Characterizing effects of tree mortality from wildfire and bark beetles using satellite observations of solar-induced chlorophyll fluorescence

Satellite observations of solar-induced chlorophyll fluorescence (SIF) represent a promising remote sensing tool for estimating disturbance impacts to forest photosynthetic productivity. However, the ability to characterize tree mortality from forest disturbance events using SIF remains unexplored, particularly across different disturbance types and mortality severities. Here, we investigated the sensitivity of SIF observed by the Sentinel-5 Precursor satellite’s Tropospheric Monitoring Instrument (TROPOMI) to tree mortality from wildfires and bark beetle outbreaks using gridded datasets of tree mortality for the western United States from 2018-2023. We analyzed TROPOMI SIF observations at 0.05° (∼5 km) spatial resolution together with vegetation products from NASA’s Moderate-Resolution Imaging Spectroradiometer (MODIS) by implementing a bootstrapping methodology to compare wildfire- and bark beetle-affected pixels with biogeographically matched controls. We found that SIF declined sharply in response to wildfire, with differences from control increasing with burn severity, and SIF declining to as low as 20% of pre-fire levels. Moderate-to-severe bark beetle-induced mortality showed declines in growing season SIF to 60-70% of pre-drought levels over multiple years. Bark beetle-related declines in SIF were 10-20% greater in magnitude than those of control areas, exhibiting statistical significance from the control two to three years prior to visible detection from aircraft surveys. Similar declines were observed in a physiological SIF_yield metric, suggesting that fluorescence efficiency responses were a key driver of SIF changes. SIF was more sensitive to bark beetle mortality than MODIS vegetation products prior to visible mortality onset as marked by aircraft surveys, suggesting that SIF may outperform other vegetation indices as an early warning tool for drought and mortality. Future efforts to characterize forest disturbance effects using SIF will likely be enhanced as forthcoming satellite missions such as the European Space Agency's Fluorescence Explorer (FLEX) provide SIF observations at substantially higher spatial resolutions.