Age-Dependent Variations in Xylem Hydraulic Efficiency and Safety of Abies fabri

The impact of aging on the hydraulic functions of entire trees is crucial for accurately forecasting the productivity and expansion of mature forests. Nevertheless, it is not well understood whether and how the hydraulic properties of subalpine conifers evolve as they age. To investigate this, we evaluated the hydraulic and embolic properties of the roots and stems of Abies fabri at three different stand ages and assessed their safety and efficiency tradeoffs and vulnerability segmentation. In the mature stand, root and stem hydraulic conductivity reached maximum values of 3.18 × 10^-2 kg m s^-1 MPa^-1 and 3.54 × 10^-2 kg m s^-1 MPa^-1, respectively. There was a clear tradeoff between hydraulic safety and efficiency in the root xylem, while this tradeoff was relatively weak in the stem xylem. Specifically, stems exhibited the strongest embolism resistance, with the lowest percentage of hydraulic loss and the highest water potential, and the water potential at 50% loss of conductivity (P₅₀) value reached a minimum of -2.55 MPa in the mature stand. In the roots, however, the age-dependent embolism resistance was strongest in middle-aged stands, with a P₅₀ value of -1.86 MPa. The hydraulic vulnerability segmentation mechanism changed as the trees grew, showing positive segmentation at young and mature ages (P_50root-stem > 0) and negative segmentation (P_50root-stem < 0) in middle-aged stands. These results imply that the vertical variation in hydraulic traits of A. fabri as they age serves an adaptive purpose, enabling trees to achieve greater heights and enhance their hydraulic thresholds, which is vital for plant health optimization.