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close this book Agro-forestry in the African Humid Tropics (1982)
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The role of trees in farming systems in the humid tropics

The role of trees in farming systems in the humid tropics

A. Getahun, G.F. Wilson, and B.T. Kang
International Institute of Tropical Agriculture, Ibadan, Nigeria


The bush fallow-food crop rotation system has been the most popular and stable arable cropping system in the humid tropics of Africa. The system's stability is attributed to the presence in the fallow of deep-rooted woody plant species that are essential to soil fertility restoration. Population increases and the associated pressures on land have been threatening the stability and productivity of the system. As more land is brought in to production, the fallow period is shortened and woody species are eliminated or become ineffective.

A recent survey in southern Nigeria confirmed that the bush fallow-food crop rotation is still the dominant land-use pattern. Other important patterns are permanent tree crops (including plantain and banana), taungya, and permanent compound farming, in which trees are major components.

The role of trees in nutrient recycling, soil organic matter buildup, and erosion control has been recognized by the traditional farmers, who have identified and have been encouraging the most effective tree species in the fallow. With these selected species, the fallow period can be effective/y shortened. To exploit the potential of selected tree species in land and soil management further, we have been perfecting a system called alley cropping. This system embodies the agro-forestry concept of combining crops and trees and ensures the dominance of effective tree species during the fallow period.


The tropics have about 50 per cent of the world's population and less than 40 per cent of the earth's land surface. It is estimated that by the Year 2000, the tropics will have nearly doubled their population as against a population increase of 20 - 30 per cent in the developed countries. BY the year 2000, 60 per cent more food will be required to feed the world population (FAO 1977). Increasing agricultural productivity to meet this demand poses a special problem, as the greater proportion of the population increase is expected in developing tropical countries where food production is relatively low and, in many cases, falls below the demands of the present population. Alternative solutions to this problem can be found by expanding the area of production, increasing the productivity of the area under production, or a combination of both.

Expanding the area under food production is possible only in areas where suitable lands are available. Unfortunately, many developing nations are already utilizing fully the area regarded as arable, and further expansion would bring into production lands of marginal productivity (under existing local practices).

Increasing productivity per unit area can be approached by:

  • Introduction of high-input management technologies based mainly on fossil fuel energy and inorganic fertilizers; the use of high-input technologies has been successful under certain soil, ecological, and management systems. These so-called modern farming methods have not generally been accepted in the humid tropics because of soil, climatic, and socioeconomic constraints. In many instances, these technologies have led to serious land and soil degradation (Uehara 19761; and
  • Development of more efficient low-input systems based on biological recycling of energy and chemical nutrients through use of fallow as already practiced by traditional farmers; this alternative may maintain naturally land and soil productivity in many areas of the humid tropics (Wilson and Kang 1980).

The bush fallow and related shifting cultivation system, which is still the dominant traditional food crop production system in many parts of the tropics, particularly humid tropical Africa, involves a few years of cultivation alternating with several years of bush following, with the main purpose of the latter being soil fertility regeneration ( Ruthenberg 1971). This system has often been criticized as being wasteful and inefficient, and the major reason for degradation of soil and rapid decline in soil fertility and crop yield (FAO 1957). However, while land has been abundant, the bush fallow system has provided the traditional farmers with an efficient, balanced, and stable system for maintaining soil productivity. Problems only arise when land becomes limited due to increasing population, as has already been observed in several countries in humid tropical Africa, where fallow periods have become progressively shorter and fertility restoration becomes correspondingly less effective (Steiner 1973). Grinnell (1975) mentioned: "shifting cultivation though apparently wasteful, if not pushed to excess, has given man his livelihood in the humid tropics for centuries and is significant even now, when after a quarter century of experiment in tropical Africa, we have failed to introduce to the forest regions any method of food production superior to the natural fallow system." Because of the importance of trees and shrubs in fallows or in association with food crop production systems in land and soil management (ICRAF 1979), and also because of their roles in providing fuel, staking material, and edible products (Bene et al. 1977; Brewbaker and Hutton 1979), attention has recently been given to improving the bush fallow system and to developing improved land-based production systems for the tropics (Wilson and Kang 1980).

FIG.1. Tropical Africa: Humid and Sub-humid Zones

Ecology of the Humid Tropics

The tropics are usually referred to as the region of the earth between 23.5 north and south of the equator. They cover 4.96 billion ha or 38 per cent of the world's land mass. Forty-three per cent of this area is in Africa (Dudal 1980). Among the agroecological zones in tropical Africa, the humid and sub-humid tropics make up over 44 per cent of the total area and extend between 8°S to 8° N latitude (Fig.1).

