1. School of Life Science, Shanxi Normal University , Taiyuan 030000 , China
2. State Key Laboratory of Grassland Agro-Ecosystem, School of Life Sciences, Lanzhou University , Lanzhou 730000 , China
3. School of Economics and Management, Zhejiang Sci-Tech University , Hangzhou 310000 , China
4. Department of Environmental Science, Policy, and Management, University of California, Berkeley , CA 94720 , USA
5. Department of Plant and Environmental Sciences, University of Copenhagen , DK-1871 Frederiksberg , Denmark