California is, increasingly, a state defined by wildfire. Fire season is now a year-round phenomenon in the state, and 10 of California’s worst fire seasons have occurred since 2000. The July 2018 Mendocino Complex Fire is currently the largest wildfire in state history, burning nearly half a million acres last summer, and the Camp Fire, which ignited just a couple of months later, is the state’s deadliest.
Understanding the mechanisms that determine whether a fire grows or fizzles out is crucial information, whether you are suppressing a wildfire or prioritizing forest restoration activities on a landscape. But wildfire systems are inherently unpredictable and difficult to study because numerous interacting factors—wind, humidity, fuels, and temperature—create feedbacks and responses that influence fire behavior.
Top-down controls, like weather and climate, generally come from outside of a given system and exert influence across large areas. Bottom-up controls include ignitions, available fuel, and topography. The arrangement of bottom-up controls will inevitably shape fire patterns across a landscape, but Povak and his colleagues, Paul Hessburg and Brion Salter, wanted to know more.
“We wanted to understand to what degree topography matters and how its influence varies from place to place,” Povak said.
The team analyzed topography and fire perimeter data from 11,000 historical fires across California between 1950 and 2012—the first study to analyze such a large number of fires over time across mixed land ownerships and broad environmental gradients. They used statistical modeling to identify spatial correspondence between the fire event and topographic patch size distributions. Their results show that topography plays a dominant role in regulating wildfire dynamics in California, particularly in mediating small and medium fires. Topography appears to have the most influence on fires in rugged, complex terrain, and less influence in arid ecoregions.
“We were somewhat surprised that our analysis showed that topography’s influence on fire was significantly stronger in valley bottoms compared to ridgetops, because ridgetops can provide a natural firebreak,” Povak said. “But we found that they provided minimal control on fire spread across all ecoregions.”
Their analysis also revealed that roads exhibited the strongest control of fire behavior on small- and medium-sized fires, similar to topography. Roads and other human-constructed landscape features may lessen the influence of topography on regulating wildfire dynamics. However, even when the influence of roads was statistically removed, the role of topography was still apparent across most regions.
“Firefighters have long understood the value of topography as a natural fire break,” Povak said. “We show this effect across very different environments and forest types and provide evidence that this effect varies across fire sizes.”
Read the study online at https://www.fs.usda.gov/treesearch/pubs/57458.