Acknowledgments This study received grants from the Natural Science Foundation of China (no. 30771720); the New Century Excellent Talents program (no. NCET-06-0715) from the Ministry of Education, China; the Furong Scholar program; and the program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province. We would like to thank Dr Xunlin Yu and postgraduate Mr Yongfu Xu for their assistance in identifying tree species. We would also like to thank postgraduates Hongxia Cai, Liyun Hong, Xiaomang Li, Xiaocui Liang, Guoxian Qing, Bin Wu, and Guobing Zhang who participated in the field plot investigation.
