Slash-and-Burn Cropping Systems of the Tropics

Cropping Systems: Slash-and-Burn Cropping Systems of the Tropics

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Ken Giller Wageningen University, Wageningen, The Netherlands

Cheryl Palm Columbia University, Palisades, New York, U.S.A.

INTRODUCTION This article describes the key features of slash-and-burn agriculture (otherwise known as shifting cultivation). Such systems are useful in demonstrating the concept of agricultural sustainability in relation to increasing pressure on land due to population growth. The largely closed nutrient cycles under natural vegetation are opened up, resulting in losses that can only be restored through long fallows or through intensification of production.

SLASH-AND-BURN IS THE OLDEST FORM OF AGRICULTURE In the hierarchy of farming systems, slash-and-burn (or shifting cultivation) is essentially the basic form of agriculture, and remains the perfect example to illustrate the concept of sustainability in agriculture in relation to intensity of land use. Shifting cultivation can be simply defined as the ‘‘alternation of cropping periods on cleared plots and lengthy periods when the soil is rested.’’[1] Although we tend to associate slash-and-burn with typical management systems in tropical rainforests, various types of shifting cultivation occur in areas of both forest and savanna, and were in fact the dominant form of agriculture practiced in the conversion of temperate-zone forests and woodlands. Various forms of shifting cultivation can be identified in pollen records from many parts of the world dating back several thousand years. From the great complexity of causes of tropical deforestation, agricultural clearance by smallholder farmers is but one force—currently a relatively minor one.[2] Currently, major forces driving the clearance of forests are the exploitation of tropical timber and land clearance for commercial agriculture. Currently about 37 million people practice shifting cultivation on 1035 million hectares of land in the tropics (Table 1). Although this is only about 3% of the agricultural population, it encompasses 22% of the agricultural land area of the tropics. Regionally, similar numbers of people practice shifting cultivation in Africa, Latin Encyclopedia of Plant and Crop Science DOI: 10.1081/E-EPCS 120010540 Copyright D 2004 by Marcel Dekker, Inc. All rights reserved.

America, and Asia. The area in Latin America is two times more than that in Africa and three times that in Asia.

THE FOUR PHASES OF SHIFTING CULTIVATION Four main phases can typically be identified in a cycle of slash-and-burn agriculture: clearing, burning, cropping, and abandonment. The abandonment phase involves the movement of activity to a new location, sometimes in very extensive systems involving movement of whole settlements. (This is essentially a long fallow phase during which the productivity of the soil (system) recovers.) Crop-fallow systems in temperate regions are generally associated with soil left bare while the land is rested, but in most tropical regions the fallow phase is associated with the rapid regrowth of vegetation. The clearance phase involves the felling of trees and slashing of the shrub layer (understorey). This phase usually occurs at or before the onset of the dry season so that the slashed vegetation can dry out to allow burning at the end of the dry season or beginning of the rainy season. Fire is the basic tool used to clear away the vegetation. This job would otherwise require significant time and labor. The burn has several positive and negative effects on the systems’ productivity. The intensity and effects of burning depend substantially on how the vegetation and fire are managed. Particularly important factors are the length of time the cut vegetation is left to dry, and whether heaps or piles are made before burning. Piling of biomass tends to achieve a more complete burn, and leaves behind little organic matter. The clearance and burning phases result in the opening up of what are relatively closed nutrient cycles under forest, where the perennial root system ensures efficient capture of nutrients into the vegetation and recycles through litterfall and decomposition.[4] Elements that are readily oxidized to gases (notably C, N, and S) are lost during the burn, although the degree of nutrient loss depends on the intensity of the fire. Other nutrients—notably 363

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Table 1 Land area and population practicing shifting cultivation in the tropics

Africa Latin America South and Southeast Asia Tropical total

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Million hectares

Agricultural population (millions of people)

263 600 172 1035

11 11 15 37

(From Sanchez and Palm, forthcoming; adapted from Ref. 3.)

