Do mountain pine beetle outbreaks change the probability of active crown fire in lodgepole pine forests? Comment.

Simard et al. (2011) have produced a comprehensive data set and analysis concerning mountain pine beetle (MPB; Dendroctonus ponderosae)-caused mortality and associated crown fire feedbacks in lodgepole pine (Pinus contorta)-dominated forests. Misapplication of the NEXUS fire modeling system (Scott and Reinhardt 2001) results in the suspect conclusion that active crown fire (perpetuation of flames through the canopy) probability is reduced in all post-mortality stages. Simard et al. (2011) assert that the loss of canopy fuel following tree mortality overwhelms the concomitant loss of foliar moisture content (FMC) but do not fully account for this drop in moisture or the resulting increase in surface fuels. Here, we show how to account for decreases in FMC and increases in surface fuels within NEXUS and report findings contrary to Simard et al.’s (2011) conclusions for the red stage (dead needles still within canopy). Overall, NEXUS is a questionable choice for this research due to its inherent lack of crown fire predictive capability (Cruz and Alexander 2010) and empirically derived crown fire models developed using living canopies (Van Wagner 1977, Rothermel 1991, Scott and Reinhardt 2001). NEXUS and related fire modeling systems (e.g., FARSITE, FlamMap, BehavePlus, and FFE-FVS) rely on the integration of Rothermel’s (1972) surface and crown (1991) fire spreadmodels with VanWagner’s (1977, 1993) crown fire transition and propagation models (Scott 2006, Cruz and Alexander 2010). The fire modeling community recognizes the need for calibration of model input variables and parameters, including custom surface fuel models, which Simard et al. (2011) employ, to achieve accurate representations of observed, and thus predicted, fire behavior. Without calibration, quantitative fire behavior output values can be wildly unrealistic, as large under-prediction biases are prevalent (Cruz and Alexander 2010). For example, necessitating 1000 km/h wind speeds to initiate crown fire in undisturbed lodgepole pine forests (Simard et al. 2011). However, relative comparisons of disturbed and undisturbed forests’ fire behavior are possible when full consideration is given to the primary drivers of fire behavior.