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).