Ponderosa pine (Pinus ponderosa) is one of the most widespread conifers in western North America and has recently experienced high levels of bark beetle-induced tree mortality. A number of previous studies address wetter forest types, but little is known about the fire/bark beetle interactions in this drier forest type. This project explored fire behavior attributes under three levels of tree mortality in a southwestern U.S. forest dominated by ponderosa pine at three stages: pre-outbreak (“green stage”), immediately post-mortality when dead needles remain on trees (“red stage”), and when needles drop to the ground (“gray stage”).
Researchers used data from 60 field plots across five national forests in northern Arizona to develop fire simulations, and a physics-based model, HIGRAD/FIRETEC that accounts for effects of spatial heterogeneity, dynamic winds, and fire behavior. They ran the simulations at three open wind speeds: 4, 8.8, and 18.7 m/sec.
Simulations indicated that fire rate-of-spread (ROS) and canopy fuel consumption varied depending on the stage of the bark beetle outbreak, the level of tree mortality and the wind speed.
As forest canopy fuels transitioned from the green to the red stage, fire ROS was generally amplified and total canopy fuel consumption was amplified at higher levels of mortality, but unaffected at low levels of mortality and high wind speeds.
However, for all levels of mortality the greatest difference in fire ROS between red stage and green stands occurred at low wind speeds, and even just 20% tree mortality resulted in a 15% increase in ROS over green stands (Fig. 1). With high levels of mortality predicted ROS was 90% higher than in green stands.
In contrast to the red stage results, simulations in the gray stage revealed a general dampening effect on ROS and either no effect or dampening on fuel consumption. The one exception was in the low wind, low mortality gray simulations, where both fire ROS and canopy fuel consumption were predicted to be higher than in green stands (Fig. 1).
Results suggest that bark beetle-caused tree mortality can either dampen, amplify, or have little effect on subsequent fire disturbances and their ecological effects, and may even lead to some ecological surprises.
C.H. Sieg, R.R. Linn, C. Hoffman, J. McMillin, F. Pimont, J. Winterkamp, R.A. Parsons. Modelling fire behaviour in heterogeneous fuels resulting from bark beetle outbreaks. VII International Conference on Forest Fire Research. Nov. 14-21, 2014. Coimbra, Portugal.
Sieg, C.H., R. Linn, C. Hoffman, J. Winterkamp, J. McMillin. Coupled Fire/ Atmosphere Behavior in Highly Heterogeneous Woodlands and the Impacts of Tree Mortality on Fire Spread. Ninth Symposium on Fire and Meteorology, Palm Springs, CA Oct. 18-20, 2011.
Linn, R., C.H. Sieg, J. Winterkamp, R. Parsons, and C. Hoffman. Examining mountain pine beetle impacts on coupled fire/vegetation/atmosphere behavior in lodgepole pine stands using FIRETEC. Ninth Symposium on Fire and Meteorology, Palm Springs, CA Oct. 18-20, 2011.
Sieg, C.H., R. Linn, C. Hoffman, J. Winterkamp, J. McMillin. Exploring how time-since-mortality influences fire propagation following drought-induced bark beetle outbreaks in piñon/juniper woodlands. Southwestern Association of Fire Ecology Conference 2012. Santa Fe, NM February 27-March 1, 2012.
Hoffman, C.M., C.H. Sieg, J.D. McMillin, P.Z. Fulé. Fuel loadings and predicted fire behavior five years after a bark beetle outbreak in Southwestern ponderosa pine forests. Southwestern Association of Fire Ecology Conference 2012. Santa Fe, NM February 27-March 1, 2012.
R.R. Linn, C.H. Sieg, C. M. Hoffman, J. Winterkamp, J.D. McMillin. 2013. Accounting for spatial and temporal variability in modeling the effect of recent bark beetle outbreaks in pinyon-juniper woodlands on potential fire behavior. Agricultural and Forest Meteorology 173:139-173.
Linn, R.R., 1997. A Transport model for prediction of wildfire behavior. LA-13334-T, Los Alamos National Laboratory, Los Alamos, NM.
Linn, R. R., Cunningham, P., 2005. Numerical simulations of grass fires using a coupled atmosphere–fire model: Basic fire behavior and dependence on wind speed. J. Geophys. Res. D: Atmos. (1984–2012), 110 (D13).
Linn, R. R., J. Winterkamp, J. Colman, C. Edminster, 2005. Modeling interactions between fire and atmosphere in discrete element fuel beds. International Journal of Wildland Fire 14(1): 37-48.
Sieg, C.H., R. Linn, C.M. Hoffman, J. Winterkamp, F. Pimont. Simulated fire hazard following bark beetle outbreaks in ponderosa pine: Amplification, attenuation, and a few ecological surprises. Manuscript in preparation; proposed for submission to Ecological Applications.