Long-term nutrient enrichment, mowing, and ditch drainage interact in the dynamics of a wetland plant community

Fertilization studies have elucidated basic principles of the role of nutrients in shaping plant communities and demonstrated the potential effects of anthropogenic nutrient deposition. Yet less is known about how these effects are mediated by interacting ecological factors, particularly in nutrient-poor wetland habitats. In a long-term experiment in a coastal plain wetland, we examined how fertilization and mowing affected the diversity and composition of a plant community as it reestablished after major disturbance. A drainage ditch in proximity to the experimental plots allowed us also to consider the influence of hydrology and its interactions with nutrient addition. Fertilization decreased species richness, with wetland specialist species showing especially great losses, and several lines of evidence suggest that the effect was mediated by competition for light. Altered hydrology via ditch drainage had effects that were similar to fertilization, with more rapidly draining plots showing lower diversity and decreased abundance of wetland species. Plant community diversity and dynamics were influenced by complex interactions between fertilization, disturbance, and hydrology. The negative effect of fertilization on species richness was initially mitigated by mowing, but in later years was more evident in mowed than in unmowed plots. In the absence of disturbance, nutrient addition increased the rate of transition to primarily woody communities. Similarly, drained plots experienced increased rates of succession compared to wetter plots. Our results suggest that these interactions need to be considered to understand the potential effects of anthropogenic nutrient addition and hydrologic alterations to wetland ecosystems.