1. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water ConservationNorthwest A&F University Yangling Shaanxi 712100 PR China
2. College of ForestryNorthwest A&F University Yangling Shaanxi 712100 PR China
3. CAS Center for Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences Xi'an 710061 PR China
4. Oregon State University Agriculture Program at Eastern Oregon UniversityOregon State University La Grande OR 97850 USA