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'Are Phragmites Invasions Dynamic Through Time?'
Eric Hazelton (eric@hazelton-ecological.com), Utah State University; Melissa McCormick (mccormickm@si.edu), Smithsonian Environmental Research Center; Matthew Sievers (mfsievers89@gmail.com), Smithsonian Environmental Research Center; Karin Kettenring (karin.kettenring@usu.edu), Ecology Center and the Department of Watershed Sciences, Utah State University; Dennis Whigham (whighamd@si.edu), Smithsonian Environmental Research Center

Biological invasions cause changes to the communities and environments they invade, and the invader itself can change during invasions. Resulting changes to the environment may include altered nutrient cycling, hydrology, substrate, or other factors. Community composition is often altered by competitive exclusion of native species previously present at the invaded site. Changes to an invader may result from either successional or density-dependent processes and may include increased reproductive output or changes in vigor. We investigated intrinsic and extrinsic changes in Phragmites invasions over several decades in two Chesapeake Bay subestuaries. We used historic vegetation maps to locate Phragmites patches that were present prior to 1970 and aerial imagery to locate patches that established after 1990. We compared Phragmites germination rates, flowering rates, clonal richness, and herbivory rates and plant community composition between old and young stands. Our results indicated that none of the Phragmites vigor metrics changed as the invasion aged, nor did the plant community composition. This implied that Phragmites invasion progressed from the initial colonization to a stable, invaded state rapidly and the new state persisted through time. Clonal richness did differ significantly between the young and old stands and was the only significant metric. The old stands had significantly lower clonal richness and genetic diversity than the young, which may be the result of intraspecific competition extirpating clones through time, or the evidence of density dependent Allee effects early in the invasion. In conclusion, our findings suggest that Phragmites invasion rapidly creates a stable invaded state that persists through time, after which only intraspecific changes occur.