Proceedings of the Third International Conference on Invasive ...

Chapter 2: Spartina Distribution and Spread<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive Spartinalinked to sedimentation consistent with S. alterniflora and S.townsendii invasions in New Zealand and S. anglica inEngland (Daehler and Strong 1996), and predictions byCallaway and Josselyn (1992).One interpretation <strong>of</strong> <strong>the</strong> spread rate differences amongsites is that invasion rates <strong>of</strong> susceptible habitat are initiallyhigh and <strong>the</strong>n slow until more susceptible habitat is createdby sediment accretion. Fringing marshes, mudflats andbeach habitat (Site Type II) experienced <strong>the</strong> highest rate <strong>of</strong>spread from 2001 to 2003, suggesting a new invasion frontwas being exploited by hybrids. Presumably, after <strong>the</strong> initialrapid colonization <strong>of</strong> suitable “empty-niche” habitat, <strong>the</strong> rate<strong>of</strong> spread would slow to match <strong>the</strong> rate at which <strong>the</strong> Spartinaaugments sedimentation and creates additional habitat toinvade.Established S. foliosa marshes and open mudflats <strong>of</strong> anewly opened restoration sites (Site Type I) are initiallyhighly susceptible to invasion by pollen and by seed,respectively. The formerly diked bayland Citation Marshexperienced a 900% increase in hybrid cover in three years.Then, <strong>the</strong> rate <strong>of</strong> cover increase slowed as suitable habitatwas filled in.Site Type IIIs may be initially slower to colonizebecause <strong>the</strong>y include deep channels and creeks, wheresediment must accrete before <strong>the</strong>y are at suitable elevationfor more extensive colonization. Once <strong>the</strong> channel beds havesufficiently accreted, colonization <strong>of</strong> remaining channelbanks and bottoms is rapid, with clonal colonies quicklycoalescing into meadows. Rate <strong>of</strong> spread in urbanizedshoreline (site type IV) may likewise be relatively low dueto poor or little sediment availability.Among <strong>the</strong> three non-native Spartina taxa, hybridsspread <strong>the</strong> most rapidly (317% in three years). Hybrids havehigh rates <strong>of</strong> vegetative spread, produce large quantities <strong>of</strong>pollen, have successful seed set, and readily backcross to <strong>the</strong>native S. foliosa. Spartina foliosa exposed to pollen <strong>of</strong>hybrids produces hybrid seeds, fur<strong>the</strong>r accelerating <strong>the</strong>hybrid invasion. The rate <strong>of</strong> colonization is particularly rapidin <strong>the</strong> early stages <strong>of</strong> invasion. For example, EmeryvilleWest showed an increase in cover <strong>of</strong> 2,175%, between 2001and 2003. All habitats in <strong>the</strong> Bay appear to be susceptible toinvasion by this S. alterniflora hybrid.S. densiflora, which has a cespitose growth form andinvests more reproductive effort in seed production thanvegetative spread, didn’t spread to <strong>the</strong> same degree as didhybrids: 52% averaged over all site types. Site Type Iexperienced <strong>the</strong> greatest increase in cover <strong>of</strong> S. densiflora(68%).S. patens apparently decreased in cover by 84%, but it isfound only at Southampton Marsh where <strong>the</strong>re is less thanan acre <strong>of</strong> total cover. The apparent decrease is likely due tomapping error not a true decrease in cover.Of <strong>the</strong> three Bay regions, <strong>the</strong> Central Bay, nearest to <strong>the</strong>original introduction sites in <strong>the</strong> Fremont and San Brunomarshes, had <strong>the</strong> largest increase in cover, 292%. Hybrids,<strong>the</strong> fastest spreading species (392%), dominate this region <strong>of</strong><strong>the</strong> Bay. Clearly, <strong>the</strong> hybrids are <strong>the</strong> dominant invasiveSpartina taxa and are well established, and both <strong>the</strong>iracreage and rate <strong>of</strong> spread is greatest in <strong>the</strong> heart <strong>of</strong> <strong>the</strong> Bay.Some new non-native Spartina populations were foundin <strong>the</strong> sites surveyed at <strong>the</strong> request <strong>of</strong> land owners,managers, and stakeholders. In <strong>the</strong> South Bay, a survey <strong>of</strong><strong>the</strong> Plummer Creek Mitigation Site found only nativeSpartina. However, 19% <strong>of</strong> seedlings from a Spartina spp.population at <strong>the</strong> Cooley Landing/Ravenswood PreserveRestoration Project were genetically tested and determinedto be hybrids. Both <strong>of</strong> <strong>the</strong>se sites are near extensive hybridpopulations, and considered to be at risk. Genetic surveyswere performed at a few small sites in <strong>the</strong> Central Bay(Crissy Field, Steege Marsh and Beach Drive) as part <strong>of</strong> amanagement regime to identify and remove newlyestablished non-natives.GISThree difficulties emerged during <strong>the</strong> 2003 monitoring:imprecision <strong>of</strong> measuring small clonal patches or small areasdue to <strong>the</strong> limits <strong>of</strong> precision <strong>of</strong> <strong>the</strong> GPS units, differencesbetween aerial photo interpretation estimates and groundtruthdata, and low power to detect small changes in areacoverage because <strong>of</strong> broad cover class intervals (Zaremba etal. 2004). Field mapping with three-meter- resolution GPSunits is imprecise for small areas such as new invasion sites,which necessarily contain small populations, and <strong>the</strong>physically smaller Spartina species, S. densiflora and S.patens. These possible sources <strong>of</strong> error could account for <strong>the</strong>apparent decrease in cover <strong>of</strong> S. patens at SouthhamptonMarsh or S. densiflora at Pickleweed Park or Blackie’sPasture and Creek.Remote sensing using aerial infra-red photography haspotential for synoptic mapping <strong>of</strong> <strong>the</strong> spread <strong>of</strong> invasiveSpartina compared with labor-intensive field mapping.However, among <strong>the</strong> five sites where we compared <strong>the</strong>semethods in 2003 <strong>the</strong>re were two sites where aerial estimateswere 8-33% less than field estimates and three sites whereaerial estimates were 9-661% greater than field estimates.Averaged over all five sites, <strong>the</strong> field measurements were170% greater than <strong>the</strong> aerial photo measurements. Aerialphoto interpretation methods may prove useful to monitor<strong>the</strong> spread <strong>of</strong> non-native Spartina in grossly invaded areas,but it is not currently precise enough to map without fieldtruthing. The examples <strong>of</strong> both higher and lower aerial photointerpretation cover estimates indicate that <strong>the</strong> currentmethod <strong>of</strong> digitizing polygons around <strong>the</strong> marsh or Spartinapatch with a cover class may be too coarse to estimate coverprecisely.-80-