Comment 1 - 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).
Comment 2 - An expansive mountain pine beetle (MPB) epidemic is currently impacting North American forests (Raffa et al. 2008). As beetle-attacked trees die, lose their needles, and eventually fall to the ground, there are substantial changes in stand structure. These fuel changes likely affect both surface and crown fire behavior, but there is not yet a consensus among experts regarding the nature and magnitude of these effects (Jenkins et al. 2008). Simard et al. (2011; hereafter referred to as SRGT) used linked crown fire models implemented in the NEXUS crown fire modeling system (Scott and Reinhardt 2001) to predict the occurrence of active crown fire across a chronosequence of increasing time since attack. They concluded that, under moderate fire weather conditions, recently attacked or red-stage stands had a lower occurrence of active crown fire than undisturbed stands. Here, we suggest that these conclusions are compromised because (1) the fire behavior modeling framework used has no mechanisms for considering highly heterogeneous fuels and (2) their use of this framework omitted critical aspects of how canopy and surface fuels change after an attack, particularly during the first two to three years.
Reply - In a recent paper, we showed that contrary to conventional wisdom, outbreaks of the mountain pine beetle (Dendroctonus ponderosae) did not increase the predicted potential for active crown fires relative to comparable undisturbed stands in lodgepole pine (Pinus contorta var. latifolia) forests of Greater Yellowstone (Simard et al. 2011). Under certain intermediate weather conditions, the potential for active crown fires was actually predicted to decrease relative to comparable stands undisturbed by bark beetles. Our study was based on a rich data set of forest fuels sampled in a robust post outbreak chronosequence of beetle-killed and undisturbed stands that were replicated in space and time and validated with dendrochronological reconstructions of pre-outbreak conditions. Moran and Cochrane (2012) and Jolly et al. (2012) have expressed concerns about certain aspects of our fire behavior modeling in recently killed stands. Although these authors do not refute the conclusions of our study, they do raise issues that are highly debated among scientists and practitioners alike. Here we explain that these concerns are largely based on misconceptions about the dynamics of mountain pine beetle (MPB) outbreaks, and we provide complementary data that strengthen our conclusions.
Comment 2 - Jolly, W. Matt; Parsons, Russell; Varner, J. Morgan; Butler, Bret W.; Ryan, Kevin C.; Gucker, Corey L. 2012. Do mountain pine beetle outbreaks change the probability of active crown fire in lodgepole pine forests? Ecology 93(4):941-946.
Reply - Simard, Martin; Romme, William H.; Griffin, Jacob M.; Turner, Monica G. 2012. Do mountain pine beetle outbreaks change the probability of active crown fire in lodgepole pine forests? Reply. Ecology 93(4):946-950.