Trekking through a Phragmites dominated area in one of our project sites.
Phragmites australis, everyone’s favorite invasive species and found on almost all wetland restoration projects. Recovery of Phragmites dominated areas, requires knowing what treatment options are available, how the selected treatment will impact recovery, and the likelihood of success.
Our Associate Ecologist, Zak Lehmann, shares new research, trends, and developments surrounding Phragmites removal and recovery presented at the 2015 Society for Wetland Scientists Annual Meeting.
We know Phragmites is more productive both above and belowground – crushing native plant communities. This increased productivity means they have a higher nitrogen demand. However, they are able to thrive in nutrient-limited ecosystems – how? Justin Meschter of Northeastern University answered this question in the session, Deeply Rooting Phragmites Australis Utilizes Deep N Pools to Circumvent Competition with Native Species and Meet Higher Aboveground Nitrogen Demands.
- The study found that Phragmites has a high nitrogen (N) content when compared with other saltmarsh plants.
- Most of the nitrogen is found in the leaves, not the stem or the roots.
- As compared to native species, the deep root biomass of Phragmites allows the plant to access deep nitrogen pools.
- Found the highest nitrogen uptake at 40cm highest (in roots) all the way down to 80 cm (over 2.5 feet) below the surface.
Installation of test plots to assess Phragmites root depth at one of our project sites.
What happens to land after removing Phragmites and how easily can the land recover itself? Eric Hazelton of Utah State University presented 5-years of monitoring data after Phragmites removal from study site in Chesapeake Bay. Key findings presented in the session, Tidal Phragmites Marshes Have Diverse Native Seed Banks: What is Limiting Recruitment After the Invader is Removed:
- Diverse native population still present in seed bank of soil samples grown in lab.
- The amount of time Phragmites dominated the land determines the ability of the land to recover to its native habitat.
- Areas that had been dominated for less time were more likely to recover than areas that had stayed a mono-culture for prolonged periods of time.
- This is likely due to changes in elevation and hydrology due to Phragmites root mats.
What this means for restoration projects: Gradual elimination of these types of habitats may be better at promoting native recovery instead of a single, holistic elimination.
- While this is not a new idea, it’s important for practitioners as the study provides empirical evidence to back it up.
- Many restoration projects artificially recover the sites through planting native species, so although it’s important to understand the science of the seed back, recovery is artificially induced.
Another study detailed the effects of various removal treatments used along the Great Salt Lake. The study applied removal treatments at two scales; smaller 1-acre plots, newer Phragmites invasion, and 3-acre plots of older well-established invasives.
Removal treatments applied to both scales (1-4):
- Untreated control
- Fall glyphosate spray, winter mow
- Summer imazapyr spray, winter mow
- Summer glyphosate spray, winter mow
- Fall imazapyr spray, winter mow (only used on the large scale plots)
Used on the smaller test plots (plus treatments 1-4 above)
- Summer mow, fall glyphosate spray
- Summer mow, then black plastic solarization
- The project size (1 or 3 acres) was unrelated to the effectiveness of the removal treatments
- The black plastic solar treatment (only used on the small plots) was not effective.
- (Treatment is as it sounds: cover an area with black plastic which shades the Phragmites seeds and rhyzomes as well as prevents additional seeding from neighboring plants, preventing it from growing back.)
- The time of herbicide application should be during the summer before seeds develop. This allows for the best natural recovery (assuming multiple herbicide applications over more than 1 year).
- Native plants can recover within one year if seedbank is still present. (Found on both scales.)
Study presented by Christine Rohal, University of Utah. Session: Phragmites Management at Multiple Scales: Method Comparisons on the Great Salt Lake.
An evaluation of the genet richness (a measure of genetic diversity) of Phragmites found in the Great Salt Lake and Chesapeake Bay wetlands found that:
- Salinity thresholds of Phragmites determine the limit of clonal expansion more than other factors, such as inundation or shading.
- Threshold is 30 ppt in porewater – which is about the same as saltwater.
- A herbicide application test found that more concentrated herbicide solutions are not more effective (in removal) than the recommended concentrations.
- The data also indicated that non-native clones of Phragmites (the Euro-invasive genotypes) spreads more by seed than other methods
Study presented by Karin Kettenring, Utah State University. Session: Patterns of Phragmites Clonal Diversity Across Space, Time and Management Regimes.
When selecting removal methods, it’s important to know and understand which techniques will promote natural recruitment. A more general discussion tied together ideas discussed above.
- The biggest issue preventing natural recovery is the litter created by Phragmites during removal.
- This is why burning appears more attractive as a removal technique but is quickly becoming harder to do (especially in urbanized areas.)
- A good hydrologic regime is critical for natural recovery
- Managing water levels is important for promoting native plant species and discouraging Phragmites.
- By manipulating inundation time and salinity we can help restrict recolonization and promote native species growth.
- Herbicide face-off: In a study in New Jersey, Imazapyr performed better than Glyphosate. An interesting result as glyphosate is used as the preferred industry standard herbicide.
- Flail mowers typically are a much more successful mechanical removal technique (as compared to other mowers, discers, or manual removal.) The goal here is to mulch (destroy) as much of the potential litter material as possible.
We’ve shared only a small fraction of the ideas, research, and developments presented during the conference which covered much more than Phragmites. View the full programand abstracts to learn more.