Monitoring the spatio-temporal dynamics of swidden agriculture and fallow vegetation recovery using Landsat imagery in northern Laos

Swidden agriculture is an age-old, widespread but controversial farming practice in Montane Mainland Southeast Asia (MMSEA). In the uplands of northern Laos, swidden agriculture has remained a predominant human-dominated land-use type for centuries. However, swidden system has undergone dramatic transformations since the mid-1990s. Debates on changes in swidden cultivation are linked to globally critical issues, such as land use/cover changes (LUCC), biodiversity loss and environmental degradation. Since the implementation of Reducing Emissions from Deforestation and Forest Degradation (REDD), much attention has been paid nationally and internationally to swidden agriculture in the tropics. However, knowledge of the explicitly spatial characteristics of swidden agriculture and the consequences of these transitions at macroscopic scale is surprisingly scarce. In this study, the intensity of swidden use and fallow forest recovery in northern Laos in 1990, 2002, and 2011 were delineated by means of Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper plus (ETM+) imagery (30 m) using a decision tree classification approach, followed by an analysis of the spatio-temporal changes in swidden agriculture. Next, annual successive TM/ETM+ images during 2000-2010 were used to delineate the dynamics of the burning and cropping phase. Subsequently, the burned pixels identified in 2000 were compared respectively with their counterparts in the following years (2001-2011) to investigate temporal trends, land-use frequency, and the swidden cycle using time-series Landsat-based Normalized Difference Vegetation Index (NDVI) data. Finally, as the swidden cycle changed from 1 to 11 years, the fallow vegetation recovery process was studied. The results showed that: (1) from 1990 to 2011, the area of swidden agriculture increased by 54.98%, from 1.54x10(5) ha to 2.38x10(5) ha in northern Laos. The increased swidden cultivation area was mainly distributed in Luang Prabang and southern Bokeo, whereas the decreased parts were mainly found in Phongsali; (2) swidden agriculture increased mainly at elevations of 500-800 m, 300-500 m, and 800-1000 m and on slopes of 10A degrees-20A degrees and 20A degrees-30A degrees. Over 80% of swidden fields were transformed from forests; (3) during 2000-2011, the frequency of swidden use in northern Laos was about two or three times. The interval between two successive utilization of a swidden ranged from one to seven years. Comparison of swidden cycles and the related proportions of swidden farming in 2000, 2003, and 2007 revealed that swidden cycles in most areas were shortened; and (4) there was a significant correlation (0.97) between fallow vegetation recovery and the swidden cycle. The NDVI of regenerated vegetation could approach the average level of forest when the swidden cycle reached 10 years.