A group of researchers from the University of Delaware teamed up with an international group of collaborators to verify the accuracy of remote sensing products that measure forest extent. Together, they looked at forest extent estimates from seven land or tree cover remote sensing products and compared them to two independent forest inventory datasets.
Using the information, they developed a forest extent agreement map for Mexico that combined the information from the seven products and two datasets and identified the areas of agreement in their estimates.
The National Aeronautics and Space Administration Carbon Monitoring System program of the United States funded the research, and the results of the study were recently published in Environmental Research Letters.
Dustin Braden, who received his master’s degree from UD in 2023, was the lead author of the paper. Pinki Mondal, assistant professor in the Department of Geography and Spatial Sciences in UD’s College of Earth, Ocean and Environment, was a co-author on the paper. Mondal and Rodrigo Vargas, professor of ecosystem ecology and environmental change in UD’s College of Agriculture and Natural Resources and director of the Water Science and Policy program, co-advised Braden during his time at UD.
Mexico’s forest surveys
Braden said that Mexico has one of the best programs in the world for gathering high-quality information about the country’s forests for the National Forest Inventory, which is run through Mexico’s Comisión Nacional Forestral (CONAFOR).
The National Forest Inventory measures canopy density and the number of trees present, among other things, Braden said. For their research, Braden and his team accessed Mexico’s National Forest Inventory and compared it to the findings from various satellite products.
“Then, we could compare the two to see where there’s areas of agreement and disagreement to support better decision-making about the National Forest Inventories and these satellite-derived products,” Braden said.
It was the first time Braden had participated in a large international research project. With a background in conservation and geography, he also didn’t have much experience related to carbon monitoring.
“One of the great things about being co-advised by both Pinki, who has more of the geographic information systems and remote sensing background, and Rodrigo, who has a little bit more of that carbon background, was getting to bring these two different fields together through my work and getting to learn how I could apply mapping and satellite data in a more specific way to ask really interesting questions,” Braden said.
Forest extent agreements and disagreements
Using the data, the researchers were able to find areas where the satellite products agreed on forest extent and where they disagreed. They found that high agreement tended to occur in heavily forested areas like the Yucatan Peninsula and mountain areas in the Sierra Madre Oriental and Sierra Madre Occidental. High disagreement tended to occur in complex ecological zones like the tropical dry forest and subtropical mountain system, where the varied tree cover can lead to inconsistencies in remote sensing analysis.
Ideally, the researchers would like to have high correlation between the remote sensing products and the on-the-ground data, Braden said, but he admitted that is a lofty goal.
“There’s always going to be differences just because they’re different methods,” Braden said. “What someone is doing on the ground all day, walking through these places and measuring these data, versus what the satellite can pick up — it’s always going to be a little bit different.”
But now that they’ve identified where the products agree and where they disagree, researchers can start to look at what exactly is causing the disagreement. The information could also be helpful to researchers on the ground who are collecting data for Mexico’s National Forest Inventory.
“The researchers collecting data might use something like this to say, ‘Here’s this region where some products from satellites are identifying it as forested and some aren’t. We should probably get some better data here, on the ground, to see what exactly is happening,’” Braden said. “There’s a whole bunch of reasons they might be getting confused. Satellites might be misidentifying shrubs as forest, or they might just not see trees where there are trees. That’s a useful insight that goes beyond the national forest inventory data.”
