Compounding Impacts of Drought on Post-Fire Vegetation Response in Western U.S

Fires modulate ecosystem evapotranspiration (ET) by directly affecting the aboveground vegetation immediately after burning, thereby reducing the total leaf surface area available to transpire. The trend toward increasing fire frequency and severity across the western United States, coincident with intensifying droughts, has a critical role in impacting the post-fire response of vegetation. Relationships between droughts and fire properties have been explored, however, it remains uncertain to what extent the compounding disturbance from droughts influence post-fire vegetation recovery. Here we use remote sensing-derived high-resolution ET observations from OpenET and burned scar maps from Monitoring Trends in Burn Severity (MTBS) program to study changes to ET and vegetation recovery as a function of landcover, drought, and burn severity. We show that ecoregions dominated by grasslands and savannas are most susceptible to drought stresses, which amplify fire-induced ET suppression and undermine net carbon uptake. In contrast, severely burnt forests respond with slow or incomplete recovery but with a lower sensitivity to dry extremes. We find that the response of burnt vegetation to droughts is a by-product of changes to post-fire fuel recovery and moisture availability. This explains why moderate to extreme droughts enhance post-fire ET suppression in grasslands and savannas, while moisture accessed by deeper roots in forests help meet the evaporative demands unless fires disrupt internal tree structure. Our results highlight the dominant control of dry climatic extremes on altering the response of vegetation to fires, with critical implications for terrestrial ecosystem stability in the face of anthropogenic climate change.