VARIATIONS IN GRASSLAND VEGETATION COVER IN RELATION TO CLIMATIC FACTORS ON THE TIBETAN PLATEAU Variation in net CO2 exchange, gross primary production and its affecting factors in the planted pasture ecosystem in Sanjiangyuan Region of the Qinghai-Tibetan Plateau of China Effects of Snow Cover on CO2 Flux of Northern Alpine Meadow on Qinghai-Tibetan Plateau Modeling Soil Respiration of Alpine Meadow on the Northern Tibet Plateau Using MODIS and Climate Data