1. College of Grassland Science, Gansu Agricultural University, Lanzhou, China;
2. College of Forestry, Gansu Agricultural University, Lanzhou, China;
3. College of Water Conservancy and Hydropower Engineering, Gansu Agricultural University, Lanzhou, China;
4. Department of Biological Systems Engineering, Hampton Roads Agricultural Research and Extension Center, Virginia Polytechnic Institute and State University, Virginia Beach, Virginia, USA