Tidal wetlands mitigation involving Phragmites control and colonization from the remnant seed bank

The recolonization enhancement approach for tidal wetland mitigation at a site on the Raritan River in Edison, New Jersey followed a four-step process. The first step involved selection of an appropriate mitigation site with a monoculture of Phragmites australis. Second, a boundary ditch was constructed to form a hydrologic barrier to inhibit the spread of Phragmites rhizomes into the mitigation area. Next, a Phragmites control program was implemented that combines the use of Rodeo^® (glyphosate) herbicide and cutting/removal of dead Phragmites stalks. These activities resulted in an exposed, non-vegetated surface across the mitigation area at the outset of the project. Finally, a monitoring program was initiated to document recolonization from the remnant tidal marsh seed bank. Annual transect/quadrat monitoring results from 1998 through 2004 document the recolonization from bare ground in 1998 to a diverse tidal marsh community with more than 17 indigenous plant species in 2004. Areas of bare ground remaining within the mitigation area provide mud flat habitat. Annual application of Rodeo^® in successively decreasing amounts has controlled Phragmites to less than 2 percent of vegetative cover. Successional species such as Pluchea purpurascens and Salicornia europaea, observed in the early stages of recolonization, were absent by 2001 as increasing numbers of low-marsh and high-marsh species present in the remnant seed bank colonized the area. This recolonization approach for enhancement of tidal wetlands yielded large cost savings (compared to traditional mitigation) because it eliminated the need to remove large quantities of marsh soil to control Phragmites and the need for plantings. It also allowed a natural regrowth and succession of tidal marsh vegetation that will be sustainable over the long term as a result of natural competition.