Abstract
Large fires account for the majority of burned area and are an important focus of fire management. However, ‘large’ is typically defined by a fire size threshold, minimizing the importance of proportionally large fires in less fire-prone ecoregions. Here, we defined ‘large fires’ as the largest 10% of wildfires by ecoregion (n = 175,222 wildfires from 1992 to 2015) across the United States (U.S.). Across ecoregions, we compared fire size, seasonality, and environmental conditions (e.g., wind speed, fuel moisture, biomass, vegetation type) of large human- and lighting-started fires that required a suppression response. Mean large fire size varied by three orders of magnitude: from 1 to 10 ha in the Northeast vs. >1000 ha in the West. Humans ignited four times as many large fires as lightning, and were the dominant source of large fires in the eastern and western U.S. (starting 92% and 65% of fires, respectively). Humans started 80,896 large fires in seasons when lightning-ignited fires were rare. Large human-started fires occurred in locations and months of significantly higher fuel moisture and wind speed than large lightning-started fires. National-scale fire policy should consider risks to ecosystems and economies by these proportionally large fires and include human drivers in large fire risk assessment.
Highlights
Large wildfires have important economic, societal, and ecological costs and threaten infrastructure, ecosystems, and human life
We identified large wildfires within each of the 84 Level III ecological regions defined by the Commission for Environmental Cooperation (Figure S1; [30]) in the continental U.S We used ecoregions as the scale for this analysis because ecoregions represent areas of common vegetation and fuels, which are an important determinate of fire regimes
Our classification of ‘large’ depends on the distribution of fire sizes within each ecoregion, rather than a fixed value across the continental U.S previous analyses have defined large fires as those burning more than a fixed area (Table 1), we argue that defining large fire size thresholds on ecologically meaningful scales allows for a more flexible definition of a ‘large’ fire that varies based on the fundamental fuel and climate constraints that limit fire size and enables us to consider the importance of smaller large fires within ecoregions that have historically had lower fire probability
Summary
Large wildfires have important economic, societal, and ecological costs and threaten infrastructure, ecosystems, and human life. The frequency of large wildfires has increased over the last several decades in the U.S [2,3,4,5,6,7], boreal forests of North America [8], central Asia and Russia [9], and in the western Mediterranean [10] More large fires are expected in the future with more severe fire danger in the U.S [7,12], Canada and Russia [13], the Mediterranean [11], and Australia [14,15]. Wildfires burned an estimated 36 million ha in the continental U.S from 1992 to 2012 and, of the total burned area, just over half was concentrated in desert and forested ecosystems of the western
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