Satellites have detected a drop in photosynthetic activity in Western U.S. forests two years before bark beetle infestations caused visible tree deaths, according to a new study led by the University of Utah. The research suggests that chlorophyll fluorescence (SIF), a measure of plant photosynthesis observed from space, could serve as an early warning signal for forest stress. Published in the October issue of Remote Sensing of Environment, the study highlights how SIF can help scientists track how events like drought, wildfire, and insect outbreaks impact forests' ability to absorb and store carbon from the atmosphere. The researchers used data from the TROPOMI instrument aboard the European Sentinel-5P satellite to compare fluorescence patterns in forests affected by bark beetle infestations and wildfires with those of unaffected areas. In forests that would later experience bark beetle mortality, a significant decline in SIF was detected about two years before the U.S. Department of Agriculture’s aerial surveys identified tree deaths. The drop in SIF was more severe in beetle-infested forests compared to healthy ones, suggesting that factors beyond drought, such as insect damage, were at play. Changes in SIF can be caused by various factors, including drought, insect infestations, canopy dieback, shifts in the timing of seasonal growth, reduced sunlight, and changes in the types of plants growing in the forest. While the researchers accounted for these variables, the complexity of forest ecosystems makes it difficult to interpret year-to-year SIF changes. However, in this case, a clear pattern emerged, showing that SIF could reliably detect early signs of forest stress. To validate their method, the researchers tested it using wildfires, which have more predictable effects on forest productivity than bark beetle infestations. They found that declines in SIF were proportional to the amount of vegetation lost in fires. Wildfires also affect a larger area, making them ideal for testing the method. This validation helped strengthen the credibility of their findings on bark beetle mortality. Additionally, the study showed that SIF can be used to monitor how forests recover after wildfires, highlighting its potential for tracking long-term changes in forest productivity and carbon cycling. Since forests store large amounts of carbon, understanding these changes is crucial for assessing how disturbances like wildfires and insect outbreaks affect the carbon balance of Western U.S. forests. SIF proved more sensitive to bark beetle damage than other remote-sensing tools and detected stress earlier, about two years before traditional surveys. Future satellite missions, like the European Space Agency’s FLEX, will offer even higher-resolution SIF data, helping scientists uncover more patterns in forest health over time.