Browsing by Author "Michaletz, Sean Thomas"

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    Biophysical processes of tree mortality in surface fires
    (2005) Michaletz, Sean Thomas; Johnson, Edward A.
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    Tree mortality and seed survival in forest fires
    (2012) Michaletz, Sean Thomas; Johnson, Edward A.
    This thesis combines process-based mathematical models with experimental and empirical data to examine and understand the mechanisms linking forest fire behavior to tree growth, mortality, and seed survival. The thesis begins with a review of the state of knowledge of post-fire tree mortality mechanisms at the beginning of the research. At that time, the mechanisms of phloem and vascular cambium necrosis in tree stems and branch, bud, and foliage necroses in tree crowns were well understood, but understanding of how these injuries were linked to post-fire tree growth and mortality was very limited. To explore how forest fires might affect whole-tree physiology, I examined heat effects on the hydraulic conductivity of xylem. I demonstrated for the first time that heating reduces the hydraulic conductivity of xylem via two mechanisms: enhanced cavitation and conduit wall deformation. This suggests that fire effects on whole-tree hydraulics may play an important role in post-fire tree growth and mortality. I then derive a process-based approach that links a computational fluid dynamics (CFD) model of forest fire behavior to the heat transfer mechanisms of heat injury in tree stems and crowns. These injuries are then linked to whole-tree growth and mortality using a dynamic mass balance approach. The model is currently under development and is presented here as a proof of concept. The focus then shifts to mechanisms of post-fire recruitment, and I couple a CFD fire spread model to heat transfer in closed white spruce cones to explore how non-serotinous, non-sprouting species can persist in fire regimes that theoretically should favor serotinous species. I demonstrate that non-serotinous cones can insulate seeds from heat necrosis in crown fires, but that recruitment in high numbers would require the co­incidence of a fire and a mast year. Nevertheless, post-fire recruitment from in situ aerial seedbanks can occur for non-serotinous species, and this may be an important cause of their persistence in fire regimes to which they otherwise seem poorly suited.