The role of forests in climate change is currently an open question, and it’s one that Fred Huemmrich, research scientist at UMBC’s Joint Center for Earth Systems Technology (JCET) and adjunct faculty in geography and environmental systems, is working to answer. Huemmrich is part of a team that provided proof-of-concept for a new technique to measure photosynthesis in evergreens, using data collected by NASA’s MODIS instruments flying on the Aqua and Terra Earth-observing satellites. The Proceedings of the National Academy of Sciences recently published the results of that work.
At the core of Huemmrich’s research are two conflicting points. Plants remove carbon from the atmosphere during photosynthesis. As the climate warms, longer growing seasons will allow more time for trees to photosynthesize, increasing total carbon uptake and slowing climate change. However, a warmer climate could bring more drought, which stresses trees, reducing photosynthesis and exacerbating climate change.
The biosphere quickly absorbs about 50 percent of the “extra” carbon humans add to the atmosphere. Plants are responsible for the vast majority of that uptake, so it’s important to understand the roles different ecosystems play by measuring their photosynthetic activity. But until now, it’s been impossible to measure how evergreens contribute to carbon uptake on a large scale.
Scientists have been measuring overall leaf cover to estimate photosynthesis in deciduous forests for more than 30 years, but that doesn’t work for evergreens. In terms of scale, existing measurement techniques can detect photosynthesis in small geographic areas, but they are geographically limited and expensive, relying on towers placed within forests.
The MODIS instrument can measure the reflectance of evergreen needles, which changes depending on which pigments are present. In the summer, chlorophyll dominates, making the needles green and creating a particular reflectance profile. But in the fall, pigments like carotenoids (oranges), xanthophylls (yellows), and anthocyanins (reds) outnumber chlorophyll as plants photosynthesize less, revealing trees’ fall colors and creating a different reflectance profile. Huemmrich and colleagues coined the Chlorophyll/Carotenoid Index (CCI), a calculation that measures the ratio of pigments in leaves.
“Plants are constantly making adjustments to a changing environment,” explains Huemmrich. “If we can detect how a plant’s reflectance is shifting, we can determine how it’s responding to those environmental changes.”
While Huemmrich analyzes the MODIS data to calculate CCI, his collaborator John Gamon, professor at the University of Nebraska, measures seasonal changes in leaf pigments by directly testing leaves as a basis for comparison. The team also compares results from their satellite-based analyses to data from existing research towers in forests. “The idea is that we can use these towers to train and test our models,” explains Huemmrich.
The results were clear: The new technique accurately reflected the photosynthetic activity of three forests of primarily evergreens in North Carolina, Maine, and Washington state, while the traditional measurement of green leaf canopy was much less accurate. This is great news for climate researchers worldwide, who will now be able to more easily measure evergreen photosynthesis on a global scale using MODIS data that is consistently pouring in.
With the proof-of-concept in hand, Huemmrich and Gamon have been awarded a new NASA Arctic – Boreal Vulnerability Experiment (ABoVE) grant to extend this satellite approach. The new project will measure the carbon dioxide taken up through photosynthesis in Alaska and western Canada using MODIS data. A second ABoVE grant will fund them to use airplanes flying above the same areas with more advanced remote sensing instruments.
“I’m excited about the airplane study, because it fills the gap between ground measurements and MODIS,” says Huemmrich.
As the climate continues to change, Huemmrich notes that it will be critical to understand mitigating factors, like photosynthesis, and use that knowledge to focus protections on those ecosystems that help slow warming. Coniferous forests cover about 17 percent of Earth’s land surface, and Huemmrich’s work to develop affordable, large-scale photosynthesis measuring techniques could have a major global impact.
Image: Cloud cover over the Eastern U.S. as recorded by MODIS on the TERRA satellite.