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Sarah Gallagher - Plant Ecology

University of Mary Washington, Virginia

The effect of small disturbances on the germination and emergence of invasive Phragmites australis in four brackish wetland plant communities

Phragmites australis is rapidly invading brackish wetlands along the Atlantic coast including the Chesapeake Bay.  Its invasion has been tied to declines in native floral and faunal diversity and alterations to marsh nutrient cycling.  In this study, our objective was to understand which native plant communities Phragmites is likely to invade and if the presence of small disturbances will increase the likelihood of an invasion within these plant communities.  We also wanted to know if Phragmites seeds or rhizomes were the better colonizer in these native plant communities and disturbances.  We looked at Phragmites seed germination and rhizome emergence in artificial disturbances in four native plant communities (Iva frutescens, Schoenoplectus americanus, Spartina patens/Distichlis spicata, and Typha angustifolia).  Seeds and rhizomes were planted in one of three disturbance treatments (40 cm diameter subplots) within each plant community: a control, an above ground disturbance (vegetation and litter removed), or a below ground disturbance (vegetation removed and soil turned over). We found that seed germination increased significantly in the high marsh plant communities (Iva, Schoenoplectus, Spartina/Distichlis) with an increase in disturbance severity.  After taking environmental characteristics data (relative seed and rhizome temperature, relative light level, salinity, and tidal flooding data), we attributed this to significantly higher light levels and temperatures as well as less flooding.  There was very little Phragmites seed germination in the low marsh plant community (Typha); this appeared to be due to low light levels, low temperatures, and increased flooding.  Rhizome emergence was consistently low (8.6%) across all plant communities and disturbance treatments.  Based on our findings, we predicted that Phragmites is most likely to invade and establish in high marsh communities that have been disturbed but the success of these seedlings to maturity requires further study.  We also conducted a survey of naturally occurring disturbances in these four native plant communities.  This survey showed that the majority of the disturbances are caused by deer, muskrats, and humans.  These disturbances appeared to have similar environmental characteristics as those we artificially created.

Funding provided by the National Science Foundation – Research Experience for Undergraduates