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Part two: The uses of Acacia in the rural economy
2.1.1 Fuel
It is generally accepted that the current use of trees for fuel nationally is exceeding incremental growth. The standing volume obtainable from some of the best 20 year-old Acacia woodlands is only 8.5 m³ ha (Vink, 1989), equivalent to about 0.42m³/ha of mean annual increment.
Wilson (1983) obtained annual values per capita of 0.50-0.62 m3 of fuelwood for urban use in Niono, Mali, and cites 0.62 m3 in northern Ethiopia and 0.63 m3 in northern Nigeria. In those areas of Ethiopia where wood is extremely limited and cattle dung (which should have been used on cultivated land) has now become the principal fuel, as in Makalle, Tigre Province, with annual fuelwood consumption per capita now as low as 0.15 m3 and, in areas where wood is still abundant, as high as 1.5 m3. Other authors give values ranging from the annual fuelwood requirements in the Sahel of one person for cooking of 0.7 m3 (Maydell, 1987) up to 1.5 m3 (Montalembert and Clement, 1983) and, in Tanzania for Dar es Salaam, 2.1 m3 (Hines and Eckman, 1993). For the higher ranges it is uncertain whether fuel consumption is for domestic use only or includes bakeries, brewing, brick kilns, potteries, blacksmiths, etc. Since the degree of drying and efficiency as a fuel also affects fuelwood consumption, any conclusions based on the above figures would be inconclusive apart from obvious deduction that fuelwood consumption is exceeding supply and must inevitably affect other forms of tree usage too.
The present trend of a 2-3% annual population increase and its concomitant increase in demands for more fuel and land for food have placed an impossible strain on existing forest resources, especially in the drier regions. The Mbeere of northern Kenya have been forced to reduce their fuel consumption, great log fires no longer burn overnight in their compounds (Riley and Brokensha, 1988). Similar economies must surely be practiced throughout Africa, where an exponential population explosion creates a demand that excedes natural regeneration of the fuel source. However, more efficient stoves could reduce fuel consumption considerably and thereby substantially reduce some of the pressure on the existing fuel resources.
Whereas it is suggested that the use of fuel by the rural population, which relies heavily on dead wood, may be more or less in balance with regrowth, the urban population places an unbearable strain upon fuelwood tree resources. Land cleared for cultivation, especially in poorly-planned development schemes, also represent a future loss of potential fuelwood supplies (M.A. Trossero, verbal information, 1993). This is well illustrated by the changes in the source of fuelwood for Khartoum, Sudan.
The original source of firewood was from the fairly dense, even stands of Acacia tortilis subsp. spirocarpa which formerly occurred to the east of Khartoum (Harrison and Jackson, 1958). Although the cutting of living trees was illegal the firewood cutters had little difficulty in making a dead tree from a live one. There were quarrels between the firewood cutters and the graziers over this destruction of living trees as the pods were a valuable dry season feed for livestock.
In order to reduce the destruction of trees in the vicinity of Khartoum, the Government, by the 1940s, was bringing in subsidised firewood by rail from the clay plains in the region of Gedaref, some 370 km to the southeast. Both A. mellifera and A. seyal were used, but A. mellifera was the preferred species since A. seyal is susceptible to the bostrychid beetle Sinoxylon senegalense and, if not used quickly enough, is soon reduced to a pile of sawdust (J.K. Jackson, pers. comm. 1994). Charcoal production has helped to overcome this problem as well as easing transport difficulties. However, since the 1960s, vaste areas of the clay plain have been more or less cleared for irrigation and mechanized agricultural schemes and much of the A. mellifera has been eliminated. Woodcutters have been equally successful in destroying all the A. tortilis in the Khartoum area.
The effect of such clearance schemes has been that charcoal from relatively local A. mellifera has now been replaced by that from the more distant A. seyal. The latter species, because of its abundance, is now considered to be the most important charcoal source in the Sudan (Badi et al., 1989). A consequence of this shrub and tree clearance is that the incidence and severity of the haboob (dust storm) have also increased. Not only have the plant communities been ruined, the local people have been displaced from their land and their social structure destroyed. Many have been dispersed to new homelands, either in marginal areas or newly opened farming areas, and so the cycle of deforestation has continued.
The once abundant natural stands along the Nile of A. nilotica subspp. nilotica and tomentosa were formerly important sources of fuel for the Nile steamers and the railways as well as timber for boat building and railway sleepers (Muriel, 1901); they were even used to pay taxes (Schweinfurth, 1893). Plantations of these species are now minor sources of fuel for Khartoum.
Other urban areas have a similar history of fuel problems, for example Niono, Mali (Wilson, 1983), Nairobi, Kenya (Kinyanjui, 1985) and Kano, Nigeria (Cline-Cole et al., 1990).
