Coupling drought resistance strategies with phylogenetics in fine root anatomical traits of northern subtropical tree species in China

Aims Fine root anatomy directly affects drought resistance through water transport efficiency and storage capacity, while its variation may be phylogenetically constrained, reflecting evolutionary selection of drought adaptation strategies. While fine roots exhibit remarkable functional plasticity, the phylogenetic imprint on their anatomical trait variation patterns remains unclear, highlighting a critical knowledge gap in plant adaptation strategies. Methods This study investigated 21 local tree species from northern subtropical China to quantify anatomical traits across first- to fifth-order fine roots. By integrating drought resistance assessments with phylogenetic signal analysis (K-value), we investigated how species identity, root order, and their interactions drive functional differentiation in fine roots. Results Our findings elucidate adaptive strategies in absorption-transport trade-offs while delineating phylogenetic constraints on trait variation. Our results demonstrate: (1) Species and root order jointly drive adaptive differentiation in fine root anatomy, where root diameter, cortical ratio, vessel diameter, and cross-sectional area emerge as key drought-resistance traits; (2) PCA revealed cortical traits (thickness and ratio) dominate water absorption (PC1), while vascular traits (vessel density and stele ratio) govern transport capacity (PC2); (3) Phylogenetic analyses showed strong conservatism in first-forth order roots, with evolutionary history accounting for significant trait variation, underscoring phylogenetic constraints on functional adaptation. This study deciphers the dual regulatory framework of fine root anatomical adaptation to drought stress, integrating eco-physiological trait networks with phylogenetic constraint analysis. These findings not only amplify the fundamental understanding of evolutionary trade-offs in plant hydraulic strategies, but also establish a mechanistic basis for precision selection of afforestation species.