Positive interactions occur between Phragmites australis lineages across short term experimental nutrient regimes.
Land use changes and greater nitrogen input into waterways have facilitated the spread of an invasive Eurasian lineage of Phragmites australis across North America. Its establishment has led to decreases in wetland plant diversity, and displacement of a native American Phragmites lineage considered to be a low-nutrient specialist. We hypothesized that carbon-rich amendments that reduced nitrogen availability would competitively favor the native lineage and nitrogen additions would favor the invasive lineage. In the greenhouse we assessed competitive interactions between native and invasive lineages following sawdust (low nitrogen) and urea (high nitrogen) additions by measuring total biomass, chlorophyll fluorescence and evaluating biomass allocation. Sawdust additions did not limit invasive Phragmites growth, while urea increased aboveground biomass of both lineages. Unexpectedly, mixtures of native and invasive Phragmites produced more above and belowground biomass than monocultures. Our findings suggest that at the level examined in this study, carbon additions would not be an effective management tool to control invasive Phragmites or restore the native North American lineage, but that facilitation between native and invasive lineages could promote their coexistence across a range of nutrient availability. Our results also provide limited evidence that displacement of native Phragmites could be due to other factors such as disturbance rather than competitive exclusion.