Proceedings of the Third International Conference on Invasive ...

Chapter 4: Spartina Control and Management<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaThe highest number <strong>of</strong> cards found within an estuaryoccurred in <strong>the</strong> case <strong>of</strong> <strong>the</strong> San Francisco September releasewhen thirty cards were recovered on <strong>the</strong> eastern edge <strong>of</strong> <strong>the</strong>bay, mainly near <strong>the</strong> Berkeley and Albany shoreline.Observed winds at <strong>the</strong> time <strong>of</strong> that release were from <strong>the</strong>west at approximately 7.7 – 10.3 m/s. Recoveries <strong>of</strong> SanFrancisco cards not blown back into <strong>the</strong> bay after <strong>the</strong>September release showed 61% were carried south.Maximum velocities for this release were approximately 2.4cm/s to both <strong>the</strong> north and south. Eighty-seven percent <strong>of</strong>October cards were found to <strong>the</strong> south. Maximum velocitieswere 5.4 cm/s to <strong>the</strong> north and 6.9 cm/s to <strong>the</strong> south.DISCUSSIONRototilling appears to have fairly uniform cutting actionon solid meadows; it produces rhizome fragments <strong>of</strong>consistent size and with similar numbers <strong>of</strong> attached culms.A high percentage (87.8%) <strong>of</strong> <strong>the</strong> S. alterniflora fragmentsproduced by rototilling in Willapa Bay had at least oneattached culm. The observed difference in culm length andrhizome diameter between quadrats was likely a result <strong>of</strong>variations in <strong>the</strong> age <strong>of</strong> coalesced clones, ra<strong>the</strong>r thanvariable tearing action by <strong>the</strong> tilling blades. Assuminguniformity in <strong>the</strong> production <strong>of</strong> rhizome fragments(approximately 312 fragments/m 2 within <strong>the</strong> top 10 cm) wecould make a conservative estimate that 0.5% <strong>of</strong> fragmentsmight be loosened by wave or tidal action, becomingsuspended in <strong>the</strong> water column. Based on those assumptions,as many as 15,600 fragments might be distributed in <strong>the</strong>open water for every hectare rototilled (~6,300 per acre).Viability <strong>of</strong> rhizomes after floating at <strong>the</strong> water surfaceseems to be primarily dependent on <strong>the</strong> presence <strong>of</strong> at leastone culm. Randall and Milne (unpublished) found thatrhizome fragments 2.5 to 15 cm long with no attached culmshad 100% mortality regardless <strong>of</strong> position on <strong>the</strong> mud-flatsubstrate or beneath it at various depths. The presence <strong>of</strong>attached culms should allow <strong>the</strong> fragment to respire and<strong>the</strong>reby increase its chance <strong>of</strong> survival. In fact, repeatedmowing to remove vegetative shoots caused a reduction <strong>of</strong>oxygen to <strong>the</strong> root system and was initially utilized as acontrol method for Spartina (Ebasco Environmental 1993;Hedge et al. 1997). Of <strong>the</strong> 116 rhizomes planted withoutattached culms during <strong>the</strong> greenhouse study, none survived.Compared to fragments planted with culms, this findingseems to support <strong>the</strong> <strong>the</strong>ory that at least one vegetative shootis needed for survival <strong>of</strong> vegetative propagules.For fragments having culms, salinity and initial rhizomesize determined survival. The 35 ppt treatment resulted innotably poorer survival rates compared to <strong>the</strong> lower salinitytreatments. Differences in 0 ppt and 15 ppt treatmentsappeared largely due to initial rhizome size. Larger rhizomeswould logically have higher chances <strong>of</strong> survival since <strong>the</strong>ywould be likely to have more nodes, a greater number <strong>of</strong>established roots and more non-structural carbohydrates t<strong>of</strong>uel new growth.The length <strong>of</strong> immersion was a less importantdeterminant <strong>of</strong> survival and establishment or rhizomefragments than salinity or rhizome size. Longer immersiondurations may increase viability, although this effect was notconsistent. Three to four months <strong>of</strong> wet, cool conditions mayhelp break seed dormancy and increase germination rates byleaching a germination inhibitor. Similarly, <strong>the</strong> conditionsthat fragments are exposed to while floating in open watermay retard growth <strong>of</strong> pathogens, encourage shoot productionor elongation or o<strong>the</strong>rwise increase chances <strong>of</strong> survival.Repeated monitoring <strong>of</strong> treated sites in Willapa Bay hasshown that mechanical treatments such as rototilling anddisking have higher efficacy during <strong>the</strong> period <strong>of</strong> Decemberthrough February (Patten & Stenvall 2002). All <strong>of</strong> <strong>the</strong> plantsused for this study were collected four to five weeks after <strong>the</strong>normal rototilling period in Willapa Bay. Increased culmlength, as well as higher air, soil and water temperatures at<strong>the</strong> time <strong>of</strong> collection, may have increased survival rates.Additionally, <strong>the</strong> vigorous action <strong>of</strong> rototilling producesmore ragged edges and somewhat damaged culms than werereproduced in <strong>the</strong> greenhouse. This might also elevate rates<strong>of</strong> survival shown here.Preliminary results from <strong>the</strong> first two months <strong>of</strong> <strong>the</strong> driftcard study suggest propagule deposition from infestedestuaries lessens with increased distance. Flow over <strong>the</strong>coastal shelf is predominantly poleward in <strong>the</strong> winter andearly spring, with mean current velocities <strong>of</strong> 20 cm/s.Summertime flow is typically southward with meanvelocities <strong>of</strong> 10 cm/s. Dispersal patterns seen from <strong>the</strong>sepreliminary findings may be due to a seasonal transitionperiod between <strong>the</strong>se predominant currents. Recoverypatterns may also reflect wind forcing and local eddies from<strong>the</strong> mouths <strong>of</strong> <strong>the</strong> release estuaries. Frequent recoveries inbeaches along Long Beach peninsula, where Spartina wrackis commonly found in <strong>the</strong> fall, suggests that <strong>the</strong> cardssimulate wrack dispersal with some accuracy.CONCLUSIONSThe estimate <strong>of</strong> 15,600 fragments per hectare may seeminconsequential when compared to estimates <strong>of</strong> seedgermination rates for S. alterniflora which range from nineto nineteen million seeds per hectare (3.7 million to 7.7million seeds per acre) (Callaway 1990; Daehler and Strong1994). However, understanding <strong>the</strong> risks associated with allcontrol methods is necessary when site-specific treatmentdecisions are made. Rototilling or o<strong>the</strong>r mechanicaldisturbance that produces fragments larger than 2.5 cmshould be used with caution in areas with fresh tomoderately brackish (mesohaline) waters. Mechanicaldisturbance following some o<strong>the</strong>r treatment method, such asherbicide application, may pose less <strong>of</strong> a risk <strong>of</strong> starting new- 260 -