Tidal wetland historical change by habitat class, Oregon coast, USA from 1800s to 2019

How have tidal wetlands on the Oregon coast, USA changed over the past 150-200 years? We mapped and quantified a striking and previously unrecognized coastwide alteration: the near-eradication of once-prevalent tidal forested wetlands (TFWs). We argue that this finding calls for a paradigm shift in management of Oregon’s outer coast estuaries. By combining elevation-based estuary maps and historical (1800s) vegetation data, we found that TFWs made up a majority (54%) of the coast's historical tidal wetland area, while emergent tidal wetlands ("tidal marsh") occupied a smaller area (42%). However, diking and vegetation conversion resulted in the loss of 95% of historical TFW, compared to 59% of historical tidal marsh. Tidal marsh loss was offset by a gain of 1770 ha of tidal marsh through progradation, but no similar gain occurred for TFW. The extreme loss of TFW has most likely reduced the provision of key TFW ecosystem services, such as carbon sequestration and complex, productive salmonid habitat. Restoring TFW is a high priority and can help re-establish these habitats and ecosystem services, benefiting coastal economies and ecosystems. Further research is needed to identify restoration sites with environmental conditions suitable for the establishment and long-term persistence of TFW. Identification and prioritization of potential restoration sites should account for climate change and emphasize the availability of landward migration areas and other landscape setting factors that could affect long-term persistence of TFW.