Response of wetland vegetation to the post-1986 decrease in Lake St. Clair water levels: Seed-bank emergence and beginnings of the Phragmites australis invasion

Water-level fluctuations are critical for maintaining the diversity and resultant habitat value of wetland plant communities in the Laurentian Great Lakes. However, activation of the seed bank can also provide an opportunity for invasive species to displace native species, as occurred when common reed, Phragmites australis, expanded across many wetlands after lake levels receded following highs in 1997. Timing of the invasion process is not clear, however, as Phragmites propagules had to be present to exploit the exposed soils. A data set from Dickinson Island on the St. Clair River delta collected in 1988–1991, 1996 during a previous lake-level decline was analyzed to document prior Phragmites growth, as well as overall seed-bank response. Above-ground biomass was determined for all plants each year in randomly placed quadrats in a 5-ha area exposed when lake levels decreased by 0.65 m from 1986 to 1988. A total of 38 taxa were identified in 1988, but the number decreased, along with biomass of many species, as canopy-dominating Typha angustifolia and Phragmites increased in later years. Although Phragmites did not expand greatly until after the decline from the 1997 high, it likely inoculated the area with viable seed during the previous low. Because post-1997 lake levels were lower than those post-1986, they exposed a greater area for Phragmites colonization from seed; lake levels also remained low for a longer time. Differences in bathymetry below the 1986 and 1997 lake-level elevations likely played a role in greater post-1997 spatial expansion of Phragmites at other sites in the Great Lakes also. The next high lake level will likely be required to displace Phragmites, but the effect will be temporary.