Trees maintain broad hydraulic safety margins in their trunks during drought in a temperate forest

Background/Question/Methods

The vulnerability of trees to drought is a potential threat to the structure and function of forest ecosystems. Recent research suggests that many trees maintain narrow hydraulic safety margins and are vulnerable to hydraulic failure by cavitation, a process that has been hypothesized to contribute to forest die-off during drought. But, the dynamics of tree hydraulic status during drought are generally unknown because in-situ observations are difficult to obtain and are weakly connected to conditions throughout the hydraulic pathway, especially in irreplaceable organs like the main trunk. We modeled hydraulic status in the trunks of twelve trees from four species in the Duke Forest (Orange County, NC) over two years, one year with a late-summer drought and one with average moisture conditions. We fit the model to observations of sap flow and soil moisture in a Bayesian state-space framework and used published values of cavitation vulnerability as prior information, allowing us to simultaneously draw inference on the hydraulic characteristics and changing water status of the trees.

Results/Conclusions

Trees maintained broad safety margins in their trunk throughout the observation period. This finding was likely due to lower cavitation vulnerability and increased water status in the trunk compared to peripheral branches and leaves. During the drought, trunk water potential increased for most trees. The dynamic regulation of water status could be due to reductions in canopy stomatal conductance or cavitation in leaves and/or peripheral branches. We validated vulnerability estimates with correlated observations of xylem anatomy and compared trunk water status predictions to observations of leaf water potential during the drought. These results indicate that trees maintain their water potential during drought, either through hydraulic segmentation or stomatal regulation. We suggest that catastrophic hydraulic failure is not likely in the main trunks of trees in this ecosystem under the range of observed moisture conditions.