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close this book Agroforestry in the West African Sahel (1984)
View the document Acknowledgements
View the document Preface
View the document Overview
View the document Chapter 1: Desertification in the Sahel
View the document Chapter 2: Traditional Land Use Systems
View the document Chapter 3: Uses and Potential of Agroforestry
View the document Chapter 4: Agroforestry Applications
View the document Chapter 5: Sahelian Agroforestry: Institutional Considerations
View the document References
View the document Bibliography
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Chapter 2: Traditional Land Use Systems

ENERGY FLOW AND PRODUCTIVITY IN TRADITIONAL SAHELIAN AGRICULTURAL SYSTEMS

In determining the potential productivity and sustainability of an agricultural system, it is often useful to analyze the energy flows that connect the farmer with his crops and livestock. Energy flow, as used in this report, refers to the transformations and sequential dependencies of solar energy as it passes through the food chain.

Solar energy is the ultimate source of all life on earth. The basic source of energy for man is the chemical energy derived from food. The ultimate source of this food is vegetation. In energy analysis, this vegetation is termed "primary production." It is based on the capacity of plants to transform solar energy into chemical matter through photosynthesis. Man exploits solar energy by eating plants or the animals that eat plants. This relationship is often expressed 'in terms of the following unidirectional energy flow: primary production (vegetation) > herbivore (livestock) >carnivore (man), with subsequent recycling through decomposers such as bacteria and fungi (Bayliss-Smith 1982).

The amount of solar energy available for primary production is a function of geographical position. This energy is conventionally measured in joules (J). One joule equals 0.239 calories. A larger unit, megajoules (MJ), is often used to describe inputs of solar energy. One MJ equals 239 kilocalories, Approximately 10.5 MJ/day are required in human diets to maintain basic metabolic processes and regenerate tissue. In the Sahel, the annual input of solar radiation is on the order of 0.63 MJ/cm2--much higher than inputs in humid tropical or temperate regions.

Only a relatively small proportion of incoming solar energy, 1 J/cm2/hr, is converted into chemical energy as a result of photosynthesis by plants. Because photosynthesis is inefficient in its use of solar energy, it is necessary for farmers to increase vegetative cover and to arrange it in such a way that its foliage can achieve optimal productivity. The amount of foliage is often measured through the leaf area index--the total leaf area (in cm2) per unit of soil surface (in cm2). Experiments with many crops have shown that a considerable amount of solar radiation is wasted by falling in the gapsbetween leaves and between plants (Bayliss-Smith 1982). Too much plant cover can also be harmful because many leaves will be shaded and will therefore function inefficiently.

Although variation in leaf area index, availability of water and nutrients, and other factors will account for most differences in crop performance, the efficiency of photosynthesis is also influenced by the proportion of visible light that can be converted into carbohydrate by different plants. These differing capabilities relate to the so-called C3 and C4 pathways for carbon dioxide use; the former generally associated with temperate species, the latter often associated with plants in tropical and arid regions. The C4 species, such as millet and sorghum, are able to convert very high light intensities into chemical energy more efficiently than C3 species.

Temperature, moisture stress, and nutrient availability also impose constraints on plant growth. Moisture stress is of particular importance in the Sahel. Approximately 100 mm of rainfall per month is necessary to sustain crop growth. Hence, the hydrological growing season in the region between the 400- and 750-mm isohyets would generally be from 2-4 months. Nutrient deficiency is both an important constraint and the only one that is not, in practical terms, beyond the farmer's ability to control. Nitrogen, phosphorus, potassium, calcium, and magnesium are particularly important nutrients.

