Vegetation-fire feedback reduces projected area burned and emissions

Across the western US, the area burned by wildfire has been increasing as a result of higher temperatures and earlier spring snowmelt.  When temperature goes up, ecosystems dry out and become more available to burn.  This relationship has formed the basis for projecting future area burned as a result of climate change.  With the projected increase in area burned, the expectation is that wildfire emissions will also increase.  In a prior study, we estimated a 19-101% increase in emissions from wildfires burning in California by the end of this century.  However, the majority of fire projections, including our future work makes the assumption that there will be vegetation available to burn if a fire occurs.  The problem with that assumption is that fire is a self-limiting process, meaning for some period of time after a wildfire occurs, there is not enough vegetation available to support another fire.  Further, even if enough vegetation is present to support a second fire, the amount of vegetation may be lower than the first fire and result in fewer wildfire emissions.  To determine the effects of prior wildfires on future wildfires, we modeled this vegetation-fire feedback by simulating forest growth and wildfire under future climate across three transects in the Sierra Nevada (Figure 1).  We re-estimated wildfire size distributions at each decade from 2010-2100 to account for the effect of prior wildfires on future fire size.  We used the area burned and the type of vegetation that it burned to estimate the emissions from wildfire.  

Figure 1: Location of the three transects simulated along the Sierra Nevada.

We found that when we accounted for the vegetation-fire feedback, the cumulative area burned was 9.8-21.8% lower than our simulations that only accounted for the effect of climate on wildfire (Figure 2).  Scaled to the entire mountain range, this equals a 14.3% reducing in cumulative area burned through 2100.

Figure 2: Cumulative area burned for the three transects in the Sierra Nevada under projected climate. The dynamics simulations (solid lines) account for the effects of prior fires limiting future fires and projected climate. The statics simulations (dashed lines) only account for projected climate.

​The largest wildfires are typically the most impactful to both society and ecosystems because large wildfires typically occur under extreme weather conditions, which cause them to spread rapidly.  When we accounted for the effects of prior wildfires on fire size (dynamic), we found that by mid-century the largest wildfires were significantly smaller than the largest wildfires in the climate-only (static) simulations (Figure 3). However, by late-century, vegetation recovered in the previously burned areas and the largest wildfire sizes were no longer statistically different. 

Figure 3: Maximum fire size distributions by time period. The early period is 2010-39, mid is 2040-69, and late is 2070-99. An asterisk indicates that the distributions are significantly different.

​The data in Figure 3 are plotted on a log-scale to meet the assumptions of the statistical test we used to compare them.  By mid-century, the dynamic simulations had a median fire size of 24,053 acres and the static simulations had a median value of 53,389 acres.  The biggest fire we simulated in the dynamic scenario occurred during early-century and was 210,452 acres.  Whereas, in the static scenario, the biggest fire we simulated occurred in late-century and was 440,754 acres.  For comparison, the 2013 Rim Fire burned 257,314 acres in the Sierra Nevada and estimated to have emitted the equivalent of 12 Tg of carbon dioxide, equivalent to 2.7% of California’s total emissions.  When we calculated the emissions that our simulated wildfires would emit, we found that even when we account for the effects of previous wildfires limiting future wildfire size, total emissions in the Sierra Nevada are equivalent to one Rim Fire occurring every 3.8 years.  This could have significant implications for air quality in California.