Nitrogen further promotes a dominant salt marsh plant in an increasingly saline environment

Aims Human alterations of the environment are combining in unprecedented ways, making predictions of alterations to natural communities a difficult and pressing challenge. Estuarine systems have been subject to a high degree of modification, including increased nitrogen (N) inputs and altered salinity, factors important in shaping estuarine plant communities. As human populations increase and the climate changes, both N and salinity levels are likely to increase in these coastal marshes. Our objective was to evaluate the interactive effects of N and salinity on US West Coast salt marsh species; in particular, the performance of the dominant species Sarcocornia pacifica (pickleweed) alone and in mixed species assemblages. We expected increased salinity to favor S. pacifica but that N enrichment could help maintain greater species richness through use of N in salinity tolerance mechanisms. Methods We crossed treatments of N (added or not) and salinity (salt added or not) in a field experiment at a salt marsh in the San Francisco Estuary, California, USA, in each of three habitats: (i) monotypic pickleweed on the marsh plain, (ii) monotypic pickleweed along channels and (iii) mixed assemblages along channels. In a greenhouse experiment, we crossed treatments of N (added or not) and salinity (at three levels to simulate brackish to saline conditions) in (i) pots of pickleweed only and (ii) the same species mix as in the field. Important Findings N addition doubled S. pacifica biomass and branching in both channel and marsh plain habitats regardless of salinity and greatly increased its dominance over Distichlis spicata and Jaumea carnosa in mixed assemblages along channels. In the greenhouse, S. pacifica biomass increased 6to 10-fold with N addition over the range of salinities, while D. spicata and J. carnosa biomass increased with N addition only at lower salinity levels. Thus, while localized management could influence outcomes, expected overall increases in both N and salinity with human population growth and climate change are likely to enhance the production of S. pacifica in US West Coast marshes while reducing the diversity of mixed species assemblages. This decline in diversity may have implications for the resilience of marshes already subject to multiple stressors as the climate changes.