Optimal remote sensing method for estimating green-up date of Leymus chinensis steppe in Inner Mongolia

1 ) College of Geosciences and Survey Engineering ,China University of Mining &Technology ( Beijing ),100083 ,Beijing ,China ;
2 ) College of Resources Science and Technology ,Beijing Normal University ,100875 ,Beijing ,China ;
3 ) State Key Laboratory of Earth Surface Processes and Resource Ecology ,Beijing Normal University ,100875 ,Beijing ,China