In terms of calorific value, multiple use, natural regeneration, fast growth and yield under Sahelian conditions A. nilotica subsp. adstringens and tomentosa are regarded by Maydell (1986) as superior to all other indigenous candidates; subsp. nilotica, which differs from subsp. tomentosa in having glabrous to nearly glabrous pods and glabrous to shortly puberulous twigs, is not mentioned but should also be included.
See Table 2.1.1 for the use of Acacia species for fuelwood and charcoal. In the absence of alternative fuelwood sources, all Acacia species are probably used for firewood, even though the references cited do not mention their use, even A. oerfota with its slender branches and obnoxious smell. Conversion to charcoal has certain specific requirements, the species listed are therefore more likely to be a more accurate portrayal of usage.
2.1.1.1 Firewood
The desirable criteria for firewood species are (Riley and Brokensha, 1988; Hines and Eckman, 1993):
1. Rapid growth, high volume production, ability to coppice or sprout readily when cut. an requiring minimum management.
2. Wood dense with a low moisture content, relatively easy to cut, easy to handle, with few or no thorns, splitting easily and readily transportable; wood collected during the rainy season should not absorb moisture (the scandent or non-climbing shrub A. ataxacantha and A. brevispica are preferred by the Mbeere in Kenya for this reason despite their small dimensions)
3. Slow burning with high calorific value, producing very little smoke without objectionable nor toxic fumes and neither spits nor sparks.
These are criteria that are found in many Acacia species; they are also to be found in many of the Combretaceae. Although no comparative studies have been made, personal observation suggests that some Combretum species are slower growing than those of Acacia. This is supported by the preference of members of the Combretaceae by the Mbeere for charcoal, the hardness being equated with slow growth (Riley and Brokensha, 1988); in West Africa one of the most prized woods for charcoal is from Anogeissus leiocarpus (Combretaceae), while other members of the family, Combretum spp. and Guiera senegalensis provide most of the firewood as they are the most readily available coppice in fallow lands
The preferred Sahelian species in Burkino Faso and Niger are, because of their high calorific value, freedom from smoke and sparks, Acacia ehrenbergian, A. tortilis subsp. raddiana and A. nilotica subspp. adstringens, nilotica and tomentosa, while the wood of A. macrostachya and A. erythrocalyx is not highly regarded and are little used (Guinko, 1991). By their occurrence in large pure stands, offering easy harvesting and processing conditions, Acacia seyal formations have played a special role in the supply of charcoal to growing cities in Sudano-Sahelian Africa such as Dakar, N'Djamena as well as for many other cities in the Sudan where mechanized farming and the needs for fuel have contributed to the depletion of extensive areas of pure Acacia seyal stands.
Because of decreasing availability there is an increasing gap between preference and actual use. The deciding factor is availability and ease of collection (Riley and Brokensha, 1988). The use of such an obnoxious fuel as A. oerfota in the Yemen can only be tolerated because all other fuelwood sources have already been exploited; its local abundance in the lowlands is attributable to overgrazing and the replacement of more palatable species by the less palatable (Al-Hubaish and Müller-Hohenstein, 1984).
2.1.1.2 Charcoal
Charcoal, whether by desire or availability, is often the preferred or only fuel used in urban areas. Its manufacture can be a lucrative industry but inefficient practices can often result in a conversion efficiency that can be as low as 15% so that it is consequently a wasteful process.
The desirable criteria for charcoal are similar to those for fuelwood but requiring large, uniform stands producing wood of suitable dimensions and a high specific gravity (0.7-0.9) for efficiency and ease of manufacture. The characteristics of the finished product are: moisture <7%, ash <3 %, fixed carbon >75 % and apparent density >0.3 g/cc (FAO, 1962).
2.1.2 Timber
See Table 2.1.2 for timber properties and uses of Acacia species. According to Hines and Eckman (1993) the desirable criteria for construction and pole timber are:
1. Rapid growing, with straight stems of uniform size and small branches, naturally pruning and rapidly self-healing..
2. Possess good physical, mechanical, seasoning, preserving, and processing properties and insect, fungus and rot resistant.
For domestic use and handicrafts, such as platters, spoons, shuttles, mortars, tool handles, musical instruments, carvings, bows, arrows, etc. shape, strength, density, ability to peel, work or carve easily, freedom from splinters and, for such items as spoons and bowls, low permeability to liquids, are additional requirements.
While a number of species can provide pole timber, which is widely used in the domestic economy for construction purposes, commercial sawn timber is largely from A. nilotica subspp. indica, nilotica and tomentosa, A. polyacantha and A. sieberana.