Floristic Development in Three Oligohaline Tidal Wetlands after Dike Removal

Abstract

The decline of salmon populations has intensified tidal wetland restoration efforts throughout the Pacific Northwest, but few results are available monitoring the trajectory of these efforts over time. In three oligohaline tidal wetlands, dike removal restored tidal influence to provide juvenile salmon rearing habitat in the South Fork Skagit River Delta, Washington, USA. This study compared up to 13 years of vegetation development in these restoration sites to reference tidal marsh sites using remote sensing and transect surveys. While native emergent plant communities and open water dominated the most recently restored site (41.6% and 39.5% cover), invasive species present prior to restoration dominated the earlier restored sites. Typha angustifolia (narrow leaf cattail) overran one site (60.7% cover), and Phalaris arundinacea (reed canarygrass) the other (40.0% cover). Typha angustifolia also covered 37.5% of the reference sites. Combined elevation distribution of invasive species overlapped that of native species, suggesting direct competition in this environment. Furthermore, the ability of pre-established invasive species to persist in the subsided restoration sites at elevations outside of reference occurrence ranges affected the native species elevation distributions. The authors hypothesize that despite sufficient native propagule dispersal, competition from persistent invasive species resulted in simplified community structures with reduced native herbaceous and scrub-shrub cover. In potential restoration sites dominated by non-native T. angustifolia and P. arundinacea, managers should consider their control to facilitate native species colonization. In new restoration sites where plant communities are still evolving, they should monitor invasive species cover and composition to keep levels below the that of the reference site condition.