The impacts of wildfires of different burn severities on vegetation structure across the western United States rangelands

Authors: LI, ZHENG - Texas A&M University; Angerer, Jay; WU, X. BEN - Texas A&M University

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/3/2022
Publication Date: 7/8/2022
Citation: Li, Z., Angerer, J.P., Wu, X. 2022. The impacts of wildfires of different burn severities on vegetation structure across the western United States rangelands. Science of the Total Environment. 845. Article 157214. https://doi.org/10.1016/j.scitotenv.2022.157214.
DOI: https://doi.org/10.1016/j.scitotenv.2022.157214

Interpretive Summary: During the last 3 decades, the number of large wildfires has increased on rangelands across the western US. These wildfires have had differing levels of severity on the vegetation that burns, which can impact plants in different ways. This study was implemented to examine the impacts of wildfires, having different levels of severity, on the cover of different groups of plants, which included grasses, forbs, shrubs, and trees using a new, satellite-derived, plant cover product available from the Rangeland Analysis Platform (RAP). Relationships between the plant cover and weather conditions before and after the fire were also evaluated. Statistical methods were used to group wildfires into clusters having similar vegetation characteristics prior to the wildfires. Nine clusters were identified and evaluated for changes in plant cover after the wildfire. Results of this evaluation indicated that grasses and forbs (both annual and perennial) decreased most during the year of the fire. Shrub cover decreased most 1 year after the fire and tree cover decreased most two years after the fire. As severity of the wildfires increased, the plant cover for all groups of plants generally decreased. Impacts of wildfire were more severe on shrubs and trees compared to grass and forb cover. The severity of drought prior to wildfires also impacted shrub and tree cover after wildfires with increasing severity of drought leading to greater decreases in shrub and tree cover after the fires. These results highlight how vegetation conditions prior to the fire, wildfire severity and drought conditions can impact vegetation cover changes after wildfires. This information will assist land managers in their evaluation of conditions that could lead to severe wildfire and assessment of potential impacts. In addition, these results provide insights for adaptive management on rangelands under changing climate.

Technical Abstract: Large wildfires have increased in western US rangelands over the last three decades. The area burned with different severities, which have diverse ecological consequences on rangelands, has received less attention. This study assessed the impacts of large wildfires on rangeland fractional cover including annual forbs and grasses (AFG), perennial forbs and grasses (PFG), shrubs (SHR) and trees (TREE) across the western US, and further identified the relationships between changes in fractional cover and weather conditions with different lag effects. The Expectation Maximization (EM) algorithm was used to cluster wildfires into different groups based on the prefire rangeland fractional cover extracted from the Rangeland Analysis Platform. The Standard Precipitation Evapotranspiration Index (SPEI) with various lag scales from Gridded Surface Meteorological (GRIDMET) was used to represent antecedent weather conditions. Nine clusters were identified, and results showed that AFG, PFG, SHR and TREE cover generally decreased most at the fire year (FY), one year postfire (FY+1) and two years postfire (FY+2), respectively. High severity wildfires led to the highest decrease in cover of all plant functional types, while low severity wildfires caused the lowest decrease in the cover though some variations existed. Furthermore, the impacts of wildfires on vegetation cover were higher in woody types (i.e. SHR and TREE) compared with herbaceous types (i.e. AFG and PFG). Significant negative correlation existed between percent changes in AFG and PFG cover and SPEI indicating that lagged moisture conditions led to larger decrease in AFG and PFG cover following wildfires. Significant positive correlation existed between changes in SHR and TREE cover and SPEI with longer lag scales (i.e., 6,9 and 12 months) indicating that drier prefire conditions resulted in larger decreases in SHR and TREE cover following wildfires. These results can help better understand the impacts of wildfires on rangelands, and provide insights for rangeland management under changing climate.