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Crop mixtures in traditional systems

Akinola A. Agboola
Professor of Soil Fertility and Farming Systems, Department of Agronomy, University of Ibadan, Ibadan, Nigeria

Abstract

The traditional cropping system is stable because it is adapted to the farmers' level of technology and the soils' capability. It incorporates mixed cropping and bush fallow, and it gives a high total return per unit area of land. Furthermore, growing crops in mixtures is consistent with the farmers' goal of security. Their present systems have evolved naturally as an answer to the challenging environment in which they live.

Researchers have been hesitant to tackle multiple cropping experiments in general, and agro-forestry in particular, because of the infinite combinations possible, lack of knowledge about existing systems, and the traditional separation between agriculture and forestry. Also, multiple cropping is associated with unmechanized farming and low productivity; research in intercropping and multiple cropping should be geared to increasing the productivity and returns in both arable crops and forest products. The peasant farmers' system of agro-forestry should be improved upon, and researchers should evolve a combination of arable crops and fast-growing trees that can be easily adapted by smallholder farmers.

The traditional cropping systems will continue until an alternative is evolved that can fit into present technology, environmental constraints, and at the same time maintain high crop yield. My feeling is that agro-forestry research has the potential of offering an early and viable alternative.

Introduction

Farming systems can be defined as the distribution of plants and animals in space and time and the combination of inputs believed to give maximum production in socioeconomic, political, and cultural contexts.

In Africa, a farmer or farm family usually operates a small, diversified agricultural enterprise. According to Okigbo (1978), farmers with a homestead in an upland, well-drained soil may operate a compound farm or garden close to their homestead and maintain two or more plots in cropping systems involving natural or planted fallow and in the flood plain of a nearby river or steam. They may also keep pigs, goats, sheep, and poultry for manure, meat, sales, etc.; they may be palm wine tappers, basket-makers, musicians, or priests in the traditional religion. The cropping mixtures on the farms often involve major staples, vegetables, and condiments in multiple, double, relay, and patch intercropping patterns of annuals, perennials, or both. The compound farm or homestead garden usually carries more species of cultivated plants than bush fallow farms.

The most common tradition in African cropping systems is the spatial arrangement of crops on the field. The crops are established haphazardly in mixed culture (Okigbo and Greenland 1975), the objectives being to take advantage of local topographic features and micro-relief; disperse species at wide enough spacings so that they do not compete for nutrients and light; ensure that crop cover is adequate to control soil erosion and weeds; and ensure that each species's requirements for sunlight are met. Where annual staples are uniformly planted among tree crops, heavy pruning of the tree crops is usually carried out to ensure that adequate light reaches the ground level. Whether crops are grown on mounds, beds, ridges, or the flat, their spatial arrangement and frequency in mixtures usually indicate their importance in the diet and sometimes their uses.

Intercropping and Multiple Cropping

The simultaneous cultivation of different crops on the same piece of land has been described interchangeably as mixed cropping or intercropping by Webster and Wilson (1966) and Norman (1971). Ruthenberg (1976), however, distinguishes between mixed cropping and intercropping on the basis of the pattern of the intermixture.

The term intercropping has been used rather generally in the literature as referring to the practice of growing two or more crops simultaneously in different but proximate stands (Okigbo 1978). Grimes (1963) defined a common practice in intercropping: the system of growing different crops in alternate rows, which he terms alternate row cropping. Row intercropping is common in filled areas, annuals often being planted under perennials. For instance, tall-growing crops such as cassava or bananas are planted in young coffee, cocoa, or rubber plantations (Sanchez 1979).

In multiple cropping, Herrera and Harwood (1973) indicated that each of the crop mixture patterns has different physiological characteristics and different advantages. For example, Norman (1974) showed that although there were at least 156 crop mixtures and many different spatial arrangements among Hausa farmers near Zaria (Nigeria), the most popular arrangement was a systematic spatial pattern on ridges. With intensification of cropping, interactions among plants become critical. The most widespread multiple cropping systems practiced in the humid tropics are mixed intercropping and relay intercropping.

Mixed intercropping is common when cereals, grain legumes, and root crops are grown together and when little or no tillage is practiced. For example, farmers in southern Nigeria plant simultaneously maize, cassava, vegetables, and cocoyam. In Abakaliki, Nigeria, mixed cropping is practiced in mounds or ridges of soil constructed with hoes. Several crops are planted on different parts of the mounds. For example, an Abakaliki farmer plants yams on the mound, rice in the furrow, and maize, okra, melon, and cassava on the lower parts of the mound. Mounding is beneficial because it increases the volume of soil available to root crops.

