Science Serving Maryland's Coasts

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Phragmites Australis Invasion in the Chesapeake Bay: Implications of Nitrogen Pollution, Elevated CO2, and Genotypic Variation for Tidal Marsh Management

Principal Investigator: 

Patrick Megonigal

Start/End Year: 

2012 to 2014

Institution: 

Smithsonian Environmental Research Center

Co-Principal investigator: 

Thomas J. Mozdzer, Melissa K. McCormick, Smithsonian Environmental Research Center

Topic(s): 

Strategic focus area: 

Regional effects of climate change and sea level rise

Description: 

OBJECTIVES Our goal is to forecast how Phragmites invasion rates will respond to two fundamental global change factors, elevated CO2 and N pollution. The results of our work will be directly applicable to land managers currently dealing with Phragmites invasion and the resulting displacement of native plant communities. We expect the proposed research to generate clear insights about how Phragmites invasions will respond in the future to elevated CO2, N pollution, and sea level rise, the consequences of such invasions for key tidal wetland ecosystem services, and insights into controlling Phragmites invasion in the future.

METHODOLOGY In an ecosystem-level field experiment, we will expose both Phragmites and the native plant community it is invading to predicted changes in CO2 and N, thereby simulating present and future environmental conditions. Elevated CO2 will be achieved with open top chambers (OTC). Plant communities will be assigned to four treatments: (1) ambient CO2, (2) ambient CO2+N (+25 g N m-2 yr-1), (3) elevated CO2 (ambient+340 ppm), and (4) elevated CO2 + N (n=3 per treatment). A novel feature of the study is that the chambers are designed to measure the rate of Phragmites invasion into native marsh as the primary response variable. To evaluate ecosystem level responses, we will measure invasion progress, genotypic diversity, ecophysiological responses, and nutrient availability. Surface Elevation Tables (SETs) will provide soil elevation data.

RATIONALE Our preliminary greenhouse data demonstrate that Phragmites growth will likely increase with rising concentrations of CO2 and N. However, it is unclear whether enhanced Phragmites growth or the invasion process will be affected in real, intact marsh ecosystems field where elevated CO2 has been demonstrated to decrease N availability. Thus, in the field, elevated CO2 may actually help limit future invasions driven by N eutrophication. In addition, intraspecific genetic diversity that increase niche breadth may further increase the probability and/or rate of invasion. Our work is highly relevant to the Chesapeake Bay where Phragmites invasions threaten to alter carbon and energy exports to the estuary, and alter habitat for aquatic species of significant economic interest. Given our preliminary data, there is hope that if N loading can be limited, it may be possible to limit future invasions in Chesapeake Bay, especially in combination with elevated CO2.

Impact/Outcome: 

This section describes how this project has advanced scientific knowledge and made a difference for coastal residents, communities, and environments. Maryland Sea Grant has reported these details to the National Oceanic and Atmospheric Administration (NOAA), one of our funding sponsors.

Summary: Researchers completed the first multi-factor field study indicating that elevated levels of nitrogen and carbon dioxide in Chesapeake Bay marshes directly facilitate increased rates of invasion of Phragmites, the common reed. The findings indicate that land use managers may limit future Phragmites expansion by limiting nitrogen availability. In 2015, the researchers presented these results at a four-day Phragmites symposium they organized at the Society of Wetland Scientists meeting.

Relevance: In the Chesapeake Bay, invasions of a non-native type of Phragmites australis have altered native marsh habitat. The researchers studied how rates of Phragmites growth and invasion in intact marsh ecosystems would be affected by likely future increases in atmospheric carbon dioxide (CO2) and elevated levels of nitrogen (N) caused by human activities. In Chesapeake Bay, Phragmites invasions threaten to alter habitat for aquatic species like the mummichog (Fundulus heteroclitis), an abundant fish on Bay marshes and a food source that supports many commercially important fisheries.

Response: Maryland Sea Grant supported research by principal investigators Patrick Megonigal and Melissa McCormick of the Smithsonian Environmental Research Center and Thomas Mozdzer of Bryn Mawr College. In a field study, they exposed tidal marsh plots to combinations of carbon dioxide gas and nitrogen-rich flood water. Each plot straddled a non-native Phragmites plant community and the native plant community it was invading. The primary response variable of interest was the rate at which non-native Phragmites invaded.

Results: The researchers found that N, CO2, and N+CO2 clearly increased Phragmites invasion rates in marsh habitats. This suggests that future increases in N and CO2 may make non-native Phragmites more capable of displacing native marsh habitat. However, the research team also found that in the low nutrient marsh under ambient conditions, Phragmites density and biomass did not change significantly. This suggests that altering management practices to limit N and/or CO2 may slow future Phragmites invasions. These data can also be used to help prioritize which marshes can and should be managed. The scientists have advised the U.S. Fish and Wildlife Service's (USFWS) Region 5 (Northeast) about applying these findings to improve management of Phragmites in National Wildlife Refuge marshes managed by USFWS. The researchers’ data also indicate that ecosystems dominated by Phragmites may gain surface elevation more quickly than those dominated by native plants, which may make the Phragmites-dominated marshes more resilient to the effects of sea level rise. Dr. Mozdzer organized a four-day Phragmites symposium held during the 2015 Society of Wetland Scientists meeting. About 100 participants, who included land managers, policy makers, federal agencies, and scientists, heard 42 talks, seven of which featured research supported by Maryland Sea Grant.

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