The upland soils of low-altitude humid and sub-humid tropical Africa are dominated by low activity clay soils characterized by low cation exchange capacity (CEC) and low moisture-holding capacity (Moormann and Kang 1978). The soils can be grouped into two groups. The first is Ultisols and Oxisols (Sols ferrallitiques by INRA classification or Acrisols by FAO World Soil legend) and associated soils. These are strongly acidic and leached upland soils occurring in the perudic to udic moisture areas (with rainfall > 1800 mm). Ultisols are dominant in coastal regions. They are mainly coarse-textured, kaolinitic Typic Paleudults. The Ultisols (and some Oxisols) derived from basement complex rocks are mainly coarse-textured Plinthudults and Tropudults. The second group is Alfisols (Sols ferrugineux tropicaux by INRA classification or Luvisols by FAO World Soil legend). These are slightly acidic and less leached soils from the humid and subhumid zones. Most of these soils are derived from basement complex rocks or sandstone. In the drier forest zone, Oxic paleustalfs are the dominant soil types.

The biological environment of the humid zone, which consists mainly of infertile and fragile or easily degradable soils combined with abundance of moisture and radiation, provides a suitable and continuous growing season for the trees, which can also accumulate the largest amount of biomass. The traditional bush fallow system and its related land and soil management system is a direct adaptation to the prevailing physical and biological environments. However, the traditional system, which relies heavily on forest or woody fallows, is undergoing significant changes as a consequence of increasing population, agricultural land-use pressure, and over-exploitation of forest resources. The result is a steady decrease in the number of trees and shrubs in fallows and a steady increase in grasses and weeds which are less effective in soil rejuvenation.

Dominant Farming Systems

A recent survey of the humid and subhumid zones of southern Nigeria (Getahun 1979a; IITA 1978 and 1979a) showed that the traditional agriculture is predominantly upland and consists mainly of: (1 I bush fallow-food crop rotation; (2) permanent tree crop farming; (3) taungya; and (4) permanent compound farming in the case of eastern Nigeria.

All are characterized and dominated by a tree component. In the bush fallow, the climax of the fallow is indicated by the presence and size of certain woody species. These woody species with their deep root systems play an important role in enriching the upper soil layers by depositing litter on the soil surface. The litter decomposes and releases nutrients brought up from lower soil depths. The importance of trees or woody species in the fallow cannot be over-emphasized. Wherever the fallow has been shortened to a period inimical to the development of trees, land productivity in terms of food crop yields has declined. Shifting cultivation, in the classical sense of the moving of entire villages, may still be observed in isolated cases.

The reasons for the shifts have not been made clear and may not be directly related to factors influencing land productivity.

Tree crops, including plantain and banana (Muse spp.), accounted for up to 67 per cent of the land under cultivation, whereas arable crops accounted for only 25 per cent. Combinations of tree and arable crops, in mixtures, were common but were usually associated with farmers operating on small areas.

The taungya system is found only where forestry departments are establishing tree plantations. Food crop production takes place during the period between land clearing and plantation establishment phase. The system is not attractive to farmers and persists only where there is population pressure or when the diversion of land to plantations reduces the area available for land rotation as practiced in the bush fallow system.

Permanent compound farming represents an intensive management system in which soil fertility is maintained by addition of crop residues and household refuse. Both trees and arable crops are usually found in mixtures in this system. Plantains and bananas are of special significance in the system, as the high organic matter and mulching effect of the refuse increase the yield and productive longevity of these crops (Wilson and Braide, 1978). Fruit trees are generally found in these gardens and are readily accessible to the farm dwellers.

The overall picture reveals that outside permanent compound farms each farmer operates an arable crop (multiple cropping) and a tree crop enterprise. The arable crops supply food, and the tree crop provides cash income. The cash returns to labour from tree crops such as cocoa are often more than twice those from the food crop (Grinnell 1975). Land and labour with cash/credit as an occasional modifier are the important production factors in traditional crop production. Shortage of land invariably leads to reduction in the fallow period and expansion of the area under arable crops. Shortage of labour increases the area under tree crops but reduces the area under arable crops. Thus shortage of land appears the most serious factor threatening the survival of trees and ultimately that of human beings in the humid tropics.

The productivity of arable crop farming in traditional agriculture is positively correlated with the duration of the bush fallow. Thus, as the fallow length declines, it becomes more and more difficult to produce the food necessary to sustain the population. Therefore, to reduce the role of woody species in soil fertility maintenance and erosion control is to reduce the food output of the traditional food production system (Grinnell 1975). Tree crop production systems are suited to the humid tropics, but food is essential for human survival. Thus, if self-sufficiency is deemed necessary, as in the subsistence systems common in the tropics, arable crops are essential in the farming systems.

