Environmental determinants of Phragmites australis expansion in a New Jersey salt marsh: an experimental approach

Interdependence among disturbance events, ecosystem properties, and biologi cal invasions often make causal relationships difficult to discern. For exa mple, Phragmites australis invasion in mid-Atlantic salt marshes is often a ssociated with disturbances that create well-drained features as well as wi th low sulfide concentrations, but explanations of these associations have been elusive. We tested experimentally: 1) that disturbances increasing wet land drainage facilitate Phragmites invasion by altering sulfide concentrat ions and salinity; 2) that translocation allows plants to spread beyond dra inage areas; and 3) that plants can then lower edaphic stress through press ure ventilation of the rhizosphere and promote further expansion. At the in vasion front, treatments of 1) severing rhizomes to halt translocation and 2) combined severing with clipping dead culms to limit ventilation of the r hizosphere killed most culms, but did not affect pore water chemistry. In a lready invaded areas, severing and clipping reduced culm height and panicle production, severing alone and in combination with clipping also raised su lfide and ammonium concentrations in the root zone. There were no treatment effects on plant performance or pore water chemistry along mosquito ditche s, where sulfide concentrations were negligible. Small-scale hydrological a lterations such as ditches appear to provide suitable sites for the establi shment of Phragmites because soils are well-drained and are low in free sul fides. Subsequent expansion into more hostile areas occurs through transloc ation, with well-drained areas acting as sources for essential substances. Once established, the plant increases rhizosphere oxygenation and lowers su lfide concentrations.