In a traditional Sahelian agroforestry system, trees, shrubs, and annual crops are often integrated in such a manner that leaf area indices are relatively high, permitting correspondingly high levels of productivity during the short growing season. The prominence of C4-pathway staples, such as millet, sorghum, and maize, further contributes to the productivity of traditional agricultural systems. The deep-rooted trees and shrubs in the systems provide a wide range of products of importance to local populations, forage for livestock, and microclimates that support greater crop diversity. They also recycle critical nutrients, including the important nitrogen contributions of leguminous species such as Acacia albida. Research in Senegal has revealed that yields of millet and groundnuts grown under A. albida trees on infertile soils increase from 500 +/-200 kg/ha to 900 +/- kg/ha. In addition to increased crop yields, there are 50-100 percent increases in soil organic matter, improved soil structure, increased water-holding capacity, and a marked increase in soil microbiological activity beneath the trees (Felker 1978). In addition to nitrogen-fixing rhizobial bacteria, there are larger populations of other important soil organisms, such as the mycorrhizal fungi that are so important in facilitating phosphorus uptake in plants. The increased stability and productivity of these systems reduces the need for extended fallows.

The perennial components of traditional Sahelian agroforestry systems are also effective in intercepting rainfall, promoting ground water recharge, and preventing erosion. According to some climatologists, there is also a possibility that increased perennial ground cover increases or maintains higher levels of rainfall (National Research Council 1983).

Departures from traditional systems of agroforestry in the Sahel have often included the clearance of trees and shrubs from agricultural lands. Land is cleared for a variety of reasons: the perceived need to reduce competition with crops for water, to eliminate roosts for granivorous birds such as Quelea quelea (black-faced diochs), and to permit the use of mechanized farm equipment. Unfortunately, the clearance of woody perennials also results in lower potential productivity: the leaf area index and ground water recharge are often reduced, soil temperatures and soil-water relationships become less stable, soil ecology is disrupted, nutrient cycling breaks down, and erosion increases.

In many instances, complex mosaics of traditional C4 food crops and legumes have been replaced by mono cropped systems of comparatively less efficient C3 cash crops such as rice and cotton. One result is that a much narrower range of food and other plant products is available for local use, with a corresponding reduction in the self-reliance of rural populations. In the absence of leguminous field trees, these crops require heavy applications of costly commercial fertilizer. Fertilizer requirements are further increased as the mineral nutrients contained in the crops (approximately 5-10 percent of the dry weight of the plants) are lost to the system with the export of the crops. It has been estimated that in some areas the shift to open-field cultivation has reduced the human carrying capacity of the land from 25-40 individuals/km2 to 10-20 individuals/km2 (Felker 1978).

 

TRADITIONAL STRATEGIES OF LAND USE

Two strategies of land use have been employed so far and will, to some extent, continue to be essential in the Sahel. The first is based on nomadic pastoralism and is characterized by:

· Extensive use of vast areas for grazing

· Mobility of people and livestock, as a flexible reaction to even small environmental and climatic changes

· Year-long engagement in land use; seasonality is met by rhythmic migration, adapted development of herds

· Multiple exploitation of plant cover but no cultivation of plants

· Little regular market sale of production

· Animals (cattle, camels, donkeys, goats, and sheep, as well as wildlife) are at the center of all traditional life.

The second strategy is based largely on sedentary rain fed agriculture, characterized by:

· Intensive use of small favorable sites

· Relative immobility of residents; if applicable, shifting or rotation of fields within easy reach of the family compound

· Short season of concentrated agricultural efforts, dependent on rains (July-October), seasonal "unemployment" frequently taken up by nonagricultural jobs in towns and cities

· Cultivation of a few plant species for specific products; supplementary multiple use of natural vegetation

· Only minimal livestock (e.g., small ruminants such as goats) kept for products used in the household (milk, meat, hides)

· Field crops beyond subsistence demand are regularly sold or exchanged for various goods.

Additional strategies of regional importance, but more localized application, would include the highly productive recessional (decrue) agriculture practiced along the margins of seasonally fluctuating lakes and rivers.

Sedentary agriculture is expanding northward, and there is increasing overlap and competition as well as a mixture of the original two basic types of land use. The mayor problems and conflicts arise in the transition zone between pastoralism and rain fed agriculture.