The survey indicates that, under high population pressure, systems are evolved in which special woody species capable of restoring soil productivity with relatively shorter fallow periods are relatively encouraged to dominate the bush fallow (table 1). Farmers recognize the potential of these species, which can achieve in two to four years that which requires six to eight years under uncontrolled regeneration.

There are no clear indications that the farmers are deliberately planting special species as fallow (Benneh 1972; Okigbo and Lal 1979), but it is encouraging to know that their awareness of the effectiveness of these species may enhance their acceptance of using woody species to reduce fallow duration while increasing land productivity in the humid tropics.

The Natural Fallow

A natural fallow is regarded as one in which recolonization by plants occurs without the interference of humans. The normal sequence of fallow development is defined by soil and climatic factors, local plant species, the agricultural technology in use, and the duration of the fallow. The general fallow regeneration pattern in the humid regions of West Africa has herbaceous grasses and broadleaved species as dominant during the first two to three years, during which they are interspersed with seedlings, root shoots, or coppice regrowth of trees and tall shrubs. According to Okigbo and Lal (1979), where the fallow period exceeds 15 years the climax vegetation includes large trees such as Albizia gummifera, Anthocleista vogelli, Diospyros con fertifolia, Funtumia elastica, Nauclea (Sarcocephalus) diderrichii, Lophira alata, Brachystegia spp., Khaya ivorensis, Triplochiton scleroxylon, Ficus spp., Cola spp., Celtis spp., and Antiaris spp. Elaeis guineensis occurs frequently in the less-crowded spaces. This type of fallow occurs only in areas of low population density where the cropping period is short and the fallow period long.

TABLE 1. Vegetation Density and Botanical Composition of Three- and Seven-year-old Bush Fallows in Eastern Nigeria


Density/ha (%)

Density/ha (%)

3-yr. 7-yr. 3-yr. 7-yr.
Dialium guineense 48.4 41.0 0.8  
Anthonotha macrophylla 27.0 34.6 53.4 33.7
Pentaclethra macrophylla 2.4 - - -
Acioa barter) 12.7 23.9 - 3.6
Alchornea cordifolia 9.5 0.5 15.3 51.2
Napoleona imperialis - - - 3.6
MM (unknown) - - 30.5 7.8
Total 100.0 100.0 100.0 99.9
Total stems/ha 1,008 1,504 1,048 1,328

Source: Amaza, Akukwe, Getahun, Okafor, et al. (unpublished),

TABLE 2. Effects of Clearing Techniques on Maize Yield, Soil Erosion, and Water Runoff

Clearing Method Grain yield maize
Soil erosion
Water runoff (mm) Soil loss/ grain yield
Traditional (partial clearing of  
secondary forest) 0.5 0.01 2.64 0.02
Manual, complete clearing 1.6 4.64 54.30 2.90
Mechanical, complete clearing 1.8 19.57 250.33 10.87

Water runoff and soil erosion losses before forest clearing were minimal.
Source: IITA 1979 (b)

Where population density is high, the fallow is short, and a new set of species is dominant in what may appear as a climax vegetation. Obi and Tuley (1973) listed the dominant species as Alchornea cordifolia, Acioa barter), and Anthonotha macrophylla. Other abundant species include Harungana madagascariensis, Dialium guineense, and Crestis ferruginea. These are interspersed with many other species.

Among the many species that dominate natural fallows, farmers have recognized the superiority of certain species in restoring soil fertility and have encouraged these species.

The preference given to some of these plants results in almost pure stands in some fallows. Benneh (1972) and Okigbo and Lal (1979) have reported planted fallows of Acioa barter), Anthonotha macrophylla, and Alchornea cordifolia in eastern Nigeria. Observations by Kang (unpublished) at Onne in eastern Nigeria showed varying populations of Anthonotha ranging as high as 1,000 - 2,500 plants/ha. In Oyo State in south-western Nigeria Gliricidia septum dominates the fallow and could be regarded as an indirect planted fallow.

The effectiveness of trees in regenerating soil nutrients and land productivity is ascribed to their ability to absorb plant nutrients from the deep soil layers and contribute them to the upper layers through leaf litter (Lundgren 1978b; Nye and Greenland 1960). The conclusion drawn from these data is that tropical people have not yet devised systems of harnessing the nutrients derived from fallow without destroying or setting back the fallow.

The importance of trees in land management and particularly in soil-erosion control is clearly demonstrated by the results of a land clearing trial that was carried out at IITA (1979b) (Table 2). Complete removal of the tree cover and mechanical clearing greatly increased runoff and erosion.

