Monitoring Alpine Grassland on the Tibetan Plateau: its Phenological Change and Climatic Dependencies with Satellite Time Series

The Tibetan Plateau, with an area over 2.5 million km2 and an average elevation higher than 4000 m, is a unique cold and dry highland recognized as the Earth's 3rd Pole. Its fragile ecosystems are sensitive to climate change on the plateau that has been experiencing a distinct warming trend in past decades. The plateau is primarily composed of alpine grasslands (>60%) with an east-west transition of alpine meadow, alpine steppe and alpine desert grass, respectively. Due to geographic difficulties to access, the bio-properties of these alpine ecosystems and their phenological variations in the changing climate are not well documented.

This study extracts the biophysical and phenological features of alpine grasslands from satellite time series and examines their relationships with climate and permafrost conditions on the plateau. The 8-day, 500-m MODIS surface reflectance products (MOD09A1) in 2000-2010 are downloaded to extract the normalized difference vegetation index (NDVI). The annual NDVI time series are then processed to extract peak NDVI, growing-season NDVI and a set of pheno-phrases of alpine grasslands. The MODIS snow cover products (MOD10A2) at the same scale are used to extract snow accumulation in this period. At 25-km grid size, daily soil moisture distributions are available with the Advanced Microwave Scanning Radiometer-Earth Observation System (AMSR-E) Level-3 Soil Moisture products in 2003-2010. Permafrost maps on the plateau are also available via previous collaborative research. A non-parametric Mann-Kendall trend analysis is performed to find the trends of all these metrics. Their correlations are then examined to identify the environmental drivers that influence the change of alpine grasslands.

Opposite trends of phenological change are observed between the east and west of the plateau, with delayed Start of Season, Peak Date and End of Season in the west and advanced pheno-phases in the east. The correlation analysis indicates that precipitation, with a decreasing trend in the west and increasing in the east, may serve as the primary driver of the onset and peak dates of greenness. Temperature increases all over the plateau. While the delay of the End of Season in the west could be related to higher late-season temperature, its advance in the east needs further investigation in this unique cold region. Impacts of permafrost thawing on the plateau are still under investigation. This study demonstrates that frequent satellite observations are able to extract phenological features and environmental conditions of alpine grasslands, which provides spatio-temporally detailed base information for long-term monitoring on the Plateau under the rapid climate change.