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Part one: Acacia in the ecosystem

1.1 Acacia in Africa and the Near East
1.2 The ecosystems
1.3 Biomass production

1.1 Acacia in Africa and the Near East

There are c. 1250 species of Acacia, of which 134 species (represented by 170 taxa) are native of Africa, with 20 species (26 taxa) extending into Asia and 6 species (7 taxa) native to East Asia (Ross, 1979; Hassan and Styles, 1990; Lock, 1989, 1991). For the purposes of this present study Faidherbia albida (formerly known as Acacia albida) is also included here since its role in the rural economy is so similar to that of the Acacia that its omission by what are relatively minor taxonomic distinctions does not appear to be justified; never-the-less it is still treated throughout this document as Faidherbia albida. The accepted names of species cited in the text and their synonyms found in the consulted literature are listed in Annex A.

Despite this richness of Acacia species, relatively few appear to have been investigated. They are Acacia nilotica, A, senegal, A. tortilis and Faidherbia albida; their taxonomy and distribution are dealt with in Brenan (1983) and are further discussed in Part 4. This does not imply that the other species are worthless. Indeed, some are extremely important locally; all species make some contribution to the environment and rural economy by way of shade, shelter, soil stabilization and fertility, browse, fuel, etc. (Table 1.1).

1.2 The ecosystems

Approximately 55 per cent of the land surface of Africa is arid or semi-arid (Jahnke, 1982, cited by Seif el Din, 1991) and is illustrated in Annex B. These areas are characterized by annual rainfalls of less than 100 mm to c. 600 mm in a short season of 2-4 months. In some parts of Ethiopia, Kenya and Uganda a bimodal regime prevails and although the total annual rainfall may be higher, the distribution of the rainfall and the intensity of the dry season imposes a semi-arid regime. The total precipitation in the arid and semiarid regions varies widely from year to year and its distribution within a year is also variable. This low, erratic and poor distribution of rainfall means that the productivity of annual crops, especially in areas with less than 400 mm, is unpredictable, placing even greater value on perennial vegetation, especially multipurpose trees such as the Acacia.

The semi-arid ecosystems of tropical Africa are, with few exceptions, either thorn scrub or thorn savanna, with Acacia as the dominant species, either forming pure stands, e.g. Acacia mellifera thorn scrub, A. seyal thorn savanna, Acacia albida riparian woodland (now more correctly known as Faidherbia albida riparian woodland), etc. These ecosystems are important rangelands for the livestock industry, especially in those areas where A. tortilis is widespread.

Grasses and herbs alone cannot support a livestock industry in the semi-arid regions; browse, especially from Acacia species, plays an essential part. The dietary requirements of herbivores requires an intake of 20-25% browse, of which some 5% is consumed during the rainy season when herbage is relatively plentiful and 30% in the long dry season. Consequently the loss of these Acacia and other browse species would mean that herbivores would no longer be able to live through the dry season without costly and currently uneconomic food supplements. Livestock production is undoubtedly the most suitable industry for the utilization of the rangelands but full regard must be made for sustainability not exploitation. It has been suggested that in the Sahel, if the present trend in overexploitation and desertification continues unhindered, most of the browse species would disappear within the next 20 to 25 years and the livestock industry, which is the main economic activity would be seriously jeopardized (Le Houérou, 1989).

Pure stands of A. mellifera or, in higher rainfall areas, A. seyal often occur on clay plains. Such areas can be suitable for irrigation, as is the case of the so-called 'cotton soils', the Quaternary alluvial clay plains of the Blue and White Nile in northern Sudan. In recent decades large areas have been clear felled for the mechanized cultivation of irrigated crops of cotton, sorghum, groundnuts, sugarcane, etc. with disastrous results with regard to fuel wood resources and wind erosion - the latter being associated with an increase in both number and severity of dust storms (Wickens, 1968). It should be noted that the black 'cotton soils' of the Lake Chad basin are far less tractable than those of the Sudan despite the similarity in the vegetation and are better suited to the growing of rice and not wheat.

Riverine communities of Acacia nilotica subsp. nilotica or subsp. tomentosa, such as those growing on the alluvial soils of the Blue and White Nile are also widely used for the growing of such irrigated crops as cereals, beans, cucurbits, tomatoes, onions, etc. on both a peasant and commercial scale. As a result of the influx of displaced nomadic tribesmen into the Nile valley following the recent prolonged drought the trees of these communities are in danger of extinction due to excessive cutting for fuelwood (Wickens, in ed.).

Open stands of Faidherbia albida, especially in West Africa, are highly valued for the enhanced yields of crops grown beneath their canopy as well as being a source of highly nutritious browse during the dry season. The riverine stands along the Wadi Aribo in western Sudan probably present the largest concentration of this species in Africa and the Aribo basin is rightly regarded as a prime dry season grazing area. The trees are similarly valued to those in West Africa although the stands are now endanged following the heavy and indiscriminent lopping for browse during the past two decades by the camel nomads who have moved into the area following the desertification of their former grazing areas.

