Plant invasions in the wetlands of the Great Lakes Region are known to be strongly influenced by nitrogen loading. The invaders Phragmites australis and Typha x glauca are often managed using fire, herbicide, and mowing with varying degrees of long-term success, likely due to the persistence of N loading. However, even if nitrogen loading decreased to pre-invaded levels the wetland ecosystem could potentially stay in an invaded state because a regime shift has occurred in nitrogen cycling. We studied the potential for such a regime shift using the MONDRIAN model, an individual based model that simulates growth and competition among individual ramets. We explored the success or failure of simulated invasion into a 3-species native community by both invaders across a range of N loading scenarios. These included constant (4 to 30 g N m-2 yr-1) or decreasing N loading where the system starts in a eutrophied state (30 g N m-2 yr-1) and ends in a low N loading state (4 g N m-2 yr-1) across a 50-year time period. We compared these scenarios to determine if invasion into a eutrophied wetland produced a regime shift, keeping N cycling higher even after a decrease in N loading.
Results/Conclusions
In the constant N loading scenarios, Typha and Phragmites were unsuccessful at low N loading, but formed monotypic stands at high N. An invasion threshold was observed between 8 and 12 g N m-2 yr-1 for both species. At a constant but low rate of N loading (4 g N m-2 yr-1), invaders were able to persist, but were a minor part of the modeled community at 115 g biomass m-2 yr-1 (~20% of community biomass). Interestingly, in the decreasing N loading scenarios, where the ending N loading rates were 4 g N m-2 yr-1 for ten years, the invader biomass was stable and intermediate between the 30 and 4 g N m-2 yr-1 N loading rates. Invader biomass was 410 g biomass m-2 yr-1 at the end of these simulations, which was 3.5 times greater than the constant N loading scenarios. These results suggest that a regime shift occurred and removing the main driver of invasion, N loading, did not return the system to a pre-invaded state, though invasion was reduced. The regime shift was likely caused by internal N cycling ramping up due to invasion.