Tracheid diameter is the key trait determining the extent of freezing-induced embolism in conifers

We tested the hypotheses that freezing-induced embolism is related to conduit diameter, and that conifers and angiosperms with conduits of equivalent diameter will exhibit similar losses of hydraulic conductivity in response to freezing. We surveyed the freeze-thaw response of conifers with a broad range of tracheid diameters by subjecting wood segments (root, stem and trunk wood) to a freeze-thaw cycle at -0.5 MPa in a centrifuge. Embolism increased as mean tracheid diameter exceeded 30 microm. Tracheids with a critical diameter greater than 43 microm were calculated to embolize in response to freezing and thawing at a xylem pressure of -0.5 MPa. To confirm that freezing-induced embolism is a function of conduit air content, we air-saturated stems of Abies lasiocarpa (Hook.) Nutt. (mean conduit diameter 13.7 +/- 0.7 microm) by pressurizing them 1 to 60 times above atmospheric pressure, prior to freezing and thawing. The air saturation method simulated the effect of increased tracheid size because the degree of super-saturation is proportional to a tracheid volume holding an equivalent amount of dissolved air at ambient pressure. Embolism increased when the dissolved air content was equivalent to a mean tracheid diameter of 30 microm at ambient air pressure. Our centrifuge and air-saturation data show that conifers are as vulnerable to freeze-thaw embolism as angiosperms with equal conduit diameter. We suggest that the hydraulic conductivity of conifer wood is maximized by increasing tracheid diameters in locations where freezing is rare. Conversely, the narrowing of tracheid diameters protects against freezing-induced embolism in cold climates.