Temporal patterns of area burned and burn severity of large wildfires in the rangelands of western United States

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Zheng Li and X. Ben Wu, Department of Ecology and Conservation Biology, Texas A&M University, College Station, TX, Jay Angerer, Fort Keogh Livestock and Range Research Laboratory, USDA Agricultural Research Service, Miles City, MT

Background/Question/Methods
Wildfires have become more frequent and larger in the western United States over the last 30 years. Trends in large wildfires have been well studied in forest ecosystems, but less attention has been given to large wildfires in rangeland ecosystems. Many rangelands are fire-dependent ecosystems but the burn severity, a scaled index gauging the magnitude of ecological change caused by fire, of rangeland wildfires is often low to moderate in contrast to forests due to relatively low to moderate fuel loads. This study evaluated temporal trends of large wildfires in rangelands from 1984 to 2017, in total area burned and area burned at different levels of burn severity, for all rangelands in the western United States (WUS) and each of the three rangeland ecosystem types including herbaceous-dominated, shrub-dominated and tree-dominated rangelands. The area burned and associated burn severity data were obtained from Monitoring Trends in Burn Severity (MTBS) program which included wildfires greater than 405 ha (1000 acres) in the western United States. The Autoregressive Integrated Moving Average (ARIMA) analysis, which accounted for spatial autocorrelation, were used to evaluate these trends.
Results/Conclusions
Results showed significant increasing trends existed in total area of rangeland burned, but the rate of increase was lower than that for forests. The area of all rangelands burned increased more than 5-fold from approximately 208,000 ha burned in 1984 to almost 1163,000 ha in 2017, and the rangeland areas burned at both low and moderate severity had significant increasing trends. The area burned in herbaceous-dominated and tree-dominated rangelands had significant increasing trends, but no temporal trends existed for area burned in shrub-dominated rangelands. The temporal trends for all rangelands, in total area burned and in areas burned with specific levels of burn severity, were similar to the trends for herbaceous-dominated rangelands but largely different from the trends for shrub-dominated rangelands. These findings can aid in the development of management strategies for reducing wildfire risk and enhancing rangeland conservation and sustainability.