Native wetland plants provide biotic resistance against non-native cattail invasion in oligotrophic and eutrophic wetlands
Coastal wetlands exist at the interface between terrestrial and aquatic habitats. This landscape position makes these wetlands biologically diverse and active, but also vulnerable to fluctuating water levels, eutrophication, and other stressors. These stressors may individually or interactively reduce wetland ecosystem functioning or promote exotic invasive species. Recent low water levels in the Great Lakes have created large bare-ground areas as shorelines recede, likely contributing to a large-scale regional increase in aggressive non-native invaders such as Typha x glauca and Phragmites australis. Eutrophication during the same time period has also likely promoted the spread of these aggressive species. To test the individual and interactive effects of N loading and bare ground, we constructed 100 artificial wetlands in two locations (northern and southern Michigan) and experimentally manipulated N and the presence or absence of native plants. Wetland mesocosms were 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 cattails (Typha sp.). We measured native plant community response, N cycling and retention, herbivory, and invasion success for two subsequent years.
As expected, Typha biomass was significantly greater in fertilized wetlands without competitors (p < 0.05), approximately tripling in biomass along the N gradient. However, Typha biomass was low and changed relatively little in the presence of native competitors. The native plant community represented a strong biotic barrier to invasion, strongly limiting Typha growth even in highly eutrophic mesocosms. We found evidence of both direct and indirect competition. The native plant community significantly increased in biomass along the N gradient (p < 0.05), which directly reduced available light and soil N. The native plant community also attracted an active insect herbivore community, resulting in significantly higher rates of herbivory on Typha where natives were present. As a result, invasive Typha were poorer competitors at high N compared to low N. This result was unexpected since Typha is often dominant in eutrophic wetlands. Our results suggest that Typha may be a relatively poor invader in the absence of disturbances, requiring at least small areas of bare ground to establish, such as those created by fluctuating water levels. However, since Typha spreads largely through clonal growth via underground rhizomes, barriers regulating its initial establishment may differ from those regulating its eventual spread.