Relay intercropping is a practice where a second crop is planted after the first crop has entered the reproductive growth phase but prior to harvest. A common example is the maize-beans system used in most of Central America and much of tropical South America. Maize is planted in rows, usually at the beginning of the rainy season; when the ears are well formed, farmers break the stalks just below the ear and plant climbing bean varieties. Relay intercropping is also very common in rice-based systems in Taiwan. Up to five crops per year can be harvested by two relay successions, rice-melons followed by rice again relayed with cabbage and maize. At present, the maize-cassava relay is being developed at the University of Ibadan, with researchers studying the effects on soil nutrients.

Advantages and Disadvantages

Baker and Yusuf (1976) wrote that the almost universal practice of traditional cropping systems by subsistence farmers throughout the world is an indication that the system has evolved naturally as an answer to the challenging environment.

The rationales for crop mixtures are that they may be relatively more profitable than sole cropping (Chandra 1978), the difference between the marginal value product of resources and the opportunity cost of the resources being insignificant (Norman 1974); they are consistent with the goals of security and year-round subsistence needs (Andrew 1972); they may alleviate adverse conditions in the ecosystem; and they may maximize the space, water, and nutrients available. Some of these benefits can be further promoted by good tillage practices, based on the principle of minimizing disturbance of the ground and vegetative cover. The practice of minimum tillage, with ample crop residues left on the soil surface, has great potential.

Although monocropping tends to attract fewer diseases and insects, these are more likely to be highly prevalent and to cause considerable damage. Cropping mixtures may reduce the abilities of pests and diseases to spread. For instance, interplanting has been shown to reduce insect problems in groundnut-sorghum and cassava-maize mixtures and has reduced the incidence of bacterial blight.

The denser plant population usually found in crop mixtures may also help control weeds (FAO 1968). In addition, because crops mature at different times, mixtures may extend the period of the year during which the soil is protected by leaf cover and root systems (Igbozurike 1971).

The disadvantages of traditional systems are that there is reduced yield of the component crops (Chandra 1978; Webster and Wilson 1966; Agboola and Fayemi 1972); there may be competition for light, nutrients, and water (Dalal 1974; Willey 1979; Webster and Wilson 1966); there may be allelopathic effects due to excretion of toxic substances by one or more crops (Dalal 1974); the practice is not well suited to modern agriculture or mechanization and, thus, research on traditional systems has been inadequate (Ahmed and Gunasena 1979); and suitable methods for investigation are difficult to define (Haizel 1974).

Research

Unfortunately, research workers appear to have been hesitant to tackle multiple cropping experiments because of the many crop combinations in use, and because multiple cropping by the peasant farmer is associated with non-mechanical farming and low levels of productivity. Doubts have been expressed as to whether any of the positive benefits of multiple cropping can be exploited at more advanced levels of farming. Attempts to improve production by the application of technology developed in temperate cropping systems have failed in Nigeria and in most other tropical countries, not because of farmers' conservatism but because the approach is inappropriate.

Farmers in the tropics have been noted to grow complex crop mixtures on compound farms, especially in the rainforest zones, where staples, vegetables, and perennial fruit trees are interplanted. As early as 45 years ago, Leakey (1934) observed that the relay and mixed cropping practices had many obvious advantages, and he recommended that those involved in agricultural development should give serious attention to research on traditional food production systems, especially those involving intercropping. This recommendation has largely been ignored, despite the fact that peasant farmers have repeatedly refused to adopt monocropping practices recommended by extension agents.

The main objectives of research into the productivity of mixtures might be to screen mixtures for high-yielding combinations; to test alleged advantages of traditionally grown mixtures; and to gain an understanding of the processes that lead to advantages so that, in a specific environment, a rational choice of components may lead to higher yields than are possible in monocultures.

Sturdy (1939) noted in East Africa that intercropping Crotalaria with millet, and groundnuts with sorghum, helped in the maintenance of soil fertility. Lambers (1940) reported that coffee intercropped with bananas in Kuri provided a mulch that improved the fertility of the soil. Results of experiments carried out by Agboola and Fayemi (1972) showed that legumes intercropped with early maize gave a maize yield equal to that obtainable with 55 kg/ha of nitrogen supplied as mineral fertilizer.

