Tipping the Tipping Point: After a Regime Shift to Typha Dominance can Management or High Water Levels Push a Wetland Plant Community Back to a Pre-Invaded State?

Abstract:

Dominance of Typha x glauca (hybrid cattail) in Great Lakes coastal wetlands strongly increases with nitrogen (N) level and can be difficult to manage in part due to a regime shift to an invaded state reinforced by high internal ecosystem N cycling. However, after a regime shift occurs it may be possible to “reset” the system by introducing disturbances that reduce the impacts of invasion on soil processes, which is the underlying cause of the regime shift. We used MONDRIAN, a wetland community-ecosystem model, to explore the dynamics of Typha invasion into a native community across a range of N loading scenarios to determine if management practices or a period of high water level could return the system to a native-dominated state. Great Lakes water levels are predicted to become more variable due to climate change and include periods of historically high water levels where most of the emergent coastal vegetation is eradicated. We studied nitrogen loading scenarios that were either constant or decreasing where the system starts with high N loading and ends with low N loading across a 65-year time period. Management (herbiciding + burning applied once annually for 3 or 6 years) or high water levels (>1.2 m for 3 years) was started 15 years after N loading decreased. In the constant N loading scenarios, an invasion threshold was observed between 8 and 12 g N m^-2 yr^-1, in which the invader proportion went from less than 20% to greater than 80%. Typha dominance continued after N loading was reduced, embodying in a 79% regime shift for invader proportion and 31% regime shift for invader NPP (% regime shift refers to how much Typha dominance remained at original, high N loading levels after N reduction). While management was not able to fully return the N-reduced wetland community to a pre-invaded state, it was partially successful, leading to a 38% reduction in invader NPP and 24% reduction in invader proportion compared to pre-treatment, post-regime shift levels. High water level in the N-reduced wetland was less effective in returning the system to a native state, causing only a 23% reduction in Typha NPP and a 10% reduction in invader proportion. These results suggest that in historically eutrophic wetlands, management and/or high water levels may help to partially, but not fully return the system to a pre-invaded plant community even after high N loading is removed.