Stability of Juncus roemerianus patches in a salt marsh

In a Virginia (USA) marsh undergoing transgression due to rising sea level, we examined the stability of the boundary between nearly monotypic patches of Juncus roemerianus and adjacent plant communities for 6 years. Patch stability was evaluated by examining interannual changes in 5 cover classes: J. roemerianus, Spartina alterniflora, S. patens, Distichlis spicata, and wrack. Patches were chosen at four sites ranging from a low marsh site with mineral soil that receives semidiurnal estuarine flooding to a high marsh site with organic rich soil that receives only storm-induced flooding from the estuary. The semidiurnally flooded site in the low marsh was the only one losing J. roemerianus cover and had largest interannual variations in wrack and bare soil. At a high marsh site near a tidal creek, incidence of wrack cover was high and patchy but apparently insufficient to reduce cover of J. roemerianus. At the two sites with infrequent estuarine flooding because of distance from tidal creeks, wrack was virtually absent and patches of J. roemerianus expanded. Patch expansion at one of these sites occurred even though the marsh subsided, and coverage by the three other species decreased over the course of the study. Overall, J. roemerianus patches appeared quite stable, with declines associated with wrack disturbance at the site that had the deepest and most frequent tidal flooding. Patch stability is due in part to the wide tolerance of J. roemerianus for hydroperiod and salinity.