Gold Creek drainage, Okanogan National Forest (Washington State). Photo by John Marshall.
Wildfires continue to challenge our capacity to control them. In California, fire season is now year-round, and 10 of the state’s worst fire seasons have occurred since 2000.
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. Many interacting factors—wind, humidity, fuels, temperature—create feedbacks and responses that influence fire behavior.
Research ecologist Nicholas Povak led a study addressing one of the biggest sources of debate in fire science: are fires regulated by top-down or bottom-up controls?
Top-down controls, including weather and climate, generally come from outside 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. To what degree does topography matter? How does topography’s influence differ from place to place?
The team analyzed topography data and fire perimeter data from 11,000 historical fires across the state of California. 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.
The authors were somewhat surprised that their analysis showed that topography’s influence on fire was significantly stronger in valley bottoms compared to ridgetops. Ridgetops can provide a natural fire-break, but Povak’s team found that they provided minimal control on fire spread across all ecoregions.
They also found 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.
This study is the first to analyze such a large number of fires across a relatively long time period (1950-2012), over mixed land ownerships and broad environmental gradients. It is also the first to take a multi-scaled approach to contrasting topographic controls across fire sizes. Importantly, it supplies quantitative evidence of topography’s central role in scaling behavior over several orders of magnitude.