Large conduits typically embolise first, but are not more vulnerable to embolism than narrow ones
Wide and long vessels in the midrib and secondary veins were found to embolise before the high vein orders (3rd – 5th vein orders) in the three experiments conducted. This pattern confirms various studies based on the optical method and microCT observations (Klepsch et al. , 2018; Scoffoni et al. , 2017; Brodribb et al. , 2016a). However, the observation of local spreading of embolism in minor veins near cut vessels in experiment 2, supports the hypothesis that embolism spreads from pre-existing gas sources, which happened in minor veins prior to embolism formation in large vessels of major veins (Fig. 3). This also suggests that proximity to a gas source is the main driver for embolism spreading, and not the conduit diameter per se.
Nevertheless, wide and long vessels are more likely to be connected to a pre-existing embolism than narrow, short conduits, as discussed above. It is also possible that large vessels may show a higher amount of intervessel pit membrane area than narrow, short vessels. Although this relationship has been examined with respect to the rare pit hypothesis (Wheeler et al. , 2005; Hacke et al. , 2006), no support for this idea was found in Acer (Lens et al. , 2011). Yet, large and wide vessels are likely to embolise first, and this temporal difference could be caused by their connectivity to a gas source, and may not reflect any inherent difference in embolism resistance per se. No alternative mechanism is known why wide conduits would be vulnerable to embolism, since pit membrane thickness, which is strongly associated with embolism resistance (Li et al. , 2016), was not related to conduit diameter (Kotowska et al. , 2020; Wu et al. , 2020).