Much work still has to be done to quantify the nutrient level of soils under traditional crop combinations. In a study conducted in 1981 composite soil samples were taken from eight farms and their adjoining fallows, and in only one case was the fallow land significantly higher in nutrient status than the adjacent cultivated land. P, K, Fe, Zn, and Ca levels in the top 0-15 cm layer were higher in the cultivated fields than in the adjoining fallows, and this was attributed to the effect of burning after clearing. These data support the view that the length of the fallow has been drastically reduced, thereby reducing also the nutrient build-up. They imply that most farmers are actually cropping infertile lands and that the nutrient status of the field does not influence the farmers" choice of crop combinations. The choice of crop combinations seems instead to be influenced by the food staples usually planted in the area, and the inclusion of vegetables is related to the economic value of the crops and food preferences. No definite effect of the crop mixtures on the soil nutrient status could be established.

Summary and Conclusions

The best cropping system in the tropics, once the soil is considered as the main factor sustaining crop production, is one that will not expose the soil to erosion hazards Therefore, more research is needed to identify better combinations, including agro-forestry schemes, so that the best type of rotation can be developed for each ecological zone. Although zero tillage has been advocated by IITA, this cannot be practiced ail over the humid tropics because it is dependent on the soil, particularly the clay content, and the prevailing weather conditions. In some areas, the temperature is low during the harmattan, and the Fadama soils cake, necessitating extra soil preparation in the following rainy season. Thus at the University of If e zero tillage has proved to be unsatisfactory.

Besides protecting against erosion, an appropriate cropping system must guard against the breakdown of the soil structure and nutrients. A good system needs to take advantage of the tonnes of human, plant, and animal refuse being produced. It should be emphasized, however, that traditional farmers will continue with their existing system until an alternative is found that will maintain higher yields, conserve the soil, control weeds, and generally fit into their present technology.

Acknowledgements

The author appreciates the contributions from J.M.A. Tocunana and C.F. Yamoah, both postgraduate students of the University of Ibadan's Department of Agronomy.

Agricultural tree crops as a no-tillage system

R.D. Bowers
International Institute of Tropical Agriculture, Ibadan, Nigeria

Abstract

The crisis in tropical agriculture is demonstrated by falling food production and migration to the towns It is argued that this is an inevitable process resulting from the inability of tropical agriculture to compete with the industrialized agriculture of the temperate zones. Industrial agriculture is a high-input agriculture, and success or failure depends on the input:output ratio. In the humid tropics, the input: output ratio is unfavourable, and industrial agriculture therefore impossible; hence the only viable form of production is subsistence farming. The obvious alternative to subsistence farming is mixed tree cropping, in which the characteristics of the natural forest cover are copied as closely as possible. Only in this way can the productive potential of the environment be realized and the fertility of the soil maintained. Crop mixtures may be selected from oil palm, coconut palm, breadfruit, plantains, coffee, cocoa, cola, citrus, and other trees. A plea is made for a research programme to be devoted to mixed tree cropping as one of the possible ways to improve the agriculture of the region lying between 10°N and 10°S.

Introduction

The growth of temperate agriculture over many centuries has involved the destruction of forests and the planting of annual crops in their place. Land clearing and progress are seen to be linked. When developers come to tropical Africa, they assume that clearing the forest is progress; likewise, African people, anxious of emulating progress seen in other countries, follow the same course.

As an agricultural engineer, I suppose that I should be in the camp of forest destroyers, and I certainly would be if I thought that this policy could be successful in economic terms and acceptable ecologically. But all the evidence indicates that the widespread clearing of tropical forest and the large-scale planting of annual crops leads to financial and ecological disaster. The worst possible combination of circumstances for Nigeria would be for the fertility of the soil to run out at the same time as the oil runs out-a possibility which could well arise unless great care is exercised. A new type of agriculture, such as that based on mixed tree cropping, must be developed with all urgency, as it offers, perhaps, the only hope for the future.

Proposition

I propose that there is a crisis in the agriculture of the region Iying between 10°N and 10°S and that there must be a reason for it.

The evidence for this crisis is falling food production and an increase in food imports. Further evidence is the mass migration from the rural areas to the towns. For those who stay in the country, the standard of living remains low, and there is little hope of improvement. The great majority of the people in the countryside are still subsistence farmers, just as they were before and during the period of colonialism, depending for the most part on the unaided strength of their bodies to wrest a living from the soil. They would seem to live in a world with which Western technology is unable to communicate.

