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In 1850, the island of Nahlapenlohd in Micronesia was sprawling enough to support a great battle among warring chiefdoms within its sizeable coconut forest. But that was then. Today, Nahlapenlohd and seven nearby islands in the western Pacific are no more, having been swallowed by the rising sea.

Color photo of researchers surveying model of Pohnpei, Micronesia
Researchers from the USGS Earth Resources Observation and Science (EROS) Center took part in work with the Pohnpei Mangrove Resilience Project in 2019. The group is working to assess the vulnerability of mangrove forests on the Micronesian island. 

Scientists measuring ocean levels in the Federated States of Micronesia say a natural trade wind cycle combined with climate change have led to sea levels climbing there by as much as 12 millimeters a year since the early 1990s. Islands are disappearing, and low-lying atolls have been abandoned. Ancient cultures reliant on oceanic and coastal resources for their health and continued existence are now battling to keep their homes.

Against that backdrop, staff from the Earth Resources Observation and Science (EROS) Center have joined others within the USGS, the U.S. Fish and Wildlife Service, the U.S. Forest Service, the Micronesia Conservation Trust, and the University of Tasmania to study the resiliency of what may be the islands’ greatest natural defense against the associated challenges of rising sea levels—their mangrove forests.

The Pohnpei Mangrove Resilience Project is assessing the vulnerability of mangroves on the Micronesian island of Pohnpei. Sensitive to impacts from slowly rising sea levels, the mangrove forests ringing the island provide important barriers to the wind and wave impacts brought on by increasing storms and tidal surges. The trees are also a source of timber and fuel wood. They provide habitat for healthy fisheries and help to filter coastal waters as well. And Pohnpei’s mangrove forests are among the world’s most efficient habitats for sequestering atmospheric carbon.

Along with assessing the situation, the project intends to work with local non-governmental organizations (NGOs) to not only bolster the resiliency of Pohnpei’s mangrove forests, but to create a community-level mangrove adaptation plan that could be transferred to other Micronesian islands with significant mangrove stands.

To help accomplish that, the vulnerability assessment requires data on ground elevation, sedimentation rates, the mapping of current mangrove extent, the change of both shoreline and mangrove cover over time and, if possible, indicators of mangrove health and stress. The project also wants to see a preliminary assessment of the feasibility of marketing mangrove carbon credits to help pay for conservation.

To assist with one of the project’s most immediate and critical data needs—accurate digital elevation models—EROS Geographer Jeff Irwin and Research Physical Scientist Dean Gesch spent 2½ weeks on Pohnpei in April mapping elevation points across the island.

Assessing elevation in the Pohnpei environment is difficult because of muddy tidal flux, and because remote sensing doesn’t work well penetrating the dense forest canopy, Gesch said. So, a lot of field work was required to determine elevation based on such things as sampling soil cores that can help measure sedimentation rates over time.

Color photo of Pohnpei, Micronesia
Mangroves of Pohnpei, Micronesia. 

Those cores help tell how much accretion—or the build-up of soil—occurs over time. Sand from the nearby ocean or sedimentation from surrounding watersheds are flushed across mangrove soil and build it up vertically. Roots, detritus, and other decomposing organic matter build up soil as well.

“Together, these two processes help the mangroves,” said Kevin Buffington, an ecologist with the USGS Western Ecological Research Center in Davis, CA, and a lead on the Pohnpei project. “Mangroves will build vertically and offset some of the effects of sea level rise.”

Buffington and his colleagues have adapted what’s called the Wetland Accretion Rate Model of Ecosystem Resilience (WARMER)—a model tied to elevation that’s used for coastal marshes and other types of wetlands—to apply to the mangrove forests. For WARMER to work effectively, very precise elevations for different points identified on the island were needed. That’s where the EROS expertise comes in.

Again, because of the difficulty of using remote sensing to penetrate the forest canopy cover for the Digital Elevation Models (DEM) that EROS staff are good at creating, Buffington designed a map two years ago that identified sampling points across the island to capture the full elevation regime. EROS staff had traveled to Pohnpei in February 2017 to do similar field work—gathering elevation measurements at points along the ocean’s edge of the island, along the landward edge, and in places in between.

Good existing DEMs for the island would have made the work much easier, Gesch said. “But there just aren’t any,” he added. “So really, the only way to get the super accurate elevation information you need for this to calibrate the sea level rise model is to physically go out there and measure it.”

Buffington said the EROS participation is important because of its history of working in coastal environments developing DEMs. Irwin’s work with differential leveling—in this case, measuring vertical distances using survey grade GPS from a known elevation point outside the forest to determine elevations of unknown points within the forest—was also key to what they were doing, Buffington said.

“The value of the EROS staff has really been the expertise in measuring elevation in a robust way,” he said. “Particularly with the differential leveling, the experience that (Irwin) brought to the table was really invaluable in getting the work done in an accurate and timely way.”

Gesch said they were able to map 50 to 60 elevation points on Pohnpei during their 2½ weeks in April. Along with providing highly accurate elevation data, that work will enable them to analyze the DEMs that do exist for the island, which he characterized as poor and thus of little use in doing detailed mapping of the mangrove forests.

A project summary meeting is scheduled for Pohnpei sometime early next year to present the findings.

“I think the whole community on Pohnpei is certainly the most interested and have been quite eager for our results,” Buffington said. “As part of next year’s presentation and workshop, we really want to educate the community on the importance of mangroves, the importance of mangrove conservation and restoration, and really how to protect their island against the effects of sea level rise.”

Video Transcript
Research Physical Scientist Dean Gesch shares an overview of USGS/EROS surveying efforts in the mangrove forests of Pohnpei.Brian Hauge (Contractor), USGS EROS Center (Public domain.)


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