Abstract: We compared branch and leaf functional traits of Ulmus pumila trees inhabiting different climatic zones (sub-humid, dry sub-humid and semi-arid zones), aiming to investigate the role of trait plasticity and coordination in tree acclimation to different water conditions. The results showed that leaf drought stress of U. pumila increased significantly from sub-humid to semi-arid climatic zones, as indicated by a 66.5% reduction in leaf midday water potential. In the sub-humid zone with less severe drought stress, U. pumila had higher stomatal density, thinner leaves, larger average vessel diameter, pit aperture area and membrane area, which could ensure the higher potential water acquisition. With the increases of drought stress in dry sub-humid and semi-arid zones, leaf mass per area and tissue density increased, and the pit aperture area and membrane area decreased, indicating stronger drought tolerance. Across different climatic zones, the vessel and pit structural characteristics were strongly coordinated, while a trade-off between xylem theoretical hydraulic conductivity and safety index was found. The plastic adjustment and coordinated variation of anatomical, structural and physiological traits may be an important mechanism contributing to the success of U. pumila in different climate zones with contrasting water environments.

Key words: functional trait, intraspecific variation, water availability, hydraulic architecture, water use strategy