1. Institute of Grassland Science, and Key Laboratory of Vegetation Ecology Ministry of Education Northeast Normal University Changchun 130024 China
2. Division of Biological Sciences and the Institute on Ecosystems The University of Montana Missoula MT 59812 USA
3. Northeast Institute of Geology and Agroecology Chinese Academy of Sciences Changchun 130012 China
4. Department of Plant and Environmental Sciences University of Copenhagen 1871 Frederiksberg Denmark