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Acacia tortilis (Forssk.) Hayne |
   
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LeguminosaeSynonyms
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Author: Le Houérou |
DescriptionGregarious, wide-spreading, flat-topped or umbrella-shaped tree, up to 4 m high; branchlets pubescent, red-brown; spines mixed, some white, straight, slender, and up to 7.5 cm long, others grey with black or brown tips, sharply curved, very small. Pinnae in three to ten pairs, leaflets in 7 to 15 pairs. Flower heads white to cream. Pod yellow-brown, pubescent, spirally twisted, slightly constricted between the seeds, circular in cross-section, 7.15 to 15 cm long, 0.6 to 0.8 cm thick (Andrews, 1952). As for A. nilotica the main differentiating traits are: the size and shape of the pods, the presence or not of glands on them, their pubescence or glabrescence and the hairiness or glabrescence of twigs. The situation is further complicated by the existence of geographic varieties within each subspecies, e.g. a glabrous pods subsp. may have a pubescent variety, e.g. subsp. raddiana var. pubescens that may result from hybridization between subsp. tortilis and subsp. raddiana in the areas where their distribution overlap. From the indications given above the Sahel tree belongs to subsp. raddiana, characterized by glabrous pods, but subsp. tortilis, characterized by pubescent pods, is also present in the eastern Rep. of Sudan, the Nile valley and the Red Sea shores (Brenan, 1983). Description for A. tortilis subsp. raddiana : A tree that may reach 20 in height with an umbrella-shaped and flat top canopy in the adult, but an intricate bush growth in the young, stem and branches are dark brown in the mature and reddish brown with grey lenticels in the young. Thorns in axillary pairs, straight, 2-10 cm long, white, some rare recurved prickles are sometimes present aside of the axils. Leaves are bipinnate with 2-5 pairs of pinnulae and 6-15 pairs of dark-green leaflets on each. Flowers are axillary,white to pale yellowish, very fragrant, set in globose heads about 1 cm in diameter, on a peduncle ca 3 cm long. Pods are spiralled 10-15 cm long and 5 mm wide with a circular section and glabrous or nearly so, green when immature and pale-orange to light-brown at maturity, containing up to 10 seeds with a dark oval depression on both lateral faces, seeds are often attacked by weevils, which do not prevent germination, quite the opposite, as long as the embryo is not damaged. Key to identifcation of subsp.
TemperatureFound in areas with "m" > 5°C. ["m" refers to the mean daily minimum t° of the coldest month]. WaterFound in areas with MAR < 400 mm. In the desert (MAR < 100 mm) A. tortilis behaves as a phreatophyte, with aquifers as deep as 40 -50 m or more, e.g. the former famous tree of the Ténéré, SW of Bilma, in NE Niger was in an area under a MAR < 20 mm, no runoff, with a water table ca 45 m below the ground surface, this tree was unquestinably getting its water from this aquifer, this famous tree, unique over several hundred km of absolute desert, was broken down by a backing military truck in the 1960's. DistributionOccurs in the drier areas of northern Africa, Senegal to Nigeria, in the Sudan, Kenya and Tanzania; often on flat alluvial areas. In Kenya it occupies the desert grass-bush zones at altitudes of 1200 m. Distribution of subsp.; subsp. tortilis: East Sahel and Nile Valley, the Horn of Africa, Israel, Jordan, Arabian Peninsula; subsp. heteracantha (Burch.) Brenan : Southern Africa; subsp. raddiana (Savi) Brenan : Sahara, Sahel, North Africa, West Asia; subsp. spirocarpa (Hochst. ex A. Rich.) Brenan) East and South East Africa and Namibia.
"m" connotes the mean minimum temperature of the coldest month, which is ca 10°C above the absolute minimum under shelter. PropagationLamprey (1967) found in the Tarangere Game Reserve in Tanzania that seedlings of Acacia tortilis could only be found growing from the faecal pellets of the impala. He obtained no germination of seed from fallen pods, but 7 percent from seed collected from faecal pellets. Goat enclosures at Maswa, Tanzania, also have seedlings germinating from the dung, and Lamprey believes that such indehiscent pods must pass through an animal for regeneration to take place. Plantation by direct seeding of treated seeds should always be preferred in order to avoid the trauma of transplanting and disturbing the tap-root that grows about ten times faster than the stem (this principle applies to many desert trees and shrubs that share this fast root growth characteristic). VariabilityThe taxonomy of A. tortilis is just as complex as that of A. nilotica ; A. tortilis has 4 recognized subspecies in Africa (Brenan, 1959, 1983 ) 3 are tetraploid, 4 n = 52 whilst subsp. raddiana is octoploid, 8 n = 104 (Fagg, 1991, Wickens, et al. 1995). Products & usesLeaves of young trees are browsed by goats and sheep, but the main value of this species is in its pods, which can be very numerous and are picked up from the ground and eaten by all African livestock. At the time when pods are mature (usually in January-February in Kenya), they are often the main source of food for cattle, sheep and goats. Timber, poles, posts, fuel, charcoal. Medicinal uses : vermifuge, skin infections, oedema and allergic dermatoses, pods, bark and roots for tanning. Nutritional Quality and Animal ProductionExcellent browse ca 16 % CP / DM, NE ca 6.0 MJ / kg DM, P ca 0.20 % DM ; see the remarks on pod feed value under Prosopis juliflora. Links:
Links for the genus:
ReferencesBrenan 1959, Brenan 1983 ; Catinot 1967 ; Ross 1979 ; Geerling 1982/88 ; Baumer 1983 ; El Amin 1973 ; El Amin 1990 ; Giffard 1974a, Giffard 1975 ; Delwaulle 1978 ; Delwaulle 1979 ; Le Houérou 1980a ; Le Houérou 1980c ; Le Houérou 1980d ; Von Maydell 1983/86 ; Fagg 1991 ; Fagg & Stewart 1994 ; Burkill 1995 ; Wickens et al. 1995 ; Dommergues et al. 1999. |