Stem heating results in hydraulic dysfunction in Symplocos tinctoria: implications for post-fire tree death

Fire-induced heating of stems can impair plant water transport by deforming xylem and increasing vulnerability to cavitation, but it is not clear whether these effects can result in tree death, or how quickly this may occur. In field experiments, we heated stems of Symplocos tinctoria (L.) L'Hér saplings to 90 °C using a thin-film resistive heater, and we monitored stomatal conductance, leaf water potential, sap flow and hydraulic conductivity until stem death. Sap flow and stomatal conductance declined quickly after heating, while whole-plant hydraulic conductance and leaf water potential remained high for the first week. In fact, leaf water potential increased during the first days after heating, indicating that stomatal closure was not initially caused by leaf water deficit induced by impaired water transport. After 1 week, leaf water potential and whole-plant conductance declined below unheated controls, while stomatal conductance and sap flow continued declining, approaching zero after 2 weeks. To better understand the cause of these declines, we directly measured hydraulic conductivity of heated stems. Stems underwent a progressive decline in conductivity after heating, and by the time that samples were severely wilted or desiccated, the heated portion of stems had little or no conductivity. Importantly, conductivity of heated stems was not recovered by flushing stems to remove embolisms, suggesting the existence of physical occlusions. Scanning electron micrographs did not reveal deformed cell walls, nor did it identify alternative causes of blockages. These results reveal that stem heating can result in xylem dysfunction and mortality, but neither response is immediate. Dysfunction was likely caused by wound responses rather than embolism, but improved understanding of the mechanisms of heat-induced hydraulic failure is needed.