These are but a few examples of the valuable ecosystems being destroyed; without due regard to their role in the local rural economy, further examples will be given elsewhere in the text. In general the ecosystems represent a state of equilibrium between the rainfall and soils in terms of available moisture and anthropogenic factors such as fire (for land clearance and hunting), grazing, cultivation, fuelwood etc. Excessive anthropogenic pressure will eventually lead to desertification, an insidious process which, in arid and semi-arid areas is ultimately more damaging than deforestation of the rainforests in that even partial recovery is almost always impossible.

Desertification can be defined as the destruction through man's activities of the natural vegetation around such focal points as watering points, habitation, livestock routes, etc. Through time the destruction expands centrifugally until the various areas of devastation coalesce, presenting a false impression of a frontal advance of the desert. Desertification is independent of drought, which is a climatic phenomenon, although desertification can certainly be aggravated by drought, hence the present dramatic devastation in the Sahel. Whether the albedo effect of desertification affects the climate and prolongs the drought remains debatable.

Although it has been repeatedly stated, e.g. Seif el Din (1991), etc. that the farmer and pastoralist appreciated the value of the natural vegetation, this is considered a rather sweeping and somewhat inaccurate generalization. It is apparent from the recent SOS Sahel Oral History Programme that practically all those questioned did not appreciate until too late the role trees had played in preventing desertification (Wickens, in prep.). The value of trees certainly appears from the examples cited above not to be appreciated by desert nomads, perhaps understandably so since they inhabit an area where trees are very few and far between and are consequently unfamiliar with their role in preventing erosion.

The boundaries between the different plant communities are not static. With the exception of abrupt pedological boundaries, such as occur in the central Sudan between A. senegal and A. mellifera where a dune system abuts onto a clay plain, boundaries are generally accepted as the point of interdigitation where there is a noticeable change in dominance, such as on clay soils where A. mellifera gives way to A. seyal. During the present extended period of drought, the former boundary of the more mesophytic A. seyal has retreated before the more xerophytic A. mellifera. Should the rainfall return to its pre-drought 'norm', the process will be reversed. However, where the desert has encroached onto the formerly lightly wooded A. tortilis grass savanna, the recovery is likely to be very slow indeed. A raw soil and low seed availability are not conducive to a rapid recovery. Indeed, according to Le Houérou (1989), the planting of indigenous woody species in francophone Sahel as part of recovery programmes has proved largely unsuccessful in areas with less than 400 mm annual rainfall. The raw soils resulting from the desertification processes are generally inhospitable to plant life and it is the writer's opinion that ground cover should be established before trees, in line with the seral stages of succession.

The major mapping regions and their subdivisions recognized by White (1983) which are relevant to this present study are given in Table 1.2

Table 1.2 Vegetation regions of Africa and the major subdivisions containing dry zone Acacia species (White 1983).

Acacia species are the dominant trees or shrubs in most of the mapping units shown in the above Table. Unfortunately, while the areas covered by the various regions is known, those of the mapping units have not been recorded although they probably occupy some 4,500,000 km².

1.3 Biomass production

As Fagg and Stewart (1994) have pointed out, there have been very few quantified studies of fodder production from the acacias (Tables 1.3.1 and 1.3.2). Preliminary results from A. tortilis subsp. heteracantha in the Tugela valley, South Africa (Milton, 1983), provide an estimated yield of 1 ton/ha/yr shoot and leaf dry weight, a figure that agrees favourably with foliage production in India from A. tortilis of 2.5 kg/tree/yr from young plantations (at 400 trees/ha). Booth (1966), working in Kordofan Province, Sudan, also obtained an oven-dry weight of 2.5 kg of leaf litter from a seven-year old A. senegal.

Table 1.3.1 Annual foliage and pod biomass production of A. senegal in Senegal (Bille, 1980)

Table 1.3.2 Annual foliage production (kg/DM) of Acacia species (Bille, 1980)

The average foliar dry matter from Faidherbia albida in Mali over a number of sites was c. 300 kg/ha/yr (Cissé and Koné, 1992), while pod yields varied from 200 to 600 kg/ha/yr (Montgolfier-Kouevi and Le Houérou, 1983).

Pellew (1983a) working in the Serengeti National Park, obtained dry weight browse productivity rates of 5000 kg/ha/yr for A. xanthophloea, and 1725 kg/ha/yr for A. tortilis, with 90 and 49% of available canopy volume and a giraffe offtake of c. 80% and c. 60% respectively per annum, the upper browse level being 5.75 m.

The measurement of leaf biomass production is extremely complex and is discussed in some detail by Bille (1983). There does, however, appear to be a highly significant correlation between between trunk circumference and annual foliage production for which there is an exponential relationship per species (Cissé, 1983). Not only is the productivity of browse poorly known, it is also difficult to assess with any degree of accuracy. It is known to be related to rainfall and ecological environment and ranges from 100 kg to 1000 kg of edible DM/ha/yr. The feed value of the foliage in terms of energy is generally low to medium, ranging from 0.25 to 0.40 FU per kg of DM, i.e. 400 to 700 Kcal per kg of DM, while digestible protein is usually high, normally above 4% and often above 10% and may even reach as high as 30% in the Capparaceae (Le Houérou, 1983c).

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