Tip-to-base water storage strategies and their relationship to hydraulic safety in two temperate conifer species.

The axial co-variation of xylem anatomical traits is well documented, but lacks a deeper understanding of the tip-to-base dynamics of wood capacitance and resistance to embolism formation for assessing the performance of forest trees under drought stress. For the first time, relative water loss (RWL) curves were generated from wood sampled along the entire length of two mature conifer trees, spanning from the tip of the canopy to the base of the trunk. These measurements were conducted alongside hydraulic vulnerability curves. Parameters related to wood water retention capacity and safety/efficiency of the hydraulic system were extracted. The results revealed significant changes in wood capacitance, resistance to embolism formation and maximum hydraulic conductivity along the gradient from the tree tip to the base, with the most pronounced variation occurring within the first 200 apical centimetres. Resistance to embolism formation and wood capacitance were notably greater at the crown periphery compared to the stem base, with lower water potentials (Ψ) driving 20%, 50%, and 80% loss of hydraulic conductivity, accompanied by a higher release of wood capacitive water volume at the P50 threshold. The strong correlation between relative water loss and conductivity loss highlights the promising potential of traits derived from RWL curves as efficient and rapid indicators for assessing drought sensitivity. This research sheds light on the potential link between axial variation in xylem anatomical traits, drought-induced embolism vulnerability, and wood capacitance, with important implications for investigating plant responses to climate change.