Over the past 30 years, there has been no lack of attempts to improve agriculture in the humid tropics. That none of these efforts has significantly changed the methods of production, even in a limited area, cannot reasonably be held to be a result of bad luck or bad management. There must be fundamental economic reasons to explain why these attempts have failed so catastrophically. The competitiveness of tropical agriculture has seriously declined in relation to temperate agriculture; the reasons for this decline include:

• The development of bigger and more effective machines. Just as it is no longer possible for a person with a headpan to shift earth competitively against one with a Caterpillar D9, so it is no longer possible for a person with a sickle to compete against one with a combine harvester; 30 years ago, however, when machines were smaller and less efficient, it was still possible for hand labour to compete;
• All major crops have now been mechanized; 30 years ago, machines had not been perfected for the harvesting of cotton, groundnuts, or sugar cane. Production of these crops can now only be competitive (and provide the producer with a reasonable standard of living) if they are mechanized; and
• Temperate region substitutes have been found for tropical crops. Soybean, sunflower, and oilseed rape have greatly reduced the demand for groundnut oil. Cotton has been displaced to a considerable extent by artificial fibres. Cane sugar has been threatened by sugar beet and, more recently, by high-fructose corn syrup.

Agriculture has gone through three phases: subsistence farming, commercial farming, and industrial farming. This last phase is still in full evolution and is rapidly displacing all forms of small-scale production because it is economically more efficient. Never in the history of human beings have the main food items been produced so cheaply in real terms as they are now in the temperate countries.

Industrial agriculture is, of necessity, a high-input agriculture because it depends on inputs of machinery, fertilizers, herbicides, insecticides, etc. Because labour forms a small part of production costs, the system cannot be made to work by a substitution of labour and land for capital.

Success or failure of industrial agriculture depends on the input:output ratio expressed as the cost of the various inputs that are needed to produce a given value of produce. By using this ratio, one can compare the efficiency of production at different places (table 1).

A profit of $0.05 is made on every dollar of produce in the USA, whereas a loss of $1.20 per dollar of produce occurs in the tropics. The loss has to be covered, for example by subsidizing inputs to subsidize the price, or by introducing import restrictions that keep the price artificially high. Support for the reliability of such input:output ratios is provided by the comparative farm gate prices of maize- approximately $1 50/t in the USA vs. $450/t in Nigeria.

Generally, profitability increases as expenditures on machinery and chemical inputs increase, up to a limit of about $0.55 per dollar. If the inputs cost more than about $0.65, commercial production is no longer possible. Production becomes economically absurd if the foreign exchange cost of the inputs exceeds the foreign exchange cost of the product.

TABLE 1. Indicative Input:Output Ratio of Maize Production in the USA and Nigeria (cents input to produce $1 output)

Input:output ratios are generally unfavourable in the humid tropics. Low yields are a result of leached soils, heavy runoff, loss of nutrients, and weed competition. Drying and storage in a hot, humid environment present further problems. Generally, unfavourable input:output ratios are found where yields are low. Approximately the same inputs per hectare will produce about 2.5 tonnes of maize in the humid tropics as against 7 tonnes in the USA. In other words, only 0.15 ha of land in the US, as compared with 0.4 ha in the humid tropics, is required to produce 1 tonne of maize. All land, machinery, herbicide and insecticide costs are directly related to area. Thus, they are all three times more expensive in the humid tropics per tonne of maize produced.

Increasing energy costs make the possibility of high-input agriculture recede even further. When inputs are expensive they can only be used where the return per unit used is high; only when inputs are cheap can they be used extensively. Since the oil price rise of 1973, hopes of using expensive inputs in marginal areas have declined.

Subsistence farming is the inevitable consequence of the unfavourable input:output ratios associated with the production of annual crops in the humid tropics. All other forms of annual crop production are economically impossible until such time as the agricultural scientists develop varieties that give input:output ratios comparable with those of similar crops grown in more favourable climates. This conclusion accords with the observable fact that subsistence agriculture is the universal form of annual crop production in West Africa.

But subsistence farming has been shown to have already failed: it cannot produce sufficient surplus to feed the large towns; it cannot supply cheap basic foodstuffs; and it cannot retain the young people on the land. The impasse appears total-the only system that is economically viable is incapable of supporting the modern economy for which developing countries are striving.

