Nitrogen loading in Great Lakes coastal wetlands affects N retention, plant community composition, and non-native invasion success

Background/Question/Methods

Coastal wetlands exist at the interface between terrestrial and aquatic habitats, and mediate many of the linkages between these ecosystems.  This position in the landscape makes wetlands biologically diverse and active, and they are highly valued for the many ecosystem services they provide.  However, wetlands are also vulnerable to pressure from multiple stressors in the watershed, such as eutrophication, exotic plant invasion, and climate change.  Understanding how these multiple interacting stressors affect wetlands is therefore important for predicting responses to these changes in the landscape.  To test the interactive effects of N loading, invasion, and climate change on wetland plant community structure and function, we constructed 100 artificial wetlands in two locations with contrasting temperature regimes, and experimentally manipulated N loading and invasion.  Wetland mesocosms were constructed in northern and southern Michigan, fertilized with one of 12 fertilizer treatments ranging from 0 to 45 g N m^-2 yr^-1, and either planted with native emergent vegetation or maintained as bare ground. After one year of native plant establishment, all tanks were invaded by native and non-native cattails (Typhasp.).  We measured plant community compositional response, N cycling and retention, and invasion success.

Results/Conclusions

The biomass and composition of the native plant community exhibited significant non-linear responses across the fertilization gradient.  Net primary production (NPP) increased approximately two-fold along the gradient from 0 to 15 g N m^-2 yr^-1 (p < 0.05), but did not substantially increase above that threshold.  Community composition also changed significantly along the gradient (p < 0.05).  As expected, native plants increased N retention and strongly decreased both soil N and light availability (p < 0.05), thereby decreasing invasion success.  Contrary to predictions however, invasive Typha were poorer competitors at high N compared to low N.  Typha biomass was approximately tripled along the N gradient in the absence of native competitors, but changed relatively little in the presence of native competitors.  Thus, while native plants responded to fertilizer as expected, the response of invasive Typha was unexpected and did not match published empirical observations showing higher Typha dominance in eutrophic wetlands.  We theorize that maternal subsidies stored as reserves in Typha rhizomes drive these unexpected results, suggesting that short-term and long-term invasion outcomes may differ both quantitatively and qualitatively.  These results also suggest that Typha may be a relatively poor invader in the absence of disturbances, requiring at least small disturbances to establish.