Tree mortality following wild and prescribed fires is of considerable interest to researchers and managers. While some models exist that can predict post-fire mortality, process-based models that incorporate physiological mechanisms of mortality are still being developed and improved. One hypothesized mechanism of delayed mortality in trees is disruption of water transport in xylem due to atmospheric and xylem heating. For example, a fire’s heat plume rapidly increases the vapor pressure deficit (VPD) in the tree canopy, quickly increasing the tension on water in the xylem, potentially leading to cavitation and eventual tree death. Cavitation in the roots could potentially be exacerbated by heating from smoldering of the duff layer. We conducted laboratory experiments examining the degree to which heating branches and roots increases their vulnerability to cavitation. We placed longleaf pine (Pinus palustris) branches and roots in a water bath at realistic exposure temperatures (~54°C) and applied pressure in a cavitation chamber to simulate a range of xylem tension levels (0-5MPa) and measured the subsequent percent loss of conductivity. Resulting vulnerability curves combined with outputs from a plume model were used to parameterize a hydraulic conductance model to assess if trees are likely to experience VPDs sufficient for xylem cavitation.
Results/Conclusions
After comparing the resulting vulnerability curves of heated branches and roots to those pressurized at room temperature, we observed increased vulnerability to cavitation in the heated samples, especially at lower pressures. In branches, P50 (the pressure at which 50% of conductivity has been lost) decreased from -3.6MPa in unheated branches to -2.8MPa when the branches were pressurized while heated to 54°C, a decrease of 22%. Longleaf pine roots appeared to be even more vulnerable to cavitation under heated conditions, experiencing a decrease in P50 of 34% relative to unheated samples. Model results indicate canopy heating substantially increased the canopy area likely to experience conditions resulting in 50% loss of branch conductivity. The height at which branches would be expected to experience considerable cavitation extended 3 – 5 m higher into the canopy than would have been expected based on ambient temperature vulnerability curves. Wind plays an important role in dissipating heat during fires, and our models suggest that wind can help to limit the conditions leading to extreme decline in stem water potentials, thereby helping to protect mature trees. Continued advancement in understanding of the mechanisms leading to delayed mortality will improve models predicting tree mortality.