Key Laboratory for Vegetation Ecology of Ministry of Education Institute of Grassland Science Northeast Normal University Changchun 130024 China;
Chinese Research Academy of Environmental Sciences Belting 100012 China;
College of Animal Science and Technology China Agricultural University Beijing 100193 China;
Langfang Normal College Langfang 065000 China;
Key Laboratory for Vegetation Ecology of Ministry of Education Institute of Grassland Science Northeast Normal University Changchun 130024 China;