*Article* **Divergent Hydraulic Strategies Explain the Interspecific Associations of Co-Occurring Trees in Forest–Steppe Ecotone**

**Jingyu Dai 1, Hongyan Liu 1,\*, Chongyang Xu 1, Yang Qi 1, Xinrong Zhu 1, Mei Zhou 2, Bingbing Liu 2,3 and Yiheng Wu 2**


Received: 11 July 2020; Accepted: 27 August 2020; Published: 28 August 2020

**Abstract:** *Research Highlights:* Answering how tree hydraulic strategies explain the interspecific associations of co-occurring trees in forest–steppe ecotone is an approach to link plant physiology to forest dynamics, and is helpful to predict forest composition and function changes with climate change. *Background* and *Objectives:* The forest–steppe ecotone—the driest edges of forest distribution—is continuously threatened by climate change. To predict the forest dynamics here, it is crucial to document the interspecific associations among existing trees and their potential physiological drivers. *Materials* and *Methods:* Forest–steppe ecotone is composed of forest and grassland patches in a mosaic pattern. We executed two years of complete quadrat surveys in a permanent forest plot in the ecotone in northern China, calculated the interspecific association among five main tree species and analyzed their hydraulic strategies, which are presented by combining leaf-specific hydraulic conductivity (Kl) and important thresholds on the stem-vulnerability curves. *Results:* No intensive competition was suggested among the co-occurring species, which can be explained by their divergent hydraulic strategies. The negative associations among *Populus davidiana* Dode and *Betula platyphylla* Suk., and *P. davidiana* and *Betula dahurica* Pall. can be explained as the result of their similar hydraulic strategies. *Tilia mongolica* Maxim. go<sup>t</sup> a strong population development with its e ffective and safe hydraulic strategy. Generally, hydraulic-strategy di fferences can explain about 40% variations in interspecific association of species pairs. Oppositely, species sensitivity to early stages of drought is convergen<sup>t</sup> in the forest. *Conclusions:* The divergent hydraulic strategies can partly explain the interspecific associations among tree species in forest–steppe ecotone and may be an important key for semiarid forests to keep stable. The convergen<sup>t</sup> sensitivity to early stages of drought and the suckering regeneration strategy are also important for trees to survival. Our work revealing the physiological mechanism of forest compositions is a timely supplement to forest–steppe ecotone vegetation prediction.

**Keywords:** drought tolerance; forest–steppe ecotone; hydraulic strategy; hydraulic trait; interspecific association; interspecific relationships; species co-occurrence; semiarid forests
