What do changing climate suggest about future fire frequency in California

Background/Question/Methods

The Sierra Nevada experiences routine summer time drought coupled with periods of extended seasonal drought driven by low winter precipitation, often for several consequtive years. Moderately high historical fire return intervals are the norm for these montane conifer forests. Twentieth century fires suppression has resulted in changing forest composition and increasing stand density. The consequence has been more frequent, larger, more intense fires, with ten of California’s largest fires have occurred since 2000. Extreme droughts represent windows of vulnerability where stand clearing fires may reset large tracts of vegetation. Understanding how climate change will impact the already increasing frequency of large and severe fires is critical in developing climate change adaptation strategies for the Sierra Nevada. We used two Global Circulation Models (GFDL, PCM) under a high (A2) emissions scenario to model the degree to which vegetation is vulnerable to change driven by fire. These same models were used to predict changes in future fire likelihoods based on climate.

Results/Conclusions

Climate models uniformly exhibit significant increases in fire likelihoods as a consequence of changing climates. In addition, empirical observations demonstrate increasing conifer stand densities at high elevation. Increasing stand densities are likely to further increase fire probabilities. Low elevation ignitions are more likely to carry to high elevation with increased mid to high elevation stand densities. Further, high elevation lightning strikes are the predominant natural ignition source in the Sierra Nevada. With increased stand density, the probability of high elevation ignitions carrying fire to lower elevations increases. Although much of the Sierra Nevada has not been burned for at least one expected fire return interval, recent increases could rapidly “catch up” and bring fire return interval back into alignment. These fires, however, are likely to have increased impacts through increased density of ladder fuels that foster stand clearing fire. As fire is a likely driver of forest cover change in response to climate change, we predict that areas primed to burn, as a consequence of a combination of high fuel loads and close proximity to human ignition sources (e.g., roads), are likely to experience the highest impact of changing climate early on in the 21^st century. These also happen to be areas most frequently visited by people (e.g., Tahoe basin, Yosemite Valley, Sequoia and Kings Canyon National Parks). Parks management must take advantage of moderate wildfire events to foster fuels reduction without catastrophic outcomes.