1. School of Earth System Science Tianjin Univ. Tianjin 300072 China
2. Key Laboratory of Ecosystem Network Observation and Modeling Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences Beijing 100101 China
3. College of Resource and Environment Univ. of the Chinese Academy of Sciences Beijing 100049 China
4. Nanjing Institute of Environmental Sciences Ministry of Ecology and Environment of the People's Republic of China Nanjing 210042 China