Vegetation Shifts from Forests to Shrublands or Grasslands in the Western U.S. due to Warming, Droughts, and Fire

Abstract Anthropogenic greenhouse gas emissions have caused significant climate changes, leading to increased drought and warming, which have resulted in more frequent and severe wildfires in the western United States. This study uses a biogeochemical and biogeography model to predict change in PFT dominance, that will occur in response to increasing temperatures, droughts, and fire intensity. Climate model projections indicate that these changes will continue throughout the 21st century, with the western U.S. expected to experience significant impacts. Specifically, I focused on the changes that will result in the dominant plant functional types (PFTs) shifting from forests to grasslands or shrublands in the Western U.S. forested grids by the end of the century (2070 – 2100). Based on the BIOME4 approach, my biogeography model uses bioclimatic thresholds to determine the PFTs that can exist in each grid cell and then optimization of Net Primary Productivity (NPP) to determine the resultant dominant PFT (NPP bake-off). However, forests will not decline unless conditions allow for a replacement fire, so fire occurrences are based on biomass thresholds, soil moisture, and relative humidity. My results indicate that 54% of the originally forested grids will convert to either shrubland (29%) or grassland (25%) by the end of the century, with most of the shifts occurring in grids with the highest temperature increases and reduced soil moisture. However, the remaining forests are more productive (increased NPP) compared to historical ones due to warming, CO2 fertilization, and increased precipitation in these grids. The overall change in carbon storage as a result of the changes in dominant PFTs is a net loss of carbon equal to 9.6 PgC. These findings have important implications for managing forested ecosystems in the western U.S. and highlight the need for proactive measures to mitigate the impact of climate change on forested and grassland and shrubland ecosystems.