Projected increases in western US forest fire despite growing fuel constraints

The western United States has experienced a tenfold increase in the annual area burned by forest fires over the past half-century, culminating in the record-breaking 2020 fire season. This dramatic escalation is attributed to several factors, including historically high fuel loads from a century of fire suppression and the cessation of Indigenous burning practices; less direct fire suppression tactics prioritizing firefighter safety; and increased fuel aridity due to human-caused climate change. While the influence of these factors varies geographically, climate variability, specifically fuel dryness, is the primary interannual driver of large-scale burned area across fuel-rich biomes in the western US and globally. Increasing fuel aridity and extreme fire weather, projected to worsen with continued climate change, portend increased wildfire activity where biomass is abundant and flammability is a constraint, particularly in western US forests. Warming directly enhances fuel aridity by increasing vapor pressure deficit (VPD) and reducing snowpack, intensifying and lengthening the fire season. Reduced summer precipitation in parts of the region has further exacerbated this trend. Most climate projections indicate increased fuel aridity across the western US and longer fire-weather seasons, leading to more frequent extreme fire-weather events. One crucial uncertainty in projecting future fire activity is the extent to which decreasing available fuel will limit fire growth, potentially introducing nonlinearities in the climate-fire relationship. Statistical models that solely consider climate often predict massive future increases in burned area. However, these models assume sufficient fuels will always be available. Evidence suggests that increased forest-fire area can trigger fire-fuel feedbacks that limit future burned area by reducing fuel biomass and extent, altering the ability of forested environments to carry fire, and modifying post-fire vegetation patterns and climate, potentially hindering tree regeneration. This study investigates these fire-fuel feedbacks and their influence on future fire activity at the scale of the entire western US forest area.

Prior research has established strong relationships between fuel aridity and burned area in western US forests, with each incremental increase in aridity leading to a disproportionately larger increase in burned area. Studies using statistical models that only consider climate as a driver predict substantial future increases in wildfire activity. However, these studies often neglect the potential moderating effects of fire-fuel feedbacks. Other research has demonstrated that fire-fuel feedbacks can influence future burned area at local scales by reducing available fuel. However, studies have not comprehensively evaluated the impact of these feedbacks and climate change on future fire activity at the broad scale of the entire western US forest area or the sensitivity of these projections to uncertainties in the duration and strength of the fire-fuel feedbacks. The existing literature provides a range of potential feedback strengths and durations, influenced by factors such as post-fire tree regeneration failure, fuel limitations from fire history, and the modification of fuel limitation longevity during drought events.

This study utilizes several dynamic models that incorporate various fire-fuel feedbacks, along with a static model that assumes constant fuel extent, to project changes in near-term (2021–2050) western US forest-fire area. The study also assesses changes in the interannual variability of forest-fire area and the likelihood of exceeding the 2020 fire season's record. Projections are limited to 2050 due to increased uncertainty in vegetation dynamics, human behavior, and climate trajectories beyond that timeframe. The researchers use several forms of dynamic models that account for various fire-fuel feedbacks based on the ecological literature, including post-fire tree regeneration failure, fuel limitations due to recent fire history, and the impact of drought on the duration of fuel limitations. The study uses monthly climate data (temperature, precipitation, vapor pressure deficit, etc.) from PRISM and ERA-5, along with CMIP6 climate model projections for 1950-2050, and burned area data from MTBS and MODIS. Three proxies of aridity were calculated: mean vapor pressure deficit (VPD), Penman-Monteith reference evapotranspiration (ETo), and climatic water deficit (CWD). A static model is used initially (log(FFA(t)) = α₁ + β₁F(t) + εt), where FFA represents forest-fire area and F represents fuel aridity, to establish the climate-fire relationship during the observation period (1984-2020). Dynamic models are then developed to incorporate fire-fuel feedbacks through a term 'L', which represents the fraction of forested land ineligible to burn due to recent fire history or regeneration failure. The study evaluates various forms and strengths of fire-fuel feedbacks and their impact on projected forest-fire area, accounting for semi-permanent limitations from regeneration failure (L₁) and temporary limitations from recent fire history (Lt). The models are run using both constant and fading feedback formulations, with varying feedback strengths and longevity parameters, and the results were compared with both historical and projected climates. Model cross-validation is performed using data from 1984-2020, assessing model skill by bias, coefficient of efficiency (CE), and the correlation between modeled and observed log(FFA). Three statistical metrics of annual FFA variability are calculated: recurrence intervals for FFA exceeding the 2020 fire season, the interquartile range (IQR) of modeled FFA, and the percentage of years with modeled FFA below the 1991-2020 observed median.

