Plant diversity and complexity often lag behind other measures of long-term vegetation development in restored US Pacific coast tidal marshes

Tidal wetland restoration is an important tool to enhance estuarine ecosystem functions. Despite substantial investments, ecological evaluation of the long-term outcomes of wetland restoration is uncommon, and such studies are seldom conducted across multiple sites, hindering understanding of restoration efficacy and design. We assessed vegetation similarity between 16 legacy tidal marsh restoration projects aged 22 to 62 yr and paired reference wetlands along the US Pacific coast as well as the influence of key environmental drivers on vegetation community structure. Across this large geographic region, restored sites generally had relatively high total plant cover dominated by native species. However, many restored sites had lower plant species richness than reference marshes at both the assemblage and site scales, and had less complex canopies. Moreover, there were commonly differences in species composition between restored and reference sites, often driven by differences in the abundance of dominant or subdominant native species. Across all marshes, elevation and salinity gradients strongly influenced community structure more than restoration status. Restored-reference differences were also driven more by site elevation than site age. Overall, our results suggest that restoration can effectively recover tidal marsh native plant communities, but that differences in species composition and diversity are likely to persist in marshes not restored to optimal elevations even after decades of development. In future projects, careful planning for site elevation and active intervention such as plantings for floristic diversity may improve biodiversity recovery. Our multisite approach can serve as a model for regional-scale monitoring to identify strategies to enhance future restoration success.