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

Abstract

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 DFA0.2. 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 3 MPa from the stem apex to base, with the largest variation occurring within 1.5 m from the stem apex. C_LIO_LIAxial anatomical patterns should be accounted for when analyzing hydraulic properties at individual, intra- and inter-specific scales. C_LI