the basic cations Ca, Mg, and K (and to some extent P)— are returned to the soil in the ash and serve as fertilizer for the subsequent crops. As many of the soils on which shifting cultivation is practiced in the tropics are strongly acidic (often with high saturation of aluminium ions), the addition of substantial amounts of base cations can have an important liming effect and ameliorate conditions for plant growth by decreasing aluminium toxicity. Burning also increases the direct impact of rain hitting the soil. With the loss of plant cover and destruction of the litter layer, erosion can result, causing drastic loss of highly enriched surface soil. Cropping is the third phase of shifting cultivation and is extremely variable. It can be as short as two cropping seasons on inherently infertile soils, or as long as five cropping seasons on more fertile soils.[4] The first crops are typically fast-growing and nutrient-demanding, such as cereal crops (including maize, upland rice, sorghum, or millet). Subsequent crops tend to be slower-growing and less nutrient-demanding (such as cassava, bananas, or legumes). Characteristics of the cropping phase are a rapid decline in soil organic matter and soil fertility, described in the classic text of Nye and Greenland.[5] This decline is often accompanied by an increase in weed pressure. Eventually the investment of labor in weeding exceeds the return in crop productivity, so that moving and clearing new plots is more favorable than continued cropping. At this point, the vegetation is left to regrow into the fallow phase. The length of fallow required to restore the original productivity of the land depends on many factors, including the length of the preceding cropping phase. Periods of at least 15–20 years appear to be necessary in West Africa.[4,6] Ruthenberg proposed a useful classification of systems based on the intensity of land use,[1] where a value R is assigned for the proportion of land cultivated annually, or the proportion of time any given piece of land is held under cultivation. If the proportion of land cultivated R = 0.15, then the dwellings also move; if R = 0.30, then a greater portion of dwellings tend to be static.

When R rises above 0.33, such land uses are no longer considered to be shifting cultivation, and instead are classified as fallow systems. Systems with values of R above 0.70 are regarded as permanent farming.

SHIFTING CULTIVATION SYSTEMS ILLUSTRATE SUSTAINABILITY The concept of sustainability of the natural resource base and agricultural production is illustrated in Fig. 1, based on the early analyses of Guillemin.[7] The first case represents a situation where land is abundant, so that there is ample time for the land’s productivity to recover to its original status before cropping (Fig. 1a). Under this scenario there is a period during which the land is rested unnecessarily. Greater productivity can be achieved when land is cropped as soon as soil productivity is restored to its earlier status (Fig. 1b). The time required to recover after each cropping phase is indicated as increasing, presumably as the rate of vegetation recovery decreases with repeated clearance. As population pressure on land increases, the length of the fallow restoration period is shortened, so that the land is cleared and cultivated before it reaches its prior soil fertility status, resulting in a productivity decline to a new equilibrium value (Fig. 1c). Although recovery of soil fertility is often stated as one of the main reasons for the fallow phase, the level of nutrients in the soil often decline’s during the fallow phase as nutrients are transferred from the soil to the regrowth vegetation.[8] As such, it is the total nutrient stocks in the soil plus vegetation system that is important to the recovery of fertility. A fairly well documented example of shifting cultivation is the Chitemene system found in northeast Zambia.[9,10] The whole region is covered by open savanna miombo woodland dominated by nonnodulating legume trees belonging to the subfamily Caesalpinioideae of the genera Brachystegia and Julbernardia. Circular areas of land are opened by lopping the high branches from the trees, leaving trunks 2–4 meters in length in the fields. The branches are heaped into the center of the opened circle, typically an area 10 times that of the cultivated area needed to provide sufficient vegetation to obtain adequate crop yields. The predominant soils in northern Zambia are strongly acid Oxisols and the large amount of ash is important in ameliorating aluminium toxicity in the soil. The land is typically sown to the relatively nutrient-demanding crop of finger millet (Eleusine coracana) followed by a crop of longer duration of cassava, often intercropped with Bambara groundnut (Vigna subterranea). The land is then abandoned and left fallow, with trees fairly rapidly regrowing from the lopped stumps. The population pressure in this region long ago exceeded the carrying capacity of the land,[11] so that although

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CONCLUSION By combining data into simple mathematical models, substantial insights have been gained in our understanding of shifting cultivation. The first such attempt can be traced to the classical work of Nye and Greenland,[5] who described the basis of declining soil C content, depending on the relative lengths of cropping and fallow phases. Trenbath[13] used a wide variety of data sources from Southeast Asia to demonstrate how the length of the fallow recovery phase increased with longer periods of cropping, to the point that after six crops, vegetation recovery was deflected to anthropic savanna dominated by dense grass cover, typically of Imperata cylindrica. This analysis has been further developed by van Noordwijk to explore the relationships between population pressure, productivity of agriculture, and other ecosystem services such as C stocks, biodiversity, and clean water provision.[14,15] Fig. 1 The theoretical relationship between the length of fallow and soil productivity. (From Refs. 1 and 7.)

