A test of the hydraulic vulnerability segmentation hypothesis in angiosperm and conifer tree species

Background/Question/Methods

We understand very little about the entire hydraulic pathway in adult trees, mostly because of our lack of knowledge of trunk hydraulic properties.  Many process-based and dynamic vegetation models use branch hydraulic parameters to predict physiological function during drought and species distributions under climate change. But the degree to which branch hydraulic properties are related to leaf, trunk and root hydraulic properties is unknown for most species. The hydraulic vulnerability segmentation hypothesis (HVSH) is a long-standing idea that stipulates that distal portions of the plant (leaves, branches, roots) should be more vulnerable to embolism than trunks, which are non-redundant organs that require a massive carbon investment. However, we have limited data with which to evaluate the HVSH.

In the current study, we tested the HVSH by comparing hydraulic vulnerability to embolism in leaves, branches, trunks and roots of three angiosperm and four conifer tree species. We also measured leaf and stem xylem water potentials in the field and the resulting hydraulic safety margins (in relation to the water potential causing 50% loss of hydraulic conductivity).

Results/Conclusions

Branches were consistently more resistant to embolism than any other portion of the plant, including trunks. Conifers tended to have a stronger hydraulic segmentation than angiosperms (i.e, larger differences in the water potentials resulting in 50% loss of hydraulic function between organs). Leaves and roots had narrow or negative hydraulic safety margins (experienced water potentials that would result in greater than 50% loss of conductivity) but trunks and branches maintained positive safety margins. Thus, our data both did and did not support the HVSH.

By using branch-based hydraulic information as a proxy for the entire plant much research may have over-estimated the embolism resistance, and potentially the associated drought-tolerance, for many species. This study highlights the necessity to reconsider past conclusions made on plant resistance to drought based on branch xylem only. This study also highlights the necessity for more whole-plant hydraulics research for a better understanding of plant drought tolerance and which plant organ is more likely to induce whole-plant drought-induced mortality.