Over the past century, humans have extensively diked and drained tidal wetlands in delta regions throughout the world. In an attempt to recovery these lost habitats, large scale tidal reconnection restoration efforts are now common. In the Pacific Northwest, non-native plant invasions have been found to impede recovery of native wetland plant communities in the high marsh zones (>2.5 m) of tidally reconnected oligohaline wetlands in the Columbia River Estuary. The aim of this study was to evaluate the possible mechanisms driving these high marsh non-native plant invasions. To do this seed bank samples were collected from the dominant native (n=20), Carex lyngbyei and Schoenoplectus lacustris, and non-native (n=20), Phalaris arundinacea and Juncus effusus subsp. effusus, plant communities within two restoration sites. These seed bank samples were then evaluated for composition through direct seed counts and for viability under simulated tidal flooding and salinity conditions in the greenhouse, designed to mimic those observed in the field and those anticipated with future sea level rise. This included three flooding treatments representing the high-marsh (1 hour daily), mid and low marsh (3 and 6 hours twice daily) and three salinity treatments including freshwater (<1 ppt), oligohaline (3 ppt), and brackish (10 ppt).
Results/Conclusions
Seed bank investigations within the plant communities found that non-native species, P. arundinacea and J. effusus, were the most abundant seeds identified across both native and non-native plant communities. In the greenhouse these species were also found to germinate at significantly greater densities under high marsh flooding and freshwater treatments as compared to mid-low marsh flooding oligohaline and all brackish treatments, mirroring their observed in-situ abundances. Native species, C. lyngbyei and S. lacustris, on the other hand, were found to germinate at similar densities across all flooding/salinity treatments. These results indicate that newly created salinity and flooding gradients in restored tidal wetlands act to suppress these non-native species’ germination in the low-mid marsh but not in the high marsh, where they are likely able to outcompete with the native species in-part due to their overwhelming dominance in the seed bank. These results further indicate that sea level rise increases in flooding and salinity may reduce these non-native species abundances, however this may come at the cost of losing existing high marsh habitat. This research highlights the importance of restored wetland seed bank compositions, tidal flooding, and salinity regimes to the overall recovery and sustainability of these critical ecosystems.