Cortical lacunae, not xylem embolism, drive interspecific differences in freeze–thaw hydraulic dysfunction of conifer fine roots

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.