Toward a better understanding of overstory tree mortality: a study of fire-atmosphere interactions inside a forest canopy

Point ignition operations for prescribed surface fires in pine forests can sometimes lead to forest overstory mortality. The possible causes for the mortality include (i) increases in ignition density that lead to larger flame convergence zones, (ii) reduced heat transport in dense canopies leading to hot gases lingering in the canopy, and (iii) large vapor pressure deficits leading to an embolism in branch vascular systems. New fire-atmosphere interaction modeling and monitoring research is needed to determine the primary mechanisms by which surface wildland fires can lead to increasing levels of tree mortality. In this study, we use the ARPS-CANOPY model to explore the sensitivity of meteorological variables potentially relevant to tree mortality [e.g., air temperature, vapor pressure deficit] to fire environment factors such as the shape and density of canopy vegetation, the presence of gaps in forest cover, ambient and fire-induced TKE regimes, and aspects of fire behavior including spread rate and direction (i.e., backing vs. head fires).

In this talk, results are presented from three sets of experiments. In the first set of experiments, we introduce a stationary surface heat source in a horizontally-homogeneous forest canopy, and vary the vertical canopy structure between two profiles, one with canopy density evenly distributed in the vertical, and a second with vegetation concentrated in the upper half of the profile. In the second set of experiments, we introduce a stationary surface heat source in a forest represented by a single canopy profile, but vary the spatial pattern of the forest canopy between uniform forest coverage and a forest plot with gaps. A third set of experiments are conducted with a moving fireline to assess the sensitivity of results from the first and second set of experiments to fire spread (both head and backing fires). The numerical experiments in this study are expected to (i) help guide the development of additional experiments with a more advanced, higher resolution fire-atmosphere model and (ii) assist the interpretation of field experiment data as it relates to tree mortality.