Ling Wang1, Deli Wang2. 1. Key Laboratory of Vegetation Ecology of Ministry of Education, Institute of Grassland Science, School of Environment, Northeast Normal University, Changchun, Jilin 130024, China. 2. Key Laboratory of Vegetation Ecology of Ministry of Education, Institute of Grassland Science, School of Environment, Northeast Normal University, Changchun, Jilin 130024, China wangd@nenu.edu.cn.