The Tree Crop Alternative

The tree crop alternative offers some hope. It is logical in both economic and ecological terms. The climax vegetation of the humid tropics is high forest, which produces the greatest sustainable rate of biomass formation. It captures all solar radiation year-round by virtue of the different layers of foliage. The layers of leaves, twigs, and branches absorb the incoming energy of tropical rainstorms, thus protecting the soil. Organic matter on the forest floor is protected from the direct sun. Runoff is reduced so that more water is available for plant growth, and extensive root systems explore the whole soil profile for nutrients. If society demands that the high forest be removed, it must be replaced by economic crops that copy as closely as possible the characteristics of the natural forest, and that afford equal protection to the environment. If mixed tree cropping is to be freely chosen in preference to annual crops it must show economic advantages.

There are reasons to think mixed tree cropping will significantly outperform annual crops as the cost of highenergy inputs rises because of its low labour requirements; low fertilizer, herbicide, and insecticide requirements; the prospects for an ideal no-tillage system; and rediced weed competition.

Conclusion

Tree crops appear to be so well-suited to the environment and have so many economic advantages that a research programme of tree crop development is urgently required. The potential for tree crops exists and in many cases is already being realized. The oil palm, for example, is by far the most efficient and cheapest source of vegetable oil. Under good conditions it will produce up to 6 tonnes oil/ha. It will, therefore, always hold a competitive advantage over the annual oilseed crops. It may be expected to constitute an important part in any mixed tree crop system. The coconut palm produces a good yield of both oil and protein. Further, the open canopy makes it very suitable for mixed cropping with plantains or bananas. The breadfruit tree (Artocarpus altilus) appears to produce up to 8 tons carbohydrates/ha. Accurate yields are not available because the tree has not been seriously studied. The best cultivars should be assembled to determine its productive capacity under a range of environmental conditions. The shea butter tree (Butyrospermum parkii) should be studied and improved so that it may replace groundnuts in the drier areas.

For the wet tropics, it is suggested that mixed tree cropping is likely to be most effective, as it copies as closely as possible the natural forest. Oil palm or coconut palm would be interplanted with breadfruit to form the top gallery. Beneath this would be planted plantains, cocoa, and coffee, and, in places, as a bottom layer of vegetation, small plots of maize, sweet potato, and cocoyams. The aim would be not to get the maximum yield of one particular crop per unit area but, rather, to maximize total production.

The objective must be to give the people of the wet tropics crops that will outyield and undersell the main food crops grown in the temperate zones. Oil from palm oil already does this; it is to be hoped that carbohydrate from breadfruit would become cheaper than maize as the cost of high energy inputs increases. If the people of the wet tropics attempt to grow temperate crops competitively, they will always be losers, and, if they cannot produce cheap food, industrialization may well be impeded.

Long-term tree research is a most urgent requirement, It is a field that has been totally neglected except in the plantation context. Trees have all the right characteristics for smallholder farmer production in an energy-hungry world, although in some cases breeding programmes must be undertaken to combine desired properties.

It is essential that one get away from the idea that trees only grow in forests or in plantations. This concept has meant that the people of the wet tropics have been (and still are) denied the help of modern research. Economic trees grow just as well on smallholdings as on plantations, and there is little loss of economic efficiency. On has only to think of the struggle of the Chagga people in Tanzania before they were allowed to grow coffee to realize how obstructive the plantation mentality can be.

One cannot know what the future holds but surely one must plan for several different possibilities. One of these is a world of increasing energy shortage, where energy-related products become more and more expensive. In such a situation low energy-input tree crops have a potential that is unmatched. Prudent people do not put all their money on one horse, and scientific researchers and government policymakers should likewise spread their bets. Mixed tree cropping may be a favourite for some, or an outsider to others, but it looks to be so well-suited for the next 20 years that it would be unwise not to put some money on it.

Traditional agro-forestry systems in the central African republic

Etienne Yandji
Head, Forest Management Service, National Forestry Bureau, Bangui, Central African Republic

Abstract

The Central African Republic is a land of forests, and yet its agrosilvicultural balance is being jeopardized by the lack of an afforestation policy. This paper contains a detailed description of the seven types of trees cultivated on many types of plantations and combined with food and other crops. The report also describes the traditional systems in use and proposes a series of measures aimed at improving their productivity. Finally, it is recommended that multidisciplinary research on soil protection end restoration be conducted, with the understandiny that human end social factors are of paramount importance in the restoration of tropical forests.

Introduction

The Central African Republic is located between latitude 2° 16' end 11°20' N and longitude 14 20' end 27 45' E In the extreme north, between Sudan and Chad, is the Birao region, with which this report is mostly concerned. Here the climate is Sudano-Sahelian to the north and Sudano-Guinean to the south, and nowhere does the yearly rainfall exceed 1,000 mm.

