Effects of Phalaris arundinacea and nitrate-N addition on the establishment of wetland plant communities

Nutrient enrichment may adversely impact plant species richness in wetlands and enhance their susceptibility to colonization and dominance by invasive species. For North American prairie wetlands, enrichment by nitrate-N (NO₃-N) from agricultural runoff is thought to contribute to the increasing colonization and dominance of Phalaris arundinacea (reed canary grass), especially during restoration. If true, P. arundinacea might compromise the re-establishment of sedge meadow vegetation on sites reflooded with agricultural drainage water. We tested this hypothesis using a fertilization experiment in wetlands with controlled hydrology. A community mixture comprising 11 species from native sedge meadow was seeded in mesocosms and grown under one of three NO₃-N levels (0 g m^-2 year^-1, 12 g m^-2 year^-1, 48 g m^-2 year^-1) with or without P. arundinacea. Above- and below- ground biomass were measured after two growing seasons to assess the response of vegetation to NO₃-N and P. arundinacea treatments. The total shoot biomass of the native community was suppressed in the presence of P. arundinacea at all NO₃-N levels, but shoot suppression was significantly greater at the highest NO₃-N dose level (48 g m^-2). Shoot growth of the native community was reduced by nearly one-half under these conditions. The total root biomass of the community was also suppressed by P. arundinacea when no NO₃-N was added. As NO₃-N increased, the relative abundance (shoot biomass) of native graminoids declined while native forbs increased in communities with and without Phalaris. The most common graminoid, Glyceria grandis, was suppressed by P. arundinacea at all levels, with suppression enhanced at the 48 g m^-2 NO₃-N level. Three other species were suppressed at the highest NO₃-N level, in the presence of Phalaris. The two most common forbs, Asclepias incarnata and Sium suave, exhibited a continual increase in growth with NO₃-N additions along with overall suppression by P. arundinacea. Community diversity and evenness declined with increasing NO₃-N levels, whether or not P. arundinacea was present. Our results demonstrate that if P. arundinacea is present, the restored sedge meadow community will not achieve levels of abundance that are possible when this species is absent, regardless of NO₃-N enrichment conditions. At the same time, the increased suppression by P. arundinacea at the 48 g m^-2 NO₃-N dose level supports the hypothesis that the dominance of this species over the native sedge meadow community is enhanced by NO₃-N inputs at levels that are common in agricultural landscapes. Our results carry two implications for achieving biodiversity conservation in agricultural landscapes. First, reducing nitrate loads to wetland reserves is essential for minimizing declines in community diversity. Secondly, the use of P. arundinacea for soil conservation and other agri-environmental purposes should be curtailed because of the likelihood of off-site impacts to wetland biodiversity.