基金Acknowledgements We thank Yongcheng LIU for his assistance in the field survey and Ting LIU for her edit English writing of the whole draft. Weizhao YANG and Janak Raj KHATIWADA also provided valuable quantitative advice. This manuscript benefited from discussions and insightful observations made by Jie WANG and Siqi YUAN. This study was supported by the National Natural Science Foundation of China （No. 31372174）.