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276 Tulbure et al.FIG. 3. PCR/RFLP analysis of non-nativePhragmites population from Point au Sauble wetland.Lanes 1 and 2 contain the rbc primer withthe HhaI digested DNA and undigested DNA,respectively. Lanes 3 and 4 contain the trn primerwith the RsaI digested DNA and undigested DNA,respectively. Lane 5 contains a 100 bp ladder(Promega, Madison, WI). The rbc productdigested and the trn product did not, indicatingthe source DNA was from the invasive genotype.perennials. The decreased cover of Schoenoplectustabernaemontani and Sagittaria latifolia was presumablydue to shading by taller successionalplants.Prior authors have identified the dynamic natureof Point au Sauble and other Green Bay wetlands inresponse to such water level fluctuations (Epstein etal. 2002, UWGB 2004). The difference between ourobserved changes at Point au Sauble and previousreports, which point out the dominance of Typhaspp. as the water level increases (UWGB 2004), isthe dominance of Phragmites. The fact that Phragmitesexpanded so rapidly in only 3 years is a causeof concern. Moreover, Phragmites from Point auSauble was found to belong to the more aggressive,introduced genotype. Therefore it may disrupt thenatural cycles of vegetation replacement that occurunder native plant communities in healthy coastalwetlands.Although there was an increase in species richnessbetween 2001 and 2004, we believe that this istransient based on our data collected at this site. Weexpect to see a decrease in species richness in plotswith the continued expansion and densification ofPhragmites, similar to what we have seen at thefour plots where Phragmites had a 100% cover.Phragmites is an invasive taxon (sensu Richardsonet al. 2000) that is a clonal dominant (sensuBoutin and Keddy 1993). Under the predictions ofthe hump-backed model (Grime 2000, Moore andKeddy 1989), the large standing crop of this clonaldominant species would be expected to reducespecies richness because the greatest species richnessis usually reached at moderate standing crops.Phragmites forms monospecific stands with a consequentreduction in species diversity (Marks et al.1994). Such a reduction did occur in the four plotswhere Phragmites had a 100% cover in 2004,species richness being significantly lower in thesefour plots than in the plots where Phragmites wasnot present or had a low cover.“Mean coverage by invasive species” was an indicatorthat suggested a degradation of the lagoon.Species richness and other metrics commonly usedas wetland indicators (i.e., FQI and mean coefficientof conservatism) did not point out a degradationof this wetland. However, greater values of allthese three metrics are expected as the site evolvedfrom a newly established community dominated byannuals to a more mature community dominated byperennials. The greater height of the dominantherbaceous plants in 2004 can be explained by thedominance of Phragmites or Typha in almost all ofthe plots. These are invasive species in the U.S. andgrow taller than native species (Galatowitsch et al.1999). Plant size larger than that of associatespecies is a key trait that allows plant species todominate, such that tall stature is a characteristic ofcompetitive species (Grime 1973, Grime 2000).Monoculture is achieved by larger species throughtheir ability to compete for light (tall shoots), forwater and nutrients (high below ground biomass),and through the presence of a high density of herbaceouslitter throughout the year (Grime 2000). BothTypha and Phragmites form mono-dominant stands,and both are well adapted to sustained inundation(Haslam 1971, Shay and Shay 1986). These fea-
