Freezing in Conifer Xylem

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Abstract
To determine whether freezing causes wide-spread cavitation in the xylem of freezing trees, pressure and temperature were measured inside freezing conifer sapwood blocks. Pressure rises of up to 3.3 MPa were recorded and average radial growth velocities of ice were between 1.75 and 2.3 μm s−l. These growth velocities of ice are less than the minimum growth velocity for bubble nucleation during freezing. To complement this experimental study finite difference models of freezing in a single tracheid and freezing in an idealized tree stem were constructed. The single tracheid model predicts pressure rises similar to those measured experimentally. This model also predicts that 5% to 8% of water in a tracheid lumen migrates out of the tracheid during freezing. The tree stem model predicts growth velocities of ice three times faster than the values measured experimentally. These results are compared with previous contradictory theories of freezing in conifers.