Phragmites australis invasion and expansion in tidal wetlands:: Interactions among salinity, sulfide, and hydrology

Through their physiological effects on ion, oxygen, and carbon balance, respectively, salinity, sulfide, and prolonged flooding combine to constrain the invasion and spread of Phragmites in tidal wetlands. Initial sites of vigorous invasion by seed germination and growth from rhizome fragments appear limited to sections of marsh where salinity is < 10parts per thousand, sulfide concentrations are less than 0.1 mM, and flooding frequency is less than 10%. In polyhaline tidal wetlands the invasion sites include the upland fringe and some high marsh creek banks. The zones of potential invasion tend to be larger in marshes occupying lower-salinity portions of estuaries and in marshes that have been altered hydrologically. Owing to clonal integration and a positive feedback loop of growth-induced modification of edaphic soil conditions, however, a greater total area of wetland is susceptible to Phragmites expansion away from sites of establishment. Mature clones have been reported growing in different marshes with salinity up to 45parts per thousand, sulfide concentration up to 1.75 mM, and flooding frequency up to 100%. For Phragmites establishment and expansion in tidal marshes, windows of opportunity open with microtopographic enhancement of subsurface drainage patterns, marsh-wide depression of flooding and salinity regimes, and variation in sea level driven by global warming and lunar nodal cycles. To avoid Phragmites monocultures, tidal wetland creation, restoration, and management must be considered within the context of these different scales of plant-environment interaction.