title: Experimental evidence for heat plume-induced cavitation and xylem deformation as a mechanism of rapid post-fire tree mortality.
abstract: 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.
authors.affiliation: Department of Biological Sciences, University of Cape Town, Rondebosch, 7700, South Africa.
authors.forename: Adam G
authors.initials: AG
authors.lastname: West
authors.affiliation: Department of Biological Sciences, University of Cape Town, Rondebosch, 7700, South Africa.
authors.forename: Jacques A
authors.initials: JA
authors.lastname: Nel
authors.affiliation: Department of Biological Sciences, University of Cape Town, Rondebosch, 7700, South Africa.
authors.forename: William J
authors.initials: WJ
authors.lastname: Bond
authors.affiliation: Department of Biological Sciences, University of Cape Town, Rondebosch, 7700, South Africa.
authors.forename: Jeremy J
authors.initials: JJ
authors.lastname: Midgley
mesh_terms: D000704:Analysis of Variance; D003198:Computer Simulation; D005052:Eucalyptus / Q000502:physiology; D006358:Hot Temperature; D008954:Models, Biological; D018515:Plant Leaves / Q000502:physiology; D013997:Time Factors; D014197:Trees / Q000502:physiology*; D052584:Xylem / Q000502:physiology* / Q000648:ultrastructure
keywords: Eucalyptus cladocalyx; Kiggelaria africana; cavitation; fire; hydraulic failure; pyrohydraulics; tree mortality; xylem deformation