The frequency of large and severe wildfires has increased over recent decades in many regions across the Western U.S., including the Inland Northwest. Fire regimes in the region typically range from fuel-limited (characterized by frequent, lower-severity fires) to climate-limited (characterized by large infrequent fires). In fuel limited ecosystems, wildfire suppression has likely increased wildfire activity by promoting fuel accumulation, while in climate limited ecosystems, anthropogenic climate change may play a larger role by increasing fuel aridity. However, the relative roles of fuel vs. climate in driving increases in wildfire activity are not well quantified at regional and watershed scales, especially in areas that experience mixed severity regimes, such as the Inland Northwest. In addition, recent studies show that climate change can shift fire regimes from fuel to drought-limited. Understanding the relative influence of climate and fire suppression is crucial for projecting the effects of climate change on future fire spread, and for developing site-specific fuel management strategies under a new climate paradigm. To quantify the extent to which climate change and fire suppression have contributed to increases in wildfire activity in the Inland Northwest, we conduct a modeling experiment using the ecohydrologic model RHESSys and the coupled stochastic fire spread model WMFire. Specifically, we used historical climate inputs from GCMs, combined with fire management scenarios to gauge the extent to which these drivers promote the spread of severe wildfires in Johnson Creek, a large (565-km2) mixed-pine dominated subcatchment of the Southfork Salmon River in North Central Idaho. Johnson Creek experienced fire exclusion over several decades due to suppression efforts following the Great Fire of 1910. We ran 500 model iterations for complete-exclusion and no exclusion fire management scenarios, both with and without anthropogenic climate change in a factorial design. We then evaluated the extent to which these two drivers influenced the size and frequency of wildfires across the study basin.
Results/Conclusions
We found that climate change had a much larger effect on fire activity than fire suppression. Thus, forest management activities like thinning, which assume that wildfire increases are fuel-driven may not be useful for reducing wildfire risk in this study system. In future studies, we will expand our analysis to evaluate the role of fire exclusion vs. climate change across a range of watersheds and fire regimes in the Western U.S. Ultimately, this research will help determine when and where management may curtail increases in wildfire activity.