Tree Injury and Mortality in Fires: Developing Process-Based Models

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.