Decoding the intra-annual xylem anatomy variation and efficiency-safety trade-off in ring-porous and diffuse-porous...

The xylem vessel size-number trade-off reflects investment in hydraulic conductance and hydraulic safety in trees. However, how this trade-off varies within a growing season, and how it mediates climate adaptation strategies across different porosity types (e.g., ring-porous vs. diffuse-porous species), remain poorly understood. This study investigated intra-annual dynamics of anatomical traits in two ring-porous tree species (Fraxinus mandshurica and Quercus mongolica) and two diffuse-porous species (Betula platyphylla and Tilia amurensis) from temperate forests in northeast China. We quantified vessel diameter, density, and theoretical hydraulic conductivity across growth stages (P1 to P4) and analyzed their relationships with climate variables and prior-stage hydraulic conductivity. The results showed that ring-porous species exhibited pronounced earlywood-latewood differentiation, with earlywood contributing most of hydraulic conductivity (Kh), while diffuse-porous species maintained more uniform anatomical traits. Segmented regression revealed that below a threshold, vessel density (VD) declined with increasing Ks in all species, but above the threshold, responses diverged by porosity type. Diffuse-porous species continued to increase VD, while ring-porous species showed a moderated decline. Climatic drivers had species- and stage-specific effects, with precipitation positively influencing earlywood vessel traits and high vapor pressure deficit constraining vessel expansion in most species. These findings suggest that xylem anatomical development is jointly regulated by climate and prior Ks, and that porosity type plays a key role in modulating these responses within the growing season.