Embolism resistance of three boreal conifer species varies with pit structure.

While tracheid size of conifers is often a good proxy of water transport efficiency, correlations between conifer wood structure and transport safety remain poorly understood. It is hypothesized that at least some of the variation in bordered pit and tracheid structure is associated with both transport efficiency and embolism resistance. Stem and root samples from three boreal Pinaceae species were collected to test this hypothesis. Tracheid and pit anatomy were studied using light microscopy as well as scanning and transmission electron microscopy. While tracheid size explained at least 90% of the variation in specific conductivity for stem and root samples, the strongest correlations with embolism resistance occurred at the pit level. Both torus thickness and depth of the pit chamber showed a linear increase with greater vulnerability to cavitation. Greater embolism resistance was correlated with increasing wood density and tracheid wall reinforcement. A thinner torus may be more flexible and better able to seal the pit aperture. The pit chamber depth is proportional to the distance that the margo needs to deflect for pit aspiration.