Increases in burned area across the western US since the mid-1980’s have been widely documented and linked partially to climate factors, yet evaluations of trends in fire severity are lacking. Here, we evaluate fire severity trends and their interannual relationships to climate for western US forests from 1985-2017. Significant increases in annual area burned at high severity (AAB_hs) were observed across most ecoregions, with an overall eight-fold increase in AAB_hs across all western US forests. The relationships we identified between the annual fire severity metrics and climate, as well as the observed and projected trend toward warmer and drier fire seasons, suggest that climate change will contribute to increased fire severity in future decades where fuels remain abundant. The growing prevalence of high-severity fire in western US forests has important implications to forest ecosystems, including an increased probability of fire-catalyzed conversions from forest to alternative vegetation types.

Fire severity: We produced raster datasets (resolution = 30m) representing the predicted composite burn index (CBI) using Google Earth Engine and the model developed by Parks et al. (2019). The CBI is a composite field measure of fire severity that rates >20 individual factors such as duff consumption, char height, and canopy mortality; as such, modeled CBI allows for improved ecological interpretations of fire effects compared to non-standardized indices such as the delta normalized burn ratio (dNBR). This procedure maps the predicted CBI using a Random Forest model developed using field data from >250 fires across North America; explanatory variables in this model include Landsat spectral indices, latitude, and 1981-2010 annual average climatic water deficit. We provide the bias-corrected CBI described in detail by Parks et al. (2019). Also included here are pre-fire NDVI rasters for each fire. Pre-fire NDVI represents the mean NDVI from one-year before the fire using the same composite imagery used to produce the CBI predictions.

CWD: Gridded monthly CWD at 1/120^th degree resolution was calculated following Dobrowski et al. (2013) using PRISM inputs for temperature, precipitation, and humidity (version LT81m), 10-m wind and downward shortwave radiation from NLDAS2 (Mitchell et al., 2004), and soil water holding capacity from POLARIS (Chaney et al., 2016).

Forest mask: We identified forest, woodland, and savanna (hereafter forest) from a combination of landscape level vegetation products that include Landfire’s (Rollins, 2009) Existing Vegetation Cover (EVC), Environmental Site Potential (ESP) and the Landsat Time Series Stacks – Vegetation Change Tracker (LTSS-VCT) (Huang et al., 2010). If you use this dataset, please cite Dillon et al. (2020).

References:

Chaney, N. W., Wood, E. F., McBratney, A. B., Hempel, J. W., Nauman, T. W., Brungard, C. W., & Odgers, N. P. (2016). POLARIS: A 30-meter probabilistic soil series map of the contiguous United States. Geoderma, 274, 54–67.

Dillon, G. K., Panunto, M. H., Davis, B., Morgan, P., Birch, D. S., & Jolly, W. M. (2020). Development of a severe fire potential map for the contiguous United States. Gen. Tech. Rep. RMRS-GTR-415. Fort Collins, CO: US Department of Agriculture, Forest Service, Rocky Mountain Research Station.

Dobrowski, S. Z., J. Abatzoglou, A. K. Swanson, J. A. Greenberg, A. R. Mynsberge, Z. A. Holden, and M. K. Schwartz. 2013. The climate velocity of the contiguous United States during the 20th century. Global Change Biology 19(1), 241–251.

Huang, C., Goward, S. N., Masek, J. G., Thomas, N., Zhu, Z., & Vogelmann, J. E. (2010). An automated approach for reconstructing recent forest disturbance history using dense Landsat time series stacks. Remote Sensing of Environment, 114(1), 183–198.

Mitchell, K. E., Lohmann, D., Houser, P. R., Wood, E. F., Schaake, J. C., Robock, A., et al. (2004). The multi-institution North American Land Data Assimilation System (NLDAS): Utilizing multiple GCIP products and partners in a continental distributed hydrological modeling system. Journal of Geophysical Research: Atmospheres, 109(D7).

Parks, Holsinger, Koontz, Collins, Whitman, Parisien, et al. (2019). Giving Ecological Meaning to Satellite-Derived Fire Severity Metrics across North American Forests. Remote Sensing, 11(14), 1735.

Rollins, M. G. (2009). LANDFIRE: a nationally consistent vegetation, wildland fire, and fuel assessment. International Journal of Wildland Fire, 18(3), 235–249.

fire.severity.zip: This file contains all of the satellite-inferred fire severity and pre-fire NDVI raster datasets used in this study. 

CWD.zip: This file contains gridded monthly climatic water deficit from 1979-2017 for the conterminuous United States.

forest.mask.zip: This file contains a gridded representation (30m resolution) of forest and non-forest for the western conterminuous United States.