This large-scale, multi-site assessment of some of the oldest tidal marsh restoration projects in California, Oregon, and Washington provides insight into the development of marsh plant communities and blue carbon functions and demonstrates the value of long-term monitoring.

The Project Tidal wetland restoration is an important tool to increase wetland area and enhance ecosystem services in estuaries. It often involves re-introduction of tidal flow to former tidal wetlands. Although millions of dollars have been spent during recent decades on tidal restoration along the West Coast of the United States, there has been less investment in tracking the success of restoration projects through time. While tidal wetlands may be monitored for either structure or function soon after restoration, many projects are no longer tracked after several years. Restoration practitioners, managers, scientists, and coastal policy makers need information on long-term project outcomes to best plan and design future projects and quantify restoration benefits.

Working with four West Coast reserves, regional blue carbon working groups, local, state and tribal organizations, and other partners, this project examined some of the oldest tidal marsh restoration projects in California, Oregon, and Washington to learn more about restoration outcomes. The team compared restored marshes (aged 22-62 years) with nearby least-disturbed reference sites. The team collected data on wetland structure, including attributes which form the basis for many of the valued functions and services of tidal wetlands. This large-scale, multi-site assessment allowed for a powerful regional analysis of restoration outcomes along the West Coast.

For elevation capital, the team found that restored sites were often lower in the tidal frame than reference marshes. However, restoration projects involving sediment addition tended to be closer to the elevations of reference marshes. Restored marsh vegetation communities resembled reference sites in part. Both types of marshes tended to have high plant cover dominated by native species, but differences in species composition often remained and many restored sites tended to have lower plant diversity. Finally, results showed that soil carbon accumulation rates in restored sites were as high on average as reference marshes, suggesting that restoring sites have recovered this important ecological function.

The project findings are helping fill important regional data gaps by providing a long-term perspective on “lessons-learned” from past restoration projects. Project outputs include peer-reviewed articles, new regional datasets, and a restoration brief designed to help restoration practitioners and coastal wetland managers. Management recommendations offered in project products may confirm existing restoration practices, while others may provide useful new insights based on newly collected and analyzed data. Although this study included only tidal marsh restoration sites from the US West Coast, many of these recommendations may be applicable to restoration in other regions, or of other tidal wetland types such as tidal swamps.