Wildfire risk climbs as trees reclaim eastern US landscapes

The eastern U.S. has more trees and shrubs than three decades ago. This growth, driven by processes such as tree and understory infilling in unmanaged forests, is helping fuel wildfires, contributing to changing fire regimes in the eastern half of the country, according to a new study.

Some parts of the eastern and southeastern United States have experienced a tenfold increase in the frequency of large wildfires over the last forty years, with Texas and the Appalachians seeing the largest increase. However, the Northeast lacked a tie between woody plant growth and large wildfires.

Wildfires thrive on woody vegetation such as trees and shrubs. The new analysis of wildfire and vegetation data shows that the eastern U.S. has seen a 37% increase in woody cover over the last 30 years. In some regions, high levels of woody cover are linked directly to a higher risk of large wildfires over the same period.

The research “is helping us narrow in on regional drivers and focus our efforts to preemptively get ahead of the growing wildfire problem here in the eastern U.S.,” said Victoria Donovan, a landscape ecologist at the University of Florida who was the senior author on the study.

The study is published in Geophysical Research Letters.

Research has shown that woody cover growing in new places or thickening within forests is directly linked to increased wildfire risk in western and central parts of the country. But whether this is true out East has yet to be explored.

To test this, Donovan and her graduate student Michaella Ivey collected data on all wildfires between 1991 and 2021 that were at least 200 hectares—around 500 football fields—or larger in eastern states. They then looked where trees and shrubs were growing in the eastern U.S over the same period. To determine if woody cover influenced wildfire risk, the researchers compared the amount of woody cover within wildfire perimeters to what would be expected if wildfires were distributed at random.

The analysis revealed a strong link between woody cover and large wildfire occurrence—but only in some parts of the country. Across the eastern temperate forest, a region that makes up nearly half of the United States, each 1% increase in woody cover led to an overall 3.9% increase in the odds of a wildfire the next year. The link between woody cover and wildfire risk was strongest in eastern Texas and in and around the Appalachian Mountains.

However, the researchers found no link between woody cover and wildfire risk in the Northeast and across some parts of the Mississippi River valley. This finding “prompts all sorts of questions about what other factors are influencing the system,” Ivey said.

Cooler and wetter conditions in the Northeast, and to some extent the Mississippi River valley, may create conditions less conducive to wildfire. However, many Northeastern ecoregions could not be included in the study due to a low number of wildfires that were large enough to meet the study’s size requirements. Wildfires in these areas may stay small because of agricultural fragmentation, the researchers said.

Because woody vegetation wasn’t tied to wildfire increase consistently throughout the study area, climate change, human actions, or a combination of the two could be more important for wildfires than vegetation in some places. But overall, the research suggests that reducing fuels is a good tactic for reducing wildfire risk in the east, Donovan said.

More prescribed fires may be necessary in southern states as climate change is expected to make the southeast drier, and potentially more prone to wildfires, the researchers caution. This research shows a path forward for states and individuals to help reduce wildfire risk in the future.

“Using fuel management to reduce wildfire risk is a lot more actionable than changing the climate pattern in the short term,” Donovan said, “though addressing climate change will likely be crucial for reducing wildfire risk in the east in the long term.”