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"The shifting cultivation, known locally as abattis, is generally practiced on small areas, less than one hectare. It leaves relatively discrete traces, if any at all at average or small scales. However, this method of land clearing is often regarded, rightly or wrongly, as one of the principal causes of tropical deforestation. To evaluate its surface extent and study the recent changes in the spatial use occurring at a local scale, we have studied the example of St.-
Georges de l’Oyapock. The study was based on three series of aerial photographies with scales ranging between 1/10 000 and 1/50 000, taken respectively in 1958, 1987 and 1997. After geometric corrections, mosaics of aerial photographies were set up and various land cover and land use types were derived from them by computer-aided photomapping, supplemented by field surveys and ground readings taken in 1998. A spatialised information data base integrated in a GIS was constituted for computer-assisted analysis. With this data base, we drew up land cover maps at different dates, and obtained statistics showing surface extent and spatial distribution of land cover and land use, and their variations in time. The results highlight the role of roads and trails in the agricultural colonization of the Amazonian forest and make it possible to appreciate better the impact of traditional shifting farming on the dynamics of the forest at a local scale. We noted that in 1998, 58% of the shifting patterns are located within a radius of 200 m on both sides of a road or trail, this proportion increases by 83% for a distance of 500 m. Around the settlement of St. Georges, 53% of agricultural parcels still under cultivation are located in a range of 2 kilometers. They represent 28% in arrange of 1 kilometer. Of the total surface area of 3,231 hectares mapped, the agricultural clearings (small clearing and other pasture lands) occupy 16% of the surfaces in 1998, against 11% in 1958. The recent forest conversion pointed out is first of all due to pastures, which thus nibbled 12% of forest cover. However, of the 708 hectares of non-forest surfaces in 1958, 273 hectares are re-covered again by forest in 1998, therefore suggesting a forest recolonization of 8% of the mapped area. Between 1958 and 1998, the rate of local deforestation was 0.26% per year. The use of aerial data integrated in a GIS constitutes an invaluable tool for the follow-up studies of deforestation dynamics at a local scale, in an area that is underpopulated and landlocked. The method used is reproducible and can be adapted to other sites of comparable nature." |
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