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 4: Spartina Control and ManagementFRAGMENT PROPAGULES OF SPARTINA ALTERNIFLORA AND POTENTIAL EASTERNPACIFIC DISPERSALV. H. MORGAN 1 AND M. SYTSMA 21, 2Portland State University, Dept <strong>of</strong> Environmental Science and Management, Center for Lakes & Reservoirs, P.O. Box 751,Portland, OR 97207-0751;1vhoward@pdx.eduCommonly used mechanical control methods for Spartina alterniflora involve varying levels <strong>of</strong>disturbance to rhizomes and roots. We examined <strong>the</strong> viability <strong>of</strong> rhizome fragments and <strong>the</strong>irpotential role in dispersal. Production <strong>of</strong> rhizome fragments by rototilling in Willapa Bay,Washington, USA was studied. The top 10 centimeters (cm) <strong>of</strong> <strong>the</strong> sediment contained an average <strong>of</strong>310 fragments per square meter (m 2 ). Median rhizome length was 3.7 cm. Eighty-seven percent <strong>of</strong><strong>the</strong> rhizome fragments had at least one vegetative shoot attached. Survivorship <strong>of</strong> S. alterniflorarhizome fragments from Willapa Bay and San Francisco Bay populations was investigated using athree-way factorial design. Treatments included two fragment sizes, approximating those found inWillapa Bay, immersed in ei<strong>the</strong>r freshwater, 15 parts per thousand (ppt) saltwater, or 35 pptsaltwater for 3, 8 or 15 days. Fragments were <strong>the</strong>n individually planted and grown in greenhouseponds for four months. Rhizome survivorship was low (8.6% or less) in all 35 ppt treatments.Survivorship was 37.3% and 87.5% in 15 ppt and freshwater treatments, respectively. Largerhizomes had higher survivorship than small rhizomes at all salinities, and <strong>the</strong> length <strong>of</strong> time <strong>the</strong>rhizome fragments were immersed prior to planting had a variable effect on survivorship. Resultssuggest rototilling for control <strong>of</strong> Spartina may spread <strong>the</strong> infestation within an estuary but isunlikely to result in spread to o<strong>the</strong>r estuaries by ocean transport. Thus, tilling should be used withcaution in estuaries with small, isolated populations <strong>of</strong> Spartina.Although ocean transport <strong>of</strong> rhizome fragments appears to be a small risk, ocean transport <strong>of</strong> wrackand viable S. alterniflora seed is likely. A drift card study was begun in late September 2004 tobetter understand potential dispersal from invaded west coast estuaries. Monthly releases <strong>of</strong> cardsfrom Humboldt and San Francisco bays in California, as well as Willapa Bay, Washington will aididentification <strong>of</strong> wrack deposition sites. Data from <strong>the</strong> first two months <strong>of</strong> this year-long studyindicate that long distance dispersal <strong>of</strong> up to 270 kilometers (km) over a four-week period can occur.Keywords: Spartina alterniflora, rhizome fragment, propagule dispersal, drift card simulationINTRODUCTIONEffective invasive plant management considers potentialvectors <strong>of</strong> propagules as well as how to minimize propaguleproduction. In <strong>the</strong> case <strong>of</strong> Spartina alterniflora, earlydetection and treatment efficacy are high priorities for manystakeholders wanting to preserve historic habitat, indigenousspecies, and o<strong>the</strong>r beneficial uses <strong>of</strong> mudflats and native saltmarshes in <strong>the</strong> Pacific Northwest. Within <strong>the</strong> core infestationsites, thousands <strong>of</strong> hectares have already been colonizedincluding an estimated 790 net hectares (ha) (1,960 acres[ac]) in San Francisco Bay, California (Zaremba andMcGowan 2004) and 3,200 net ha (8,000 ac) in WillapaBay, Washington. Additionally, thousands <strong>of</strong> hectares in 31Pacific estuaries are at risk for future colonization by one ormore invasive Spartina spp. (Daehler & Strong 1996; Pfau<strong>the</strong>t al. 2003). In Oregon alone, approximately 13,622 ha(33,660 ac) <strong>of</strong> intertidal mudflats and aquatic beds arevulnerable to invasion (Pfauth et al. 2003). Understandingboth <strong>the</strong> potential risks and <strong>the</strong> efficacy <strong>of</strong> any controlmethod is critical to refining management choices and earlydetection efforts.Efficacy and cost data have been evaluated for a widearray <strong>of</strong> chemical and mechanical treatments, (Patten 2002;Hedge et al. 2003; Pfauth et al. 2003); however <strong>the</strong> riskassessments <strong>of</strong> non-target effects have focused morenarrowly on chemical controls. Mechanical treatments suchas rototilling, disking, crushing, pulverizing and digginghave been problematic due <strong>the</strong>ir slow pace, variableefficacy, and high cost per area treated (Patten 2002). Yetrototilling and disking are still used in some situations t<strong>of</strong>acilitate <strong>the</strong> decomposition <strong>of</strong> below-ground biomass aftersuccessful chemical treatment, which allows for more rapidrestoration to usable shorebird habitat (Patten & Stenvall2002); <strong>the</strong>se mechanical methods have also been employedwhere landowners oppose chemical treatment options.- 255 -