Modeling mangrove ecosystems for sea level rise vulnerability across Pohnpei, Federated States of Micronesia

Low-lying island nations are especially vulnerable to the effects of climate change and sea-level rise (SLR). Mangrove forests provide protection for human coastal populations throughout the western Pacific, in addition to food, fiber, and fuel. The future existence of these forests, the services they provide, and the human populations that rely on them are threatened by increased rates of SLR. Identifying those mangroves areas that are more vulnerable or resilient to SLR will increase our ability to more effectively restore or protect them and maintain their future existence. Working on the island of Pohnpei, Federated States of Micronesia, we developed a numerical model, the Mangrove Ecosystem Response to Climate Change (MERCC), that integrates both mangrove forest and soil dynamics. MERCC is a point elevation model that incorporates the dominant processes that control wetland soil development, including mineral deposition, root turnover, organic decomposition, and auto-compaction, and a species transition submodel controls the growth and relative distribution of mangroves in the simulation cell. Substrate accretion rates (60 cm cores, ²¹⁰Pb, n=33), long-term subsidence (6 m cores, n=2), surface elevation tables (20-year record, n=18), forest inventory plots (basal area and dendrobands), tidal water levels, and substrate elevation data were collected and used to calibrate the model. Elevation results showed mangroves to be restricted in extent to the microtidal inter-tidal range. Accretion rates from the soil cores varied widely (0.98-12.50 mm yr^-1), but on average are not keeping pace with sea-level rise (2.89 mm yr^-1 mean accretion vs 3.87 mm yr^-1 sea-level rise). This project will show how mangrove vulnerability differs spatially around the island and across a range of SLR scenarios and highlight the importance of adaptation planning for local stakeholders.