Phragmites australis Invasion 277tures promote Phragmites as a good competitor inthe stands where it occurs (Haslam 1971).While predicting invasion of a community is difficultbecause there is no unique theory to successfullyexplain plant invasions (Williamson 1999,Grime 2000, Zedler and Kercher 2004), fluctuationof resource availability may be one of the key factorsthat control invasibility (Davis et al. 2000).New substrate availability makes an area susceptibleto invasion (Grime 2000) and is one of the factorsthat make riparian areas more prone to invasion(Tickner et al. 2001).Establishment of Phragmites typically occurs onbare, unvegetated moist soils after water levels recede(Chambers et al. 1999, Galatowitsch et al.1999, Ailstock et al. 2001, Pengra et al. 2007). Thisspecies typically invades starting from the uplandfringe of the wetland via stolons and rhizomes. Alternatively,it may start on a high point within thewetland, such as a dike remnant, and then spreadinto the wetland plain. Unlike rhizomes, stolons arelocated aboveground and usually elongate morerapidly than rhizomes. Phragmites propagation byseeds is limited (Haslam 1970, 1971). Vegetativepropagation is efficient through rhizome portionstransported by water, man, or stolons from nearbycolonies (Haslam 1971). We observed long Phragmitesstolons on beach areas north of the lagoon,and believe that stolon growth was the primarymechanism of Phragmites expansion within the lagoon.The rapid invasion of Point au Sauble by Phragmitescannot and should not be considered representativeof all Great Lakes coastal wetlands.Although the extremely low water levels between1999 and early 2004 occurred throughout lakesMichigan and Huron, Phragmites stands observedelsewhere in Green Bay by co-author Tulbure as a2004 field assistant to Pengra (2005) were muchless extensive than those found at Point au Sable. Inthat study, Pengra and co-workers visited 82 GreenBay wetland locations with emergent vegetationand found that 34 of them had Phragmites presentin 2004. We believe that this could be because theAu Sauble wetland is a lagoon protected in the interiorof a hook-shaped peninsula, whereas other sitesare sandy peninsulas and associated embaymentsthat are more open to wave action (e.g., Little TailPoint, and Long Tail Point, Wisconsin DNR 2006).Also, other sites had organic substrates (e.g., SensibaWildlife Area) rather than silty substrate asPoint au Sauble, which might have been less proneto Phragmites invasion. We believe that the exposureof unvegetated silty mineral soils at Point auSauble, which suggests the presence of a depositionalenvironment, created conditions conducive toPhragmites colonization and may be atypical forthe region. Based on these results, we predict thatsites with exposed mineral soils are most susceptibleto Phragmites invasion, and future studiesshould further test this hypothesis.The GLEI project was designed to be conductedas single-sampling event, not an analysis of changeover time. However, these results illustrate the benefitsof the sampling and documentation protocolestablished by GLEI. Without the careful recordskept by the GLEI sampling crew in 2001, documentationof this rapid Phragmites expansion would nothave been possible.ACKNOWLEDGMENTSWe thank Dr. Elizabeth Lynch and an anonymousreviewer for helpful comments on an earlier draft.This research has been supported by a grant fromthe U.S. EPA’s Science to Achieve Results Estuarineand Great Lakes program through funding to theGreat Lakes Environmental Indicators project, U.S.EPA Agreement EPA/R-828675. Although the researchdescribed in this article has been fundedwholly by the U.S. EPA, it has not been subjectedto the agency’s required peer and policy review andtherefore does not necessarily reflect the views ofthe agency and no official endorsement should beinferred.REFERENCESAilstock, M.S., Norman, C.M., and Bushmann. P.J.2001. Common Reed, Phragmites australis: controland effects upon biodiversity in freshwater nontidalwetlands. Resour. Ecol. 9(1):49–59.ASTM (American Society for Testing and Materials).1997. ASTM E 1923, Standard Guide forSampling Terrestrial and Wetlands Vegetation. ASTMInternational, West Conshohocken, PA, USA.Bart, D., and Hartman, J.M. 2000. Environmental determinantsof Phragmites australis expansion in a NewJersey salt marsh: an experimental approach. Oikos89:59–69.———, and Hartman, J.M. 2003. The role of large rhizomedispersals and low salinity windows in theestablishment of common reed, Phragmites australis,in salt marshes: new links to human activities. Estuaries26(2B):436–443.Bernthal, T.W. 2003. Development of a floristic qualityassessment methodology for Wisconsin. Final reportto the U.S. Environmental Protection Agency. Region
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