Change in Dominant Plant Functional Types: Transitions from Forests to Shrublands or Grasslands in the Western U.S due to Warming, Droughts, and Fire

Abstract This study utilizes a biogeographic model integrated within a biogeochemical model to predict change in plant functional type (PFT) dominance that will occur in response to increasing temperatures, droughts, and fire intensity in the Western U.S, based on the RCP8.5 scenario. The dominant PFT determination employs the BIOME4-based biogeography module within the Terrestrial Ecosystem Model (TEM), using bioclimatic thresholds and Net Primary Productivity (NPP) optimization. Results indicate that 54% of the originally forested grids will convert to either shrubland (29%) or grassland (25%) by the end of the century. Despite the change in PFT dominance, remaining forests show increased productivity due to warming, CO2 fertilization, and favorable climate. Yet, the overall change in carbon storage is a net loss of carbon equal to 9.6 PgC. These results highlight the critical need for effective forest management and adaptation strategies to mitigate the impacts of climatic changes on Western U.S. forests.