Abstract
Increases in burned area and large fire occurrence are widely documented over the western United States over the past half century. Here, we focus on the elevational distribution of forest fires in mountainous ecoregions of the western United States and show the largest increase rates in burned area above 2,500 m during 1984 to 2017. Furthermore, we show that high-elevation fires advanced upslope with a median cumulative change of 252 m (-107 to 656 m; 95% CI) in 34 y across studied ecoregions. We also document a strong interannual relationship between high-elevation fires and warm season vapor pressure deficit (VPD). The upslope advance of fires is consistent with observed warming reflected by a median upslope drift of VPD isolines of 295 m (59 to 704 m; 95% CI) during 1984 to 2017. These findings allow us to estimate that recent climate trends reduced the high-elevation flammability barrier and enabled fires in an additional 11% of western forests. Limited influences of fire management practices and longer fire-return intervals in these montane mesic systems suggest these changes are largely a byproduct of climate warming. Further weakening in the high-elevation flammability barrier with continued warming has the potential to transform montane fire regimes with numerous implications for ecosystems and watersheds.
Highlights
Increases in burned area and large fire occurrence are widely documented over the western United States over the past half century
We explore changes in the elevational distribution of burned forest across the western United States and how changes in climate have affected the mesic barrier for high-elevation fire activity
We focus on changes in high-elevation forests that have endured fewer direct anthropogenic modifications compared to drier low-elevation forests that had frequent low-severity fires prior to European colonization and have been more subject to changes in settlement patterns as well as fire suppression and harvest [23, 24]; we pose the following questions: 1) Has the elevational distribution of fire in the western US forests systematically changed? and 2) What changes in biophysical factors have enabled such changes in high-elevation fire activity? We explore these questions across 15 mountainous ecoregions of the western United States using records from large fires (>405 ha) between 1984 and 2017 [Monitoring Trends in Burn Severity (MTBS) [25]], a 10-m–resolution digital elevation model, and daily high-spatial–resolution surface meteorological data [gridMET [26]]
Summary
Increases in burned area and large fire occurrence are widely documented over the western United States over the past half century. Point to changing fire characteristics across many ecoregions of the western United States [15], including high-elevation areas of the Sierra Nevada [16], Pacific Northwest, and Northern Rockies [12, 17] These studies complement documented changes in montane environments including amplified warming with elevation [18], widespread upward elevational shift in species [19], and increased productivity in energy-limited high-elevation regions that enhance fuel growth and connectivity [20]. We assess the interannual relationships between Z90 and vapor pressure deficit (VPD) and compare the upslope advance
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