Natural stands of trees and shrubs have always played a supportive role for both pastoralism and agriculture. With important exceptions such as oasis agricultural systems, however, they have seldom been regenerated or newly planted by local people. In most areas of the Sahel, continuous selective exploitation and land use practices that are incompatible with tree growth have changed the original climax, eliminated many species of trees and shrubs, and resulted in degraded stands on the one hand and in local concentrations of certain species on the other. For example, such highly esteemed trees as Prosopis africana, Lannea spp., and Ximenia americana survived only in the southern savanna lands; apart from relic stands, they have disappeared from the Sahel proper. Other species, such as Adansonia digitata (baobab), Butyrospermum paradoxum (shea-butter), Parkia clappertoniana (West African locust, or nere), and Calotropis procera, concentrate in disturbed areas around human settlements. Bush fires have also played and still play a decisive role in species selection, resulting in a "fire-climax" in which only a few resistant species survive.

Tree culture as a preliminary stage of real "forestry" never had a place in traditional land use systems. Therefore, in discussing the following three agroforestry systems, the multiple and sometimes intensive use of naturally growing trees and shrubs largely represents the forestry contribution. These three systems are variants of the two land use strategies involving trees: first, mainly with livestock (silvo-pastoral systems); second, with agriculture (agro-silvicultural systems); third, with both in combination (agro-silvo-pastoral systems).

 

SILVO-PASTORAL SYSTEMS

Silvo-pastoral land use is predominant in all areas of the Sahel not used for agriculture. There are two main types of silvo-pastoral systems. The first predominates in the northern part of the Sahel where there is little rainfall and where nomadic or transhumant pastoralism is the main land use. Trees and shrubs, which form a more-or-less open canopy over the grasslands, according to site conditions, supply a substantial part of the livestock's fodder needs.

H. N. Le Houerou (1980) has suggested that, under prevailing conditions, up to one-third of all forage could be supplied by trees and shrubs. Leaves and fruits account for about 5 percent of average livestock fodder needs during the rainy season and up to 45 percent toward the end of the dry season. Their importance, however, is not only to be measured in kilograms of available dry matter but, even more importantly, in terms of fodder quality (for example, content of digestible protein, minerals, and vitamins). Research in northern Senegal has shown that an average tropical livestock unit (corresponding to one 250-kg bovine) consumes about 1,200 kg of grass and herbs and more than 500 kg of woody forage between November and July (dry season). As the net annual increase of grass and herbs in this region is between 300 and 1,300 (average 700) kg per hectare (ha), and that of accessible and digestible forage from woody plants is about 120 kg/ha, empirical research has shown that during the entire year, about 5 ha of average bush land are needed to feed one tropical livestock unit. There is considerable variation, however, subject to soil and rainfall conditions. Only 1 ha will be needed on highly productive sites--for example, in seasonally flooded depressions; over large parts of the drier northern Sahel, however, at least 10-20 ha or more will be required.

Trees and shrubs of the Sahel have thus played a significant role in livestock management. Governments in the past often responded to apparent demand by establishing vast silvo-pastoral reserves, now under the supervision of the national forest and range management services. In addition to forage, trees and shrubs furnish important supplies of edible fruits and leaves to the many people who do not grow agricultural crops. They also provide wood and many other products, including medicinals from the "ever-present pharmacy.

Natural grasslands with a relatively high proportion of trees and shrubs continue to form the basis of a more-or-less sustainable silvo-pastoral land use system over millions of hectares and for millions of people. Growing populations and steadily increasing numbers of livestock, however, have led to critical and destructive developments during the past few decades. Without improvement and controlled management, the carrying capacity of northern Senegal appears to be just below 1 person and 25 livestock units per square kilometer (100 ha). This would result in the annual removal and consumption of over 125 kg of woody forage and around 5-7 kg of wood and minor forest products per hectare, compared with the annual increment of 120 kg of accessible and digestible woody phytomass and 123 kg of wood and bark as measured at Fete-Ole, Senegal. In other words, pressure on woody forage has reached its critical level in most areas, whereas wood consumption for household use becomes destructive in concentrations of more than 25 people per square kilometer. These figures, although provisional and of rather local validity, not only indicate the definite limits to further growth but also explain the existing threat of desertification in some parts of the Sahel.

