Experimental evidence for heat plume‐induced cavitation and xylem deformation as a mechanism of rapid post‐fire tree mortality

Recent work suggests that hydraulic mechanisms, rather than cambium necrosis, may account for rapid post‐fire tree mortality. We experimentally tested for xylem cavitation, as a result of exposure to high‐vapour‐deficit (D) heat plumes, and permanent xylem deformation, as a result of thermal softening of lignin, in two tree species differing in fire tolerance. We measured percentage loss of conductance (PLC) in distal branches that had been exposed to high‐D heat plumes or immersed in hot water baths (high temperature, but not D). Results were compared with predictions from a parameterized hydraulic model. Physical damage to the xylem was examined microscopically. Both species suffered c. 80% PLC when exposed to a 100°C plume. However, at 70°C, the fire‐sensitive Kiggelaria africana suffered lower PLC (49%) than the fire‐resistant Eucalytpus cladocalyx (80%). Model simulations suggested that differences in PLC between species were a result of greater hydraulic segmentation in E. cladocalyx. Kiggelaria africana suffered considerable PLC (59%), as a result of heat‐induced xylem deformation, in the water bath treatments, but E. cladocalyx did not. We suggest that a suite of ‘pyrohydraulic’ traits, including hydraulic segmentation and heat sensitivity of the xylem, may help to explain why some tree species experience rapid post‐fire mortality after low‐intensity fires and others do not.