Water Level Decline Promotes Typha X glauca Establishment and Vegetation Change in Great Lakes Coastal Wetlands

Climate change is predicted to reduce Laurentian Great Lakes water levels, altering coastal wetland ecosystems and potentially stimulating invasive macrophytes, like Typha X glauca. Recent prolonged low water levels, which climaxed in 2007, created conditions comparable to those predicted by climate change science. In 2008, we examined ecosystem and plant community properties in 14 intact northern Great Lakes coastal wetlands and compared community data with data from a 1987–1989 high-water period, before T. X glauca invasion. In 2008, T. X glauca occurred in 50% of wetlands and 16% of plots; was associated with reduced Floristic Quality and increased soil organic matter, soil nutrients, and leaf litter (all p < 0.05); and plant community composition had shifted and was more homogeneous than in 1988 (both p < 0.05). Additionally, T. X glauca was more dominant when growing behind barrier beach ridges, which form in high-water conditions and persist in low-water, than in lake-exposed marshes (p < 0.05), revealing a physiographic mechanism for increased dominance. Beach ridges protect T. X glauca from wave and seiche energy, and as water levels decline, these energy-insulating microtopographic features will likely stimulate further invasion and dominance by T. X glauca, even in high quality wetlands.