Rapid Invasion of a Great Lakes Coastal Wetland by Non ... - BioOne

270 Tulbure et al.stall 2003). For example, Lynch and Saltonstall(2002) documented the expansion of the nativePhragmites genotype at a Lake Superior wetland.Coastal wetlands along Lake Michigan’s GreenBay are known to be dynamic, experiencing vastchanges in the extent of emergent vegetation aswater levels fluctuate over decadal cycles (Epsteinet al. 2002). Emergent plants germinate on lakebedexposed during low-water years, persist during averagewater level conditions, and decline after a seriesof high-water years, the marshes reverting toopen water (Harris et al. 1977, Bosley 1978). Typhais commonly mentioned as a key genus involved inthese vegetation cycles, but Phragmites is not.Data about plant species in wetlands of GreenBay on Lake Michigan were collected by the GreatLakes Environmental Indicators project (GLEI)during mid-June 2001, when the water level ofLake Michigan was 54 cm below average (USCOE2006). During a repeat visit to wetlands of theGreen Bay area three years later, large stands ofPhragmites were observed in the Point au Saublelagoon where little or no Phragmites had occurredin 2001. During the interim between the two sampledates, Lake Michigan experienced a prolonged periodof below-average water levels.The goal of this paper is to document the rapidchange in wetland vegetation of a Green Bay lagoonover a 3-year period associated with the rapidinvasion of Phragmites. Specific objectives were tocompare vegetation structure and community compositionbefore and after Phragmites invasion, evaluateedaphic conditions within areas that were andwere not colonized by Phragmites, and determine ifthe expanding areas of Phragmites are of the invasivegenotype.METHODSStudy AreaThe 81 ha Point Au Sauble lagoon is located onthe eastern shore of Green Bay, 10 km northeast ofthe city of Green Bay in Brown County, Wisconsin.It is currently owned by the University of Wisconsin-GreenBay and has been preserved in a naturalstate by The Nature Conservancy and duck huntinggroups for many decades (UWGB Cofrin Center forBiodiversity 2004). Point au Sauble is covered by avariety of plant communities such as floodplain forest,shrub-carr, sedge meadow, emergent aquatic,and Great Lakes beach, but the area sampled wasdescribed by earlier accounts as an open-water lagoon(Epstein et al. 2002). Although wetlands arecommon along the western shore of Green Bay,Point au Sauble is one of only a few coastal wetlandson the eastern shore of Green Bay.Abiotic Data CollectionThe soils at each plot were examined to a depthof 30 cm below the litter layer using a soil probe,and assigned to one of the following broad categories:organic, sand, silt, or clay. “Organic” soilswere those composed of organic soil material (peator muck) in a histic epipedon (Soil Survey Staff1999); undecomposed plant litter overlying the soilsurface was excluded when making this determination.Only the surface 30 cm was considered. Wedid not attempt to discern if soils in the organic categorywere true histosols. The texture of mineralsoils (i.e., sand, silt, clay) was determined by feelusing standard field methods (Soil Survey Staff1951). The hand texturing was done by co-authorJohnston, who is a professional soil scientist certifiedby the American Society of Agronomy and experiencedat wetland soil characterization usingboth field and laboratory methods. Soil texture byplot in 2004 varied from silty to organic soils(Table 1).Lake level data recorded every 6 minutes atGreen Bay, Wisconsin (Station ID 9087079) wereobtained from the National Oceanic and AtmosphericAdministration (NOAA 2006), and were averagedover the time period from 0800 hours to1800 hours local standard time to determine averagewater levels on the days sampled. Monthly averageand monthly minimum lake levels, based ondata collected from 1918-2005, were obtained fromthe U.S. Army Corps of Engineers (USCOE 2006).Vegetation SamplingThe Point au Sauble lagoon was initially sampledon 19 Jun 01 as part of GLEI, a large, multi-investigatorproject designed to identify biotic indicatorsof human disturbance (Danz et al. 2005). The GLEIvegetation sampling protocol (Bourdaghs et al.2006, Johnston et al. 2007 this issue) was used inboth 2001 and 2004, when a repeat visit to the lagoonwas made on 7 Jul 04. A north-south trendingtransect was placed across the lagoon at Point auSauble, and 11 sampling locations (plots) were establishedat approximately 20 m intervals along thetransect (Fig. 1). At each sampling location, a 1 m 2plot was established and its geographic coordinates