Despite various botanical explorations, the vegetation of the area is still not very well known. The following divisions for the Sudanian forest were proposed by Sillons in 1950.

• District XI, between the Chari and Logone rivers, north of a line running north of Batangafo and south of Ndélé and Ouadda. This is characterized by the alternation of wooded savanna with Uapaca sp., Isoberlinia sp., and Monotes sp. (Oxytenanthera abyssinica may also be present) and shrub savanna with Terminalia laxiflora, Grewia mollis, and Combretum hypopilinum.
• District XII, along the Upper Aouk River, between 9° and 10 N. This is characterized by the alternation of parkland savanna populated with rachitic trees (with Piliostigma [Baubinia] reticulate, Combretum glutinosum, and various acacias dominating! and easily flooded grass savanna with Andropogon grasses etc
• District XIII, north of Birao, betwen 10° and 11 N. Thornbush tropical steppe covers this entire region, with acacia shrubs, jujubes, and various mimosas dominating.

All of these formations may be grouped together in three regions: the first being that of the Bahr Aouk River (west of 21 E); the second that of the plains of Chad between the Koumbala River and Birao; and the third that of the areas of rocky relief south of Birao, around Ouanda Djallé and the Ouandjia Mines and along the Sudanese border. In the Bahr Aouk Zone, on quartzitic crests of the Old Precambrian period (sometimes with surface induration) grows woody vegetation composed of Daniellis oliveri, Anogeissus sp., Butyrospermum parkii, Parkia filicoidea, and Pterocarpus lucens.

In the Aluk-Aoukale zone one finds tree savannas where the most widespread woody plants are Terrninalia laxiflora, Hymenocardia acida, Prosopis africana, and Anogeissus. Actually, this is not a homogeneous zone, as there are also the main types of graminaceous pastures and dry termite forests with a tree layer of Khaya sp., Tamarindus indica, and Anogeissus and a shrub layer of Combretum spp., Cacia sp., and Bascia sp. Around the grassy plain of Lake Manoun, one may observe Borassus aethiopium, and Hyphaene thebaica, with Anogeissus, Butyrospermum, Tamarindus, Balanites, and Isoberlinia dominating. In the villages, alongside planted Ceiba pentandra, Moringa oleifera is found, which indicates a former Chadian occupation of the area. Between Dahal Hadjer and Tissi, Combretum savanna grows on the gaz (ancient aeolian sand fields).

Traditional Associations between Crops, Fodder Species, and Trees

Forest Zone

The Central African Republic is a land of forests, and thus its inhabitants have no difficulty in obtaining wood. The Republic's forest belt covers the prefectures of Sangha, Sangha Economique, Lobaye, and the southern part of Ombella-Mpoko. Bananas, cassava, coffee, taro, some groundnuts, and some sesame are grown in these regions. The cultivation of all these crops entails the destruction of forests. All trees are cut down, with the largest trees being killed either by burning or girdling. After the forest has been cleared and all the plant material burned, the plot is ready for sowing. For all except two or three of the above-mentioned crops, the soil must be turned before seeds are planted. Since the crops must have full sunlight, not a single tree is left standing and the forest is irreversibly destroyed.

Moreover, since the tropical forest is rich in wood products, the people do not spare any trees when cultivating their fields. Coffee is one of the country's main cash crops, and a modern system of cultivation is followed. If, in this type of cultivation, one finds trees still standing here and there, especially on plantations belonging to the village people, it is because they feel that allowing the trees to remain will protect the coffee plants from over-exposure to the sun, thus increasing production. Generally speaking, no trees are left standing on large-scale coffee plantations. In cocoa cultivation, trees are preserved for their shade. This type of cultivation is not widespread in the Central African Republic because of the relatively unfavourable growing conditions for cocoa . Other crops are raised on land that has been cleared and burned, although sesame is often cultivated in pockets of savanna in the midst of the forest. Production per hectare is superior to that found in typical savanna regions.

Intermediate Zone

The intermediate zone is located between the forest zone and the Vakaga zone. Its climate permits the cultivation of different types of crops. In this zone, where the majority of the population lives, cotton, sesame, groundnuts, and many other subsistence crops are cultivated. In Basse-Kotto and Mbomou, even coffee is grown. The crops are raised on Sudanian savanna where tree density per hectare is considerable. This means that the forest must be completely destroyed except where trees are of some use because they provide edible fruit, shade, or some other byproduct for which the farmer feels there is a need.

