Recent wildfires are larger and more intense than they’ve ever been in the historical record. If you’ve been watching the news at any point in the last decade, that’s no surprise.
The reasons for that are complex, though not surprising—hotter, drier weather; more fuel built up from decades of fire suppression; and increasing human ignitions from campfires to powerline malfunctions.
What is surprising is the role that spaceborne lidar is playing in fire management and what having a unique view of the Earth can tell us about reducing fire risk and hazard. Recent research from Northern Arizona University found that when conditions are intense—particularly with hot, dry winds—fires can roar through landscapes burning at high severity in dense and sparse forests alike.
The one exception to this trend is with ladder fuels, which are shrubs, small trees and lower branches that can carry a fire from the forest floor to the canopy, where it can then grow exponentially and spread more quickly. Unlike the total amount of fuel in a forest, it turns out the vertical location of those fuels, especially when they act as ladder fuels, may be the most important factor in predicting wildfire severity even in extreme conditions. Further, management efforts that focus on ladder fuels can help reduce the size and severity of wildfires in the American Southwest no matter what the weather is doing.
“This research uses large-scale satellite data to show that if fire weather is extreme and there’s ignition, let’s say from burning embers blown by the wind, a fire is more likely to result in a high-severity burn due to factors like low humidity and wind speed versus the sheer fuel volumes,” said Chris Hakkenberg, an assistant research professor in the School of Informatics, Computing, and Cyber Systems (SICCS) at NAU and lead author of the study. “That said, we were surprised to find a critical exception to this trend—specifically with ladder fuels. Put simply, even in extreme weather conditions, forests with fewer ladder fuels tended to experience less severe fires.”
That’s good news for land and fire managers, though nothing new for those practicing cultural burns for millennia.
The important role of fire—and NASA—in fire management
Land managers have long known fire is useful in land management. Controlled or prescribed fires can burn dried grass and other undergrowth, so when a wildfire tears through a forest, it doesn’t move as quickly up into the tree crowns and is more likely to die out on its own.
NASA’s spaceborne lidar Global Ecosystem Dynamics Investigation (GEDI), which was used in the study to characterize pre-fire fuel for 42 large California fires from 2019-2021, is uniquely able to peer into forests, providing consistent data of fuel structure for large areas and over multiple years. The team was able to determine that, among all measures of forest fuel, such as canopy height or volume, the presence of ladder fuels was the single most consistent determinant of high severity fire. Likewise, fewer ladder fuels were associated with reduced burn severity.
That means in the future, land and fire managers can use satellite data on weather and fuels to find out well before a fire which areas are at greatest risk and how to reduce that risk by working with managers on the ground to implement pre-fire ladder fuel treatments and guide suppression efforts.
What extreme weather means for fire severity
Forest fuels are not a great predictor of severity, especially when extreme weather is part of the equation, Hakkenberg said.
“While wildfires are a natural component of western U.S. fire-adapted ecosystems, this trend from mixed-severity toward large, high-severity fires has resulted in wildfire regimes that are more destructive to forest ecosystems and more dangerous to human communities,” he said. “These high-severity fires also run the risk of altering our forestlands to a new normal: converting them to shrub or grasslands where trees are unable to establish due to high-intensity fire’s impact on microclimate, soil conditions and the seed bank.”
The need to develop ways to adapt to a new climate normal across landscapes with historically dense forest fuels is great, but focusing management on ladder fuels is one promising approach.
NAU senior research scientist Patrick Burns and Regents’ professor Scott Goetz, both from SICCS, co-authored the study, which was done in collaboration with researchers in California.
