Frost fatigue and spring recovery of xylem vessels in three diffuse‐porous trees in situ

Frost has been shown to cause frost fatigue (reduced cavitation resistance) in branch segments in the lab. Here, we studied the change in cavitation resistance and percent loss of conductivity (PLC) from fall to spring over 2 consecutive years in three diffuse‐porous species in situ. We used the cavitron technique to measure P₂₅, P₅₀ and P₉₀ (the xylem pressure causing a 25, 50 and 90% conductivity loss) and PLC and stained functioning vessels. Cavitation resistance was reduced by 64–87% (in terms of P₅₀), depending on the species and year. P₂₅ was impacted the most and P₉₀ the least, changing the vulnerability curves from s‐ to r‐shaped over the winter in all three species. The branches suffered an almost complete loss of conductivity, but frost fatigue did not necessarily occur concurrently with increases in PLC. In two species, there was a trade‐off between conduit size and vulnerability. Spring recovery occurred by growth of new vessels, and in two species by partial refilling of embolized conduits. Although newly grown and functioning conduits appeared more vulnerable to cavitation than year‐old vessels, cavitation resistance generally improved in spring, suggesting other mechanisms for partial frost fatigue repair.