Objective  Populus tomentosa is an excellent afforestation species in northern and northwestern China. Seasonal and occasional severe droughts occur in this region, and it is the most severe area of drought stress in China. P. tomentosa may fall a little leaves or even all leaves in the dry season, which can effectively reduce the evaporation of water to preserve the core organs, and grow the leaves again to survive the drought after rehydration. In this study, P. ‘Beilinxiongzhu 1’ and P. ‘Beilinxiongzhu 2’, the varieties of P. tomentosa, were used as plant materials. Based on the plant segmentation hypothesis, the causes of leaf shedding in the dry season were analyzed from the hydraulic relationship of plant organs between hydraulics segmentation and the vulnerability segmentation.

  Method  The hydraulic characteristics and functional traits of stems, petioles and leaves of two varieties of P. tomentosa were compared.

  Result  (1) The order of Ψ₅₀ values of the vulnerability curves of the two trees was leaf > stem > petiole, which was in consistent with the plant segmentation hypothesis to some extent, and the leaves were more vulnerable than stems. (2) Specific leaf mass and stem wood density were carbon storage indicators. The specific leaf mass was related to leaf life and the wood density was related to the ability of anti-cavitation and embolization. Compared with the general values of other trees, P. ‘Beilinxiongzhu 1’ and P. ‘Beilinxiongzhu 2’ had smaller specific leaf mass, indicating that the leaves had a shorter life and the wood density was general, indicating that the resistance of the stem to cavitation was general.

  Conclusion  The data of hydraulics and vulnerability of P. tomentosa can be explained by plant segmentation hypothesis, which showed that it can defoliate under the condition of keeping water in the stem. The lower life span of leaves and the weaker resistance of stems to cavitation and embolism also need defoliation to deal with severe drought. It reveals the hydraulic reasons for the rapid growth of P. tomentosa under the condition of sufficient water and fertilizer, and the ability to survive through the defoliation under the occasional extreme drought conditions. Of course, the regrowth of leaves may also involve special physiological and biochemical functions, which needs further study.