基金
We would like to thank the undergradu- ate students from Guangxi Normal University, Yueni Huang, Chun Lan, and Caigui Nong, for their assistance in our experiment. This study was financially supported by Guangxi Science Foundation （2011GXNSFE018005）, Guangxi Key Laboratory of Environmental Engineering, Protection, and Assessment
Kadoorie Farm ＆ Botanic Garden Corporation, Hong Kong
and the Creative Team Project of the universities of Guangxi province, China.
