Current climate models predict an increase in both the intensity and frequency of drought, which may expose many tree species to stress levels beyond physiological thresholds and result in mortality. As trees desiccate, the probability of embolism formation in the xylem increases, which can lead to significant hydraulic dysfunction. Trees, however, have evolved various mechanisms that prevent the systemic spread of emboli through their hydraulic networks. One measure of tissue-specific drought tolerance is air-seeding threshold: the negative pressure of the xylem sap that exceeds the surface tension and adhesion of water in vessel pit membrane pores, which leads to embolism propagation in the vascular network. The vulnerability segmentation hypothesis posits that tissues connecting distal, low-investment tissues (e.g. leaves and fine roots) to high carbon investment tissues (e.g. trunks, stems) should be more vulnerable to embolism, with weaker pit membranes to act as hydraulic fuses, thereby protecting more permanent tissues. We tested this hypothesis by measuring air-seeding thresholds in 1,126 xylem vessels of four northern hardwood tree species of the northeastern United States. In addition, hydraulic safety margins for each tissue type were calculated by comparing mean air-seeding pressures to midday water potentials experienced by that tissue.
Acer rubrum had the highest resistance to air seeding of the four study species, and exhibited some evidence that trunk xylem vessels were more resistant to air seeding than petiole and root vessels. However, Fagus grandifolia, Fraxinus americana, and Quercus rubra showed little to no variation in resistance to air seeding, regardless of tissue type, therefore providing no additional support for the vulnerability segmentation hypothesis. When air-seeding thresholds were put into the context of hydraulic safety margins, however, we found that in every species, leaf xylem operated at water potentials near or beyond their hydraulic safety margins while perennial tissues were more conservative. In particular, roots, trunks, and stems of A. rubrum, F. grandifolia, and Q. rubra, operated well within their safety margins, even during the third driest summer in the last 100 years. Using safety margins as a predictor of survival, we suggest that these species may maintain or increase their dominance if extended droughts become more common.