Surface Elevation Vulnerability of Coastal Forested Wetlands to Sea-Level Rise

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Wetlands vary in their abilities to keep up with sea-level rise; they either adjust vertically and/or move inland. USGS is working with partners around the world to measure rates of surface elevation change relative to local sea-level rise. 

Tidal creek, Ouvea, New Caledonia
Tidal creek, Ouvea, New Caledonia

The Science Issue and Relevance: The vulnerability of forested wetlands to sea-level rise is difficult to assess because different wetland types, and even similar wetland types along different coastlines, have variable capacities to build surface elevation at rates to pace sea-level rise. When sea-levels rise, wetlands either adjust vertically and/or move inland as they adjust. Natural thresholds exist whereby wetlands are likely to be lost; this loss is occurring over decadal time scales in some regions but over centuries in others. Why? The processes involved in forested wetland resilience to sea-level rise have stimulated a number of discussions and important papers in recent years, and we have much more to learn. What we know is that projecting rates of sea-level rise over a static wetland soil surface elevation is inadequate for modeling and determining future wetland loss, and that relative rates of sea-level rise as determined by tide gauges alone must be supplemented by on-site data collected in wetlands over reasonable periods of time.

Methodologies for Addressing the Issue: We currently collaborate with a number of scientists from the United States, Australia, China, Singapore, Vietnam, New Zealand, New Caledonia, and various Pacific island nations to measure rates of surface elevation change relative to local sea-level rise. Our primary tool is the surface elevation table and marker horizon (SET-MH) approach. SETs are driven to survey standards, and surface elevation change is measured repetitively relative to that datum (see below), while accretion of sediments over that same time frame is determined simultaneously using marker horizons.

Collecting surface elevation table (SET) data from mangroves
Collecting surface elevation table (SET) data from mangroves

From this, we are able to calculate vertical soil movement, accretion, subsidence, and even root zone expansion when roots are particularly vigorous. Critically, surface elevation change is rated relative to tide gages surveyed into benchmarks of similar reference depths to SETs. Our group focuses on tidal freshwater and oligohaline forested wetlands, forest-adjacent and managed marsh, mangroves, and Pocosin wetlands.

Future Steps: Global SET-MH datasets are beginning to mature, and thus are just starting to become useful for large-scale meta-data-type assessments. For example, we began collecting SET data from mangroves on the Pacific island of Kosrae in 1997, and this is now the longest continuous SET record from the entire Indo-Pacific region. In future years, we will participate in meta-analyses, continue to publish data from our individual site assessments, and install and/or assist with installations of SETs to USGS standards with collaborators globally.