Does freezing and dynamic flexing of frozen branches impact the cavitation resistance of Malus domestica and the Populus clone Walker?

Frost damage to the xylem conduits of trees is a phenomenon of eco-physiological importance. It is often documented in terms of the percentage loss of conductivity (PLC), an indicator of air filling of the conduits. However, trees that refill their conduits in spring could be impacted more by damage to the conduits that reduce cavitation resistance, making them more susceptible to future drought events. We investigated whether ice formation, dynamic flexing of frozen branches or freeze-thaw events could reduce the cavitation resistance (cause "frost fatigue") in first-year shoots of apple (Malus domestica) and clonal hybrid cottonwood (Walker). Frost fatigue was measured in terms of P-50 (the negative xylem pressure required to cause a 50 % loss of conductivity). All treatment groups showed significant frost fatigue, with the exception of the pre-flushed, constantly frozen poplar branches. The P-50 following freeze treatments was approximately 50 % of the pre-freeze values. The effect tended to be greater in freeze-thawed branches. Dynamic bending of the branches had no effect on either PLC or P-50. In three out of four cases, there was a significant correlation between P-50 and PLC. Frost fatigue occurred in both apple and poplar, two unrelated species with different drought and frost tolerances, suggesting that it may be a widespread phenomenon that could impact the ecophysiology of temperate forests.