The Planted Fallow

African farmers, renowned for mixed cropping, do not as a rule remove from their field any plants that are potentially useful. Thus a large number of the species seen in the field are not planted but are useful volunteers that are encouraged. This practice applies not only to food crop plants but also to plants that are known for their effectiveness in restoring soil fertility. The popularity of Acioa barter) and Anthonotha macrophylla in the bush fallow on acid Ultisols in eastern Nigeria does not result from direct planting but deliberate encouragement of volunteers. Farmers do not destroy the seedlings of these species, and this preference leads to their dominance in the plant population. In south-western Nigeria, especially around the city of Ibadan, farmers claim Gliricidia septum is an effective fallow species which restores land productivity for food crops after a fallow of only two years. Though the species was planted by farmers who have since accepted it as an effective soil-restoring plant, there is no evidence to suggest that it was ever deliberately planted for soil improvement. G. septum is usually established when green stems are used as yam stakes. These stakes grow, eventually providing the fallow that is maintained in a slash-and-burn system in which the base of the tree is never completely destroyed. Since G. septum readily coppices, it re-establishes itself once the pressure from cropping is removed.

Where planted forest becomes a part of the rotation, as in taungya systems in West Africa, farmers have reported their willingness to follow Gmelina arborea and Cassia siamea with food crops but have avoided lands that were planted to Tectona grandis. (See the paper by Kio, Bada, and Okali in this volume, pp. 108-110.)

Dilkman (1950) reported that prunings from the tree Leucaena leucocephah are traditionally laid among crops in Indonesia to provide nitrogen and that 1 ha of Leucaena provides nitrogen equivalent to that of 1 ton of sulphate of ammonia. Work in Hawaii (Guevarra 1976) has shown that 500 - 600 kg N/ha can be harvested from the foliage of L. leucocephala. Juo and Lal (1977) found that L. Ieucocephala was as effective as naturally regenerated fallow in restoring soil organic carbon and exchangeable cations. In addition to the nitrogen yield, the dry matter yield has an important role in preventing soil erosion when it is used as mulch. The nitrogen supplied by the leaves is the result of biological nitrogen fixation, as neither leaf nor nitrogen yield was affected by applied nitrogen (Kang et al. 1981). In a study of a single harvest at the start of the rains, the tree Gliricidia septum was shown to produce less leaf dry matter and nitrogen than the shrubs Cajanus cajan and Tephrosia candida (Wilson and Kang 1980). The low yield from G. septum resulted from its deciduous tendency, as the leaves are shed toward the end of the dry season.

Though there are many reports on the effectiveness of herbaceous fallow in the tropics (Webster and Wilson 1966), very little scientific information exists on the use of tree fallows. Jaiyebo and Moore (1964) have demonstrated that a bush fallow is more effective than legume or grass cover crops in nutrient recycling and in increasing soil organic matter.

Guevarra (1976) examined the possibility of intercropping L. Ieucocephah with maize and concluded that reasonable maize yields could be obtained when the L. Ieucocephala prunings were used as fertilizer. We (Wilson and Kang 1980) have developed the "alley cropping" concept in which food crops, usually cereals and legumes, are grown in narrow alleys formed by fallow species (usually shrub or tree legumes). During the cropping period, the fallow is suppressed by regular pruning. This system is regarded as an improved bush fallow in which the fallow is formed by selected species and arranged to facilitate easier planting and crop maintenance, especially with mechanization. Though maize yields have not been as high as they are when inorganic fertilizers are added, crop performance in alley cropping indicates that sustained yields are obtainable in what appears to be a relatively stable system.

Maize-leucaena alley cropping at I ITA has thus far proved to be successful on the Alfisols. On the acid Ultisols, where Leucaena has not grown well, other tree fallow species are being tested for inclusion in the alley-cropping system.

Though alley cropping appears feasible, there are fears that farmers accustomed to cleared land may not accept trees in their fields. In addition it may be some time before equipment is developed for the mechanization of the operations associated with alley cropping. To overcome these problems, IITA is looking into the development of a cut-and-carry form of fallow management. In this system the fallow species would be grown on land unsuitable for arable cropping. The leaves and other prunings would be harvested regularly and transported to arable lands where they would be used as mulch and nutrient sources.

The alternative concept of establishing forest trees among food crops and continuing to crop the interrow spaces between the trees until the tree canopy closes could be regarded as a form of alley cropping in which suppression of the tree species occurs only after certain periods, say five to ten years, and regular pruning is not done during cropping.

With the shear-blade method of clearing land under tree fallow and with minimum tillage planting techniques, it is now possible to recover land from tree fallow and to establish certain crops without disturbing the soil. With these innovations, trees in the rotation may no longer be regarded as serious limitations to mechanization and largescale crop production in the humid tropics.