Hydraulic traits of trees growing in wet places: Do lessons learned from arid biomes translate to mesic forests?

Plant hydraulic traits are increasingly recognized as key regulators of carbon and water transport from land to the atmosphere. Much of our understanding of plant drought responses has been developed from studies in arid ecosystems, which experience frequent and highly damaging drought. Hydraulic mechanisms by which plants respond to drought in more mesic regions is less understood, but important to quantify given the potential for substantial losses in productivity in mesic regions entering new drought regimes. In this study, we compiled and measured plant hydraulic trait data of L. tulipifera, Q. alba, and A. saccharum, three common eastern deciduous tree species, in seven relatively mesic forested sites across central Missouri, southern Indiana, and western North Carolina. Across sites, we investigated how seasonal leaf water potential (LWP), branch cavitation vulnerability, and xylem anatomical characteristics varied across forest age and climate. We further explored the relationship between seasonal LWP and branch cavitation vulnerability, to test hypotheses emerging from prior work (largely conducted in arid regions) that low mid-day LWP is experienced primarily by species with xylem that are highly resistant to cavitation. We found that xylem lumen area, xylem density, and branch cavitation vulnerability varied substantially across species, but within species, these traits were not affected strongly by age and/or climate. Within each site, we found that vulnerability to hydraulic failure was not associated with more conservative LWP dynamics. Specifically, during periods of moisture stress, Q. alba consistently showed greater declines in leaf water potential than neighboring L. tulipifera and/or A. saccharum, despite having branch tissues more vulnerable to cavitation than the L. tulipifera and/or A. saccharum trees growing in the same sites. As such, the lack of coordination between these traits found in our study sites suggest that plant hydraulic regimes observed in arid regions may not directly translate to drought responses of the mesic eastern US forests.