G.E. Wickens
Herbarium, Royal Botanic Gardens, Kew Richmond, Surrey, U.K.
The flora of tropical Africa is estimated at about 30,000 species Brenan (1979), of which probably slightly more than 7,000 but certainly less than 10,000 are trees or shrubs. Of these at least 75%. are browsed to a greater or less extent (Whyte, 1947) and probably about 50% have been recorded as being useful to man. Thus, the full documentation of the alternative uses of browse species would indeed be a monumental task.
For this present paper the browse species selected (Table 1) are those recorded, from a limited selection of readily available literature, as being browsed by cattle, sheep, goats, camels, horses or donkeys. Where necessary suitable adjustments have been made for synonomy even though the synonomy has not been cited, neither has it been found practicable to cite infraspecific taxa. In some instances opinions were found to differ as to whether a species was browsed or not, but doubtless when given no alternative choice even the most unpalatable species will be eaten.
Apart from the encyclopaedic revision of Dalziel's Useful Plants of West Tropical Africa now being prepared by my colleague, H.M. Burkill (in press) the information regarding the various uses to which plants are put is widely scattered and has not been perused in detail. In my opinion it is not necessary to do so since it is quite clear from the literature already studied that a great deal of the information has been passed on from one author to another, sometimes without acknowledgment or in a garbled form. Much of the information is now outdated. For example, bark cloth has now been largely replaced by imported cotton or man-made fabrics.
In the self-contained, low-economy, peasant communities the chief role of the plants, apart from their values as fodder, is for the provision of food, medicine, fuel, timber and other domestic needs such as rope, baskets, mats, etc. As the communities develop economically so will their need to depend on wild sources for these commodities change, with the possible exception of fuel, for which there is, in many areas, no cheap alternative available.
With improved communications accompanying economic development, higher birthrates and longer life expectancy, ever-increasing areas are being opened up for commercial exploitation. The more valuable timber trees are removed and more land is brought under cultivation. The increased pressure on the land has often led to a reduction or even omission of the period under bush fallow. Attempts to obtain increased production through mechanisation has led to large-scale clearance schemes with their attendant erosion hazards. In the arid and semi-arid areas this can lead to accelerated desertification. In the higher rainfall areas, loss of land due to gullying becomes a problem on sloping lands, while in the level areas the removal of the trees can result in a rising water table and waterlogging (Cormack, 1953).
The utilization of browse and fodder plants by man is very much dependent upon availability, especially in relation to the presence or absence of more suitable alternatives, including imported materials. The main uses are for food, human and veterinary medicine, agriculture fuel, timber and domestic economy and, depending upon the area and economic standing, not necessarily in that order of priority. Each aspect will be discussed in turn.
Sedentary people rely on their cultivated crops for their main source of food, with additions, from wild sources as a supplement, a source of alcohol or flavouring. The use of wild sources for food is, however, extremely important in time of food scarcity and for the nomadic tribesmen or travellers generally. The browse species used for food are shown in Table 2.
In West Africa especially the leaves of the baobab, Adansonia digitata are popular as a spinach or dried and powdered and made into soups or sauces (Burkill (in press), Wickens, 1979, Williamson, 1975). The fruits are also greatly appreciated. The association of the baobab with habitation has led some ecologists to suspect that its distributions is due to man: the reverse is also a possibility. The fruits of the palms Borassus aethiopum and Hyphaene spp. may be buried and allowed to germinate and then eaten as a delicacy (Hunting Technical Services, 1964, Pardy, 1957-56, Williamson, 1975). The palms are often protected and have been known to become abundant on arable lands for this reason (Hunting Technical Services, 1968). In general however the food from wild sources is dependant upon the natural distribution of. the plants concerned.
The use of plants for treating various diseases etc. in man and stock are given in Tables 3 and 4. Some treatments may be of value, many are worthless. However it is a subject that should only be discussed by those with appropriate medical and veterinary qualifications.
The two main uses of trees and shrubs in agriculture are to provide shade for plantation crops such as tea, coffee, cocoa, etc. and for the protection of crops generally by means of live or dead fences (Table 5).
The shade-providing species are often quick-growing, non-indigenous species such as Grevillea spp. (Protaceae) and are outside the scope of this paper. Indigenous species include Cordia abyssinica (Burkill, in press, Williamson, 1975) and Parinari excelsa (Burkill, in press) grown as shade for coffee and cinchana respectively.
In peasant agriculture the increased yields obtained by growing crops under Acacia albida are well documented (Wickens, 1979). The tree has the great advantage of shedding its leaves at the start of the rainy season so that the crops are able to take advantage of the humus provided by the fallen leaves and dung produced by the browsing stock yet suffer no disadvantage from shade; new leaves are produced at the start of the dry season. The use of this species to provide shelter belts for areas under mechanical cultivation is now under investigation.
The use of live fences for arable lands has made very little progress despite the availability of species suitable for this purpose. This is largely due to the absence of secure land tenure as practised in the temperate regions. Thus there is no incentive for the farmer to provide permanent stockproof hedges around his arable lands. In most areas there are species of Commiphora, as well as other genera, that strike readily from truncheons, and are excellent hedge-forming material.
In arid and semi-arid areas the live fences have the additional advantage of reducing wind erosion. This can give rise to ridging at the base of the fence.
