ACKNOWLEDGEMENTS
We would like to thank professor Azhaz Von Hardonverg for his valuable comments on earlier draft of the manuscript. We thank Wu Yue for his assist during the field study in 2020. We are very grateful to the staff of agriculture, livestock and water supplies bureau of Ruoergai country. This study was supported by the National Natural Science Foundation of China 30180509 and the State Forestry and Grassland Administration Bureau of China.