Abstract
Wildland fire managers are often required to predict tree injury and mortality when planning a prescribed burn or when considering wildfire management options; and, currently, statistical models based on post-fire observations are the only tools available for this purpose. Implicit in the derivation of statistical models is the assumption that they are strictly applicable only for the species or conditions for which they were developed. The result has been a profusion of separate models of uncertain generality. A parallel research effort, the process approach, has been directed at modeling tree injury and mortality by directly simulating the energy-transfer process from the fire to the exterior surface of the plant, and thence into roots, stems, and foliage. Process models can currently predict stem or tree death if certain injury thresholds are reached. We present a brief review of the current understanding of the biophysical processes causing fire-induced plant injury, and focus on the challenges associated with defining boundary conditions, initial conditions, and thermal and physical properties required for modeling plant heating and tissue necrosis. We argue for integration of statistical and process approaches to predicting tree injury and mortality wherein process models provide inputs for statistical models. Research gaps that hinder the application of process-based tree injury and mortality models include linkage of fire effects models with combustion models (especially coupled fire-atmosphere models) through the boundary conditions required for simulating tissue heating, descriptions of live tree thermal and physical characteristics, and better understanding of the physiological basis for delayed fire-caused mortality and the interactions between fire injury and second-order causes of mortality such as diseases and insects.
Highlights
As a fire burns through a forest or shrub community, energy released in the combustion process can increase the temperature of the plant canopy, stem, and roots
Plant mortality as a direct result of heating by the fire typically manifests within two to three years post fire (Ryan and Reinhardt 1988, Fowler and Sieg 2004, Hood et al 2007), recent work by Harrington suggests that mortality in some species occurs over much longer periods
Process models could provide tree injury input to statistical tree mortality models. As their development proceeds, process models will be valuable as a point of comparison with statistical tree mortality models, perhaps in the sense of ensemble forecasting
Summary
As a fire burns through a forest or shrub community, energy released in the combustion process can increase the temperature of the plant canopy, stem, and roots. A predictive system combining process models of plant injury, tree physiological response to injury, and fire behavior would be more widely applicable across species, sites, and climatic conditions than statistical models, and could provide increased capability for predicting fire-induced mortality before a fire occurs (Jones et al 2004, and 2006, Michaletz and Johnson 2007). It was reasoned that because thick outer bark provides more insulation, the rate of heat transfer to the cambium may be roughly the same through both plates and fissures, suggesting that, while the exterior surfaces of some plant stems are relatively rough, smooth surfaced models may be an acceptable approximation (Jones et al 2004). Additional measurements and characterization of the thermal properties of stem and root components as a function of species, temperature, and moisture content are needed. Coupled fire-atmosphere models can be used to produce look-up tables of fire and plume characteristics from multiple simulations over relevant ranges in fuel, weather, and topographical conditions
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