Ecological Applications, 17(7), 2007, pp. 1886–1898 Ó 2007 by the Ecological Society of America NITROGEN INPUTS PROMOTE THE SPREAD OF AN INVASIVE MARSH GRASS ANNA CHRISTINA TYLER, 1 JOHN G. LAMBRINOS, 2 AND EDWIN D. GROSHOLZ Department of Environmental Science and Policy, University of California, One Shields Avenue, Davis, California 95616 USA Abstract. Excess nutrient loading and large-scale invasion by nonnatives are two of the most pervasive and damaging threats to the biotic and economic integrity of our estuaries. Individually, these are potent forces, but it is important to consider their interactive impacts as well. In this study we investigated the potential limitation of a nonnative intertidal grass, Spartina alterniflora, by nitrogen (N) in estuaries of the western United States. Nitrogen fertilization experiments were conducted in three mud-flat habitats invaded by S. alterniflora in Willapa Bay, Washington, USA, that differed in sediment N. We carried out parallel experiments in San Francisco Bay, California, USA, in three habitats invaded by hybrid Spartina (S. alterniflora 3 S. foliosa), in previously unvegetated mud flat, and in native S. foliosa or Salicornia virginica marshes. We found similar aboveground biomass and growth rates between habitats and estuaries, but end-of-season belowground biomass was nearly five times greater in San Francisco Bay than in Willapa Bay. In Willapa Bay, aboveground biomass was significantly correlated with sediment N content. Addition of N significantly increased aboveground biomass, stem density, and the rate of spread into uninvaded habitat (as new stems per day) in virtually all habitats in both estuaries. Belowground biomass increased in Willapa Bay only, suggesting that belowground biomass is not N limited in San Francisco Bay due to species differences, N availability, or a latitudinal difference in the response of Spartina to N additions. The relative impact of added N was greater in Willapa Bay, the estuary with lower N inputs from the watershed, than in San Francisco Bay, a highly eutrophic estuary. Nitrogen fertilization also altered the competitive interaction between hybrid Spartina and Salicornia virginica in San Francisco Bay by increasing the density and biomass of the invader and decreasing the density of the native. There was no significant effect of N on the native, Spartina foliosa. Our results indicate that excess N loading to these ecosystems enhances the vulnerability of intertidal habitats to rapid invasion by nonnative Spartina sp. Key words: estuary; eutrophication; invasive species; nitrogen; salt marsh; Spartina. INTRODUCTION Invasion by nonnative species is one of the most serious threats to the integrity of natural systems and the maintenance of ecosystem services (Drake et al. 1989, Vitousek et al. 1997b). Invasions can cause the extinction of native species, changes in nutrient cycling and storage, and loss of habitat. Wetlands are particu- larly vulnerable to invasion by monotype-forming plants (Zedler and Kercher 2004) and this susceptibility appears to increase under disturbances (Bertness et al. 2002, Silliman and Bertness 2004, Zedler and Kercher 2005). To gain a better predictive understanding of the vulnerability of an ecosystem to invasion, it is important to understand how variability among recipient regions may alter the potential for invasion and spread of nonnative species (Carlton 1996), because in some cases the abiotic characteristics of the local environment may contribute to the success of an invader, even at small scales (Dethier and Hacker 2005). As a result of human development, nitrogen (N) input from watersheds to estuaries has increased to the extent that eutrophication of the coastal zone is one of our most pressing environmental concerns (National Re- search Council 1993, Vitousek et al. 1997a, Howarth et al. 2000, Cloern 2001). Also, nutrient loading may facilitate species invasions (Dukes and Mooney 1999). Invaders that can capitalize on the increased availability of N in disturbed systems may outcompete native species adapted to previously low-nutrient conditions (Vitousek et al. 1997a). Changes in N inputs appear to promote the invasion of nonnative plants in a variety of ecosystems, including, for example, ombrotrophic bogs (Tomassen et al. 2004), grasslands (Olson and Blicker 2003), deserts (Brooks 2003), tropical forests (Ostertag and Verville 2002), freshwater wetlands (Green and Galatowitsch Manuscript received 17 May 2006; revised 1 March 2007; accepted 6 March 2007; final version received 16 April 2007. Corresponding Editor: B. A. Hungate. 1 Present address: Department of Biological Sciences, College of Science, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, New York 14623-5603 USA. E-mail: actsbi@rit.edu 2 Present address: Department of Horticulture, Oregon State University, Corvallis, Oregon, USA. 1886