Insights from <i>in vivo</i> micro‐<scp>CT</scp> analysis: testing the hydraulic vulnerability segmentation in <i>Acer pseudoplatanus</i> and <i>Fagus sylvatica</i> seedlings

The seedling stage is the most susceptible one during a tree's life. Water relations may be crucial for seedlings due to their small roots, limited water buffers and the effects of drought on water transport. Despite obvious relevance, studies on seedling xylem hydraulics are scarce as respective methodical approaches are limited. Micro-CT scans of intact Acer pseudoplatanus and Fagus sylvatica seedlings dehydrated to different water potentials (Ψ) allowed the simultaneous observation of gas-filled versus water-filled conduits and the calculation of percentage loss of conductivity (PLC) in stems, roots and leaves (petioles or main veins). Additionally, anatomical analyses were performed and stem PLC measured with hydraulic techniques. In A. pseudoplatanus, petioles showed a higher Ψ at 50% PLC (Ψ₅₀ -1.13MPa) than stems (-2.51 MPa) and roots (-1.78 MPa). The main leaf veins of F. sylvatica had similar Ψ₅₀ values (-2.26 MPa) to stems (-2.74 MPa) and roots (-2.75 MPa). In both species, no difference between root and stems was observed. Hydraulic measurements on stems closely matched the micro-CT based PLC calculations. Micro-CT analyses indicated a species-specific hydraulic architecture. Vulnerability segmentation, enabling a disconnection of the hydraulic pathway upon drought, was observed in A. pseudoplatanus but not in the especially shade-tolerant F. sylvatica. Hydraulic patterns could partly be related to xylem anatomical traits.