Plasticity And Coordination Of Branch And Leaf Functional Traits In Ulmus Pumila Along A Precipitation Gradient

White elm(Ulmus pumila L.)is widely distributed in various regions of Northeastern,Northern,and Northwestern China and plays an important role in wind prevention,sand fixation,water conservation,and climate regulation.U.pumila has strong adaptability to water stress and can grow naturally under contrasting water conditions.However,the physiological mechanism by which it adapts to different water environments is not yet clear.In the present study,we compared key functional traits and non-structural carbohydrate reserves of U pumila trees inhabiting different climatic zones(sub-humid,dry sub-humid,and semi-arid zones)representing contrasting precipitation regimes,aiming to investigate the roles of trait plasticity,coordination,and carbon storage strategy 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.Compared with relatively humid areas,the leaves,branches,roots,and tree cores of U pumila grown in climatic zones with more severe drought stress have higher non-structural carbohydrate contents,and the ratio of soluble sugars in total non-structural carbon carbohydrates increases.This study also found that there is a strong correlation among key functional traits during the process of plastic adjustment to changes in environmental moisture.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.Under relatively humid conditions,U pumila has relatively efficient water transport and acquisitive water use,which promotes its potential growth rate and competitiveness.Correspondingly,under semi-arid conditions,U pumila has relatively conservative water use,relatively safe hydraulic transport,and strong drought resistance,along with more non-structural carbohydrate reserves,improving its drought survival ability.This study highlights that the plastic adjustment and coordinated variation of shoot functional traits and the alteration in carbon storage strategy may be important change contributing to the success of U.pumila in different climate zones with contrasting water environments.