Factors contributing to the removal of a marine grass invader (Spartina anglica) and subsequent potential for habitat restoration

Abstract

Our goal is to understand how removal regime and habitat type interact to influence removal success of a marine plant invader and the subsequent potential for restoration. In particular, we investigate the management program designed to eradicate the English cordgrass,Spartina anglica C. E. Hubbard, in marine intertidal habitats of Puget Sound, Washington, United States. Observational and manipulative experiments were used to measure the regrowth (vegetative growth), reinvasion (seedling recruitment), and restoration potential (return to native condition) of invaded habitats. Removal regime (consistent: yearly removal; interrupted: yearly removal with the last year missed) and habitat type (low salinity marsh, mudflat, cobble beach, and high salinity marsh sites) were considered. The response to removal regime was dramatic. Under consistent removal, cordgrass slowly declined but under interrupted removal, there was substantial regrowth of the invader. This pattern results from the resiliency of belowground biomass and the subsequent high aboveground productivity and seedling growth ofS. anglica. We also found that removal success depended on differences among sites that represent different habitat types. Cordgrass regrowth and reinvasion were substantially higher in the low salinity marsh sites where soils have lower salinity. We also found that at the low salinity marsh sites, some restoration of native plants and soil conditions was evident. At mudflat, cobble beach, and high salinity marsh sites, colonization of native vascular plants and algae not normally present, in the absence of the invasion, occurred. Whether these habitats will eventually revert back to the pre-invasion conditions over a longer period of time is unknown.