The decline of ecologically important and economically valuable salt marsh systems represents a global concern. Nutrient enrichment has been identified as a potentially important driver of this decline, with nutrient influxes from anthropogenic activities altering community dynamics and ecosystem stability. However, relatively little is known about the effects of nutrient enrichment on plants in the high marsh relative to plants in the low marsh near the creek edge. Consequently, we assessed the short- and long-term effects of nutrient enrichment on above- and below-ground production of short-form Spartina alterniflora, a dominant plant species in the high marsh. We conducted a field reciprocal transplant experiment of Spartina genotypes from fertilized and reference creeks in a long-term, whole-ecosystem, nutrient-enrichment experiment in Plum Island, MA. We reared plants in the greenhouse for one year before transplanting to the field, measured stem density, height, and flowering during the growing season for two years, and then harvested the plants to determine above- and below-ground production. We looked at the independent and interactive effects of long-term fertilization (i.e., plant origin) and short-term fertilization (i.e., transplant location) on performance of Spartina genotypes to understand the effects of nutrient enrichment on high marsh plant communities.
The effects of nutrient enrichment on primary production in the high marsh were evident after only two years: aboveground biomass of Spartina transplants was significantly higher in fertilized than reference gardens. While total belowground biomass did not differ across gardens, allocation to root and rhizome biomass varied depending on nutrient enrichment, with transplants in fertilized gardens having higher rhizome biomass, lower root biomass, and a smaller root:rhizome ratio than transplants in reference gardens. Transplants in fertilized gardens also had a greater aboveground:belowground ratio. This matches earlier studies of tall-form Spartina in response to fertilization, and indicates that, despite its high marsh location, short-form Spartina is also being affected by anthropogenic nutrient input. Further, these effects manifested after only two years, suggesting that fertilization may impact plant community structure and ecosystem function of both the low and high marsh on a short time scale. We also found significant differences in production between transplants originating from fertilized and reference sites: the relative performance of transplants from the fertilized site was strongly reduced in reference gardens, indicating that long-term fertilization may also compromise the ability of plants to perform under ambient nutrient conditions.