Scientists recently noticed a puzzling pattern they could not explain in satellite-derived vegetation greenness, a measurement that is used as a proxy for the amount of leaves in an area and for vegetation productivity. The puzzle was that vegetation-greenness measures over the US Corn Belt are higher than the same measures over the Amazon rainforest, despite the rainforest having a greater amount of leaf area. Specifically, the Amazonian forests have about two times more leaves per unit area than the US Corn Belt (5.7±0.6 m²/m² vs. 2.6±0.3 m²/m², respectively). So, while you may think that it’s common sense that the Amazon’s greenness measure would be significantly higher than the greenness measure of the Corn Belt—it’s not! A new study published in Nature Ecology & Evolution finally provides an answer to the cause of this paradoxical pattern and unveils a startling insight, which suggests that we may have been viewing the Earth through a distorted lens.

Spatial pattern of solar-induced chlorophyll fluorescence (SIF, Left) and leaf area index (LAI, Right). SIF is an innovative measurement that serves as a proxy for plant photosynthetic activity. LAI is the green leaf area of a canopy or plant community per unit ground area. SIF and LAI data presented here are from ESA Sentinel-5 Precursor TROPOMI and NASA Terra/Aqua MODIS, respectively. Image: Fujiang Ji.

A team of researchers from 12 institutions across five different countries, including University of Wisconsin–Madison, NASA’s Ames Research Center, and Pacific Northwest National Laboratory (PNNL), found that vegetation-greenness measures are often shadowed—literally. Shadows, especially those cast by complex canopy structures, can introduce pronounced negative biases into satellite-derived greenness measures, thus misleading our understanding of global vegetation and how it’s changing.

“We hypothesized that the shadows cast by complex canopies might contribute to this paradoxical phenomenon. The complex canopy of the Amazon rainforests, in contrast to the relatively simple and homogeneous canopies of the US Corn Belt, provides the ideal natural experiment for testing our hypothesis, because varying canopy-structure complexity creates contrasting shadowing conditions,” said Yelu Zeng, a lead author of this study and professor at China Agricultural University (previously a research scientist at University of Wisconsin–Madison when this work was first undertaken).

To prove that canopy-cast shadows can lower satellite vegetation-greenness measures, the researchers explicitly tracked how light interacts with the vegetation canopy. They did this investigation with state-of-the-art radiative transfer modeling. They incorporated multi-angular, hyperspectral, and lidar data from several NASA missions and programs. The team placed particular emphasis on the importance of the geometry between the satellite, the Sun, and the Earth’s vegetation.

This is a black spruce forest in the NASA-supported BOREAS (Boreal Ecosystem-Atmosphere Study) field campaign region in Canada. (Left) Trees are viewed with the Sun behind the observer. All shadows are concealed, and only the bright side of the trees is visible. (Right) Trees are viewed with the Sun opposite the observer. Large shadows are visible. Image: Don Deering.

“Imagine you stand looking at trees with the Sun behind you. You will see brighter leaves with fewer shadows. Now, consider the opposite scenario, where the Sun is in front of you. You will observe more shadows and darker leaves,” said Taejin Park, a research scientist at NASA Ames Research Center / Bay Area Environmental Research Institute (BAERI) and co-first author of this study. “A more complex canopy structure results in more shadows, which in turn makes the impact of those shadows more significant.”

The consequences of the shadow effect extend far beyond the technical interpretation of satellite data. In their paper, the team investigates what this means for global vegetation studies. In one example, they showcase how land conversion from forest to crops reveals an unexpected increase in vegetation greenness in satellite data, even though the land loses green leaves when forests are replaced by crops.

“A significant portion of deforestation in the Amazon rainforest is due to cropland expansion. Our results, which demonstrate an increase in vegetation greenness after the conversion of forest into cropland, are thus important and striking.” said Dalei Hao, a research scientist at PNNL and a co-first author of this study.

The authors stress that the shadow effect needs to be taken into account for a variety of satellite-based greenness measures, including solar-induced chlorophyll fluorescence (SIF). SIF is an innovative and popular way to measure the photosynthetic activity of plants and is a measurement that many Earth observing satellites are collecting for use in global vegetation studies.

“It is important to clarify that shadows are a real component of vegetation canopies, and that the shaded portion usually represents a large percentage of the canopy’s total carbon, water, and energy budget. However, our understanding of these shadows is limited, which highlights the need for in-depth future studies,” said Min Chen, a professor at University of Wisconsin–Madison and a co-author of this study.

Understanding the true state of Earth’s vegetation is crucial for everyone. Misinterpretations could lead to heightened uncertainty in climate models, misguided agricultural policies, and ineffective conservation efforts. To avoid misinterpretations, the researchers propose adjustments that could offer a more accurate picture by minimizing the shadow effect, specifically by shifting the satellite’s viewing condition to what is known as the “hotspot direction,” where the Sun is positioned behind the satellite. The team suggests that scientists need to exercise caution when using satellite vegetation greenness for biome-to-biome comparisons or when assessing rapid land-conversion processes. Such usage may introduce unexpected biases or errors when evaluating regional and global vegetation changes and growth trends.

