© 2003 Heron Publishing—Victoria, Canada
Tracheid diameter is the key trait determining the extent of freezing-induced embolism in conifers
Jarmila Pittermann (1, 2) and John Sperry (1)
1. Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City, UT 84112, USA / 2. Author to whom correspondence should be addressed (pittermann@biology.utah.edu) / Received January 10, 2003; accepted March 18, 2003; published online August 1, 2003
Summary
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 µm.
Tracheids with a critical diameter greater than 43 µm 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 µm) 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 µm 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.
Keywords:
cavitation, freeze–thaw, hydraulic conductivity, temperature.
