the problem

Characteristics of slash-and-burn

Slash-and-burn (or shifting cultivation), the traditional farming system over large areas of the humid tropics for centuries, still remains the dominant land-use practice in about 30% of the arable soils of the world and provides sustenance for an estimated 250 million of the world's poorest people and additional millions of migrants from other regions (Hauck 1974).

Throughout the world, traditional shifting cultivation practices are remarkably similar. Small forested areas are cleared by axe and machete during periods of least rainfall and are burned shortly before the first rains. Without further removal of debris, crops such as com, rice, beans, cassava, yams and plantains are planted in holes dug with a planting stick or in mounds for root crops in Africa. Intercropping is very common, and manual weeding is practised. After the first or second harvest the fields are abandoned to rapid forest regrowth. The secondary fallow may grow for 10 to 20 years before it is cut again (Nye and Greenland 1960, Sanchez 1976).

Traditional shifting cultivation is based on nutrient cycling and weed and pest suppression during the fallow period. In regard to nutrient cycling, burning releases to the soil about half of the nitrogen and phosphorus of the burning biomass and practically all of the other nutrients in the form of ash, which also causes a liming effect. Higher soil temperature following burning also accelerates the decomposition of soil organic matter. These factors provide high nutrient availability for one or two years to grow food crops, depending on the inherent fertility status of the soil (Seubert et al. 1977, Smyth and Bastos 1984, Lai et al. 1986). Burning also helps to control pests and diseases and enables cultivators to clear land quickly and efficiently with minimal labour requirements. (Alternative forms of clearing land are usually too expensive and labour intensive, especially for poor farmers.) As nutrients are removed by crop harvests or lost to leaching, overall fertility declines. Nutrient deficiencies as well as increasing weed pressure impede further cropping, and the fields are abandoned to a secondary forest fallow. The secondary forest grows rapidly, tapping nutrients remaining in the soil, including those released slowly by un-burned decomposing forest biomass, accumulating them above ground for 10 to 20 years, until the cycle begins again (Nye and Greenland 1960, Szott and Palm 1986).

Biodiversity is another unique feature of this form of cultivation. Most indigenous shifting cultivation systems consist of complex polycultures. They have a diversity of crops and species, including trees and food crops in 'traditional' agroforestry systems. This diversity helps to reduce the risk of disease and pests, and it provides a diverse source of foods and products for families. In addition, indigenous systems are usually tied to complex cultural norms and socioeconomic traditions and involve unique cultural knowledge of diverse species. Also, the vegetation of the fallow stage serves as a genetic reservoir for many important plants and is a refuge for invertebrate and vertebrate animals.

In relation to environmental problems promoted by slash-and-burn cultivation, the burning releases half of the nitrogen and most of the sulphur and carbon into the atmosphere, thus contributing to greenhouse gases. Similarly, the accelerated organic matter decomposition associated with intensified agriculture increases carbon loss; the higher nutrient availability can increase gaseous nitrogen emissions.

The fallow period therefore does not improve soil fertility per se. Except for some reaccumulation of carbon in the soil organic matter and fixation of atmospheric nitrogen, fallows mostly accumulate nutrients in the plant biomass, which can be tapped by future crops upon slash-and-burn. The essential mineral nutrients (phosphorus, potassium, calcium, etc.) are extracted from lower soil layers during regrowth and brought to the surface by plants. However, unlike N2 fixation, this is essentially a slow process, which concentrates nutrients where they can be used to grow a crop but which does not add nutrients to the system. These slowly accumulated nutrients are then removed in the harvested crop, increasing the net loss of nutrients from the whole system.

The traditional systems are ecologically and environmentally sound at low (or very low) population densities. For millions of peoples, shifting agriculture in traditional forms has provided and still provides a basis of subsistence, cultural values and social stability, particularly under low population density. These systems are in various forms, ranging from classic swid-den systems to altered forms, such as the taungya system. These systems also have features, such as nutrient cycling and diversity, that are useful for understanding sustainable land uses in the humid tropics.

