*This research was recently published in the journal Wetland Science and Practice. The full article is available here: PhragmitesGreenhouseWSP
Among invasive, non-native wetland plants in North America, Common reed (Phragmites australis); commonly just called Phragmites is king; forming dense monocultures and crowding out native plants. A variety of this species is native to North America, but the non-native variety has been invading and choking out native vegetation in wetlands all across the country. Nantucket is no exception with large stands of Phragmites seen at Hummock Pond, Long Pond, the Creeks and various smaller wetlands around the island.
Once Phragmites becomes established, it can be very difficult to eradicate, particularly without more impacts to the wetland. Direct, judicious application of herbicide to cut stems can be effective and reduces the amount of herbicide used and minimizes impacts to other native plants but it is also very time and labor intensive. Plants can be dug up and removed although this causes severe destruction of the wetland soil. Phragmites is typically a freshwater plant and some studies and projects have shown a dramatic impact just by applying salt water to the plants. Phragmites can be very variable in its response to increased salinity with some plants seeming to survive ate moderate salinity levels and some dying back immediately upon exposure.
In 2008, NCF initiated a large-scale salt marsh restoration project at our Medouie Creek property with the goal to reestablish salt marsh hydrology to a wetland that had been historically diked by a road (Figure 1). Previous blog posts have talked about what this restoration looked like and it's impacts on native species, including the spotted turtle. One key driver in planning this restoration project was to reduce a large population of Phragmites (~ 3.9 acres) that had invaded the diked, freshwater dominated area of the marsh (Figure 1).
Restoring salt water to Medouie increased salinity in the soil and dramatically decreased the density and health of the Phragmites population over the past seven years. Even though the Phragmites has been diminished, it still covers a large area at Medouie (~2.9 acres) so we decided to conduct an experiment to determine at what level of salinity Phragmites stems specifically growing at Medouie are most impacted. Understanding the response of these local Phragmites plants to different salinity levels in a controlled environment provides us with better understanding of target salinity levels that need to be achieved at Medouie Creek to effectively control and/or drastically impact Phragmites.
Phragmites plants from Medouie were grown in pots at the NCF greenhouse and exposed to one of five different salinity levels (0, 10, 20, 30, or 40 ppt salinity - ocean water is typically between 30-32 ppt). The treatments were continued for two growing season and NCF staff monitored plant height, stem diameter, leaf number and leaf health over that time.
Increased levels of salinity dramatically impacted both the size and health of Phragmites stems with a significant reduction in stem height and leaf number, particularly at salinity levels of 30 ppt and higher. The impacts of high levels of salinity were even more dramatic the second year of growth indicating that salinity effects on Phragmites growth are cumulative over time. Plants exposed to water with a salinity of 30 ppt or higher were much shorter and less robust than the other plants as can be very obviously seen in the above photo.
In the field, at Medouie Creek, salinity levels have been observed between 15-32 ppt (compared to 0-5 ppt before restoration); comparable to the experimental salinity levels that we saw negatively impacting the Phragmites. Currently these salinity levels are variable across the marsh and not consistent. Seven years after opening a culvert under the dike road to increase salt water flow, the observed salinity levels are likely at their maximum and the chance of decreasing the Phragmites population even more is slim. Without additional dramatic increases in soil salinity, further impacts to the current Phragmites population are unlikely.
Salinity alone is not likely to be an effective control strategy unless the entire Phragmites population can be consistently exposed to adequate, increased salinity levels. At sites like Medouie Creek where the Phragmites population extends across a natural gradient of soil salinity levels, there will likely always remain a portion of the marsh favorable to Phragmites. Therefore, further management at Medouie Creek to control the Phragmites population could include opening up additional tidal access creeks to increase salinity throughout the marsh as well as targeted herbicide treatments to decrease and eliminate Phragmites located at sites exposed to lower, more tolerable salinity levels.
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