Sea-level Rise Vulnerability of Mangrove Forests in Micronesia and the Pacific

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The USGS and partners are studying how mangrove forests in the Federated States of Micronesia may respond to sea-level rise over the coming century. Their projections will help Micronesian communities plan for the future.

Introduction

The mangrove forests across the Federated States of Micronesia provide critical resources and contribute to climate resilience. Locally, mangrove forests provide habitat for fish and wildlife, timber, and other cultural resources. Mangrove forests also protect communities from tropical cyclones and tsunamis, providing a buffer against powerful waves and winds. Mangrove forests in Micronesia can store 700-1800 metric tons of carbon per hectare1, contributing to the estimated 5-10 billion metric tons of carbon stored by mangroves around the world2. This carbon storage is essential for global climate resilience.

People work together to install a water logger in the mangrove forest

USGS and partners work together to install water loggers in the mangrove forest.

(Credit: Karen Thorne, USGS Western Ecological Research Center. Public domain.)

Mangrove forests and the benefits these ecosystems provide are threatened by accelerating sea-level rise and human land change activities. Healthy mangrove forests are resilient systems and have maintained their presence with some amounts of sea-level rise, but rapid sea-level rise may outpace their ability to adapt. Degraded mangrove forests are at greater risk where natural processes have been altered. Overharvest and clearing of timber, infrastructure development, and altered hydrology are just a few of the human activities that can damage mangrove forests.

A great deal of uncertainty remains as to how quickly and how much sea-level will rise over the next century. Understanding how different mangroves may respond to a range of sea-level rise scenarios will allow stakeholders to develop plans and management actions to: 1) identify and preserve mangrove forests that are resilient to sea-level rise over long periods of time, 2) develop strategies to increase the resilience of mangrove forests that are vulnerable to sea-level rise, and 3) provide educational outreach to local communities about the importance of conservation.

Modeling Mangrove Dynamics

Scientists and land managers from the USGS, USDA Forest Service, Pacific Islands Climate Adaptation Science Center, Micronesia Conservation Trust, the Conservation Society of Pohnpei, and Pohnpei Forestry teamed up to study how mangroves in Micronesia will respond to sea-level rise over the coming century, beginning with the islands of Pohnpei and Kosrae. The researchers developed a mathematical model to project the future of mangrove forests under different sea-level rise scenarios to inform local planning and decision making.

The model incorporates the relationships among the ocean, mangroves, and soils, showing how a change in sea level translates to changes throughout the ecosystem.  Elevation, relative to sea level, determines which tree species will thrive at a particular location and influences the biomass of the forest.  When trees drop leaves, turn over roots, or die, their biomass decays and becomes part of the soil. Along with mineral inputs, biomass builds up the soil (accretion) and raises the elevation of the soil, counteracting the effects of sea level rise. Mangrove forests can be resilient to sea-level rise if accretion rates outpace sea-level rise, or if they have room to migrate inland to high ground.

The model incorporates these processes into equations that predict how elevation and mangrove species composition will change under a range of global sea-level rise scenarios: 37 cm (RCP 4.5), 52 cm, 67 cm (RCP 8.5), or 117 cm of sea level rise by 2100.  The sea-level rise scenarios are based on possible global CO2 emission scenarios outlined by the Intergovernmental Panel on Climate Change, reflecting the resulting rise in sea level due to thermal expansion and the melting of ice as global temperatures rise. The modeling framework was calibrated with extensive field datasets from Pohnpei’s mangroves, including accretion rates derived from soil cores, over 300 forest inventory surveys, water level monitoring, and elevation surveys.

Diagram showing how sea-level and species cover in the mangrove forest influence one another

Conceptual model of a mangrove forest.

(Public domain.)

 

Model Projections for Pohnpei

So far, the model has been used to project how sea-level rise scenarios will influence the island of Pohnpei3. The approach will be modified and applied in the future to mangrove forests on the island of Kosrae and elsewhere in the region.

The model projections for Pohnpei indicate that the mangroves are likely to be resilient to low and moderate rates of sea-level rise over the coming decades if they remain intact and healthy. Under 37 cm or 67 cm of sea level rise, predictions showed limited changes in species composition over the next 30 years, with some expansion of low elevation species and possible losses of high elevation species by 2100 in some parts of the island.

However, under the most extreme sea-level rise scenario, 117 cm of sea-level rise, mangrove elevation decreased substantially relative to mean sea level, with more drastic changes in the tree species composition starting in 2060 and loss of mangroves by 2100. Vulnerability to sea-level rise varied across the island, with mangroves on the leeward side of the island generally the most at-risk to high rates of sea-level rise.

The model’s predictions are limited to the effects on elevation and tree species composition. The potential effects of sea-level rise on other parts of the mangrove ecosystem, such as bird and fish communities, are currently unknown.

Expanding the Approach Across Micronesia and the Pacific

The new modeling approach provides projections of sea-level rise impacts for the island of Pohnpei. The approach is transferable across the Pacific region and is currently being applied to the Micronesian island of Kosrae.  Maintaining the current and relatively undisturbed state of mangroves around Pohnpei and other island communities is important to maintain forest resilience to sea-level rise and preserve the benefits that these forests provide to Micronesia and people around the world.

 

Partners

USDA Forest Service

U.S. Fish and Wildlife Service

Pacific Islands Climate Adaptation Science Center

Micronesia Conservation Trust

Conservation Society of Pohnpei

Pohnpei Forestry

 

 

 

[1] Donato, D. C., J. B. Kauffman, et al. (2011). Mangroves among the most carbon-rich forests in the tropics. Nature GeoScience 4: 293-297.

[2] Alongi D.M. (2018) Mangrove Forests. In: Blue Carbon. SpringerBriefs in Climate Studies. Springer, Cham. https://doi.org/10.1007/978-3-319-91698-9_3

[3] Buffington, K.J., MacKenzie, R.A., Carr, J.A., Apwong, M., Krauss, K.W., and Thorne, K.M., 2021, Mangrove species’ response to sea-level rise across Pohnpei, Federated States of Micronesia: U.S. Geological Survey Open-File Report 2021–1002, 44 p., https://doi.org/10.3133/ofr20211002.