A second type of traditional silvo-pastoral system is found mainly in the southern Sahel (largely in areas with more than 500 mm annual rainfall) and in the transition zone between pastoralism and rain fed agriculture. Here, on the same plot of land, crops of millet, cow peas, and groundnuts are grown during the rainy season. After harvest the fields are opened to livestock to feed on agricultural residues and weeds; during the rainy season the livestock are maintained on the surrounding grasslands. This form of rotation is well established and adapted to the ecosystem and to local needs, and is practiced by sedentary or semi sedentary farmers, often by agreement with migrating herders (Gallais 1975). The system comprises a phase of green fallow over several years, where trees cut at stump level recover. These fallows usually offer much richer grass and herb vegetation than surrounding bush lands. Woody plants play a subsidiary role in this system; trees are lopped, and leaves and pods are collected as additional fodder. Leaves of bushes and trees sprout 6-8 weeks prior to the arrival of rain, supplying a valuable protein source at the time of greatest forage scarcity.

 

AGRO-SILVICULTURAL SYSTEMS

Agro-silvicultural systems combine tree and shrub utilization with rain fed or, occasionally, irrigated plant production. Most include a livestock component as well. Perhaps the best-known examples are the oases of the desert and sub desert zones. These systems are described in detail in the early literature of the northern Sahel.

Travelers' chronicles dating from the Middle Ages to the late nineteenth century describe agro-silvicultural systems in the northern Sahel that contained a far greater diversity of crops than is found in the region today. There was frequent mention of northern grains, such as wheat and barley, as well as Mediterranean vine and tree crops such as grapes, olives, and figs. Many of these crops were associated with Berber and Arab settlers from the north and were apparently supported by summer rains, the more frequent winter rains received in the past, and by supplemental irrigation (Robert 1976).

Other major grain crops included sorghum, millet, and maize. Garden crops grown in the oases included broad beans, lubia, peas, carrots, okra, turnips, swedes and cabbage, cucumbers, various kinds of melons, pumpkins and squash, purslane, eggplant, tomatoes, radishes, beets, onions, garlic, and chili peppers. Sugar cane was also widely grown. Important exports from the oases included mallow, sesame, saffron, cumin, coriander, "Sudan" chilies, raisins, cotton, indigo, esparto grass, and henna. Noteworthy among the trees and shrubs associated with these systems were date palms, lemon trees, Seville orange trees, mulberry trees, pomegranates, and almond, peach, and apricot trees. Wild species exploited for food included colocynth, Zizyphus spp., Hyphaene thebaica (doum palm), truffles, Nitraria tridentata, and wild grains such as the seed of Aristida pungens (Nachtigal 1879, Levtzion and Hopkins 1981).

Representative of the agro-silvicultural systems of the southern Sahel would be that of the Yatenga region of Upper Volta. In the Yatenga systems, the areas nearest the settlements are under almost permanent cultivation. Fertility is maintained by nutrient cycling through trees--particularly Parkia clappertoniana, Tamarindus indica (tamarind), and Butyrospermum paradoxum; short, 2-year fallows; and by dressing the fields with ash and manure collected in the pasturelands beyond the zone of cultivation.

The crop rotations employed in the region largely reflect soil qualities. For example, the following rotation would typically be associated with hydromorphic soils:

Year 1

Maize associated with cotton

Years 2-6

Sorghum and cow peas

Years 7-9

Groundnuts

Year 10

Grazed fallow

On better agricultural soils, the fallow period is frequently eliminated:

Years 1-6

Maize

Year 7

Sorghum and cow peas

In this rotation, cotton is associated with cereals every 2 years. On lands that are degraded or difficult to cultivate, millet and sorghum are cultivated for a period of 5 years, followed by grazed fallow or the cultivation of groundnuts (Food and Agriculture Organization 1981).

