Xylem embolism resistance varies across species influencing drought
tolerance, yet little is known about the determinants of the embolism
resistance of an individual conduit. Here we conducted an experiment
using the optical vulnerability method to test whether individual
conduits have a specific water potential threshold for embolism
formation and whether pre-existing embolism in neighbouring conduits
alters this threshold. Observations were made on a diverse sample of
angiosperm and conifer species through a cycle of dehydration,
rehydration and subsequent dehydration to death. Upon rehydration after
the formation of embolism, no refilling was observed. When little
pre-existing embolism was present, xylem conduits had a conserved,
individual, embolism resistance threshold that varied across the
population of conduits. The consequence of a variable conduit-specific
embolism threshold is that a small degree of pre-existing embolism in
the xylem results in an apparently more resistant xylem in a subsequent
dehydration, particularly in angiosperms with vessels. While our results
suggest that pit membranes separating xylem conduits are critical for
maintaining a conserved individual embolism threshold for given conduit
when little pre-exisiting embolism is present, as the percentage of
embolized conduits increases, gas movement, local pressure differences,
and connectivity between conduits increasingly contribute to embolism
spread.