1 Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, China; Department of Geography, University of California, Los Angeles, CA 90095, USA.
2 Department of Geography, University of California, Los Angeles, CA 90095, USA. Electronic address: yxue@geog.ucla.edu.
3 Department of Geography, University of California, Los Angeles, CA 90095, USA.