Low intra-tree variability in resistance to embolism in four Pinaceae species

Variability of embolism resistance within individual trees was assessed in four Pinaceae species by using a single method of measurement: the Cavitron. Contrary to what has been previously observed, our findings show a small variability in embolism resistance within and between organs. Indeed, we found (i) a lack of variability between branches within the crown, and (ii) that roots and trunks are either equally resistant or slightly more vulnerable to embolism than branches. This contradicts the vulnerability segmentation hypothesis proposed in the early 1990s. This paper also demonstrates that only few branches are necessary to determine the embolism resistance of a given tree. Embolism formation in xylem has an important impact on plant growth and survival. Since most studies on xylem embolism resistance focus on branches, it remains questionable how the entire plant deals with embolism across organs. In this study, we aimed to evaluate the variability of embolism resistance within a given organ and between different organs within a single tree. Based on the Cavitron method, we estimated the intra-organ and the intra-plant variability of embolism resistance for four Pinaceae species. In addition, we compared pit anatomical characters for wood of all organs and species. We found no variability of embolism resistance for a given organ within a tree. At the tree level, trunks and roots were either equally or more vulnerable to embolism than branches. For all species, organs that showed a similar range of embolism resistance presented similar torus-aperture overlap values. However, the least negative P (50) value for roots of Pinus pinaster was associated with the lowest torus-aperture overlap value. Our findings suggest that P (50) values are constrained within a particular organ and that intra-tree variation in embolism resistance is less substantial than previously reported. Moreover, our data do not support the vulnerability segmentation hypothesis which suggests that distal organs are more vulnerable to xylem embolism.