Received date: 2010-07-12; accepted date: 2010-10-20 Foundation item: Under the auspices of Sino-British Fellowship Trust by British Academy (No. SG-47266), National Natural Science Foundation of China (No. 49571031), Foundation of China Scholarship Council (No. 2008619067) Corresponding author: ZHANG Jie. E-mail: jiezhang@nju.edu.cn © Science Press, Northeast Institute of Geography and Agroecology, CAS and Springer-Verlag Berlin Heidelberg 2011