News and ResourcesPhragmites Invasion; Detrimental or Beneficial - February 26, 2017
Maryland’s wetlands are under attack; not from an animal or human activity but from a single, aggressive grass species. Phragmites australis is an invasive species of grass in Maryland that has spread across the state; from the Coastal Bays behind Ocean City and Assateague to the tidal creeks of the Chesapeake Bay. Phragmites has rapidly changed many of our valuable wetlands from areas of high productivity and biodiversity to areas dominated by a single plant species with questionable productivity. With the damage to our wetlands from sea level rise, land development, and pollution, the stress added to these ecosystems by Phragmites infestation is an additional problem wetlands must contend with.
Phragmites is an aggressive grass species that targets disturbed wetlands that have been altered from their original state by natural or anthropogenic causes. After a disturbance event, such as a hurricane or nor’easter, many wetlands are altered significantly by having soil washed away or having trees and plants torn from the wetland itself. This creates an ideal situation for Phragmites to establish itself in the disturbed wetland system. This can happen by seeds landing on the disturbed area or even living sections of root washing onto the wetland and creating new plants. Once a small population is established, the plant will reproduce via wind seed dispersal or more commonly, by extending its rhizome, which is a horizontally growing root that grows laterally and has adventitious roots. These can grow into a new area of the wetland and create new plants off the extended rhizome. A major problem with Phragmites is that it will continue to expand throughout the wetland until it is a monoculture of Phragmites. Monocultures decrease the overall health of wetlands by reducing biodiversity, changing wetland hydrology and nutrient cycles, increasing fire risk, and decreasing productivity compared to mixed vegetation wetlands.
There are two main subspecies of Phragmites in Maryland; an invasive subspecies and a native subspecies. The invasive subspecies was brought over accidently from Eurasia in the 1800s. Both subspecies grow to a maximum height of 15 feet and grow in clusters. The invasive subspecies grows in denser clusters than the native, which can kill surrounding plants by blocking sunlight and access to nutrients. The native subspecies lives in saturated to occasionally flooded soils; however, the introduced subspecies can thrive in permanently flooded soils that the native cannot. The native is also more inclined to live in a mixed-vegetation wetland while the invasive creates a monoculture, an area in which only one plant species thrives. The invasive subspecies has a greater photosynthetic rate and a higher nitrogen uptake rate, which combines to make it a better competitor than the native. The invasive subspecies requires almost four times more nitrogen to outcompete the native species; however, due to excess nutrients in our waterways this is not a hindrance.
Due to the invasive nature of Phragmites, there have been numerous efforts to rid our wetlands of this plant; however, none of them have been very effective. The most common forms of removal and control are herbicides, mowing, draining, burning, and grazing. Consistent treatment is more effective than single applications. Plants can readily regrow from their rhizomes or latent seeds in marsh soil and the population sky rockets again due to disturbance in the wetland caused by the various extermination methods. Consistent treatment over a number of years has proven effective in some cases, especially in small areas. So perseverance is necessary.
New research indicates that Phragmites might have a hidden benefit for wetlands. The research done by the Smithsonian Environmental Research Center (SERC) has found that the invasive subspecies has high carbon storage capabilities. In conditions with high nitrogen availability, Phragmites absorbed three times more carbon than native plants. The excess of nitrogen in our waterways allows the invasive subspecies to grow for a longer period of time and create more chlorophyll; the green pigment necessary for plants to photosynthesize. This combination allows for the storage of large amounts of carbon in Phragmites which means there is less carbon in the atmosphere. Excess carbon in the atmosphere increases sea level rise and creates more frequent and intense storms. Invasive Phragmites also builds up more soil belowground compared to the native subspecies. This will help protect wetlands from sea- level rise and erosion.
As we continue our battle with Phragmites, it is imperative to consider what is more important; a diverse species of wetland plants or a wetland that helps combat sea-level rise. In the future compromises may be possible, where invasive Phragmites is allowed to remain in wetlands that are more vulnerable to sea-level rise. Further studies may be necessary to determine whether the benefits of invasive Phragmites outweigh the costs.
Phillips is the Program Manager for the Maryland Coastal Bays Program. Jackson is the former Education Coordinator for the Maryland Coastal Bays Program and current graduate student at Clemson University.
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