1. Department of Ecology; Key Laboratory for Earth Surface Processes of the Ministry of Education; Peking University; Beijing; 100871; China
2. College of Life Science; Inner Mongolia University; Hohhot; 010021; China
3. Xinjiang Institute of Ecology and Geography; Chinese Academy of Sciences; Urumqi; 830011; China
4. Institute of Biological and Environmental Sciences; University of Aberdeen; Aberdeen; AB24 3UU; UK