Abstract:

Aims: Fine root hydraulic anatomical traits profoundly influence water uptake and transport in woody plants and the stability of community succession in arid environments. This study investigated Caragana korshinskii (common name: ning tiao) shrubs of different stand ages (11, 16, 22, 32, 47, and 60 years) in the loess hilly region of northwestern Shanxi Province. By analyzing the hydraulic anatomical traits of 1st- to 5th-order fine roots, this study aimed to clarify the synergistic effects of stand age and root order on fine root hydraulic anatomical architecture. 

Methods: Fine roots were classified into five orders using the root-order classification method. Paraffin sectioning and microscopic image analysis were used to measure key anatomical parameters, including xylem area (A_xyl), vessel density (VD), hydraulic diameter (D_h), root specific hydraulic conductivity (K_hp), and the ratio of vessel area to xylem area (A_ves/A_xyl). Two-way ANOVA, principal component analysis (PCA), and structural equation modeling (SEM) were used to examine the effects of stand age and root order on fine root hydraulic anatomical architecture. 

Results: (1) Root order was the main source of variation in fine root hydraulic anatomical traits, accounting for 88.16% and 76.32% of the variation in A_xyl and K_hp, respectively. With increasing root order, D_h, A_xyl, and K_hp generally increased, whereas VD decreased, indicating that lower-order roots had more conservative vessel configurations, while higher-order roots exhibited greater structural plasticity. (2) Stand age mainly affected vessel configuration in higher-order roots. At stand ages of 11 and 16 years, higher-order roots only maintained the basic hydraulic structure. At stand ages ranging from 22 to 60 years, higher-order roots increased D_h and Khp by adjusting the combined relationships among VD, A_ves/A_xyl, and A_xyl, thereby enhancing water transport capacity. (3) The interaction between stand age and root order had extremely significant effects on VD, K_hp, A_xyl, and D_h (P < 0.001), indicating that root-order effects varied with stand age. Structural equation modeling further revealed that stand age mainly affected fine root hydraulic structure by regulating vessel space allocation within the xylem, whereas D_h responded more directly to soil water content. 

Conclusion: The fine root hydraulic anatomical architecture in C. korshinskii followed a root-order hierarchical response pattern. Lower-order roots were constrained by their primary structure and maintained relatively conservative vessel configurations, whereas higher-order roots utilized the structural space provided by secondary growth to improve water transport capacity through phenotypic plasticity. This mechanism may help maintain the balance between water supply and demand during shrub succession. In vegetation restoration practices, water management should receive particular attention in 22- to 32-year-old stands. For older stands (> 47-year), moderate stumping or thinning may be considered to reduce the risk of excessive deep soil water depletion.

Key words: fine roots, stand age, hydraulic anatomical structure, phenotypic plasticity, spatial constraint