Introduction

High population growth rates in the tropics and the increasing demand for land to produce food and raw materials, is resulting in high rates of deforestation at the forest margins in the tropics. Traditional shifting cultivation systems that previously sustained people without degradation of the natural resource base, are rapidly being replaced by unsustainable slash and burn agriculture.

In traditional shifting cultivation, farmers cleared and burned small parcels of forest, cropped the land for two to four years and then moved on to clear a new patch of forest. The previously cropped land was rapidly colonised by forest species and as population densities were low and land was plentiful, the farmers could leave the land fallow for up to 60 years. The short cropping period followed by a long fallow, restored carbon and nutrients to the system and facilitated the reinvasion of flora and fauna that had been negatively impacted by the short cropping period. Farmers used these regrowing forests and harvested a variety of fruits, nuts, medicinal plants, and wild game.

Sequence 01 Sequence 02 Sequence 03 Sequence 04

The photo sequence above shows the clearing, burning, cropping and secondary forest fallow stages of traditional shifting cultivation [Photos © E.C.M. Fernandes]

As the area of primary forests continues to decline worldwide, many governments have established protected forest areas and parks and farmers are now clearing, burning and cultivating land with 5 to 15 year forest forest fallows. As land scarcity increases, farmers cultivate the cleared land for longer periods than under shifting cultivation and the soil is depleted of nutrients and the seed pools required for future regeneration of the vegetation. These degraded lands become dominated by aggressive, weedy species (e.g. Imperata cylindrica, Lantana camara) that prevent the regeneration of forest species. In Latin America, the large-scale clearing and burning of forests for pasture establishment, often leads to degraded pastures which are abandoned within 6-8 years of forest clearance.

Between 1981-1990, the average global deforestation rate in the humid tropics was estimated to be 0.1 to 0.14 million-km2 yr-1 (FAO 1993). The rapid clearing and burning of forests and the use of unsustainable agricultural practices, is resulting in millions of hectares of degraded lands, increased fluxes of greenhouse gases, and major loss of biodiversity. In 1994, the CGIAR with funding from the Global Environment Facility (GEF) and the United Nations Development programme (UNDP), initiated a Systemwide programme on Alternatives to Slash and Burn (ASB) agriculture. The ASB programme is coordinated by the International Center for Research in Agroforestry (ICRAF) and its goal is to reduce deforestation caused by unsustainable slash-and-burn agriculture. The long-term aims are to reduce greenhouse gas emissions from tropical deforestation and unsustainable agriculture, enhance agrobiodiversity and conserve forest biodiversity, and to alleviate human poverty by promoting the development of ecologically sound, economically feasible and culturally acceptable agroecosystems.