Xylem heating increases vulnerability to cavitation in longleaf pine

Improved understanding of the physiological mechanisms of tree mortality following fires is important with the predicted increase in wildfires under climate change, as well as continued use of prescribed fire for forest management. Disruption of water transport in the xylem from exposure to the heat plume of a fire has been hypothesized as a mechanism of delayed tree mortality. This heat plume rapidly increases vapor pressure deficit in the canopy, increasing transpiration and tension on the xylem causing cavitation, thus reducing water transport and leading to eventual tree death. We aimed to increase understanding of the mechanisms behind such unintended mortality by determining whether branches and roots of longleaf pine are more vulnerable to cavitation when exposed to temperatures expected to occur during prescribed or wild fires. Additionally, we modeled expected branch cavitation under fire conditions based on measured cavitation vulnerability. We heated branch and root segments in a water bath to 41 degrees C and 54 degrees C and simulated the negative xylem water potentials experienced during exposure to a heat plume using a double-ended pressure chamber. When branches and roots were pressurized under elevated temperatures, xylem in both organs was more vulnerable to cavitation. In branches, as temperature was increased from 23 degrees C-54 degrees C, the pressure at which 50% conductivity was lost (P-50) increased from -3.55 MPa to -2.79 MPa, while in roots, P-50 increased from -2.08 MPa to -1.36 MPa. When the P-50 values measured under elevated temperatures were included in plume and hydraulic models, branches were predicted to experience conditions leading to 50% loss of conductivity up to two meters higher into the canopy than under ambient temperatures. Overall, these results suggest that heating of branches and roots during fires can increase vulnerability to xylem cavitation, potentially leading to hydraulic disruption and delayed tree mortality.