**5. Conclusions**

Our results revealed the linkage between plant hydraulic strategies and the interspecific associations of co-occurring trees in forest–steppe ecotone. Though missing some environmental data to fully investigate the relationships between the tree species and environment water resource supply, this study pointed out the important rule of plant physiology in the forest composition and dynamics, laying the foundation for further scientific exploration and data collection.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1999-4907/11/9/942/s1, Figure S1: The relationship between stem hydraulic transportation efficiency and safety (as measured by the water potential of stems at 88% loss of stem conductivity, P88) of all species, Figure S2: Spearman correlation between the interspecific association pattern and the hydraulic tradeoff differences among species.

**Author Contributions:** Conceptualization, H.L. and J.D.; methodology, J.D.; investigation, J.D., C.X., Y.Q., X.Z., B.L. and Y.W.; resources, M.Z.; data curation, J.D.; writing—original draft preparation, J.D.; writing—review and editing, J.D.; visualization, J.D.; supervision, H.L.; project administration, H.L.; funding acquisition, H.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China, Grant Number 41530747, 41790422.

**Acknowledgments:** We are grateful to Guangyou Hao for his teaching in laboratory measurements and permission to use his laboratory facilities.

**Conflicts of Interest:** The authors declare no conflict of interest.
