The relationship between xylem conduit diameter and cavitation caused by freezing.

The centrifuge method for measuring the resistance of xylem to cavitation by water stress was modified to also account for any additional cavitation that might occur from a freeze-thaw cycle. A strong correlation was found between cavitation by freezing and mean conduit diameter. On the one extreme, a tracheid-bearing conifer and diffuse-porous angiosperms with small-diameter vessels (mean diameter <30 µm) showed no freezing-induced cavitation under modest water stress (xylem pressure = -0.5 MPa), whereas species with larger diameter vessels (mean >40 µm) were nearly completely cavitated under the same conditions. Species with intermediate mean diameters (30-40 µm) showed partial cavitation by freezing. These results are consistent with a critical diameter of 44 µm at or above which cavitation would occur by a freeze-thaw cycle at -0.5 MPa. As expected, vulnerability to cavitation by freezing was correlated with the hydraulic conductivity per stem transverse area. The results confirm and extend previous reports that small-diameter conduits are relatively resistant to cavitation by freezing. It appears that the centrifuge method, modified to include freeze-thaw cycles, may be useful in separating the interactive effects of xylem pressure and freezing on cavitation.