COS 263-4 - Biotic resistance across a nutrient gradient in experimental coastal wetlands

Abstract: The dispersal, colonization, and establishment of non-native species is strongly influenced by biotic and abiotic factors, with prevailing interactions between resource availability, competition and invasive species traits. In the Great Lakes region, wetlands are experiencing invasion by several non-native plants, especially under high nitrogen (N) loading from the surrounding agricultural and urban landscape. However, biotic resistance to invasive plants through competition with native plant species could play a key role in reducing the rapid spread of these non-natives. We tested the biotic resistance exerted by a native species community against three taxa of cattails with contrasting invasiveness: Typha latifolia (native, least-invasive), T. angustifolia (non-native, invasive), and T. glauca (hybrid-species, most-invasive), across a nitrogen input gradient. Typha species performance was evaluated in a 5-year mesocosm experiment conducted at two locations (northern/southern Michigan). We tested two competition treatments: 1. Typha-only, only Typha species present, and 2. Invasion, where Typha species were introduced to a pre-established community of thirteen native species. The competition treatment was crossed with 12-levels of nitrogen input, ranging from 0 to 45 gN/m2/year. At the end of the experiment, ramets of Typha species were identified, measured, counted, and weighed to evaluate the composition and biomass of the final communities. We found less biomass and number of invaders when native plants were previously established, although this impact depended on both site and nitrogen level. In the southern site, invaders were almost completely outcompeted by natives under all nitrogen levels, while in the northern site invasive plants were able to succeed at high nitrogen in the presence of native species, but with a 61% overall reduction in biomass when compared to the Typha-only treatment (across all N-levels). Furthermore, when comparing biomass under the highest N input to the lowest, in the Typha-only treatment there was a >400% increase along the N gradient. The proportion of Typha species changed to a lesser extent along the nutrient gradient. Typha latifolia and T. glauca accounted for the most biomass overall, while T. angustifolia peaked at a medium nitrogen input. The observed patterns could have strong consequences on cattail establishment rates in the Great Lakes and consequently on its invasive success. We found that higher nitrogen increases the susceptibility of wetland invasion, but the presence of native species can locally mitigate invasion rate or magnitude. Our study highlights the role of biotic resistance of the native community to invasions in variable environments.