Variability of Primary Production and Surface Albedo along Desert-steppe Ecotone based on Satellite Observation

Vegetation in desert-steppe ecotone is sensitive to fluctuation of ecohydrological conditions and human disturbances such as increasing pressure of grazing by domestic herbivores. Temperatures are increasing, and higher associated rates of evaporation will likely bring drier conditions that lead sparser and less productive vegetation in those areas if summer rainfall doesn’t increase. Meanwhile, increasing variability of vegetation cover in ecotone region may have important feedback to local climate, even to summer monsoon system. Regional climate model projects that severer water shortage, intensifier land degradation, and more frequent dust storms will likely occur over monsoon Asia in coming decades, as consequences of climatic change and increasing human pressure. To examine how primary production and energy balance were changing along the transitional zone, we analyzed satellite data time series of annually accumulative normalized difference vegetation index and spectral reflectance from several sensors. We detected the decadal trends and extremes of primary production and surface albedo, and identified areas with the greatest variability along gradients of bioclimate regimes and grazing intensity in desert-steppe ecotone in northwestern China and southern Mongolia. Here we examined satellite time series derived and fused from two satellite sensors (AVHRR and MODIS) to investigate interannual changes of primary production over the region. We applied a meta analysis of field ANPP data collected from various locations to scale up the patterns with fractional analysis of fine resolution satellite data. Increased biological production was evident in large portion of the study region, but declining vegetation growth was also observed in some areas over lowlands and edge of sandy desert. There were strong interannual variability throughout the region, with the greatest fluctuations occurred along the ecotone between semi desert and desert steppe. There was generally low production and high albedo in 1980s, coincide with declined summer rainfall, strong fluctuations in 1990s, and a slight greening trend since 1999, which is likely contributed by the combination of relatively favorable water availability and recent grazing ban policy that closed 20% of rangeland and put another 30% in rotation. There is a clear evidence of grassland recovery in enclosed rangeland, however, vegetation degradation around lowland areas and oasis due to dropping water table and increasing human pressure is also detected.