**4. Discussion**

Divergent hydraulic strategies can partly explain interspecific associations among tree species co-occurring in the temperate forest–steppe ecotone. The overall species associations within the forest were not significant in either years, suggesting the absence of species mutual exclusion caused by intensive interspecific competition in the plant community. The divergent hydraulic strategies illustrated that different water-use strategies may be a way trees avoid competition over water absorption, which was the primary limitation for the forests in forest–steppe ecotone [8,27]. The strong negative association within the forest occurred between *P. davidiana* and *B. platyphylla*, as well as *P. davidiana* and *B. dahurica*, indicating the strong interspecific competitions. The hydraulic strategies of these two species pairs were the least divergent ones. The efficient and safe hydraulic strategy of *T. mongolica* can explain the increase in abundance of *T. mongolica* during the studied years. In general, the positive relationship between interspecific association and hydraulic differences supported the previous hypotheses.

The divergent hydraulic strategies can reduce water stress among co-occurring species thus maintain the stability and species diversity of the forest community in drying forest–steppe ecotone. At the species level, divergent hydraulic strategies can reduce the interspecific competition for water resources. Species with higher water-transport efficiency can adopt more aggressive water absorbing strategies: trees can absorb and store as much water as possible after rainfall. Alternatively, species with strong water deficit tolerance can make use of small amounts of soil water in the long term. At the community level, the co-occurrence of species with divergent hydraulic strategies in forest–steppe ecotone can enlarge the moisture niche for tree species and increase the maintenance of semiarid forests. Thus, the divergent hydraulic strategies allow semiarid forests to maintain the same level of tree diversity with very large regional water supply differences. For example, the tree diversity in our study area with mean annual precipitation of 350 mm is quite similar to the ones with mean annual precipitation of 500–650 mm [19]. The maintaining of species diversity is increasingly important given that the forest–steppe ecotone is facing a drier and more variable climate in the future [28,29]. Divergent hydraulic strategies allow species to occupy moisture niches with many kinds of physical conditions such as slopes, aspects and soil textures [30]. However, we still lack the knowledge to determine water absorbing, storing and utilizing patterns for each species, which is urgently needed for forest dynamic studies and forest managements. Thus, enhanced long-term monitoring of plant physiological traits is needed in the forest–steppe ecotone in the future.

In contrast to the divergent hydraulic strategies, co-occurring species have convergen<sup>t</sup> sensitivity to early stages of drought. The convergen<sup>t</sup> *P12* values of all the species except *Q. mongolica* indicate that the threshold for plant water uptake in response to early stages of drought [26] may be determined by the environment instead of the species. The convergen<sup>t</sup> drought sensitivity can be explained by the hypothesis that plants tend to maximize their physiological functions at the edge of their environmental water limitation, which was put forward in a global meta-analysis for the hydraulic safety margin [31]. Only the "riskiest" approach does allow trees to maximize their carbon gains and biomass increase under normal water supply [31]. Moreover, the species with less safe hydraulic strategies must have some other strategies for drought tolerance or resilience improving, as all the species are facing the same level of drought and are similarly sensitive to the early stages of drought. The suckering regeneration strategy [21,32,33] seems to be a more important strategy for *P. davidiana* than the hydraulic strategies for its long-term survival in forest–steppe ecotone. The severe tree mortality during 2011 and 2012 can be explained by the neither safe nor highly efficient hydraulic strategy of *P. davidiana* in our results, while the rapid regeneration after drought events ensured the regrowth phase of *P. davidiana* instead of the succession to other species [21].
