Acknowledgements
We thank Ping Wang, Guang-Di Li, Yue-Jun He, Guang-Qi Zhang and Shi-Xiong Wang for comments and advice on earlier versions of the paper, and Bao-Tian Zhang and other staff at the research station for assistance in managing our experiment. This work was supported by the National Natural Science Foundation of China (NSFC, grant number 31800335), and Guizhou Province Science and Technology Planning Program ([2019]1089), NSFC-Cultivation-Program Priority Funding Program (2017-5788), Ecology Domestic First-Class Discipline Construction Program (GNYL2017-007) to S.W..