LTREB: Thirty-Four Years of Tidal Marsh Response to Environmental Change

Coastal wetlands are some of the most productive ecosystems on Earth, and are critical transition zones between terrestrial and marine habitats. Salt marshes, hotspots of biological activity, have a remarkable ability to sustain themselves, largely due to their vegetation, and maintain coastlines as sea level changes. However, the annual loss of thousands of hectares of low-lying coastal wetlands to open water each year threatens fisheries, wildlife habitat, water quality, soil carbon stocks, and adjacent human infrastructure.

The objective of this project is to learn how coastal wetlands, salt marshes in particular, will respond to intensifying environmental change. This project plans to extend experiments examining the influence of rising atmospheric CO2, nutrient pollution, invasive species, and sea level rise on the structure and persistence of salt marsh vegetation. It aims to take advantage of an unparalleled thirty-four year record of plant, microbe and soil responses to global change to identify the most important factors for marsh survival.

These results should be relatable to other coastal ecosystems, and are likely to improve predictions of future marsh loss as well as inform management strategies for maintaining wetlands. Data from the project will be incorporated into undergraduate classes at two institutions, undergraduates will assist with data collection and use data from the project for senior theses, and the general public will be given tours of research site and given the opportunity to participate in citizen science initiatives.

This project includes three complementary, long-term field experiments that manipulate

atmospheric CO2 and N enrichment in different plant communities, that leverage the inter-annual variability in other critical factors such as sea level, salinity and precipitation. The team plans to test the hypothesis that sea level rise, as the ultimate driver of change in coastal wetlands, will cause the two plant species with the highest flood tolerance (native sedge and invasive Phragmites australis) to replace those with lower flood tolerance (C4 grasses) over the next decade.

Because the sedge and Phragmites also respond strongly to elevated CO2, the team predicts that plant community composition will initially shift and increase the ecosystem-scale CO2 response, but it will ultimately decline due to increasing flooding stress. The marsh is expected to cross a threshold in the next decade as rates of relative sea level rise accelerate to the point that the marsh becomes unstable due to declining productivity.

However, the areas dominated by Phragmites, particularly plots with added N, yield the greatest productivity on the marsh, and have the best chance to keep up with accelerating sea level. Finally, the team hypothesizes that an increase in methane emissions expected with increased flooding will be offset initially by higher rates of rhizosphere methane oxidation from the flood-tolerant sedge.

The results from this experiment can continue to inform models and global change syntheses. The site and extensive dataset are used as a hands-on case study in ecology courses at Villanova University and Bryn Mawr College. This project plans to train researchers at all levels, to engage the public in the long-term and forward-looking science that arises from this LTREB site, and to promote the application of this basic science to support coastal wetland protection, conservation, and restoration.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.