The most common method of protecting the growing crops is by means of brushwood fences using thorny material obtained during the process of land clearing. Unfortunately the life of such fences rarely exceeds the growing season and repairs during the following years of cultivation is generally from material gathered from outside the cultivation area. This is regarded as an inefficient use of the material resources.
There are two species that are of particular interest for irrigated areas. They are Balanites aegyptiaca and Swartzia madagascariensis. The fruit of the former are known to contain certain saponons that are lethal to the freshwater snails that act as the intermediary host of bilharzia, and also the waterflea, Cyclops that harbours the guinea worm (Fanshawe,1948, Suliman et al, 1959). The pods of Swartzia have also been used for the control of bilharzia (Williamson, 1975). The use of these species to give some control, no matter how slight, would undoubtedly be beneficial to man. The growing of trees along canal banks is usually disapproved of because of possible root damage to the banks. However, Balanites has rather bunched roots with few subsurface laterals (Hunting Technical Services, 1964) and is believed unlikely to cause damage.
The use of trees such as Bridelia micrantha and Tamarindus indica for silkworm farming have so far proved unsuccessful (Burkill, in press) nevertheless the commercial possibilities should be investigated further.
Many species, including Acacia spp. are used locally for honey production. The commercial possibilities again require investigation.
Although the majority of browse species are probably used as fuel, relatively few have been so mentioned in the literature (Table 6). Naturally, depending upon the availability, some species will be preferred to others. Thus, because of its high moisture content and spongy tissue that causes the axe to bounce off rather than cut, the baobab, Adansonia digitata has a very low priority as a fuel (Wickens, 1979).
Species with a high latex content, such as Calotropis procara (Hunting Technical Services, 1964) and the figs Ficus spp. are also considered undesirable. In contrast the ironwoods are difficult to cut and maybe ignored; Prosopis africana is hard enough to blunt an axe (Dalziel, 1937) and Pericopsis angolensis is so hard it will twist the African hand axe (Williamson, 1975). Some timbers may have undesirable characteristics when burning. Cassia sieberiana is reported to cause headaches (Dalziel 1937); Maerua crassifolia burns with a nauseating smell (Brown and Massey, 1929) while the smoke from Stereospermum kunthianum is locally believed to be conducive to leprosy (Brown and Massey,1929), its wood is also unsuitable for charcoal since it turns to ash, while in Malawi Psorospermum febrifugeum is believed to have protective properties and is therefore not used for firewood (Williamson, 1975).
There are also conflicting opinions about the value of certain species. In the Sudan Balanites aegyptiaca is regarded as poor fuel because it burns with an oily smoke, although making excellent charcoal (Suliman et a1,1959), while in Nigeria it is regarded as a good fuel for use in huts because it produces no smoke (Burkell (in press), Dalziel, 1937).
It takes several years for a tree to produce branches of suitable dimensions for use as fuel and excessive cropping will severely reduce the tree's vitality and ability to produce fresh growth. In the rural areas the increasing population pressure and the general breakdown of the bush fallow has created such a demand for fuel that the requirements from natural sources can only be met by cropping the trees to destruction within an ever-increasing radius from the centre of habitation. Inevitably browse will suffer and stock will be forced to travel further. Alternative sources of fuel or firewood reserves are usually available for the established urban areas, but there is need for a stronger awareness of the requirements of smaller settlements.
The various uses of browse species for timber purposes are shown in Table 7. In general there are three major categories; pole timber for building frames etc., timber for local domestic use such as doors, furniture, boxes etc., and commercial timber for mining, railway sleepers, bridge and wharf construction, etc.
In many instances the cropping results in the destruction of the trees. Forestry protection by rotational and selective felling, protection orders etc. is obligatory for the preservation of the tree as a natural resource.
The multitude of uses to which browse species are subjected in the domestic affairs are shown in Table 8. This ranges from cordage, tanning, dyeing, soap making, toothbrushes, fish poisons, oils, gums, etc. Some products, such as gum arabic from Acacia senegal are sold on the world market although it has been calculated that the whole of the Sudan's annual production could be met by a properly managed plantation of 55 sq km. (Hunting Technical Services, 1964). Bark cloth, dyes, soaps etc. have been or will be replaced by imported products; bark cordage however is so convenient that it is likely to have a use for many years to come.
The unrestricted use of roots for cordage, medicine, etc., can however lead to the death of the tree. In Malawi Boscia angustifolia has been eliminated from some areas because of excessive root cutting in order to make 'lover' amulets (Williamson, 1975). In West Africa the potential use of the floss from Bombax costatum for stuffing matresses is limited because many of the trees are unproductive because of fire damage (Burkill, in press).
As has already been stated earlier, it is the availability of the species rather than its suitability that dominates its usefulness to man. In general greater reliance is placed on obtaining the maximum utilisation of a species in areas of low economic development, low or uncertain rainfall or where the population is largely nomadic.
It would be invidious perhaps to select any one species as being more useful to man than another, but for diversity of utilization I would select the baobab, Adansonia digitata, which provides practically all the necessities for survival except fuel and timber (Wickens, 1979). For priority of investigation, however I would suggest the possible use of Balanites aegyptiaca and Swertzia madagascariensis to provide a form of biological control of bilharzia, the scourge of the tropics.
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