Rainfed agriculture in Northern Laos – identification of land use cycles in slash-and-burn agriculture by satellite imagery

In mountainous regions in northern Laos, shifting cultivation, namely slash-and-burn agriculture, has been practiced regularly. However, the crop-fallow rotation cycle has tended to shorten due to forest conservation policies and population pressure. It is considered that such kind of change causes deterioration of productivity and affects farmers' livelihood in the region. As a large part of shifting cultivation is practiced in remote, steep mountainous areas, field investigation faces some constraints. To address this problem, we developed a method to identify crop-fallow rotation cycles, cropping intensity, and fallow length spatially using Landsat/Thematic Mapper (TM) and Enhance Thematic Mapper+(ETM+) which have abundant data archives. The impact of the identified cycles on plant production was assessed using Normalized Difference Vegetation Index (NDVI). The study site was an area between N20°00′ and N19°30′ latitude and E101°45′ and E102°30′ longitude in Luang Prabang province. This area is a major production area for upland rice using slash-and-burn agriculture. Using 8 scene data acquired annually from 1995 to 2003, the actual agricultural land use in each year was classified as crop fields or fallow plots based on the presence of vegetation in late dry season, and represented by an 8-digit code developed to track land use year by year and represent each unique rotation pattern over the 8 year period. Accordingly, land use pattern in the duration was represented by 28=256 patterns. A cropping intensity index was also developed. Analysis using this index indicated that, 66 000 ha (17.3%) had never been cropped in 8 years, but 41 000 ha (10.7%) had been used for cropping every year. The areas of crop intensity with 1-30, 31-60 and 61-90% were 88 000 ha (22.9%), 74 000 ha (19.3%), and 114 000 ha (29.8%), respectively. The process of plant regeneration in fallow was tracked by NDVI, and it was estimated by regression that 11 years would be needed to reach to the same NDVI of forest. Based on maps indicating crop intensity and potential for plant regeneration, we found that in the study site, the vicinity of rivers was intensively used and fallow length was accordingly short. When regeneration of fallow vegetation showed the potentiality of plant production, the low potential area was distributed along the west basin of Mekong River and it increased as crop intensity was higher. There is a high risk that reduced vegetation regeneration and consequent deterioration of crop productivity will occur in these areas in the future.