Proceedings of the Third International Conference on Invasive ...

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 2: Spartina Distribution and SpreadTHE INFLUENCE OF INTERTIDAL ZONE AND NATIVE VEGETATION ON THE SURVIVALAND GROWTH OF SPARTINA ANGLICA IN NORTHERN PUGET SOUND, WA, USAC. E. HELLQUIST 1 AND R. A. BLACKSchool <strong>of</strong> Biological Sciences, Washington State University, Pullman WA, 991641 Current address: Department <strong>of</strong> Biological Sciences, State University <strong>of</strong> New York, Oswego, NY 13126;eric.hellquist@oswego.eduSpartina anglica colonizes mudflats, tidal channels, salt marshes, and gravel beaches in nor<strong>the</strong>rnPuget Sound, Washington, USA. We measured Spartina seedling survival and growth along anintertidal gradient from mudflat to salt marsh at Alice Bay, Skagit County, Washington. Spartinaseedlings were transplanted into plots with full and reduced competition in open mudflat, a lowmarsh intertidal zone dominated by Salicornia virginica, and a middle marsh zone dominated byDistichlis spicata. In <strong>the</strong> mudflat, where little native vegetation was present, two separate treatmentswere monitored: unmanipulated (in situ) seedlings and seedlings transplanted within <strong>the</strong> mudflat.The in situ seedlings had 100% survival whereas seedlings transplanted within <strong>the</strong> mudflat had 60%survival. The mean relative growth rate (RGR) <strong>of</strong> in situ seedlings in <strong>the</strong> mudflat was 16 milligramsper milligram per day (mg mg -1 day -1 ) compared to RGR <strong>of</strong> 8 mg mg -1 day -1 for <strong>the</strong> transplantedseedlings. In <strong>the</strong> Salicornia virginica zone, Spartina survival was 94% and RGR averaged 13 mgmg -1 day -1 with reduced competition. Spartina seedling survival was 90% and mean RGR was 11 mgmg -1 day -1 when grown in full competition with Salicornia. In <strong>the</strong> Distichlis zone, Spartina survivalwas 57% and <strong>the</strong> RGR was 5 mg mg -1 day -1 without competition. Spartina seedling survivorship waszero when grown in competition with Distichlis. In general, Spartina survival and RGR were highestat lower intertidal elevations and with reduced competition. The spread <strong>of</strong> S. anglica into upperintertidal levels appears limited by competition as well as physical conditions along <strong>the</strong> intertidalgradient.Keywords: competition, Distichlis spicata, Salicornia virginica, Spartina anglica, zonationINTRODUCTIONVegetation zonation in salt marshes is determined byboth competitive (Bertness 1991a; Pennings and Callaway1992) and facilitative (Bertness and Shumway 1993,Bertness and Hacker 1994; Callaway and Pennings 2000)biotic interactions as well as physical conditions includingsoil waterlogging, oxygen availability (Howes et al. 1981),sulfide toxicity (King et al. 1982) and porewater salinity(Callaway et al. 1990, Pennings and Callaway 1992). Innorthwestern Washington, salt marsh communities <strong>of</strong>tenform a fringe <strong>of</strong> vegetation along <strong>the</strong> upper edges <strong>of</strong>mudflats. These sites have high porewater salinities (>30grams per kilogram [g kg -1 ) and are typically isolated fromriverine freshwater. The lowest intertidal zones in <strong>the</strong>semarshes are typically colonized by mats <strong>of</strong> Salicorniavirginica with intermittent patches <strong>of</strong> Spergulariacanadensis, Triglochin maritimum, and Puccinelliamaritima. Frequently, individuals <strong>of</strong> <strong>the</strong>se species willcolonize <strong>the</strong> open mudflat several meters <strong>of</strong>f <strong>the</strong> seawardedge <strong>of</strong> contiguous salt marsh.The introduction <strong>of</strong> Spartina anglica into northwesternWashington has altered <strong>the</strong> zonation <strong>of</strong> salt marsh vegetationby creating a community <strong>of</strong> emergent vegetation below <strong>the</strong>Salicornia virginica zone on sediment that was previouslyopen mudflat. Throughout nor<strong>the</strong>rn Puget Sound, Spartinacan form dense stands in areas that were formerly open mudflats (e.g. Triangle Cove and Livingston Bay, IslandCounty). However, Spartina is only found sporadicallygrowing among native plants in higher intertidal zones (e.g.at Maylor Marsh and English Boom, Island County).Based on <strong>the</strong> tendency <strong>of</strong> Spartina to colonize openmudflats as opposed to closed mats <strong>of</strong> native vegetation, wehypo<strong>the</strong>sized that Spartina is an inferior competitor and thatsurvival and growth rates would be highest in <strong>the</strong> lowestintertidal zones and in experimental plots where competitionfrom native plants was limited.MATERIALS AND METHODSWe conducted a seedling transplant experiment in fourlocations at Alice Bay (48º 33’ 20” N, 122º 29’11” W;Skagit County). A series <strong>of</strong> 10 random plots were placedalong 30-meter (m) transects in <strong>the</strong> mudflat. We alsomonitored <strong>the</strong> growth <strong>of</strong> 10 naturally occurring,unmanipulated (in situ) seedlings in <strong>the</strong> mudflat to controlfor <strong>the</strong> effects <strong>of</strong> transplanting. Ten paired plots were-99-