Responses of an invasive grass, Phalaris arundinacea, to excess resources

Because Phalaris arundinacea L. (reed canary grass) responds strongly to increasing resources (water, nutrients), it is the most widespread invader of wetlands in Wisconsin. It is also planted to treat wastewater. Elevated nitrogen (N) levels accelerate invasions, thus Phalaris is presumed to have high nutrient uptake that contributes to higher N retention in a wetland. We compared the capability of wet prairie vegetation with and without invading Phalaris under low and high N treatments to accumulate N in plant tissues, retain N in soil, and remove N from water flowing through experimental mesocosms. We found no evidence that vegetation invaded by Phalaris had greater N-retention capacity than the wet prairie vegetation it displaced and thus suggest that treatment wetlands employ non-invasive native species instead of Phalaris.; Herbicide can reduce Phalaris biomass, but invasions tend to recur when environmental drivers persist. Efforts to reverse invasions may be more effective where Phalaris is not yet dominant and where disturbances that facilitate invasions are removed. We tested the reversibility of different invasion states, i.e., levels of Phalaris biomass:native species richness, under chronic or halted flooding and nutrient inputs. Herbicide reduced Phalaris biomass more effectively where nutrient and flooding inputs were halted than where continued. Our findings suggest that Phalaris invasions should be most reversible before a monotype has formed and where flooding and eutrophication are minimal. Phalaris is not easily eradicated, however, and continual surveillance and control measures will be needed.; Morphological plasticity can facilitate the spread of invasive wetland plants in areas of increased flooding and eutrophication. We explored canopy plasticity of Phalaris as it differentially invaded wet prairie mesocosms under 3 flooding durations and 3 levels of nutrient addition. Phalaris grew as a sward under early-season flooding but shifted to tussocks under constant flooding. Canopy ratios differed with tussocks having a ≥25% increase in shoot number:biomass, shoot height:biomass, and total shoot length:biomass, and a ≥28% decrease in leaf area:shoot height and leaf area:total shoot length compared to swards. Our findings suggest that plasticity in canopy attributes could confer greater spatial spread and persistence over time of Phalaris.