Contents - Previous - Next

Part four: Selected species of Acacia

4.1 Acacia Nilotica
4.2 Acacia Senegal
4.3 Acacia Tortilis
4.4 Faidherbia Albida
4.5 Exotic Species of Acacia

Four widely distributed, multipurpose species are discussed, A. nilotica, A. senegal, A. tortilis and F. albida. Their overall uses are presented in Annex D, of which their major uses are respectively for timber, gum arable, browse, and agroforestry and browse. The introduced species are also briefly discussed.

4.1 Acacia Nilotica

A polyploid complex (Fagg, 1992) within which nine subspecies are recognized with more or less recognizable distinctive geographical ranges (Brenan, 1983):

There is considerable variation in habit ranging from the ovoid-crowned, evergreen or semi-evergreen, subsp. nilotica, tomentosa and indica to the deciduous, flat and spreading crowns of. subsp. adstringens, subulata and kraussiana, while subsp. cupressiformis possesses a narrow, erect crown reminiscent of a Lombardy poplar and subsp. hemispherica forms a hemispherical bush. Despite this wide range of habit and other taxonomic characters authors rarely note the subspecies when reporting on their utilization. This is regrettable since there are probably differences in tannin content between subspecies which affects their value as browse and tanning agents, as well as other attributes. Their ecology also differs, including their ability to withstand waterlogging.

Subspecies nilotica, tomentosa, indica and cupressiformis are grown under cultivation, the first three largely for timber, fuel and tannin while subsp. cupressiformis is finding increasing popularity in India for windbreaks around fields, its narrow crown casting less shade than other available windbreak trees. In India subsp. indica is extensively grown on degraded saline or alkaline soils with a pH 9 and a soluble salt content below 3% (Fagg, 1992). It is also grown in agroforestry schemes despite reports of lower crop yields from beneath its canopy. This is reported to be caused by competition for water (Adjers and Hadi, 1993).

4.2 Acacia Senegal

A widespread and rather variable species within which four varieties are recognized (Brenan, 1959, 1970, 1983; Cossalter, 1991):

The gum exudate from var. senegal, is the major source of commercial gum arabic and is the variety that is being planted throughout the Sahel. According to Anderson and Weiping (1990) cited by Anderson (1990) each variety has a distinct and characteristic gum exudate: However, this does not affect the current status of gum arabic as defined by FAO (1990) and WHO (1990a) and discussed in section 2.2.2

4.3 Acacia Tortilis

A polyploid species (Fagg, 1991) within which four, more or less geographically demarcated subspecies are recognized in the arid and semi-arid lands of Africa and the Near East (Brenan, 1983):

Two varieties are recognized within both subsp. raddiana and subsp. spirocarpa, vars. raddiana and pubescens, and vars. spirocarpa and crinita respectively. Furthermore, subsp. raddiana is regarded by some authorities as a distinct species. Its recognition as a subspecies is justified on the grounds that var. pubescens would appear to be the putative hybrid between subsp. raddiana and subsp. tortilis (Brenan 1983).

A multipurpose tree whose utilization appears to be independent of any recognition of subspecific and varietal categories, although in the absence of trials it is uncertain whether the intraspecific variations have any great ecological significance.

4.4 Faidherbia Albida

The unusual behaviour foliage being present throughout the dry season and being shed at the start of the rainy season and the associated benefits of this tree for fodder and increased crop production are well documented (Wickens, 1969; Nongonierma, 1979; Baumer, 1983; Koné, 1986; Bonkoungou, 1987; CTFT, 1988; Vandenbeldt, 1992, etc.). The behaviour occurs throughout its distribution range on either side of the equator; in areas of bimodal rainfall there two changes of foliage may occur annually (Wickens, 1969).

The natural distribution would appear to be riverine and where, like A. nilitica subspp. nilotica and tomentosa, the root system is almost entirely below the water table during the rainy season (Wickens, 1966). The species also occurs inland, where it is not subjected to any seasonal inundation of the root system. It is possible that two races may have developed, with anaerobic or aerobic root respiration as the norm. It is suggested that this may be one of the factors why some provenances have failed in trials when introduced outside their normal habitat.

Faidherbia albida, which is among the five most common farmland trees around Kano, Nigeria, supplies 25% of the local firewood from regular loppings; dead wood from young trees that have died from drought is also collected; older trees were apparently less susceptible. The species is not regarded as a fuelwood source (Cline-Cole et al., 1990); however, given its availability it has been used more and more as other resources have been steadily depleted.

