1 Institute of Ecology, College of Urban and Environmental Science, and Key Laboratory for Earth Surface Processes of the Ministry of Education, Peking University, Beijing 100871, People's Republic of China; adoleung@imu.edu.cn.
2 School of Ecology and Environment, Inner Mongolia University, Hohhot 010021, People's Republic of China.
3 School of Natural Resources and the Environment, University of Arizona, Tucson, AZ 85721.