Altered microclimate may be more important than augmented fuel loads in predicting post-insect outbreak fire behavior

Background/Question/Methods

Growing numbers of invasive insect outbreaks have been cited as a key contributor increased to wildland fire activity in North American forests over the past 20 years.  Numerous studies have found strong relationships between outbreaks and increased in forest fuel loads, thought to be the causal mechanism of increased fire activity; however fire behavior models based on these relationships continue to unreliably predict variability in fire behavior following outbreaks.  Altered forest meteorology has been largely overlooked by researchers studying effects of forest insect outbreaks, yet may prime conditions for wildfires.  Recent work in the New Jersey Pinelands has also found dramatic alterations  in sensible and latent heat fluxes related to reduced canopy cover and function following gypsy moth (Lymantria dispar) and Southern pine beetle (Dendroctonus frontalis) outbreaks, and which would suggest drier and windier conditions in such stands.  We conducted two studies to assess microclimatic conditions in disturbed and undisturbed stands, and to assess forest biomass related factors that may influence fire severity.  Here we present a synthesis of the results of these studies, and propose an alternative hypothesis such that altered forest structure and microclimate are the dominant mechanisms by which insect outbreaks enhance wildfire activity. 

Results/Conclusions

Half-hourly surface temperature and wind speed measured at 3m were greatest under low canopy cover conditions, with average daytime surface temperature and average windspeed being 23% and 275%, respectively, than those measured in medium and low disturbance conditions, highlighting a dramatic departure in conditions that lead to the drying of surface fuels and greater fire spread.  Comparison of forest biomass data measured prior to burning in wild and prescribed found pre-burn mass of downed woody debris was poorly correlated with fire severity (r²= 0.02), while litter depth was negatively correlated (r²=-0.33), suggesting that increased fuel loads from insect disturbances do little to increase fire behavior themselves.  Pre-burn canopy cover, however, also was negatively correlated with fire severity (r² =-0.41).  While fire behavior was not explicitly quantified under various canopy disturbance and fuel conditions, the data presented here suggest that microclimatic variability resulting from insect disturbances may be an important mechanism in facilitating post-outbreak forest fires.  Future fire research efforts in the field and with modelling should focus more attention to the role of microclimate in fire behavior and how forest disturbances such as insect outbreaks impact microclimatic conditions.