ACKNOWLEDGEMENTS
The field survey of this study was completed in the Ningxia Habahu National Nature Reserve. Thanks to Mr. Wan Xue You and Mr. Dian Yu of the Reserve Administration for their strong support. At the same time, we are also very grateful to Professor Song Gu, Associate Professor Qing Hua Nie, Professor Hui Fen Liu and Professor Jin Long Wang for participating in some field investigations. Thanks to my graduate students Hong Juan Jing, Yu Ting Liang, Hong Tao Xie, Xuan Zhao, Meng Qi Li, Ping Yi Xu and Jia Yue Zhang for participating in some field investigations and experimental analysis. This research was supported by the National Key Research and Development Program of China (2016YFC0500706).