 5 seconds. Artificial regeneration by way of shoot cuttings is possible. With vegetative propagation, seedlings grow best when they are watered every 7th day. Some grass cover or mulch seems necessary for optimal seedling establishment. A. senegal is usually raised in the nursery in polyethylene pots, 2-4 seeds per pot, thinned to one seedling after 4-6 weeks. Direct seeding (5-8 seeds in 30 x 30 x 30 cm pits or larger) can also be used. Strict protection from fire, livestock grazing and efficient control of weed competition during at least the first two years is important to seedling survival. Minimum spacing for block planting is 4 x 4 m. At 10 x 10 m spacing, agricultural intercropping is possible, for example, interplanting with millet, beans, or groundnuts.

Weeding should be immediate in the season following planting and for the following 2 growing seasons. Protection of young trees from livestock browsing is essential. Seedlings are rather slow growing with initial annual increments of 30-40 cm. For firewood production, trees can be cut on a rotation of 20 years using coppice with standards, lopping up to 50% of the lower crown in rainy season.

The buffalo treehopper (Stictocephala bubalus) may destroy seed crops. Spiders (Cyclops sp.) can smother young growing apices. Larval stage of Coleoptera (bruchids), Lepidoptera, and Hymenoptera damage the seed. Locusts (Acridium melanorhodon) can defoliate vast areas overnight. A. senegal is also attacked by the fungi Cladosporium herbarium, Fusarium sp., Ravenelia acaciae-senegalae and R. acaciocola.

Seed should be harvested before pods have dried for easy collection and to avoid insect attack. Seed is easily extracted by hand. Freshly extracted seed should immediately be dusted with an insecticide. There are 10,000-30,000 seeds/kg. Seed storage behaviour is orthodox; 5% germination following 51 years open storage at room temperature; viability can be maintained for 6 years with air-dried seeds stored at room temperature; viability can be maintained for several years in hermetic storage at 10 deg. C with 4.5-9% mc.

Erosion control: A. senegal has been used for desertification control, re-establishment of a vegetative cover in degraded areas, sand dune fixation and wind erosion control. The rostrata and leiocarpa varieties are useful for soil stabilization because of their weediness. Nitrogen fixation: The nitrogen-fixing ability of A. senegal is questionable. Soil improver: Improves soil fertility by nutrient cycling after leaf fall. Ornamental: It is grown as an ornamental tree in Puerto Rico and the Virgin Islands. Boundary or barrier or support: Cut branches are useful for fencing in cattle because of their spines. Intercropping: It is a highly suitable tree for agroforestry systems, widely grown in combination with watermelon, millets and forage grasses. In the Sudan, A. senegal is grown in 'gum gardens' for gum as well as to restore soil fertility. A traditional bush-fallow system is followed with a 20-year rotation during which time A. senegal is grown for 15 years. Agricultural crops are grown for five years (millet, sesame, sorghum, groundnuts), followed by five years with young, unproductive A. senegal trees, which later produce gum during the last 10 years of the rotation. Corresponding to this rotation, 1/4 of the land is kept in agricultural crops, 1/4 in young unproductive trees and 1/2 in productive trees.

Food: The seeds are dried and preserved for human consumption as a vegetable. The dried seed is the main component of panchkut, a delicacy in Jodhpur, India, also containing fruits of Capparis decidua, Cucumis sativa and Prosopis cinerearia. Fodder: Leaves and pods are browsed by sheep, goats and camels. Crude protein values are 20% for leaves, 22% for green pods, and 20% for dry pods (expressed as a percentage of dry matter). Apiculture: Bees seek the nectar from the flowers. Honey amber in colour, with a mild aroma and granulates rapidly. Fuel: An excellent fuelwood-sometimes the only wood species to survive in dry areas. The calorific value is estimated at 3000 kcal/kg. Wood yields of 120-190 cubic metres per hectare, with annual increments of 0.5-1.0 cubic metres per hectare have been recorded. The dense wood also yields charcoal. Fibre: The long, flexible surface roots yield a strong fibre used for cordage, ropes and fishing nets. Timber: The heartwood is almost black and takes polish well. It is used for making carts and Persian wheels, sugar cane crushers, agricultural implements, horse girths and tool handles. Gum or resin: Gum arabic exudes from the duct of the inner bark; it is tapped in the hot season (May-June) when the trees are stressed. Tapping begins when trees are 4-5 years old. It commences after leaf fall and ceases during the colder months of the dry season. Gum nodules form in 3-8 weeks, exuding from the former broken abscission scars. Care should be taken to keep the gum clean. Annual yields stand at 188-2856 g for young trees and 379-6754 g for older trees (7-15 years). Gum production is excellent on poor soils and higher in stressed trees. Essential oil: Seeds contain fat (khakhan), which is used both for medicine and for soap making. Medicine: Roots are used to treat dysentery, gonorrhea and nodular leprosy.