Abstract: The survival of urban forests is increasingly challenged by prolonged droughts which adversely affect the function of urban trees. Drought resistance in tree species is determined by their plant-water relationship. There are significant differences in xylem structure between ring-porous and diffuse-porous species, and these structures are closely linked to their hydrodynamic functional traits. This study examined the relationship between branch xylem anatomy and hydrodynamic traits in two timber species and analyzing xylem samples from eight broadleaved species. A water-efficiency safety trade-off was observed in diffuse-porous species, while ring-porous species adapt to their environment by adjusting water transport traits and varying tissue types. Two distinct hydraulic strategies were identified: ring-porous species with high water demand, formed a large conduit area, and axial parenchyma to improve water transfer efficiency, while increasing the thickness of the conduit wall to improve the implosion resistance. Diffuse-porous species formed an independent conduit distribution pattern with greater conduit density and proportion of conduit tissue hydraulic security. The physiological roles of conduit structures system directly determine the dynamic balance between efficiency and safety of water transport in the xylem hydraulic system, spatial distribution and the allocation of resources to thin-walled and fibrous tissues. Overall, woody species in urban environments exhibit considerable variation in drought tolerance, forming complex three-dimensional systems where conduit structure, spatial distribution and functional tissue allocation work together to determine their drought resistance strategies.

Key words: Xylem anatomy, Hydrodynamic functional traits, Ring-porous species, Diffuse-porous species