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

Large wildfires have increased in western US rangelands over the last three decades. There is limited information on the impacts of wildfires with different severities on the vegetation in these rangelands. 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 explored relationships between changes in fractional cover and prefire soil moisture conditions. The Expectation Maximization (EM) algorithm was used to group wildfires into nine clusters based on the prefire rangeland fractional cover extracted from the Rangeland Analysis Platform. The Standardized Precipitation Evapotranspiration Index (SPEI) with various lag scales from the Gridded Surface Meteorological (GRIDMET) dataset was used to represent antecedent soil moisture conditions. The results showed generally that fractional cover decreased most for AFG and PFG during the fire year, one year postfire for SHR, and two years postfire for TREE. High severity wildfires led to the greatest decrease in cover for all plant functional types, while low severity wildfires caused the least decrease in the functional type cover in most cases, though some variations existed. Furthermore, the impacts of wildfires on vegetation cover were greater in woody (SHR and TREE) types than in herbaceous (AFG and PFG) types. Significant negative correlation existed between percent changes in AFG and PFG cover and SPEI indicating higher prefire soil moisture conditions likely increased fine fuel loads and led to a larger decrease in AFG and PFG cover following wildfires. Significant positive correlation existed between percent changes in SHR and TREE cover and SPEI indicating drier prefire conditions resulted in larger decreases in SHR and TREE cover following wildfires. These findings help better understand the impacts of wildfires on rangelands and provide insights for rangeland management.