Today, a number of agro-silvicultural systems appear to be practiced in the Sahel. Gardens are found within settlements where water is available, usually with a tree component that provides shade and shelter and, often, edible fruits or leaves. The same holds true for intensively managed, irrigated, and fertilized gardens near urban centers. Both subsistence home gardens and cash-generating market gardens are highly productive. Fruit and pod-bearing trees, shade trees, and hedges or living fences are the "forestry" components, sometimes supplemented by decorative woody plants. Mangoes, citrus trees, guavas, Zizyphus mauritiana (Indian jujube), cashews, palms, Ficus spp., and wild custard-apples are prominent kinds of fruit trees. Shade is often provided by Azadirachta indica or similar species, while fencing is provided by thorny species of Acacia and Prosopis, and by Commiphora africana, Euphorbia balsamifera, flowery shrubs such as Caesalpinia pulcherrima (paradise-flower), and other species.

Close to the settlements is a ring of suburban gardens, often irrigated, in which cassava, yams, maize, millet, sorghum, rice, groundnuts, and various vegetables are grown, for subsistence as well as sale, depending on the ecozone.

Outside the villages in areas of adequate rainfall are bush farms, often more than 1 ha in size, where the region's main agricultural crops such as millet, sorghum, maize, and groundnuts are grown. These farms may be and sometimes are so far away that people have to build shelters to stay there for many days without returning home. Woody plants are often cut from the fields to avoid competition with crops or to deny nesting to birds, and for use as fencing material. Where possible, however, Acacia albida or food trees (for example, Butyrospermum paradoxum, Parkia clappertoniana, Tamarindus indica, Borassus aethiopum (ronier), and Hyphaene thebaica) are left and often carefully maintained and protected. With some regional variations, a very old and sophisticated tradition of food-tree culture flourishes, which has resulted in an almost perfect combination of single-tree management and intensively managed farmlands and therefore merits detailed study.

 

AGRO-SILVO-PASTORAL SYSTEMS

These systems combine all three elements of crops, trees, and livestock in both space and time. Two systems are mentioned here. The first is a rotation system that is common in the Republic of Sudan but is also known in Chad and Niger. It is based on about 20 ha of family-owned land subdivided into 4 sections to be rotated every 5 years. For example:

Years 1-5

Plantation of Acacia senegal trees; in between agricultural land use (for

example, where millet is cultivated).

Years 6-10

First harvest of gum arabic from the trees;cutting grass from fallow land.

Years 11-15

Period of full yields of gum arable; controlled grazing possible between

trees.

Years 16-20

Final harvests of gum arable; clear felling of the acacia trees (used for

firewood, charcoal,posts); grazing.

The rotation continues with replanting and agricultural use in the twenty-first year. Depending on site quality and rainfall, these 20 ha can maintain a family of 5-10 people.

The second system is found in many parts of the Sahel where rainfall and ground water level permit the growth of Acacia albida, the remarkable multipurpose tree of semiarid Africa. Outstanding examples are to be seen in Senegal (near Bambey) in parts of Upper Volta, and in southern Niger (Dallol-Maouri). Acacia albida is, in fact, often regarded as a sacred tree. Proverbs refer to it: "The tree is your better friend: it never demands but always gives. The sultans of Zinder, in former times, used to cut off the hands of people who illegally felled an Acacia albida.

This leguminous, nitrogen-fixing tree has a deep tap root system that enables it to tap lower sources of water and nutrients inaccessible to agricultural plants. It is leafless during the agricultural season and therefore does not compete with crops for light. It produces leaves and provides ample shade during the hot dry season--a phenomenon known as reverse foliation. The leaf litter of the trees contributes nitrogen, other nutrients, and much-needed organic matter to the fields. In addition to soil improvement, which results in up to 100 percent higher yields than those outside the range of Acacia albida crowns, this tree produces large quantities of qualitatively valuable forage by shedding its apple-ring-like pods during the dry season. The wood is used for local construction and as fuel. The thorny branches serve as fencing material, and many parts of the tree provide medicine for people and livestock. About 20 adult trees per hectare will guarantee an almost perfectly balanced system of agroforestry with high yields in millet production in the rainy season (Figure 4) and more than 1,500 (sometimes as much as 2,500) kg of pods for feeding cattle and small ruminants; that is, sufficient for the supply of one tropical livestock unit per hectare in the fodder deficient dry season. Therefore, Acacia albida has been given high priority in agroforestry projects in suitable parts of the Sahel.