This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. Many satellites monitor forest health using signals such as greenness and canopy structure because 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 that 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 is re-emitted 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 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 unaffected 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.
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