Relationships of native and exotic strains of Phragmites australis to wetland ecosystem properties

Background/Question/Methods

Wetlands are important ecosystems because they provide habitat, allow for flood abatement, and function as ecosystem filters, improving water quality and cycling nutrients.  Wetland degradation, loss, and/or the introduction of exotic species such as Phragmites australis can decrease their ability to perform valuable ecosystem services.  Historically, native Phragmites was found in North American high marshes of coastal and inland heterogeneous plant communities, but by the 1970s, extensive Phragmites monocultures were present in all lower 48 US states, leading to suspicions of an exotic Phragmites strain. In 2002, exotic and native Phragmites strains were genetically identified.  Despite this development however, there are currently few studies that document how each Phragmites strain (native or exotic) impact wetland ecosystems.  The goal of this study was to compare relationships of native and exotic Phragmites strains to environmental attributes, soil nutrient pools, and microbially-mediated denitrification in the soil.  We selected three sites with native P. australis, three sites with exotic P. australis, and we paired them with control sites that lacked P. australis. At each site, we quantified Phragmitesstem density, measured environmental attributes (soil moisture, soil pH, soil temperature, water level), and quantified soil nutrient pools (nitrate, ammonia, phosphate, soil organic matter) and denitrification.

Results/Conclusions

Data were analyzed using two approaches: 1) Comparison of sites with and without either strain using t-tests, and 2) Correlations of environmental attributes, soil nutrient pools, and microbially-mediated denitrification with Phragmites stem density of either strain. The presence of Phragmites (regardless of strain) was not associated with any changes in environmental attributes, soil nutrients, or denitrification relative to the control sites.  The exception was soil pH, which was higher at sites with the exotic strain than at sites with the native strain.  In addition, correlation analyses revealed no significant relationships between the exotic strain stem density and any of the variables measured. However, increasing native P. australis stem density correlated significantly with increased soil ammonium, SOM, and denitrification, and marginally so with increased phosphate.  In contrast, temperature and water level showed a significant negative correlation with increasing native P. australis stem density.  Contrary to our expectations, results suggest that the native Phragmites strain was the one that correlated significantly with environmental attributes, soil nutrient pools, and denitrification. Future experimental studies should help differentiate if correlations found in this study are a consequence of the particular Phrgamites australis strain (exotic or native).