Chitemene agriculture is still widely practiced, much of the population depends on permanent farming or on income from relatives living in cities.

UNDERSTANDING SHIFTING CULTIVATION Shifting cultivation is an extensive form of food production. Various attempts to intensify agriculture in remote areas have tended to fail for reasons clearly summarized by Ruthenberg:[1] Fallow systems exist by soil-mining, and efforts to prevent it are usually not economic (. . . .) given the price relations in the location and the preference of the people concerned. The return to soil-preserving measures (green manuring, compost, terracing, etc.) is too low in relation to the disutility of effort, and there is not yet the scarcity of land which would bring about a change in preferences.

As such, there is an almost inevitable degradation in the natural resource base with the intensification of shifting cultivation, until the point that the land becomes scare and the returns in soil-preserving measures become high. This process of land degradation and subsequent investment in and recuperation of land is well described by Boserup.[12] The search for alternative forms of sustainable livelihoods for people living on the forest margins of the tropics has been a major aim of the Alternatives to Slash-and-Burn Program (http://www.asb.cgiar.org/ ).

ARTICLES OF FURTHER INTEREST Organic Agriculture As a Form of Sustainable Agriculture, p. 846 Sustainable Agriculture: Philosophical Framework for, p. 1198 Reconciling Agriculture with the Conservation of Tropical Forests, p. 1078 Sustainable Agriculture and Food Security, p. 1183 Sustainable Agriculture: Definition and Goals, p. 1187 REFERENCES 1.

Ruthenberg, H. Farming Systems in the Tropics, 3rd Ed.; Clarendon Press: Oxford, 1980; 101. 2. Geist, H.J.; Lambin, E.F. Proximate causes and underlying driving forces of tropical deforestation. Bioscience 2002, 52, 143 – 150. 3. Dixon, J.; Gulliver, A.; Gibbon, D. Farming Systems and Poverty: Improving Farmers’ Livelihoods in a Changing World; FAO: Rome, 2001. 4. Sanchez, P.A. Properties and Management of Soils in the Tropics; John Wiley: New York, 1976; 618. 5. Nye, P.H.; Greenland, D.J. The Soil Under Shifting Cultivation, Technical Communication No. 51; Commonwealth Agricultural Bureaux: Harpenden, UK, 1960. 6. Szott, L.T.; Palm, C.A.; Buresh, R.J. Ecosystem fertility and fallow function in the humid and subhumid tropics. Agrofor. Syst. 1999, 47, 163 – 196. 7. Guillemin, R. Evolution de l’agriculture autochthone dans les savannes de l’Oubangui. Agron. Trop., Nogent 1956, 12. Nos, 1,2,3. 8. Szott, L.T.; Palm, C.A. Nutrient stocks in managed and natural humid tropical fallows. Plant Soil 1996, 186, 293 – 309.

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9.

Stromgaard, P. Biomass, growth, and burning of woodland in a shifting cultivation area of south central Africa. For. Ecol. Manag. 1985, 12, 163 – 178. 10. Stromgaard, P. Soil nutrient accumulation under traditional African agriculture in the miombo woodland of Zambia. Trop. Agric. (Trinidad) 1991, 68, 74 – 80. 11. Chidumayo, E.N. A shifting cultivation land use system under population pressure in Zambia. Agrofor. Syst. 1987, 5, 15 – 25. 12. Boserup, E. The Conditions of Agricultural Growth; Aldine: New York, 1965.

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13.

Trenbath, B.R. The Use of Mathematical Models in the Development of Shifting Cultivation Systems. In Mineral Nutrients in Tropical Forest and Savanna Ecosystems; Proctor, J., Ed.; Blackwell Scientific Publications: Oxford, 1989; 353 – 371. 14. van Noordwijk, M. Scale effects in crop-fallow rotations. Agrofor. Syst. 1999, 47, 239 – 251. 15. van Noordwijk, M. Scaling trade-offs between crop productivity, carbon stocks and biodiversity in shifting cultivation landscape mosaics: The FALLOW model. Ecol. Model. 2002, 149, 113 – 126.

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