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 SpreadDISCUSSIONThe high RGR <strong>of</strong> seedlings on mudflats corresponds tosimilar findings by Dethier and Hacker (2005).Transplanting seedlings clearly reduced survivorship andRGR <strong>of</strong> Spartina seedlings on <strong>the</strong> mudflat where <strong>the</strong>re wasno competition from native vegetation (Fig. 1). Dislodging<strong>the</strong> roots during transplantation in <strong>the</strong> mudflat madeseedlings vulnerable to uprooting via tidal disturbance. Tidaluprooting can be an important source <strong>of</strong> S. anglica seedlingmortality in open mudflats (Groenendijk 1986). In this studyonly transplanted seedlings in <strong>the</strong> mudflat experiencedmortality that could be attributed to tidal disturbance.Seedling survival did not differ between competitiontreatments in <strong>the</strong> Salicornia zone, but was greatly reducedby full competition in <strong>the</strong> Distichlis zone. Reduced survivalin <strong>the</strong> Distichlis zone appeared to be due to competition withnative vegetation as well as rodent herbivory. The apparentasymmetric affect <strong>of</strong> Salicornia and Distichlis on Spartinaseedlings may be <strong>the</strong> result <strong>of</strong> interacting substrateconditions in <strong>the</strong> lower intertidal zones that are favorable toSpartina growth.Although Spartina survival in <strong>the</strong> Salicornia treatmentswas not different, Spartina tiller production and totalbiomass were reduced in <strong>the</strong> presence <strong>of</strong> Salicornia(Hellquist 2005). Decreased survival and RGR by Spartinaanglica when grown in competition with native vegetation(e.g. Distichlis) is consistent with previous studies <strong>of</strong>Spartina. In greenhouse and field studies, Puccinelliamaritima has been shown to have a competitive effect on S.anglica (Scholten and Rozema 1990; Thompson et al. 1993;Huckle et al. 2000). Competitive suppression by neighboringvegetation also has been documented for S. alterniflora in<strong>the</strong> nor<strong>the</strong>astern United States (Bertness 1991b) and S.maritima in Spain (Castellanos et al. 1994)Spartina anglica colonizes a variety <strong>of</strong> habitatsincluding mudflats, low and high salinity salt marshes, aswell as cobble and gravel beaches in Puget Sound (Hacker etal. 2001). At Alice Bay, a mudflat with abundant Spartinaadjoins a protected salt marsh that is truncated by a dike.Spartina is a sporadic colonizer among Salicornia, but wasabsent from <strong>the</strong> virtual monospecific stands <strong>of</strong> Distichlisspicata higher along <strong>the</strong> intertidal gradient. Our experimentshows that at Alice Bay, in <strong>the</strong> absence <strong>of</strong> competitors,Spartina is capable <strong>of</strong> surviving along <strong>the</strong> entire salt marshgradient. In high salinity salt marshes Spartina may berestricted to lower intertidal zones such as <strong>the</strong> Salicorniazone due to interspecific competition and a lack <strong>of</strong> availableopen substrate (Hacker et al. 2001). Although not testedhere, propagule pressure also can play a role in speciesinvasions (e.g. Von Holle and Simberl<strong>of</strong>f 2005) and mayinfluence <strong>the</strong> distribution <strong>of</strong> Spartina across intertidal zones.Although <strong>the</strong>se data and additional growth parameterdata (Hellquist 2005) indicate that biotic factors play a rolein <strong>the</strong> distribution <strong>of</strong> Spartina in high salinity marshes(defined as > 29 g kg -1 by Hacker et al. 2001) <strong>the</strong> importance<strong>of</strong> abiotic factors such as tidal uprooting (Groenendijk 1986)and soil physical characteristics also contribute to Spartinacolonization (Hacker et al. 2001; Dethier and Hacker 2005;Hellquist 2005). Physical factors have been shown to beimportant in controlling seed germination <strong>of</strong> S. anglica inPuget Sound (Dethier and Hacker 2005). Physical factorsmay play a more important role during seed germination andinitial establishment (Dethier and Hacker 2005), but bioticinteractions may become more important during seedlingmaturation (Hellquist 2005)An understanding <strong>of</strong> <strong>the</strong> physical and biotic factorsresponsible for plant zonation in salt marshes is necessary tobetter identify anthropogenic pressures that threaten saltmarshes and to establish effective restoration programs(Bertness and Pennings 2000). These data as well as Dethierand Hacker (2005) and Hellquist (2005), suggest thatremoval <strong>of</strong> Spartina in <strong>the</strong> mudflats will be an effective wayto slow <strong>the</strong> spread <strong>of</strong> Spartina by controlling fast-growingpopulations that can serve as seed sources. Successfulcontrol in high salinity marshes is probably aided by slowerinvasion rates mediated by biotic and physical factors(Hacker et al. 2001; Hellquist 2005). Knowledge <strong>of</strong> Spartinagrowth patterns, <strong>the</strong> influence <strong>of</strong> physical factors, and <strong>the</strong>importance <strong>of</strong> biotic interactions will provide a valuablecontext to prioritize sites for Spartina control (Dethier andHacker 2005; Hellquist 2005).ACKNOWLEDGMENTSWe are very grateful for <strong>the</strong> support <strong>of</strong> <strong>the</strong> GraduateStudent Research Fellowship (NA07OR0262) from <strong>the</strong>Estuarine Reserves Division, Office <strong>of</strong> Ocean and CoastalResource Management, National Ocean Service, NationalOceanic and Atmospheric Administration, Padilla BayNational Estuarine Research Reserve (NERR), Mt. Vernon,Washington to CEH. Terry Stevens, Doug Bulthuis, andSharon Riggs (Padilla Bay NERR) provided essentialsupport for this research. Additional support was providedby <strong>the</strong> United States Environmental Protection AgencyScience to Achieve Results (EPA STAR) Graduate StudentFellowship (U-91616801: 2003-05). The Betty W.Higinbotham Trust <strong>of</strong> <strong>the</strong> Washington State UniversitySchool <strong>of</strong> Biological Sciences provided critical researchfunding as well. Richard Alldredge and Paul Rabie providedstatistical advice. Denise Howe, Sven Nelson, Ben Rhodesand Justin Snider assisted with sample processing. Thecooperation <strong>of</strong> Kyle Murphy <strong>of</strong> <strong>the</strong> Washington StateDepartment <strong>of</strong> Agriculture and Bill Rogers <strong>of</strong> <strong>the</strong> SkagitCounty Noxious Weed Control Board also are appreciated.Thanks also to <strong>the</strong> reviewers for <strong>the</strong>ir suggestions to improve<strong>the</strong> manuscript.- 101 -