Many Atlantic salt marshes have been severely degraded by structures such as roads and dikes that restrict tidal flow. Tidal restriction causes a reduction in salinity and a shift in salt marsh to brackish, freshwater, and even upland plant species (Amsberry et al. 2000; Smith 2007; Smith et al. 2009). Many of these tidally restricted salt marshes are being restored by increasing tidal exchange (Roman and Burdick 2012). However, the restoration of native salt-marsh plant communities can still be limited by the presence of the invasive common reed (Phragmites australis). Persistent stands of this salt-tolerant species, which tends to proliferate in tidally restricted systems, exclude native halophytes by impeding seed dispersal and shading the seed bank (Rand 2000; Minchinton et al. 2006).
The recovery of salt-marsh plant communities is partly dependent upon seed germination, which is influenced primarily by salinity and light availability (Rand 2000; Carter and Ungar 2004; Smith and Warren 2012). Halophyte seeds are dispersed by tides. They may be free-floating or, more commonly, mixed in with dead plant biomass (wrack) that forms large mats. These wrack mats are prevented from dispersing across marsh floodplains when they become trapped by physical barriers, such as Phragmites stands (Smith 2007). Smith (2007) showed that pathways cut into Phragmites zones allowed wrack to advance with the incoming tide, dispersing viable seeds and increasing halophyte establishment in more interior areas.
Cape Cod National Seashore manages several tidal restoration projects on Cape Cod, Massachusetts, USA. One salt marsh undergoing restoration is Hatches Harbor, which was diked for 70 years for mosquito control (Portnoy et al. 2003). The ensuing degradation of this system led to efforts in 1999 to reestablish seawater exchange by installing culverts in the dike (fig. 1). This has resulted in significant expansion of salt-marsh vegetation within the formerly restricted floodplain (table 1). However, 10 years after tidal restoration at Hatches Harbor, Phragmites stands continue to flourish where salinities are still between 10 and 25 parts per trillion (ppt) (Sun et al. 2007; Smith et al. 2009).
The goal of our study was to evaluate whether the establishment of halophytes could be enhanced by manual cutting of Phragmites and to assess relationships among salinity, elevation, and vegetation. Specifically, we evaluated the composition, abundance, and diversity of extant halophyte vegetation and the seed bank within halophyte-dominated areas, Phragmites-dominated areas, and areas where Phragmites was mechanically removed.
Wheeler, J. S., R. K. Thiet, and S. M. Smith. 2013. Enhancing native plant habitat in a restored salt marsh on Cape Cod, Massachusetts. Park Science 29(2):42–48.
Available at http://www.nature.nps.gov/ParkScience/archive/PDF/Article_PDFs/ParkScience29(2)FallWinter2012-2013_42-48_Wheeler_et_al_3639.pdf.