Will Pohnpei's Mangroves Keep Up with Rising Seas?

Mangrove forest on the coast of Pohnpei. USGS.

Mangrove forests across the Federated States of Micronesia supply critical resources and help communities weather a changing climate.

• Locally: habitat for fish and wildlife, timber, and other cultural resources — and a buffer against the waves and winds of tropical cyclones and tsunamis.
• Globally: Micronesian mangroves can store 700–1,800 metric tons of carbon per hectare, part of the estimated 5–10 billion metric tons held by mangroves worldwide.

The threat

Healthy mangroves have kept pace with some sea-level rise, but rapid rise could outrun their ability to adapt. Degraded mangroves — where overharvest, clearing, infrastructure or altered water flow have disrupted natural processes — are at greater risk.

Modeling the forest

Scientists and land managers from the USGS, the USDA Forest Service, the U.S. Fish and Wildlife Service, the Micronesia Conservation Trust, the Conservation Society of Pohnpei and the Pohnpei Forestry office studied how Micronesia's mangroves will respond over the coming century, starting with the island of Pohnpei. They built a mathematical model to project the forest's future and inform local planning.

How it works: the model links ocean, trees and soil:

1. Elevation relative to sea level decides which species thrive where, and how much biomass the forest holds.
2. Fallen leaves, dead roots and trees decay into the soil; with mineral inputs, they build it up (accretion) and raise its elevation, countering sea-level rise.
3. A mangrove forest stays resilient if accretion outpaces sea-level rise, or if it has room to migrate inland to higher ground.

Scenarios: global sea-level rise by 2100 of 37 cm (RCP 4.5), 52 cm, 67 cm (RCP 8.5) or 117 cm, based on Intergovernmental Panel on Climate Change emission scenarios — the rise coming from warming seawater expanding and ice melting.

Calibration: extensive field data from Pohnpei's mangroves — accretion rates from soil cores, surface elevation tables, more than 300 forest inventory surveys, water-level monitoring and elevation surveys.

Pohnpei's mangroves by elevation: a low-elevation species (top), a generalist (middle) and a high-elevation species (bottom). Kevin Buffington, USGS.

What the projections show

• Low to moderate rise (37 or 67 cm): if intact and healthy, the forests are likely resilient for decades. Species composition changes little over the next 30 years, with some expansion of low-elevation species and possible losses of high-elevation species by 2100 in parts of the island.
• Extreme rise (117 cm): mangrove elevation falls well behind sea level, species composition shifts sharply from 2060, and mangroves are lost by 2100.
• Where: vulnerability varies around the island; mangroves on the leeward (southwest) side are generally the most at risk from high rates of rise.
• Limits: the model covers elevation and tree species only; effects on birds, fish and the rest of the ecosystem are unknown.

Projected mangrove species in 2020 and in 2060 under 37 cm and 117 cm of sea-level rise; black marks lost mangroves. USGS.

Management takeaways

• Pohnpei's mangroves are likely resilient to low and moderate sea-level rise over the coming decades if they stay intact and healthy.
• Under high rates, expect big changes in species composition and loss at the island's edges.
• The leeward southwest is the most at risk.
• The modeling approach can be applied to other Pacific mangroves; at the time of the report it was being applied to Kosrae.
• Continued monitoring will track sea-level rise impacts and inform adaptive management.

What it means

How fast and how far the sea will rise this century is uncertain. Knowing how different mangroves could respond lets stakeholders plan to:

1. conserve mangroves resilient to sea-level rise over the long term;
2. strengthen those that are vulnerable;
3. educate local communities.

Keeping Pohnpei's mangroves in their current, relatively undisturbed state is key to their resilience — and to the benefits they give local and global communities.

Sources

Based on Sea-level Rise Vulnerability of Mangrove Forests on the Micronesian Island of Pohnpei, by Karen M. Thorne and Kevin J. Buffington, USGS Fact Sheet 2021–3030 (May 2021), U.S. Geological Survey (DOI), summarizing Buffington and others (2021, USGS Open-File Report 2021–1002) and citing Donato and others (2011) and Alongi (2018); a work of the United States government in the public domain. Photographs and maps from the fact sheet's PDF; its conceptual diagram, which uses licensed stock tree drawings, and a locator map on an Esri basemap are left out.