The eastern United States has seen an increase in the number of trees and shrubs over the past thirty years. This growth, mainly due to tree and understory filling in areas where forests are not managed, is contributing to the rise of wildfires, leading to alterations in fire patterns across the eastern U.S., as revealed in a recent study.
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.
Certain regions in the eastern and southeastern U.S. have seen a surprising tenfold rise in the occurrence of large wildfires over the last four decades, with Texas and the Appalachian area experiencing the most significant increases. Yet, the Northeast did not show a direct correlation between the growth of woody plants and large wildfires.
Wildfires thrive in environments rich in woody plants like trees and shrubs. The new study shows that the eastern U.S. has experienced a 37% uptick in woody cover over the past 30 years. In specific areas, substantial woody vegetation is directly associated with an increased likelihood of large wildfires within the same timeframe.
The research “helps us to identify regional factors and allows us to proactively address the escalating wildfire challenges we face in the eastern U.S.,” stated Victoria Donovan, a landscape ecologist from the University of Florida, who played a key role in the study.
Published in Geophysical Research Letters, an open-access journal by AGU, the study focuses on impactful, concise reports relevant to Earth and space sciences.
Previous research has indicated that the expansion of woody cover in new areas or the thickening of existing forests increases wildfire risk in western and central regions of the nation. However, this relationship in the eastern U.S. had not been thoroughly examined prior to this study.
To explore this connection, Donovan and her graduate student Michaella Ivey analyzed data on all wildfires in eastern states from 1991 to 2021 that affected at least 200 hectares (equivalent to about 500 football fields). They then investigated the growth of trees and shrubs in the eastern U.S. during that same period. To assess whether woody cover affected wildfire risk, the researchers compared the extent of woody cover within wildfire boundaries to what would be expected if wildfires were distributed randomly.
The findings uncovered a notable correlation between woody cover and large wildfire events, but this was evident only in certain regions. In the expansive eastern temperate forest—covering almost half of the United States—each 1% boost in woody cover correlated with a 3.9% increase in the likelihood of a wildfire occurring the following year. This relationship was most pronounced in eastern Texas and around the Appalachian Mountains.
Conversely, no connections were established between woody cover and wildfire risk in the Northeast or certain areas along the Mississippi River. This result “raises many questions about what other factors could be influencing this situation,” Ivey mentioned.
The cooler and wetter climate found in the Northeast and parts of the Mississippi River valley may lead to conditions that are less favorable for wildfires. Additionally, many ecoregions in the Northeast could not be included in the study due to a scarcity of large wildfires meeting the size criteria. Researchers suggested that agricultural fragmentation might contribute to smaller wildfires in these areas.
Since the growth of woody vegetation did not consistently correlate with an increase in wildfires throughout the study region, it indicates that climate change, human influences, or a combination of both may play a more significant role in wildfire occurrences in some areas than vegetation alone. Overall, the research suggests that managing fuel sources is a practical strategy to mitigate wildfire risks in the east, said Donovan.
In southern states, there may be a greater need for prescribed burns as climate change is anticipated to dry out the southeast, potentially increasing wildfire occurrences, the researchers warn. This research provides a roadmap for states and individuals to follow in order to minimize wildfire risks moving forward.
“Managing fuel sources to diminish wildfire risk is far more actionable than attempting to change climate patterns in the short term,” Donovan explained, “yet addressing climate change will ultimately be vital for reducing wildfire risk in the eastern U.S. in the long run.”