Vakaga Zone

The trees are carefully preserved in Birao (which is roughly in the middle of Vakaga prefecture) as wood is the primary fuel for cooking and lighting. As the rainy season approaches, inhabitants lay in a supply of firewood to tide them over until the next dry season. Inhabitants of this region traditionally protect "useful" trees that grow in their fields -useful because they provide fruit, oil, medications, and fodder for livestock.

There is a noticeable lack of industrial-scale cultivation in the Vakaga region. The population depends primarily on subsistence agriculture, sheep-herding, hunting, and gathering. There are approximately 15,000 inhabitants of the Vakaga prefecture, with a population density of 2 people/km². The Vakaga prefecture is divided into two subprefectures, Birao and Ouanda Djalle. The region's chief town is Birao. All Ministry of Rural Development services are represented there: agriculture, animal husbandry, and forestry. The latter is further divided into three sections (Birao Centre, Gordil, and Ouanda Djallé) with ranger stations and patrol areas.

The Vakaga region differs from other regions of the Central African Republic in that cassava has been introduced there fairly recently. Millet is grown in light, sandy soils and its abundant foliage is favoured by horses and cattle. Two varieties of Sorghum candatum are found, as well as Sorghum vulgatum, an intermediate variety, and red and white berbere (the term is used to describe heavy, compact, clayey soil and also for several varieties of Suorghum durra). Groundnuts are cultivated, but maize is not very widespread since it is simply planted around huts as a back-up cereal.

The population of the Vakaga region consumes vast quantities of roselle (Hibiscus sabdariffa), whose calyx and early fruit are used as ingredients in couscous and whose flower is used in herbal teas. This malvaceous plant is almost always planted in millet fields and around huts. Hibiscus esculentus (okra or gumbo) is one of the most common field and garden vegetables. Sesamum indicum (sesame) is planted in sandy soil, especially in fine sand, where its yield is excellent. Cucumis sativus (cucumber, or fagouss) is one of the most widely cultivated vegetables, and this is often cultivated together with squash.

Trees Recognized as Useful

Vegetation in the Vakaga region, as in most Central African regions, has been greatly affected by human intervention. Over-grazing has sometimes caused the almost total disappearance of woody vegetation in some areas. Vast stretches of desert can be seen around Birao, even though climatic conditions warrant the growth of abundant thorny vegetation. Nevertheless, six trees are recognized as being particularly useful: Balanites aegyptiaca, Butyrospermum parkii, Parkia biglobosa, Borassus aethiopium, Adansonia digitata, and Tamarindus indica. This is proof that, even when the inhabitants do not try to protect trees in their fields and incorporate them into their farming system, a certain equilibrium between agriculture and silviculture is sometimes established.

Throughout the Birao region, Balanites aegyptiaca is carefully cultivated because of its many uses. Its fruit provides an oil that is often used in cooking and some old people will eat only sauces made with Balanites oil. The sulphur-yellow flesh around the pit is eaten in much the same way as chocolate. The tree is often stripped by livestock, which favour its foliage, and also by herders, who cut off branches to build shelters for their calves. Treated in this way, the tree acquires a straight trunk. Village people use it as the main member in the frames for their huts, because the hard wood resists attacks by termites. The wood is also used to make handles for spears, axes, and other tools. Villagers are fond of eating fried beef liver on Balanites aegyptiaca leaves, which are reputed to cure liver ailments. Sometimes there are clashes between stock breeders and village people, not because livestock have destroyed millet fields but because a herder has cut branches from a tree to make a bed. This species more or less covers the northern part of the area, and three or four of these trees can be found in even the smallest fields. Its many uses cause it to be cultivated also in Bangui, the capital of the Central African Republic.

Butyrospermum parkii .(karite, or shea tree) is characteristic of the region. It provides a fleshy fruit and a stone whose oil extract is valued as a preservative. Farmers gather the bark to make beehives; this usually kills the tree. In some villages, this practice is forbidden by tradition, except for trees that have been declared unproductive.

Parkia biglobosa (nitta) is a leguminous mimosa producing pedunculate fruit and a yellow powder eaten by the local inhabitants. The seeds are boiled, fermented, and used to make a tasty sauce for eating with Hibiscus esculentus (okra). Village people often set their beehives in this tree.

Borassus aethiopium (palmyral is widely used in this region. All huts constructed by the government are made from planks from this tree, rough-hewn locally. Since the palmyra resists termites, it is also used locally in the frames of huts. The flesh of its fruit is eaten by village people; elephants are also fond of it and can sometimes even be seen in an inebriated state near large stands of palmyra. Young shoots are gathered and marketed as in other regions.