The static model, assuming constant fuel extent, projects a doubling of the mean annual forest-fire area by 2021–2050 compared to 1991–2020. This projection shows a significant increase in interannual variability and a threefold increase in the likelihood of years exceeding the 2020 fire season. Dynamic models, incorporating fire-fuel feedbacks, only modestly reduce the projected increase in forest-fire area. Even with strong feedbacks, the models project significant increases in forest-fire area, extreme fire year probabilities, and year-to-year variability. The strongest negative feedback model still shows a 46% increase in forest-fire area. The study finds that the area ineligible to burn (L) has increased following the increase in burned area, but these feedbacks are insufficient to offset the profound climate-driven increase in forest-fire area projected for the coming decades. The interannual relationship between fuel aridity (F) and forest-fire area shows a strong positive correlation (r² = 0.80). The nonlinear response indicates that each increase in F results in a larger increase in forest-fire area than previous increases. A one-unit increase in mean F from 1984–1999 to 2000–2020 led to a fourfold increase in the upper quartile of annual forest-fire area. Model cross-validation suggests that changes in fuel extent or other human-environment factors during 1984–2020 did not significantly affect macroscale climate-fire relationships, suggesting that the models have near-term predictive utility. Climate models robustly project increased fuel aridity (F) for the next 30 years. Across 30 climate models, F is projected to increase by 0.66σ (standard deviations) on average, primarily due to warming-induced increases in evaporative demand. The study finds that the fraction of forested land ineligible to burn (L) has declined over the twentieth century, but the recent increase in forest-fire area has reversed this trend. Analyses of the effects of varying fire-fuel feedback strengths and forms suggest that even the strongest feedbacks yield only a moderate reduction in the projected increase in forest-fire area. Projected increases in forest-fire area are found to be relatively insensitive to the choice of aridity index used (VPD, ETo, or CWD).

The findings suggest that the ongoing increase in western US forest-fire area due to drying fuels will only be partially mitigated by constraints on fuel extent and biomass from fire-fuel feedbacks. Even under the strongest modeled feedback scenarios, significant increases in forest-fire area, extreme fire year probabilities, and interannual variability are projected. These projections result from the thermodynamic response to anthropogenic climate change and the exponential relationship between forest-fire area and fuel aridity. The study focuses on the aggregate forest-fire area in the western US due to the strong and consistent relationship between aggregate burned area and climate, and the widespread nature of fire-season aridity patterns. While more spatially refined models may provide additional insights into complex feedback processes, they are computationally expensive for continental-scale studies. The study acknowledges that fire regime characteristics are also affected by other disturbances such as insect outbreaks, drought mortality, and human activities, but the complex interactions of these factors make their incorporation challenging. The results highlight the critical need for proactive mitigation and adaptation strategies, emphasizing that reductions in fossil fuel emissions to mitigate fuel aridity, combined with locally appropriate land management strategies to reduce fuel load or increase the area ineligible to burn, may be necessary to slow negative fire impacts. The study concludes that the significant increase in forest-fire area will persist absent effective mitigation measures.

This research demonstrates that fire-fuel feedbacks alone are unlikely to significantly curb the substantial climate-driven increases in western US forest-fire area projected for the coming decades. Even with strong fire-fuel feedbacks, significant increases in fire extent, extreme fire years, and interannual variability are unavoidable. This underscores the urgent need for mitigation efforts to reduce greenhouse gas emissions and adaptation strategies to actively manage forest fuels and reduce the risks associated with increased wildfire activity. Future research should focus on refining the understanding and quantification of the interplay of different feedback mechanisms and exploring spatially explicit modeling approaches to better understand fire dynamics at finer scales. The integration of socio-economic aspects is also critical for effective management of fire risks in a changing environment.

The study acknowledges several limitations. The models used are based on aggregate forest-fire area for the western US and may not capture the full complexity of spatially variable fire behavior and feedback mechanisms. The incorporation of other disturbance agents besides fire, such as insect outbreaks or drought mortality, was limited by the complexities of their interactions with fire. The study also assumes that human behavior and management practices will remain relatively consistent in the future, which might not accurately reflect real-world scenarios. Furthermore, the representation of fire-fuel feedbacks is simplified and may not fully capture the nuanced and regionally variable nature of these processes.