Recalcitrant vulnerability curves: methods of analysis and the concept of fibre bridges for enhanced cavitation resistance

Vulnerability curves ( VCs ) generally can be fitted to the W eibull equation; however, a growing number of VCs appear to be recalcitrant, that is, deviate from a W eibull but seem to fit dual W eibull curves. We hypothesize that dual W eibull curves in H ippophae rhamnoides   L . are due to different vessel diameter classes, inter‐vessel hydraulic connections or vessels versus fibre tracheids. We used dye staining techniques, hydraulic measurements and quantitative anatomy measurements to test these hypotheses. The fibres contribute 1.3% of the total stem conductivity, which eliminates the hypothesis that fibre tracheids account for the second W eibull curve. Nevertheless, the staining pattern of vessels and fibre tracheids suggested that fibres might function as a hydraulic bridge between adjacent vessels. We also argue that fibre bridges are safer than vessel‐to‐vessel pits and put forward the concept as a new paradigm. Hence, we tentatively propose that the first W eibull curve may be accounted by vessels connected to each other directly by pit fields, while the second W eibull curve is associated with vessels that are connected almost exclusively by fibre bridges. Further research is needed to test the concept of fibre bridge safety in species that have recalcitrant or normal W eibull curves.