1. Institute of Geographic Sciences and Natural Resources Research; Chinese Academy of Sciences; Beijing 100101 China
2. Graduate University of Chinese Academy of Sciences; Beijing 100049 China
3. School of Geography; Beijing Normal University; Beijing 100875 China
4. The Foundation for Archaeobotanical Research in Microfossils; P.O. Box 37 Fairfax VA 22038 USA
5. Department of Geography and Geoinformation Science; Center for Earth Observing and Space Research; George Mason University; Fairfax VA 22030 USA