Consistent decrease in conifer embolism resistance from the stem apex to base resulting from axial trends in tracheid and pit traits

Drought-induced embolism formation in conifers is associated with several tracheid and pit traits, which vary in parallel from stem apex to base. We tested whether this axial anatomical variability is associated with a progressive variation in embolism vulnerability along the stem from apex to base. We assessed the xylem pressure at 50% loss of conductivity ( P50 ), the tracheid hydraulic diameter ( Dh ) and mean pit membrane area ( PMA ) on longitudinal stem segments extracted at different distances from the stem apex ( DFA ) in a Picea abies and an Abies alba tree. In both trees, Dh and PMA scaled with DFA 0.2. P50 varied for more than 3 MPa from the treetop to the stem base, according to a scaling of - P50 with DFA-0.2 . The largest Dh , PMA and P50 variation occurred for DFA <1.5 m. PMA and Dh scaled isometrically (exponent b =1). Pit traits vary proportionally with tracheid lumen diameter. Apex-to-base trends in tracheid and pit traits determine a large DFA -dependent P50 variability. Such a DFA effect on P50 did not receive sufficient attention so far, although analysing the relationships P50 vs. DFA is fundamental for the assessment of embolism vulnerability at the individual level. Highlights ### Competing Interest Statement The authors have declared no competing interest.