Moving beyond the cambium necrosis hypothesis of post-fire tree mortality: cavitation and deformation of xylem in forest fires

Authors

  • S. T. Michaletz,

    1. Department of Biological Sciences and Biogeoscience Institute, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
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  • E. A. Johnson,

    1. Department of Biological Sciences and Biogeoscience Institute, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
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  • M. T. Tyree

    1. Department of Renewable Resources, 444 Earth Sciences Building, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
    2. Northern Research Station, US Forest Service, 705 Spear St, S. Burlington, VT 05403, USA
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Author for correspondence:
Sean T. Michaletz
Tel: +1 403 220 7635
Email: sean.michaletz@ucalgary.ca

Summary

  • It is widely assumed that post-fire tree mortality results from necrosis of phloem and vascular cambium in stems, despite strong evidence that reduced xylem conductivity also plays an important role.
  • In this study, experiments with Populus balsamifera were used to demonstrate two mechanisms by which heat reduces the hydraulic conductivity of xylem: air seed cavitation and conduit wall deformation. Heat effects on air seed cavitation were quantified using air injection experiments that isolate potential temperature-dependent changes in sap surface tension and pit membrane pore diameters. Heat effects on conduit wall structure were demonstrated using air conductivity measurements and light microscopy.
  • Heating increased vulnerability to cavitation because sap surface tension varies inversely with temperature. Heating did not affect cavitation via changes in pit membrane pore diameters, but did cause significant reductions in xylem air conductivity that were associated with deformation of conduit walls (probably resulting from thermal softening of viscoelastic cell wall polymers).
  • Additional work is required to understand the relative roles of cavitation and deformation in the reduction of xylem conductivity, and how reduced xylem conductivity in roots, stems, and branches correlates and interacts with foliage and root necroses to cause tree mortality. Future research should also examine how heat necrosis of ray parenchyma cells affects refilling of embolisms that occur during and after the fire event.

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