Discussion
Experiment 1 and 2 show that spreading of drought-induced embolism can
be strongly dependent on the proximity of the xylem area studied to the
cut xylem tissue. Hydraulic segmentation, however, may limit the
potential artefact caused by cut conduits due to differences in conduit
dimensions, conduit connectivity, and/or the non-random arrangement of
vessel ends. Yet, the proximity of pre-existing embolism was found to be
more important for embolism spreading than conduit dimensions per se,
with narrow vessels in minor veins embolising due to the presence of gas
in neighbouring vessels prior to embolism formation in major veins.
Moreover, no major differences were found between the optical and
pneumatic method for five species, despite minor differences in the
P12 values of both methods. Our findings throw novel
light on the driving mechanisms behind drought-induced embolism, and may
have consequences for estimations of embolism resistance based on cut
xylem tissue (Wheeler et al. , 2013; De Baerdemaeker et
al. , 2019).