Cortical
lacunae, not xylem embolism, drive interspecific differences in freeze–thaw hydraulic dysfunction of conifer fine roots
Milos Simovic, Sean T. Michaletz Summary
Winter snowpack loss exposes fine roots to freeze–thaw cycles at high latitudes and elevations, but the mechanisms of injury and reasons for species differences in vulnerability are not well understood. Motivated by yellow‐cedar decline, we evaluated competing mechanisms of freeze–thaw hydraulic dysfunction (xylem embolism and cortical lacunae) and whether root economics strategy predicts susceptibility. We subjected 96 seedlings of four co‐occurring Pacific Northwest conifers (mountain hemlock, western hemlock, western redcedar, and yellow‐cedar) to seven daily temperature cycles with minima of 3, −3, or −9°C, quantifying xylem and radial conductivity loss, cortical lacunae, and root economics traits. Xylem conductivity loss was low in all species and treatments (mean 11–14%). Yellow‐cedar averaged 58% simulated radial conductivity loss at −9°C, with lacunae occupying 20% of cortical area. Hemlocks (with pervasively lignified cortical walls) rarely formed lacunae, whereas cedars (with largely unlignified walls reinforced by phi‐thickenings) were more susceptible. More acquisitive roots were more vulnerable, among and within species. Our results indicate that cortical lacunae, not xylem embolism, drive interspecific differences in freeze–thaw hydraulic dysfunction, which was most severe in yellow‐cedar. Vulnerability corresponded to cortical wall construction and root economics strategy, providing a mechanistic basis for yellow‐cedar's unique susceptibility to snowpack loss.