However, traditional shifting cultivation systems are being rapidly replaced by shifting cultivation in disequilibrium, which turns into unsustainable forms of agriculture. These changes also lead to cultural and social disruption of traditional societies. The unsustainable form of slash-and-burn is practised by migrants from other regions, who are unfamiliar with the humid tropics and ignorant of the sophisticated practices of indigenous cultures that make shifting cultivation a sustainable system. The central concern in this project is unsustainable slash-and-burn, in areas where alternatives are needed, as distinct from the traditional systems practised by indigenous people in forest areas.

Soil erosion is seldom a significant problem in traditional shifting cultivation, because the land area is relatively small and is always covered by some sort of vegetative cover, such as fallen logs, crops, weeds or forest fallows. When shifting cultivation is practised by newcomers to the humid tropics, the land is often devoid of soil cover for considerable time. This can lead to major erosion and siltation of rivers, particularly in hilly areas (Lal et al. 1986).

Shifting cultivation is definitely not sustainable if significant increases in land productivity are required to support higher human population densities and increased demand for food and fibre. Recent increases in population growth, as well as transmigration to poorer, less fertile soils, have placed great pressure on farmers to increase the productivity of limited land resources by expanding the length and intensity of the cropping period and decreasing the fallow period. Migration is less significant in Africa, but major in Latin America and Asia.

As the time available for secondary forest fallow growth decreases, the fertility and productivity of the soils, which are mostly low, continue to decline. Furthermore, when the fallow period is shortened, it generates a disequilibrium of carbon input-output ratios and intensifies nutrient mining. Complex and often adverse ecological changes occur, such as invasion of Imperata grasslands and reduction in the number of native seeds left viable for regrowth. Re-establishment of secondary forest fallow vegetation is slowed or stopped, some soil becomes bare and erosion begins. This situation is typified by the so-called derived savannas, which occupy more than three-fourths of the previously tropical moist forest of West Africa. In turn, these declines in productivity contribute to increasing economic hardship and impoverishment for shifting agriculture under conditions of disequilibrium. Moreover, people in these situations tend to lack access to alternative economic opportunities, are isolated from development programmes and in some areas are marginalized further by the expansion of large-scale producers such as cattle ranchers. The trends towards expansion of the cropping cycle and a 'decrease in the fallow period are central to the problem of non-sustainable production. They are also the key trends affecting the contribution of slash-and-burn to global warming. In particular the net reduction in soil organic matter and plant biomass through intensification and modification of traditional systems are the main mechanisms by which slash-and-burn is, if anything, increasingly contributing to the greenhouse effect. The two problems, and their solution, go hand in hand.

Population growth and poverty are not the only causes behind increased rates of deforestation. External forces or processes such as expansion of commercial plantations or farms, ranching, logging and mining also attract or push migrants into slash-and-burn, causing considerable deforestation. In Africa the expansion of cash crops for export (e.g., groundnut, cotton, coffee, cocoa) has considerably reduced land availability for food crops, increasing forest encroachment and reducing the fallow period. Another important cause is the need for fuelwood, estimated to account for half of the wood harvested in the world. Commercial exploitation for high-value logs accounts for much of the deforestation in Central America, Bolivia, Brazil, Nigeria, Cote d'Ivoire, Indonesia, Malaysia and Philippines, mainly to supply European, American and Japanese markets.

In Latin America, clearing the forest is a way for settlers to claim title to state lands, encouraging uneconomic forest clearing and land speculation. Construction of roads and other infrastructural facilities supporting development strategies have also contributed to accelerated rates. The recent deforestation in the Brazilian Amazon can be attributed mainly to commercial logging, plantations, speculation and mining, while population pressure by small peasant agriculturalists, clearing land for their own farms, accounts for only about 10% of total deforestation.