Variation in soil salinity associated with expansion of Phragmites australis in salt marshes

Salinity is a well-known stressor of Phragmites australis (common reed), leading to reduced success in brackish and salt marshes. Although saline, many remaining salt marshes in New England are changing in structure and function due to tidal restrictions and rapid proliferation of P. australis. The poor reputation of this native plant (its dominance is used as an indicator of marsh degradation) has stimulated management and research using natural stressors for control. Our field study associated natural variability in soil salinity levels over time and space with vigor and spread rates of P. australis. Over 2 years, salinity was measured 15 times from three depth intervals (5-20, 35-50, and 65-80 cm) at five stations established in six colonies of P. australis. Our results indicated that salinity in tidal marshes varied temporally due to the extent of tidal flooding (salinity was greater during spring tides compared with neap tides) and regional freshwater runoff (salinity was lower in the spring). If the growing season is split into early (May-July) and late (August-October) periods, interesting patterns emerged (salinity increased with depth early, but decreased with depth late). Shoot height, cover, and expansion rate of the six colonies were measured twice over 3 years. In general, the stands of P. australis were expanding into salt marsh at 0.35 m per year, and increasing in cover (8% per year), even though the canopy height decreased at all but two of the sites over the study period. Salinity was lower in marshes where tides were artificially restricted (11 - 16 ppt compared with 19 - 24 ppt for the natural marshes), and one of these sites exhibited rapid P. australis expansion. At sites with natural hydrology, P. australis appeared to be expanding more slowly, shading out marsh species, and perhaps avoiding salinity stress by accessing natural sources of fresher water at different soil depths during different seasons. (C) 2001 Elsevier Science B.V. All rights reserved.