Stands of dead trees, known as ghost forests, ring the tidal marshes of the Chesapeake Bay region due to salt water intrusion. This phenomenon drew the interest of GW biology Professor Keryn Gedan and recent PHD graduate Justus Jobe, who have collaborated on a number of research projects involving saltwater intrusion in ecosystems located near Washington DC.
Saltwater intrusion research intersected directly with Jobe's PHD work in the Chesapeake Bay, examining sika deer and phragmites species. Gedan and Jobe were interested in the spread of phragmites, a widespread invasive species of grass common at marshes’ upland borders where ghost forests form. This project began as part of Jobe’s undergraduate research at University of Maryland under Professor Gedan, who taught there before she joined the faculty at GW in 2016.
Tidal marshes mitigate storm surges, providing a refuge when hurricanes hit by slowing down water and preventing large storm damage. Tidal freshwater wetlands, as emphasized by the National Park Service (NPS) and U.S. Environmental Protection Agency (EPA), play a crucial role in cleaning water through sediment trapping and pollution control, which is especially important for the polluted Anacostia River that runs through the district. Additionally, they offer aesthetic value for humans and habitats for valued wildlife. Wetlands impact storm intensity, which may be exacerbated by climate change.
Marshes can respond to sea level rise through vertical accretion, accumulating sediment and organic material to increase elevation, or lateral migration into adjacent forests.The Chesapeake Bay project aims to understand marsh migration in response to rising sea levels. This study focuses on how marshes adapt by either growing in elevation or migrating to new areas. Eastern Maryland, with its low slopes, provides an ideal setting for observing this migration.
A key aspect of this project examines the role of herbivores, particularly non-native sika deer, in promoting or inhibiting the spread of invasive plant species that impede marsh migration. The invasive grass Phragmites australis, which can grow up to 10 feet tall, poses a significant threat. Phragmites, an invasive species introduced from Asia and Europe during colonial times, has spread extensively along the U.S. coastlines and inland waters. It reproduces clonally, making it a formidable competitor in marsh ecosystems.
As saltwater intrudes, trees die off, opening the canopy and allowing marsh species to move in. Phragmites, resilient to salinity and flooding, often establish themselves before other marsh plants, complicating this natural migration. It forms dense stands between marshes and forests, outcompeting native marsh plants and preventing their migration deeper into the forests.
Sika deer, although non-native, do not negatively impact the native white-tailed deer population and are economically beneficial due to hunting. They originated from southern Japan, introduced to Maryland in the early 1900s, and have since spread, particularly in Dorchester County.
The research investigates the interactions between Phragmites, sika deer, and the marsh ecosystem. While sika deer significantly reduce plant cover and species diversity, they also eat Phragmites to some extent, complicating its spread. In forested areas, Phragmites has begun to dominate, reducing native plant communities and impacting marshes. The deer’s selective grazing on Phragmites in the forest, however, slows its expansion.
The study also highlights the broader implications of marsh migration for areas like Washington, D.C. As sea levels rise, tidal freshwater wetlands could be threatened by increased salinity, potentially leading to barren mudflats due to a lack of connectivity with marshes. These marshes provide a range of services to the D.C. population including mitigating storms, water purification, and habitats for wildlife.They continue to adapt and provide these benefits despite environmental changes.
Professor Gedan’s research network and the grant proposals she has written with collaborators in the region have allowed GW undergraduates, graduates, and postdoctoral researchers to engage with community members at local field locations and participate in regional and national research networks including LTER and Critical Zone Networks. These projects have led to the presentation of GW’s research at the 2024 Salinity Affecting Lands in Transition (SALT) Conference with the University of Maryland and the University of Delaware, which attracted 185 participants in academia, policy, natural resource management and industry. More importantly, the Gedan Lab’s research continues to contribute to our understanding of the dynamic changes to the coastline resulting from sea level rise and ecosystem changes.