Abstract
Summary 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.
Highlights
The mechanism by which trees die following exposure to a lowintensity fire is poorly understood
We suggest that a suite of ‘pyrohydraulic’ traits, including hydraulic segmentation, thermal sensitivity of xylem deformation and stomatal sensitivity to heat plumes, may be important traits that contribute towards fire survival or mortality
Branches of c. 2 m in length were sampled from individual trees, as this ensured that the segments were much longer than the maximum vessel length of each species (61 cm for E. cladocalyx and 56 cm for K. africana)
Summary
The mechanism by which trees die following exposure to a lowintensity fire is poorly understood. Low-intensity fires may result in tree mortality, maintaining open ecosystems, such as savannas, from conversion to closed canopy forest (Bond & Midgley, 2000; Hoffmann & Solbrig, 2003; Hoffmann et al, 2009). The ability of a tree to survive exposure to low-intensity fire is a key determinant of whether these open ecosystems are converted to closed forest (Bond & Midgley, 2000). It has been largely assumed that fire mortality arises through prolonged lethal heating (> 60°C) of the vascular cambium and phloem of the stem (Dickinson et al, 2004; Michaletz & Johnson, 2007). In the event of necrosis of the entire circumference of the vascular cambium and phloem, the tree is unable to transport photosynthate to the roots, resulting in starvation of these tissues and tree mortality (Bond & Van Wilgen, 1996)
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