Fertilization Favors Grasses Over Forbs

There are forb-dominated communities unique to salt marshes of the northernmost Atlantic states of the USA, the New Brunswick coast of the Bay of Fundy and the St. Lawrence Estuary that are not associated with exceptionally high soil salinity or saturation. These forb-dominated (predominantly Plantago maritima and Triglochin maritima ) and Spartina patens -dominated communities were subjected to monthly fertilization with a slow-release nitrogen fertilizer in a Bay of Fundy marsh. After 20 years forb cover significantly declined where they had been dominant and are being replaced by grasses (predominantly S. patens and Spartina alterniflora ). This shift is likely due to the increased availability of nitrogen which conferred a competitive advantage to S. patens and S. alterniflora . These grasses require nitrogen for production of osmolytes, chemicals which reduce osmotic stress due to saline soils. In contrast, Plantago maritima and T. maritima do not depend upon nitrogen for their osmolyte production. In marshes to the south forb communities are only associated with exceptionally high soil salinity or saturation. Results of our experiment suggest that absence of forb communities similar to those of higher latitudes could be due to historically greater availability of nitrogen in marshes to the south.