Adansonia digitata (baobab) figures in African mythology and is greatly revered. The local inhabitants make sacrifices under this tree. Shoots are eaten in a smoked fish sauce. The tree is found throughout the area but is practically non-existent in places where elephants have come to rest, for they strip the tree of its bark. The white powder around the seeds is eaten in the same way as chocolate.

There are several varieties of Tamarindus indica in Birao. Some do not sprout leaves until the start of the dry season; their fruit is picked toward the end of this season to feed livestock (sheep and kids). Others, however, have year-round foliage. It is this second kind that is usually found in fields and villages. It produces a very sour fruit thought to be a good cure for colds because it is rich in vitamin C. The branches are gathered, boiled, and used as a medicine for fevers, rheumatism, and fatigue.

Oxytenanthera abyssinica (bamboo) must be added to the six described above because of the role it plays in local construction. It is used for roofing and especially for fences, for the region is Moslem and each hut must have a fence around it.

Agrisilvicultural Equilibrium

The theory that some sort of balance must be established between agriculture and silviculture presupposes the maintenance of an ecosystem. The ecosystem is greatly influenced by climatic, biotic, human, and soil-related factors, but must be continuously maintained by agronomists, veterinarians, and foresters. Such multidisciplinary activity is difficult. When farmers in the Vakaga region allow trees to remain standing, the reason is that they derive some products or service from the trees. This explains why, although there is indeed a certain traditional agro-forestry system, its aim is not just to preserve trees or to ensure a certain equilibrium between agriculture and silviculture. Thus an information programme should be set up to teach agro-forestry methods in the areas concerned.

An information campaign presupposes an established plan ready for execution. Consideration should perhaps be given to the application, in this region, of all silvicultural methods used to date in wet forest zones and savanna: the taungya method, the Malayan uniform system, the tropical shelterwood system, the semi-selective management system, the cross-ride method, full-capacity planting, and restricted grazing Some of these methods can be applied only in forest zones, but taungya, full-capacity planting, and restricted grazing (which is the most economic and most important method), could be widespread. Initially, farmers in central Birao should be organized into three groups, according to the kinds of crops that suit each system. The first group would adopt the taungya method; the second and third groups would practice full-capacity planting and restricted grazing, respectively. Later, all three systems could be applied in every village of the Vakaga region, depending on the crops grown and the environment, the main factor being the dominant species of tree. A five- or six-year rotation would suffice to allow the rural population to grow crops on land enriched by fallowing to improve its yield.

The taungya method may be practiced in conjunction with full-capacity planting and restricted grazing. In heavily treed areas where the dominant species of trees are those that farmers are in the habit of preserving and growing in their fields, the taungya method will be applied. In Birao Centre, Ouanda Djalle, and the rural areas in these two subprefectures, the same method will be used but the kinds of crops will vary depending on the farmers' means of livelihood. This will give rise to a wide diversity of crops being raised in conjunction with Balanites aegyptiaca, Parkia biglobosa, Butyrospermum parkii, and Tamarindus indica, which are the only local trees suitable for the taungya method and restricted grazing. Even if the taungya method and fullcapacity planting fail, restricted grazing (which is the least costly method) will undoubtedly be successful if brush fires and human factors can be contained.

From the example of other African countries, it can be seen that farmers automatically preserve useful trees in their fields. However, it should be noted that this still does not provide the level of forestation necessary to maintain a favourable climate and soil fertility.

Other solutions may be considered, such as:

• Prolonging fallow periods;
• Planting a large number of useful trees in the fields;
• Planting useful trees (fastgrowing exotic species) alongside paths leading to fields and around the boundaries of these fields; and
• Planting fast-growing species and fruit trees along cattle and goat paths through farm areas and around water holes.

Conclusion

The immediate result of this activity will be to check deforestation around the city of Birao and the villages in that region, and to create an environment favourable to reforestation. Over the medium term, the main objectives will be to combat soil erosion and to make local inhabitants aware of the forest's usefulness and of the adverse effects of systematic deforestation on agriculture. Over the long term, the forest cover will be re-established and city-dwellers will be supplied with firewood through cuttings on a rotational basis.

This work will involve considerable multidisciplinary research on soil protection and restoration in each ecological zone, keeping in mind agriculture, animal husbandry, and forestry -without neglecting the human and social aspect. Customs are so important that nothing can be accomplished without a great information, popularization, and organizational campaign. The future of our tropical forests depends on it.

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