Phragmites australis (Poaceae) is an invasive grass in mid-Atlantic salt marshes of the United States. However, the grass is experiencing a dieback at the filled Harrier marsh in New Jersey. Measurements of pore water salinity taken for Phragmites australis, Juncus gerardii, Spartina patens, Distichlis spicata, and recent disturbance patches indicated significantly higher salt concentrations in Phragmites patches than for Spartina patens and Distichlis spicata, even though the flooding regime was similar for these patches. Furthermore, salinity readings taken after rainfall events indicate quicker recovery to high salinity conditions in Phragmites than in other species. Precipitation readings and test holes dug at the site indicate that the major input of water during the summer was tide water. We hypothesize that transpiration rates of the plants are differentially affecting pore water salinity and sulfide concentrations at the site. The resultant increase in salinity under Phragmites leads to significant demands on nitrogen due to the accumulation of osmoregulatory amino acids and inhibition of ammonia uptake. Increases in sulfide concentrations lead to sulfide toxicity and competitive inhibition of ammonia. The resultant stresses might account for the diminutive culms in the dieback areas. The results are dependent on site and time specific factors, including drought, lack of ground water due to filling, and fill characteristics.

Key words: Dieback, Phragmites australis, Salinity, Transpiration