The Wallow Fire burned over half a million acres of forest in Arizona and New Mexico in 2011. Officials consider it to be the largest wildfire in Arizona’s history, but new CIRES research finds several pre-1900 fires were just as large — if not larger — than the Wallow. 

The new study shows the Wallow Fire’s size was indeed exceptional, but not unprecedented in the last 375 years. The results suggest that although wildfires in western North America have become more severe and destructive in recent decades, they’re not necessarily larger. Rather, they’re burning hotter and spreading faster, due to a combination of climate change and a buildup of fuels from the fire suppression tactics humans have adopted over the past hundred years. 

The new study, along with other recent CIRES research, suggests that it’s also important to consider the speed at which modern fires are spreading, rather than just their size, when determining how destructive they are. 

“Part of how we describe the current wildfire crisis is you'll hear folks say that fires are getting bigger, and that this is unprecedented and extreme,” said Chris Guiterman, a CIRES paleoecologist and lead author of the new study published in Fire Ecology. “There are aspects of current fires that indeed fit some of that description. But when it comes to size and really size alone, it's not.” 

Reconstructing past fire activity 

Wildfires in western North America have burned more forest area since the 1980s, a change driven primarily by individual “megafires” such as the Wallow Fire. 

In the new study, Guiterman and his colleagues wanted to know if the Wallow Fire’s size was unprecedented or if historical fires could have grown just as large. They analyzed new and existing tree ring fire scars from the area where the Wallow Fire burned. When trees survive a fire, sometimes the burn is preserved in the annual rings of the tree, and those scars can be dated with exact precision to the year and sometimes the season of the fire. 

Using nearly 1,300 individual fire scars from 160 trees, the researchers reconstructed the extent of annual area burned back to 1650. Their results showed fires in 1748, 1847, and 1851 were just as large (and likely larger) than the Wallow Fire. 

However, related research shows that these pre-1900 fires were likely to be lower in severity than recent high-intensity burning, like some of the Wallow Fire. The lower fire intensity allowed landscape vegetation to recuperate much more quickly after pre-1900 fires, and many of the mature trees survived repeated fires. 

“To have two Wallow-sized fires in four years (1847 and 1851) shows us how low-severity these fires were,” Guiterman said. “We have dozens and dozens of trees that we know got burned by both those fires and lived to tell the tale, whereas a lot of trees died in Wallow.” 

In fact, their reconstruction shows that large, “megafire-sized” fires burned every 20 years or so in the same landscape prior to 1900. 

“The frequency of very extensive fires on this landscape that we reconstruct in this paper is pretty astounding,” Guiterman said. “So we know that those two Wallow-sized fires were occurring under very different conditions than the Wallow Fire, and therefore had very different fire behavior.”

Evolving fire management practices

Around 1900, American expansion into the Southwest led to a major decline in fire, and the new study shows that since then, the area has been in a fire deficit, with a surplus of fuel building up. For the region analyzed in this study, the most recent 20 years had less total area burned than during any other 20-year period from the late 1600s to 1900, according to the authors. 

Even with the Wallow Fire, the landscape's cumulative fire deficit continued to increase through 2024. The researchers estimate that had the historical fire regime not ended by 1900, the study area would have burned 3 to 7.5 million acres over the last 125 years. Instead, the area has seen an 85% decline in total area burned.

These findings underscore that future wildfire events could be even larger, and likely more severe, as fuels accumulate and the climate grows warmer and drier.

Despite the harm large wildfires can cause for people, property, and ecosystems, much of the acreage burned in recent decades is with low to moderate severity fire, which has ecological benefits. Low- and moderate-severity fires are very effective at reducing the severity of the next fire. That sort of burning is in line with the pre-1900 fires that Guiterman and his colleagues suggest can enhance forest resilience to future climate extremes. 

“One way to keep these ecosystems resilient to climate change and to wildfire is to burn them extensively and frequently, as long as we have the right conditions for human safety from the flames and the smoke,” Guiterman said. “We propose extensive fires maybe every 40 years in similar southwestern dry conifer forests, but in some areas even more frequently might be better.”