The American Geophysical Union Fall Meeting, the largest Earth and Space Science meeting in the world, will be held this year in San Francisco from December 11-15, 2023.

The schedule below shows the date, time, location, ARC-CREST participant(s), and topic for the presentations, as well as a link to abstracts published on the AGU website.

The schedule below shows the date, time, location, BAERI participant(s), and topic for the presentations, as well as a link to abstracts published on the AGU website.

Moffett Field, CA—The Bay Area Environmental Research Institute (BAERI), in partnership with California State University, Monterey Bay (CSUMB) and San José State University (SJSU), has been awarded a five-year, $117 million cooperative agreement from NASA Ames Research Center to advance cutting-edge research in Earth science and technology.

The award, the Ames Research Center Cooperative for Research in Earth Science and Technology (ARC-CREST), is a renewal of the original 10-year agreement awarded to BAERI and CSUMB in 2012 and is a testament to the successful partnership between the organizations. To date, ARC-CREST scientists have received 3 individual and 21 group achievement awards from NASA, supported over 30 scientific field campaigns, published 387 articles in peer-reviewed journals, created 10 critical new datasets and technologies, and trained over 500 students to help advance Earth science and natural resources management.

“ARC-CREST provides crucial support for Earth science and makes it possible for BAERI to facilitate long-lasting collaborations between NASA and outside teams,” said Robert Bergstrom, president of BAERI. “This partnership is a driving force behind the tangible progress we’re making in Earth science research.”

The increased annual funding of the new award, almost double that of the original ARC-CREST, paves the way for more researchers to engage in extensive collaborative efforts. “The ARC-CREST agreement creates vibrancy, enabling cooperation in a way that the civil servant workplace cannot always provide,” said Ames Earth Science Division Chief Florian Schwandner. “It brings a different spirit into the collaboration: providing new ideas, and training the next generation.”

Over the next five years, research groups will build on the success of several ongoing and completed projects and initiatives. These include:

• OpenET (CSUMB/NASA), a tool that provides easily accessible satellite-based  evapotranspiration data for improved water management.
• Wildfire research (SJSU), leveraging faculty and expertise of the Wildfire Interdisciplinary Research Center, including the creation and deployment of innovative remote sensing technology, such as the SJSU Wildfire Infrared Imaging System. This work aims to enhance understanding of extreme fire behavior and to improve fire modeling.
• Climate research (BAERI/CSUMB/NASA) that uses NASA Earth Exchange data to observe and manage the impacts of climate extremes (wildfires, floods, droughts, hailstorms, etc.) on various socioeconomic systems.

About NASA Ames
Founded as an aeronautical laboratory in 1939 and converted to a NASA research center in 1958, Ames Research Center has since led NASA in conducting world-class research and development in aeronautics, exploration technology, and science. Ames has evolved as a place where state-of-the-art facilities and world-class talent meld to produce cutting-edge research in fields such as aerodynamics, thermodynamics, simulation, space and life sciences, and intelligent systems. In recent decades, Ames has increasingly partnered with public and private organizations, nonprofits, and academia, to continue to advance human understanding of our planet and beyond. To learn more, visit www.nasa.gov/ames/about-ames/.

About the Bay Area Environmental Research Institute
The Bay Area Environmental Research Institute is a scientist-founded research nonprofit that enables groundbreaking work in the Earth and space sciences. Founded in 1993 and headquartered at the NASA Ames Research Park in Moffett Field, California, the Institute has supported over 350 researchers in the last 30 years and has partnered with dozens of organizations, including NASA, Stanford University, USGS, Protectores de Cuencas, and The University of Sydney. To learn more, visit https://baeri.org/. 

About California State University, Monterey Bay
Established in 1994 by educators and community leaders on the former site of Fort Ord, California State University, Monterey Bay, explores innovative ways to meet the needs of students while powering the regional economy. We make higher education accessible to traditionally underserved and low-income populations. Seventy-two percent of our students receive some form of financial aid, and fifty percent of our students are among the first generation of their family to go to college. The university’s diverse population of over seven thousand students receive personal attention in small classes while pursuing degrees in twenty-five undergraduate and nine graduate majors. Learn more at csumb.edu.

About the Wildfire Interdisciplinary Research Center at San José State University
The Wildfire Interdisciplinary Research Center  at San José State University brings together an interdisciplinary team of academic and industry leaders to address the wildfire problem faced by many regions around the nation and the world. A National Science Foundation Industry-University Cooperative Research Center (IUCRC) and an official Campus Center and Institute within the California State University system, WIRC’s mission is to conduct high-impact wildfire research to provide new predictive tools and informed strategies to communities and industry stakeholders, including first responders, those interested in risk analysis, and policymakers in communities, companies, and utilities affected by wildfire. Learn more at https://www.wildfirecenter.org/.

In addition to his significant contributions to many, many climate-change related research programs. Park was recognized for exceptional collegiality and spirit of mentorship in his collaborative work.

For the 2022 report, Park co-authored a chapter that focuses on charting the phenology, or cycles, of growing seasons and phytoplankton blooms in locations around the globe. Find out more and download the publication here.

Read about the celebration in the students’ honor in this piece by BAERI’s Milan Loiacono