Poplar species with tall columnar crowns are more prone to hydraulic limitation than congener with short oval crowns in water-limited areas

Populus trees are commonly used in the construction of shelter forests in water-limited areas of China; however, different poplar species are facing various levels of dieback risks under the increased drought associated with climate change. The objective of this study was to explore whether crown height affects the xylem hydraulics and to evaluate the suitability of different Populus species for constructing sustainable shelterbelt in water-limited regions. Xylem hydraulics and water relations of branches at upper and lower positions of tree crown, alongside radial growth rate, were compared between two species that are commonly used in shelterbelt construction but have contrasting crown types, i.e., Populus simonii Carrière with short oval crowns and Populus pioner Jabl. with tall columnar crowns. The results showed that as height increases, P. simonii exhibited enhanced hydraulic efficiency and safety, while no significant differences in these hydraulic traits across canopy layers were observed in P. pioner. In addition, the upper branches of P. pioner have lower water potential and longer water flow paths, resulting in lower hydraulic safety margin, which means that the species was more prone to hydraulic limitation and eventually dieback. Adjustments of vessel sizes and leaf mass per area along the crown of P. simonii contributed to the increase in xylem hydraulic capacity in upper branches and the homeostasis of leaf water potential within the crown. Although the adjustment of using water more conservatively potentially compromised the whole-tree carbon assimilation and thus growth rate, P. simonii seemingly showed stronger adaptability to projected drought intensification by shedding part of branches at the crown bottom and might thus be a more suitable species for establishing stable shelterbelt in water-limited areas. This study, from perspectives of tree physiology, provides an important reference for afforestation species optimization and thus the sustainable management of shelterbelts in water-limited areas of northern China.