Drought induced water stress, which may cause runaway cavitation of xylem elements, puts many forests at risk for severe mortality. This risk can be estimated through cavitation vulnerability curves that describe the decline of hydraulic conductance (K) as xylem water tension increases. These sigmoidal curves provide water stress thresholds (P50) representing the tipping point between hydraulic resilience and severe cavitation, which can be compared with in-situ xylem water tensions data to determine hydraulic safety margins.
Species’ cavitation vulnerability can vary across age and climate, as xylem elements change during development of new growth rings and in response to variable seasonal moisture. However, studies considering both age and climate effects on vulnerability are rare. We hypothesize that climate and age interact to determine phenotypic response of xylem anatomy, and predict that mature species growing in very mesic sites will be associated with the most vulnerable hydraulic systems. Cavitation vulnerability, xylem anatomy, and mid-day leaf water potential (Ψmd) measurements of two key eastern deciduous U.S. tree species, Q. alba (QUAL) and L. tulipifera (LITU), are compared across two chronosequences (15-, 35- and 85-year-old stands) in a wet Western North Carolina (NC) biome and a relatively warmer and drier Southern Indiana (IN) biome.
Preliminary data shows P50 thresholds of LITU were indistinguishable in mature stands from the NC and IN biomes. In contrast, mature QUAL trees in NC were more vulnerable than those in IN (P = 0.047, n = 5). Hydraulic regulation of Ψmd also differed between QUAL and LITU; in the NC chronosequence, LITU Ψmd was less negative with increasing age while the most negative QUAL Ψmd were observed in mature stands.
Tight stomatal control can decrease cavitation susceptibility by maintaining high leaf water potentials. Yet in the NC chronosequence, mature QUAL exhibits both risky hydraulic regulation (declining Ψmd with increasing water stress) and cavitation-prone hydraulic architecture, whereas LITU threshold values are indistinguishable across sites. We speculate that because available moisture in this mesic biome is rarely limiting, developing cavitation resistant xylem may be an unnecessary metabolic investment and could compromise hydraulic efficiency. We will test these hypotheses by comparing trends in hydraulic vulnerability from the NC chronosequence to those observed in the drier IN sites, where we expect trees to more carefully regulate hydraulic vulnerability and in-situ water potentials, particularly in older stands. Ultimately, our results are aimed at improving a priori diagnoses of forests susceptible to water stress mortality.