Changes in plant communities of low‐salinity tidal marshes in response to sea‐level rise

Abstract

AbstractAs sea‐level rises, low‐salinity tidal marshes experience greater flooding with more saline water. In the Chesapeake Bay estuary, we compared the 1980 and 2014 tidal marsh inventories (TMIs) of plant communities from James City County, Virginia, USA, with respect to the spatial distribution of two species—the invasive reed Phragmites australis and native salt marsh grass Spartina alterniflora–plus overall species richness. Since the 1980 TMI, the total area of low‐salinity tidal marshes in which P. australis occurred increased from 0.46 km2 to 6.30 km2 in 2014. Between TMIs, however, the total area of low‐salinity marshes occupied by S. alterniflora increased by only 0.02 km2. Species richness in low‐salinity tidal marshes decreased from 41 to 25 between TMIs. To assess seedling emergence under increased flooding and salinity, we completed two seed bank germination experiments using soil samples collected from six low‐salinity marshes containing established P. australis stands. In the first experiment, more seedlings emerged in the two low‐salinity (0 vs. 5 ppt) treatments after seven weeks, irrespective of flooding (water 3.75 cm below vs. at soil surface), but no P. australis or S. alterniflora germinated. For the second experiment, we added seeds of P. australis and S. alterniflora to soils exposed to the same flooding and salinity treatments to test the impact of these plant competitors on seedling emergence. No difference in number of seedlings was detected among treatments, but the number of species and their relative abundance was significantly affected by flooding (ANOSIM, R = 0.138, P < 0.001). The presence of P. australis and S. alterniflora seedlings appeared to shift the physical factor more influential on seedling emergence from salinity to flooding. For both seed bank experiments, more seedlings but not more species emerged from soils collected from marshes where P. australis coverage was <50%. High diversity plant communities of low‐salinity tidal marshes along the upper reaches of this estuary are gradually being replaced by those dominated by P. australis and S. alterniflora—a trend expected to continue here and in other riverine estuaries of the Atlantic and Gulf Coasts.