Satellite measurements detected declining photosynthetic activity in Western U.S. forests two years before bark-beetle mortality appeared in aerial detection surveys, according to new University of Utah-led research.
The first-of-its-kind study suggests that satellite-observed chlorophyll fluorescence (SIF), a measure of plant photosynthesis, could provide an early warning of forest stress. As drought, wildfire and insect outbreaks occur with increasing frequency across the American West, the technology could also help scientists understand how these disturbances affect forests' ability to absorb and store carbon from the atmosphere.
"I don't know of any other tool that can detect this type of signal before tree mortality becomes obvious at a scale large enough to assess the health of entire forests," said lead author Lewis Kunik, who recently completed his doctorate at the U. Kunik's doctorate was jointly advised by study co-authors and U professors John Lin in the Department of Atmospheric Sciences and David Bowling in the School of Biological Sciences.
"The ultimate goal isn't to predict the exact tree that will die. Rather, the technology could identify areas of concern early enough for land managers to investigate, mobilize crews, allocate funding or otherwise prepare before mortality becomes widespread."
The study is online ahead of its publication in the October issue of Remote Sensing of Environment.
Fluorescence provides an early warning signal
Many satellites monitor forest health using signals such as greenness and canopy structure because some when some trees become stressed, they might wilt or drop their leaves. But pines, spruces, firs and other evergreen trees present challenges for satellite monitoring: they can keep their needles even while photosynthetically dormant, such as during winter or under other high-stress conditions. This matters for Western U.S. forests, which are largely dominated by evergreens.
Fortunately, several next-generation satellites carry specialized instruments which detect a faint red glow that plants emit during photosynthesis, the process by which plants convert sunlight into energy. This signal is known as solar-induced fluorescence (SIF)-when a leaf's chlorophyll molecules absorb radiation, some radiation re-emits at longer, red wavelengths known as fluorescence.
When plants get stressed, they absorb more light than they can use, reducing their efficiency and dimming their red glow. By tracking SIF relative to the amount of light absorbed over time, the researchers could identify subtle physiological changes in in evergreen trees that conventional satellite metrics can miss.
The authors used SIF observations from TROPOMI, the instrument on the European Sentinel-5P satellite, to compare changes in fluorescence patterns in forests affected by wildfire- and insect-caused tree mortality with non-affected control areas with similar biogeographic characteristics in forests across the American West.
In forests that would later experience bark-beetle mortality, the researchers detected a significant SIF decline roughly two years before mortality was seen in aerial surveys conducted by the USDA Forest Service. Drought stress alone couldn't explain the signal. While nearby healthy forests experienced similar levels of drought, the decline in SIF from healthy forests was 10-20% less severe than the decline in the bark beetle-infested forests.
SIF can change for many reasons, including drought, insect infestation, canopy dieback, changes to seasonal timing of growth, reduced sunlight and changes in the mix of plants growing from the forest floor to the top of the canopy. The researchers accounted for these factors, but the complexity of forest ecosystems makes year-to-year changes in SIF difficult to interpret.
In this case, however, their analysis revealed a clear pattern. The findings suggest that SIF signal could provide an early warning of forest stress that precedes widespread tree mortality.
The researchers validated their approach using wildfire mortality as a testbed for detecting SIF changes from a wide range of mortality severities. They saw declines in SIF that scaled proportionally with the amount of vegetation lost due to fire.
"Wildfire mortality has more predictable impacts to forest productivity than bark beetle mortality. There is also a lot more wildfire-affected land to study, and we can use established tools to estimate the severity of those events. Testing our method on wildfires really helped build confidence in our bark beetle assessment," Kunik said.
The researchers were also able to use SIF to monitor how the ecosystem recovered from wildfire, highlighting the technology's potential for tracking how disturbances alter forest productivity and carbon cycling over time. Because forests store massive amounts of carbon, tracking these changes will help scientists better understand how disturbances affect the carbon balance of western forests.
Visualization showing that higher CO2 levels (left: blue regions=less CO2 than baseline, red regions=more) correlate to higher levels of photosynthetic activity (right, SIF). Credit: Christian Frankenberg/Caltech
"SIF is an emerging tool that Earth scientists can use to show the fingerprint of plant CO2 uptake at regional or global scales," said Kunik. "Drought, wildfire and bark beetle outbreaks can weaken a forests' ability to absorb carbon and may release the carbon stored in the trees. Tracking these changes will help us understand whether these disturbances potentially turn forests from carbon absorbers to carbon sources."
SIF outperformed the other satellite metrics
While other SIF-observing satellites exist, the authors used data collected by TROPOMI due to its coverage and sampling frequency.
The study tested the sensitivity of SIF measurements against several forest-health and vegetation-productivity remote-sensing measures. These included other frequently used satellite signals like land surface temperature, as well as vegetation indices like the Normalized Difference Vegetation Index (NDVI).
SIF showed greater sensitivity to bark beetle mortality than the other canopy remote sensing products tested. It also showed significant stress-related declines earlier than the other products, and roughly two years before aerial surveys first detected mortality.
"The results are exciting because they show the potential of SIF to provide information on forest health over a large spatial region," said Lin. "Future satellites, such as the European Space Agency's FLEX mission, will provide SIF at much higher spatial resolution and add to the growing record of SIF that will reveal more patterns in the future."
The authors are excited for SIF's potential. The project began through conversations with USDA Forest Service collaborators who have long wanted an early warning system to help them manage forests.
"They want to know as soon as possible when forests may cross a threshold of stress that leave it vulnerable to pests, pathogens or other drought-related impacts," said Kunik. "We still can't predict if, or where, mortality will occur just based on SIF observations, but our work shows that SIF could be another powerful tool for identifying areas of concern."
