How will future climate change impact prescribed fire across the contiguous United States?

Prescribed fire is a crucial tool for managing ecosystems and mitigating wildfire risk in the United States, accounting for over 50% of annual area burned. Its effective application depends on suitable meteorological conditions (temperature, humidity, wind speed). Climate change is altering these conditions, potentially impacting future prescribed burning opportunities. A century of fire suppression has led to fuel accumulation, particularly in western forests, increasing the risk of catastrophic wildfires. Prescribed fire reduces this risk by decreasing biomass and altering forest structure, promoting ecosystem stability and ecological function. It's a cost-effective method compared to manual or mechanical fuel reduction, especially in challenging terrains. Furthermore, prescribed fire can mitigate the effects of wildfire carbon emissions and aligns with long-standing indigenous fire management practices. The majority of prescribed burning occurs in the southeast and tallgrass prairie, with high public acceptance. However, western US application faces challenges including steep topography, heavy fuel loads, narrow burn windows due to air quality regulations, and wildfire crew limitations. Climate change is already impacting both wildfire and prescribed fire through increased temperatures and precipitation variability, leading to drier fuels and increased wildfire severity. This study addresses the need for understanding how changing weather patterns affect prescribed fire windows by using locally to regionally varying prescription window information, landscape, and climate data to assess projected changes in future prescribed fire opportunities across the contiguous US.

Existing research has largely focused on wildfire trends and effects, while prescribed fire, requiring a different research approach due to its reliance on specific prescription windows, has received less attention. Previous work highlighted potential regional climate impacts on prescribed fire windows, but often used uniform prescription ranges across broad areas, neglecting local variability. This study addresses this gap by incorporating local-scale variability in prescriptions.

The study evaluated present-day (2006–2015) and future (2051–2060) opportunities for prescribed burning using data from 83 locations across the CONUS. Prescriptions, specifying safe environmental conditions for prescribed fire, were collected from land management agencies. Data included minimum and maximum temperature, minimum and maximum wind speed, and minimum relative humidity. Maximum relative humidity was excluded because it's typically measured at night, when burning is infrequent. Broader "required" ranges were used when both broader and narrower ranges were provided in the plans. Fuel type information from LANDFIRE's Scott and Burgan fuel models was used to associate prescriptions with locations within similar EPA Level II ecoregions. This expanded the data coverage to 57% of vegetated CONUS areas. Two climate datasets were used: gridMET (4 km resolution) for present-day conditions, and downscaled CMIP5 data (4 km resolution) from 18 climate models for RCP4.5 and RCP8.5 future scenarios. For each ecoregion, daily climate data was compared against prescriptions to determine burn days. A burn day was counted only if all climate variables fell within the prescription ranges. This process was repeated for both present-day and future periods. The influence of individual meteorological variables was assessed by repeating the analysis with only one variable at a time. Statistical significance of differences in burn days was evaluated using a small sample t-test, controlling the false discovery rate (FDR). Seasonal variability was also analyzed to examine how different climate drivers influence burn days across various months and ecoregions. The Haversine formula was used to identify the closest prescription to a given LANDFIRE grid cell when multiple prescriptions with the same fuel types were present within an ecoregion.

Present-day burn days (2006–2015) were highest in the Warm Deserts, Ozark/Ouachita–Appalachian Forests, and Southeastern U.S. Plains. Both gridMET and CMIP5 data showed similar patterns, although CMIP5 generally overestimated burn days compared to gridMET, likely due to underestimates of wind speed and overestimates of minimum relative humidity in the CMIP5 datasets. Future projections (2051–2060) under both RCP4.5 and RCP8.5 scenarios showed a decrease in burn days across much of the southeastern and eastern US, with larger decreases under RCP8.5. Conversely, opportunities increased in several western US regions, mainly driven by rising minimum temperatures and declining wind speeds. Analysis of individual climate drivers revealed that wind speed was the strongest overall constraint on burn days. Decreasing wind speeds led to increased burn days across much of the CONUS, except in the Southeast. Maximum temperatures were the main driver of burn day decreases in the East, while minimum temperature increases led to increased burn days in the West and North. Minimum relative humidity was a weaker constraint, with decreases leading to small decreases in available burn days in some areas. Seasonal analyses demonstrated that while generally decreasing burn days were projected during summer, increases were seen during shoulder seasons across several different locations (White Mountain, Silver Lake, Garrison, Brown Springs, Santa Fe, Chaparral). Temperature increases contributed to an increase in winter and shoulder-season burn days and a decrease in summer burn days; wind speed decreases generally contributed to increased burn days during winter and shoulder seasons.

The findings indicate that climate change will have a complex and regionally variable impact on prescribed fire opportunities. While rising temperatures and decreasing moisture will continue limiting burning in the Southeast, opportunities may expand in the West due to changes in minimum temperatures and wind speeds. The shift in seasonal burn windows suggests the need for adaptive management strategies. The study highlights the importance of considering local variability in prescribed burn windows, as opposed to broad regional averages, in assessing future impacts. These changes in burn windows have significant implications for wildfire risk management and ecosystem restoration.

This study provides valuable insights into the future impacts of climate change on prescribed fire opportunities across the contiguous US. The findings emphasize the need for adaptive management strategies that account for regional and seasonal variations in climate projections. Future research should focus on improving the representation of fuel moisture and vegetation dynamics in climate models to reduce uncertainties in burn day projections and incorporate factors like smoke management and air quality regulations into the analysis.

The study acknowledges several limitations. It focuses primarily on meteorological constraints, omitting factors such as smoke transport, wind direction, and atmospheric stratification. Fuel type and moisture content, crucial factors, were not fully accounted for due to limitations in climate model outputs and static fuel type assumptions. Therefore, the predicted number of burn days might be overestimated. The analysis is limited to the 57% of the vegetated areas of the CONUS for which prescription data is available.