Invasion, environmental controls, and ecosystem feedbacks of Phragmites australis in coastal wetlands

The M Eurasian Phragmites australis (hereafter Phragmites) is one of the most invasive plants in wetlands along the Atlantic and the Great Lakes coasts. Its rapid expansion is of concern because it displaces native vegetation and reduces overall wetland biodiversity. Here I investigated two facets of Phragmites invasibility: environmental factors that promote Phragmites invasion and mechanisms that Phragmites employs to sustain its own invasion. The EPA-funded Great Lakes Environmental Indicators project provided baseline vegetation data in wetlands along the Great Lakes coastline and represented a unique opportunity to assess natural and anthropogenic factors that promoted Phragmites expansion once it is present at a wetland site. Once established, Phragmites hinders the vegetation replacement that is characteristic of healthy Great Lakes coastal wetlands. In a first study I found that introduced Phragmites rapidly invaded newly exposed mineral substrates in a coastal wetland along the shore of Green Bay/ Lake Michigan where water level declined. Further I revisited 14 Great Lakes coastal wetlands (2005), previously sampled for vegetation by GLEI, to test hypothesized relationships of short-term changes in Phragmites cover with several natural and anthropogenic factors. Low initial Phragmites cover, decrease in bare soil cover, higher amount of agriculture in the watershed, and decrease in water depth promoted Phragmites expansion at the revisit sites. Phragmites invaded more at wetland sites where there was a decrease in lake level from 1999 to 2001 (i.e., Michigan-Huron, and Erie), and occurred predominantly on sandy substrates at the 14 revisit wetlands. These results suggest that Great Lakes coastal wetlands with high levels of agricultural activity in their watersheds, and where water level has dropped exposing sandy substrates are more likely to experience Phragmites invasion. Monitoring these wetlands and controlling for Phragmites at early stages of invasion is essential as Phragmites is difficult to control once established. This finding is important as water levels in the Great Lakes are expected to decline with future climate change and have reached extreme lows in the past years.;I conducted a comparative study between native and introduced Phragmites stands in the Chesapeake Bay to explore pressurized ventilation, a mechanism that may impart invasiveness in Phragmites. Introduced Phragmites had greater ventilation efficiency per unit area than native Phragmites. I further collected rhizomes from the same area and grew them in a greenhouse experiment to test whether this mechanism holds true in a controlled setting. Introduced Phragmites had greater ventilation efficiency per unit area and promoted more abundant aerobic microorganisms in the rhizosphere. The ability of the introduced genotype to send more oxygen to the rhizophere may explain why introduced Phragmites is able to thrive while native plants cannot. It also suggests that only cutting the stems in well established Phragmites stands is ineffective in Phragmites control, but culm removal followed by flooding may achieve the desired effect.