There have been only a few attempts at replanting beyond traditional establishment by farmers in the Sahel (Senegal, Niger, Burkino Faso, etc.) since there is adequate natural regeneration. Reports on natural regeneration of Faidherbia albida in Senegal have mentioned seed germinating in cattle dung. This is in direct contradiction to the findings in Darfur, Sudan (Wickens, 1969) where all the F. albida seed observed germinating in cattle dung became etiolated and soon died. The Wadi Azum basin in Darfur is regarded as one of the best forage areas in the Sudan and it is suggested that the excellence of the forage provided softer faeces and more readily available plant nutrients as well as rapid moisture changes, all factors combining to prevent healthy germination and establishment; it is further hypothesized that a drier, more fibrous diet would would produce faeces more suitable for plant growth. The observations by Lamprey (1967) and Koné (1986) regarding the natural regeneration of Acacia species in dung would support such a hypothesis.

4.5 Exotic Species of Acacia

A number of phyllodenous Acacia species from Australia have been introduced into Africa and the Near East, mainly for fuelwood, soil stabilization and browse (Vercoe, 1987). More recently there has been increasing interest in Acacia seeds for food in dryland tropical Africa (House and Harwood, 1992), especially in view of their ease of cultivation and heavy yields of palatable seeds that can be easily stored for long periods. The seeds are high in protein, carbohydates and fats and with low levels of known toxic and anti-nutritional factors. Of particular interest are A. colei, A. cowleana and A. tumida (Harwood, 1993). Other Australian species with edible seeds that have been introduced into Africa and the Near East include A. adsurgens, A. ampliceps, A. aneura, A. eriopoda, A. holosericea, A. kempeana, A. ligulata, A. neurocarpa, A. salicina, A. sclerosperma and A. victoriae (Devitt, 1992). The seeds of the Central American A. farnesiana are also edible.

According to Goodchild and McMenimmam (1987) the browse from the phyllodenous species tend to have higher crude fibre, lower crude protein and phosphorous content and lower organic matter digestibility than other members of the genus. Because of their evergreen phyllodes such species are valued primarily as a drought fodder reserve for when other fodder is in short supply (Thomson et al., 1994). They are particularly valued in the drylands of Africa and the Near East.

Recent re-appraisal of the taxonomy of A. holosericea complex have resulted in the recognition of the diploid A. neurocarpa, the tetratraploid A. holosericea and the hexaploid A. colei, the putive hybrid between A. neurocarpa and A. cowleana (tetraploid) (Maslin and Thompson, 1993). The significance of this re-appraisal is that previous reports on the performance of A. holosericea sensu lato have to be reconsided. A. colei has shown outstanding growth in West Africa, being more drought tolerant than either A. holosericea and A. neurocarpa and with a vast potential as a new, highly nutritious food crop for dry, sub-Saharan Africa. There are now increasing plantings of this species for windbreaks, land rehabilitation and fuelwood production. Other promising multipurpose species with edible seeds include A. cowleana and A. tumida. The oils from the arils of both A. colei and A. cowleana have the additional economic potential as solvents of ballpoint pen ink (Thomson, 1992).

A number of Australian species were introduced into tropical Africa some 20 or more years, mainly via North Africa. The results were discouraging, in some cases resulting in the death of the plants under trial. More recent introductions into West Africa have included A. bivenosa, A. colei (as A. holosericea), A. coriacea, A. sclerosperma, A. trachycarpa and A. tumida. Trials in Senegal have shown very favourable biomass production at 40 months compared to indigenous species, the results of which also include thinning operation modifying the original spacing from 3 x 3 m to 3 x 6 m (Cossalter, 1987). The results are shown below:

Interestingly, A. trachycarpa is not recognized as a browse species in Australia yet it is readily browsed by sheep and goats in West Africa (Cossalter, 1987).

Despite the success of the trials in Senegal, results further inland in the Sahel have been disappointing. Provenance collections from inland Australia are recommended (Cossalter, 1987, Souvannavong and de Framond, 1992). However, according to Le Houérou al. (1993) species of Acacia section Phyllodineae cannot be grown in the Sahel except in the Senegal coastal strip because of the very high saturation deficit (SD) and low relative humidity (RH) irrespective of the mean annual rainfall, whereas they can be grown in the arid and semi-arid regions of North, East, southeastern and southern Africa. In such areas the SD is generally below 12 hPa and the RH above 60%, i.e. SD less than half and RH more than double that for the Sahel, with similar figures for the Sahelian coastal strip of Senegal. There is obviously a physiological problem involved concerning evapotranspiration and, if provenance collections are to be made from inland Australia, due consideration must be made to SD and RH values for that continent.

Because of their diminishing indigenous tree resources, all the North African countries now rely on exotic species for timber and other tree products (El-Lakany, 1987). A number of Australian acacias have been introduced into Africa and the Near East, including. aneura, A. cyclops, A. ligulata, A. pence, A. salicina, A. saligna, A. victoriae as well as A. farnesiana from Central America, of which A. saligna is regarded as the most important of the introduced species on account of its tolerance to moderate drought, ability to grow on poor soil and bind sand, high biomass production, high nutritive value for sheep and goats, ease of establishment and management and response to irrigation (El-Lakany, 1987).

Contents - Previous - Next