Print PageVascular cambium necrosis (girdling) is the only generally accepted mechanism of tree stem mortality in forest fires. However, several lines of recent evidence suggest that fires might disrupt water transport through stems, but this has not been directly demonstrated and the causal mechanisms are unknown. Here we use air injection experiments with physical simulation modeling to propose that reduced hydraulic conductivity (not girdling) is the primary mechanism of tree stem mortality in forest fires. Air injection experiments on Populus balsamifera L. perfused with an ethanol-water mixture having a surface tension equal to water at 95 °C are used to test the temperature-dependency of water surface tension as a mechanism of air seed cavitation. Simulations using a two-dimensional model of transient stem heating implemented in a Computational Fluid Dynamics (CFD) model of forest fire spread (the NIST Fire Dynamics Simulator) are used to ask whether the temperature-dependent air seed mechanism could reduce the cross-sectional sapwood area of tree stems in physically realistic fires.
Results/Conclusions
Air injection results show that temperature-dependent changes in surface tension significantly reduce air seed pressure as compared to controls. Thus, heating promotes air seed cavitation in sapwood. CFD and stem heating simulations show that heat transport from a moving fireline can substantially reduce the cross-sectional area of stem sapwood in the absence of girdling. This suggests that reduced stem conductivity is a more fundamental mechanism of tree stem mortality than girdling. How reduced stem conductivity correlates and interacts with crown and root injuries to cause mortality remains an open question.
