Corresponding authors:
1 Shengping Li
E-mail address:lishengping@caas.cn
2 Xueping Wu
E-mail address:wuxueping@caas.cn
Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences
No. 12 Zhongguancun South Street, Haidian District, Beijing 100081, PR China
Tel./fax: +86-10-82108665