Warming enabled upslope advance in western US forest fires

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.

Rapid Growth of Large Forest Fires Drives the Exponential Response of Annual Forest‐Fire Area to Aridity in the Western United States

“…The general conclusions would hold if another climatological aridity metric strongly related to wildfire activity were used in place of VPD. Importantly, exercises that use historical fire‐climate relationships to predict future fire activity are quite sensitive to the aridity metric(s) used as predictors because VPD is projected to change more dramatically in the future than other aridity metrics that also correlate well with AFAB (Alizadeh et al., 2021 ; Brey et al., 2021 ; Holden et al., 2018 ).…”

Climate Changes and Their Elevational Patterns in the Mountains of the World

“…There is evidence that so far mountain snowpack has acted as a buffer to moderate temperature changes in high elevation streams, an important habitat for trout in the western US, for example (Luce, Staab, et al., 2014; Isaak et al., 2016). Temperature increases may also impact mountain ecology through secondary effects such as upslope advances of forest fire lines (Alizadeh et al., 2021).…”

Megafire affects stream sediment flux and dissolved organic matter reactivity, but land use dominates nutrient dynamics in semiarid watersheds

“…Concerning the causes of change in wildfire regime, parts of the western US remain in a state of fire deficit, where past and present fire suppression has limited fire at unsustainably low levels [ 188 ]. Simultaneously, anthropogenic climate change has caused long-term increases in fire extent, especially severe wildfire, associated with dry fuels, extreme vapor pressure deficits, and extended fire seasons [ 3 , 5 , 8 , 189 ]. While mitigating climate change by reducing fossil fuel emissions is the only tenable solution to the latter problem, climate and megafires themselves make improving management more difficult in two connected ways.…”