Diffuse-porous and ring-porous xylem types did not influence branch hydraulic responses along a rural-urban gradient

Abstract

Understanding how plant hydraulic traits vary with urbanization is crucial for unravelling ecological adaptation. We examined branch hydraulic traits and radial growth of 14 subtropical tree species across rural-urban gradients (urban, suburban, rural) in Hefei, China, and classified them into diffuse-porous and ring-porous types. Despite structural differences, both wood types exhibited consistent hydraulic responses along the rural-urban gradient. Principal component analysis（PCA）revealed that urban trees developed more efficient hydraulic systems with higher xylem-specific hydraulic conductivity (K_s), leaf-specific conductivity (K_l), maximum hydraulic conductivity (K_max), saturated water content (SWC), and vessel diameter (D), whereas rural trees maintained safer structures with higher vessel density (VD) and sapwood density (WD). From rural to urban sites, the diameter growth rate (DGR) of ring-porous species increased, positively associated with K_max, K_s, K_l, and D, but negatively with VD; diffuse-porous species exhibited stable DGR without such links. Partial least squares path modeling (PLS-PM) further demonstrated that variation in DGR was influenced exclusively by K_s in ring-porous species, while in both wood types, rural-urban gradient (site), WD and D exerted no influence on DGR. Our findings imply that urban environments compel trees to enhance hydraulic efficiency, reflecting adaptive shifts toward faster resource acquisition and growth. Moreover, hydraulic traits strongly promoted radial growth in ring-porous species but showed weaker or no effects in diffuse-porous species, indicating divergent ecological strategies and hydraulic sensitivity between wood types.