Root topological order drives variation of fine root vessel traits and hydraulic strategies in tropical trees.

This article investigates the variation and co-variation of fine root vessel anatomical traits and their influence on theoretical specific xylem hydraulic conductivity (Kth) across five root topological orders in 20 tropical tree species from southwestern China. The study finds that with increasing root order, Kth increases primarily due to larger mean vessel diameter (MVD), greater vessel size heterogeneity (cvVD), stable vessel fraction (VF), and decreased vessel density (Vd), reflecting a shift in anatomical strategies for water transport from absorption to conduction. While vessel traits such as MVD and VF are positively associated with Kth across species, root diameter (RD) and stele: root diameter ratio (SDR) are poor predictors of Kth interspecifically, though stele diameter (SD) shows a moderate correlation in higher-order roots. These findings highlight the dominant role of root topological order in shaping fine root hydraulic function and suggest phenotypic plasticity in root xylem anatomy as an adaptive response to soil water availability in tropical forests.