Monitoring phenological stages of swiddening in northern Laos during the dry season

Abstract

Swidden cultivation is a unique land use category and has undergone rapid transitions in the uplands of Mainland Southeast Asia. Monitoring the scale and magnitude of changes is very challenging due to the year-to-year fluctuations of land cover, which substantially constrains our understanding of the interactions between swidden system and the implementation of Reducing Emissions from Deforestation and forest Degradation (REDD) and its environmental effects. In this study, annual time-series of Landsat TM/ETM+ images in 2005 and 2012-2013 were used to observe the temporal development of swidden practice during the dry season. Three vegetation indices including Normalized Difference Vegetation Index (NDVI), Land Surface Water Index (LSWI) and Normalized Burn Ratio (NBR) were applied to characterize the different development phases (from pre-felling, felling/slashing, sun-air drying, burning to post-burn) at both the pixel and landscape levels. The results showed that: 1) the swidden system in the uplands of Laos generally starts the felling/slashing stage in mid-late February, keeps sun/air drying in whole March, and enters into the burning phase in April. The pre-felling phase usually ends in early February. 2) NDVI and LSWI were more sensitive to detect the changes of vegetation and moisture content from pre-felling to sun/air drying phase while NBR is more sensitive to detect fire-related disturbance. 3) The differences of NBR between pre-felling and post-burn phase were much bigger than those of NDVI and LSWI, which indicates the NBR as a potentially effective tool for detecting and mapping the spatio-temporal changes of swidden farming.