Ivey et al. 2024 (Geophysical Research Letters)
RESEARCH BRIEF #53
A summary based on the following publication:
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MANAGEMENT IMPLICATIONS
Woody cover has increased across the eastern United States from 1990 to 2020.
The odds of large wildfire occurrence increases with increasing woody cover.
Study findings suggest that reducing woody cover could become an increasingly important strategy for managing large wildfire risk in the eastern U.S.
This study characterizes changes in woody vegetation cover across the eastern United States from 1990 to 2020 and evaluates its association with the occurrence of large wildfires (i.e., >200 ha (~494 acres)). Although woody encroachment and canopy infilling have been partially linked to increased wildfire risk in the western US and Great Plains, large-scale trends in eastern U.S. woody vegetation—and their potential influence on wildfire risk—remain less well understood.
Three primary data sources were used for this study:
- U.S. Environmental Protection Agency (EPA) ecoregions were used to isolate study areas.
- The Monitoring Trends in Burn Severity (MTBS) database provided perimeter data for large wildfires.
- The Rangeland Analysis Platform (RAP) supplied woody cover data.
A large-scale study area was delineated using EPA’s Level I (L1) Eastern Temperate Forests ecoregion (see map of North American L1 ecoregions). To allow for finer-scale analyses based on biophysical characteristics, the L1 study area was subdivided into 33 EPA Level III (L3) ecoregions.
The MTBS program uses satellite imagery to map the extent and severity of large fires on both public and private lands, categorizing each fire by ignition type (i.e., “prescribed,” “wildland fire use,” or “wildfire”). Only fires identified as “wildfire” were included in wildfire analyses, and L3 ecoregions with fewer than ten large wildfires during the study period were excluded due to insufficient data for assessing temporal trends. Consequently, wildfire analyses were limited to 19 of the 33 L3 ecoregions.
Woody cover data from 1990 to 2020 were obtained from RAP, which generates annual percent cover estimates for five vegetation functional groups: bare ground, annual and perennial herbaceous vegetation, shrubs, and trees. Because RAP was originally developed for western rangelands dominated by low shrubs, shrub and tree cover were combined into a single “woody cover” class to reduce potential misclassification in the eastern U.S.
To assess trends in woody cover across L1 and L3 ecoregions, annual shrub and tree cover data were extracted at both ecoregion scales and summed for each year from 1990 to 2020. To evaluate woody cover patterns within large wildfire perimeters, wildfires were grouped by L3 ecoregion. The percentage and total area of woody vegetation within each wildfire perimeter were calculated using woody cover data from the year immediately preceding the fire (e.g., 1990 woody-cover estimates were used for 1991 wildfires).
The study found that woody cover consistently increased across the eastern U.S. over the 30-year study period at both L1 and L3 ecoregion scales. Across L1 ecoregions, woody vegetation rose from 33.5% in 1990 to 45.8% in 2020, marking a 36.7% relative increase over the study period. Among L3 ecoregions (see Figure 1a), the greatest annual gains in woody cover (> 0.65% per year) occurred in the Western Allegheny Plateau, the Central Appalachians, and the Northern Allegheny Plateau. Woody cover increased more slowly in the Southern Coastal Plain relative to adjacent southeastern ecoregions, namely the Southeastern Plains and the Middle Atlantic Coastal Plain. Ecoregions situated in the western portion of the eastern U.S. exhibited lower annual woody cover gains compared to those located in the eastern portion. L3 ecoregions omitted from wildfire analyses—primarily in northern areas with fewer large fire events—displayed annual woody cover increases spanning from 0.03% to 0.70%.
Analyses of woody cover and wildfire occurrence indicate a strong relationship between increasing woody cover and the likelihood of large wildfires in the eastern U.S. (see Figure 1b). At the L1 ecoregion scale, each 1% gain in woody cover corresponded with a 3.9% increase in the odds of a large wildfire. Within L3 ecoregions, this relationship was statistically significant in 16 of the 19 ecoregions analyzed (84%). In the region’s central area (Piedmont, Western Allegheny Plateau, and Ridge and Valley ecoregions), the most significant increases in the probability of large wildfires were associated with rises in woody vegetation. A similarly strong correlation was identified in the western ecoregions of the East Central Texas Plains and Ozark Highlands. Although southeastern ecoregions widely exhibited a significant increase in the relationship between large wildfires and woody cover, this association was less pronounced compared to that observed in the central and western regions. No clear relationship between woody cover and large wildfires was observed in the North Central Hardwoods (located in the north), the Mississippi Valley Loess Plains (in the south), and the Ouachita Mountains (in the west), despite these areas experiencing increases in woody cover.
At the L1 ecoregion level, landscapes characterized by high woody cover (60–100%) exhibited a disproportionately higher susceptibility to large wildfires. Across most L3 ecoregions, regions with the densest woody cover consistently demonstrated the highest probability of experiencing large wildfires. This pattern persisted despite variations in woody cover across these regions. Interestingly, some of the ecoregions exhibiting the highest likelihood of large wildfires in areas with dense woody cover also showed an elevated risk in areas with low to moderate woody cover (10–50%).
The results of this study demonstrate that woody vegetation represents a major driver of large wildfire occurrence in the eastern U.S. These findings highlight the urgency of actively managing woody cover and associated fuel loads, as vegetation type and density are among the most readily manipulated components of fire regimes. Such insights reinforce the role of woody cover management as an essential element within short-term wildfire risk mitigation strategies.
