Conifers face increased mortality risks following droughts because of drought-induced cavitation of hydraulic water transport by the xylem. Species differences in drought sensitivity may result from differences in pit structure, since pits control the air seeding between water transport conduits (tracheid’s) but this hypothesis is poorly explored. This study aims to explore the mechanisms underlying hydraulic safety (i.e., cavitation resistance) and hydraulic efficiency (i.e., hydraulic conductivity) across 28 conifer species growing in a 50 years old common garden experiment. Such an approach allows to compare species under standardized environmental conditions, thus excluding confounding effects of environment and phenotypic plasticity. For all species we measured the cavitation resistance, hydraulic efficiency, the pit and tracheid anatomy; the possible trade-off between cavitation resistance and hydraulic efficiency; and the level of phylogenetic control of variation in these traits.
Results/Conclusions
- Conifer species with relatively small pit aperture size and large torus overlap within pits were more resistant to cavitation. This is in line with the hypothesis that air seeding occurs less frequently with decreasing pit aperture size, and that those pits are better sealed by a torus moving against the pit aperture.
- Cavitation resistance also increased with a higher number of tracheid’s per stem cross-section area (tracheid density), possibly because this allows for efficient replacement of embolized tracheids.
- Unexpectedly, variation in hydraulic conductance across species was poorly explained by tracheid diameter or pit aperture size. Instead, the hydraulic conductivity was higher in species with thinner tracheid cell walls, which may reduce the flow resistivity between adjacent tracheids.
- Pit sizes and pit sealing properties (such as torus overlap) were strongly phylogenetically controlled but tracheid wall thickness was not: this explains the stronger phylogenetic control for cavitation resistance compared to hydraulic conductivity.