Seed banks of Phragmites australis-dominated brackish wetlands: Relationships to seed viability, inundation, and land cover

In tidal wetlands of the eastern United States, buried seeds of the non-native haplotype of Phragmites australis may be a source of propagules for re-establishment after eradication efforts but factors controlling the development and expression of seed banks in non-native Phragmites stands have not been examined. We sampled surface soil at four Chesapeake Bay brackish tidal wetlands dominated by the non-native (European) haplotype M of Phragmites and used the seedling emergence method to quantity species of seedlings emerging under flooded and non-flooded soil conditions. Within each subestuary, one site was dominated by Phragmites that produced viable seeds (high viability) and the other by Phragmites that did not (low viability). We also described standing vegetation in plots, measured soil salinity, analyzed soil characteristics, and described surrounding land cover. Based on number of emerging seedlings, we found that 284 and 698 Phragmites seeds m⁻² occurred at the two high-viability sites, which was significantly higher than seed densities at the low-viability sites (10 seeds m⁻²), and greater than densities reported elsewhere. We also found that emergence of Phragmites seedlings from soil samples was prevented by continuous flooding of 3.5 cm of standing water, suggesting that colonization of deep water areas is due to vegetative clonal expansion from Phragmites in adjacent higher elevations. The density of Phragmites seeds was not related to soil salinity or abundance of other species in the seed bank or vegetation, but instead was positively related to greater wave energy disturbance (much longer fetch and more open water) and lower area of wetlands nearby. The seed bank was more species-rich (15-22 species observed) than standing vegetation (3-15 species) at all sites, meaning that the dominance of Phragmites in vegetation does not prevent the development of a diverse seed bank and implying that a species-rich community may establish rapidly following control efforts. Based on these results and our findings in related studies, we postulate that wave energy disturbance generates repeated opportunities for seedling recruitment by Phragmites, which creates stands of Phragmites with higher genotypic diversity. In turn, genetically diverse stands favor greater cross-pollination and production of viable seed. These findings suggest that, in North America, targeting control efforts on non-native Phragmites patches in areas of higher exposure to wave energy may be more effective in reducing source populations than efforts in more protected locations.