Early post-restoration recovery of tidal wetland structure and function at the Southern Flow Corridor project, Tillamook Bay, Oregon
- A substantial fraction of estuarine tidal wetlands have been lost to development or other human uses in the Pacific Northwest since the 1800s. Wetland restoration, typically through tidal re-connection, can restore normal tidal hydrology to these areas and improve estuarine capacity to support ecosystem functions and services. Restoration may initiate a cascade of ecosystem-level impacts to channel and groundwater hydrology, soils, vegetation and fauna, and carbon cycling. Construction of the large Southern Flow Corridor (SFC) restoration project (179 ha) was implemented in southern Tillamook Bay in 2016 to reduce urban flooding and to enhance other wetland ecosystem services such as fisheries production and carbon sequestration. The project occurred on former tidal wetlands (originally emergent tidal marshes and forested tidal swamp) that had been diked for over 60 years prior to restoration. During the diked period, the site was used for crop agriculture, cattle grazing, and non-tidal freshwater marsh mitigation. Much of the site had been abandoned from active agricultural use for several years prior to restoration. We conducted pre-restoration (2013-2015) and early post-restoration (2017-2020) measurements of a wide range of hydrologic, soil, and biological parameters at SFC and least-disturbed reference tidal wetlands to assess early post-restoration change in ecosystem structure. Within the SFC site, we evaluated how pre-restoration differences in elevation and land-use/land-cover zones influenced early restoration trajectories. We compared conditions at SFC with two types of reference wetlands in Tillamook Bay: low and high reference marshes. Before restoration, SFC wetlands were more comparable to low reference marsh than high marsh in elevation and had fresh and slightly acidic soils with relatively low dry season-groundwater levels. SFC tidal channels were also fresh with maximum water levels much lower than fully-tidal reference channels. SFC vegetation was a mix of freshwater-adapted native and non-native species including reed canarygrass. Pre-restoration conditions differed to some extent by land-cover/land-use zone, with the northern zone being higher in elevation while the cropped zone at the southern part of the site was relatively low in elevation. Within two years of dike removal, hydrology, soils, and vegetation changed markedly at SFC, moving towards reference wetland conditions. Soil pH, salinity, and dry-season groundwater level tended to increase and existing vegetation began to die back, creating bare ground. Reed canarygrass in particular declined considerably in the middle and cropped zones in the site. Within 2-4 years of dike removal, many brackish-tolerant estuarine species began to colonize and spread across the southern and middle regions of SFC. Early soil accretion rates at SFC were high, especially in the cropped zone which was low in elevation both before and after restoration. Changes in channel morphology were observed in some locations, including channel widening and bottom scour. Restoration at SFC also led to changes in fish and benthic invertebrate communities in tidal channels. Juvenile chinook and chum salmon increased in abundance at SFC following restoration. Other finfish species such as juvenile coho salmon, staghorn sculpin, three-spined stickleback, and juvenile surfperch were found utilizing channels within the restored site, although not necessarily increasing substantially in abundance due to the restoration. Benthic invertebrate communities shifted to include more amphipods and less insects after restoration activities. Larval and adult mosquitos were captured at sites inside and near the SFC project both before and after restoration, but mosquito numbers were very low. In one of the first studies of greenhouse gas emissions from tidal wetlands in the Pacific Northwest, we found that fluxes of methane and carbon dioxide were driven by complex interactions of groundwater table, salinity, and temperature at SFC and in reference and disturbed (diked former) tidal wetlands. Methane emissions were highly variable in reference wetlands and at SFC, but high when groundwater levels were high and salinity was low. Nitrous oxide emissions were generally very low across all the wetland types measured. Monitoring and developing mitigation strategies for methane in tidal wetland restoration projects may be desirable for restoration practitioners since it is a powerful greenhouse gas. Our data provide an early snapshot of ecosystem change across an array of physical and biological parameters at the SFC site shortly after restoration of tidal flows at the site. Our findings suggest that several parameters, processes and functions at the SFC site are well on their way towards becoming similar to reference tidal wetland conditions. Processes and parameters that were already similar to (or exceeded) reference conditions two years after restoration included groundwater level, channel maximum water level, soil salinity and pH, soil accretion rate, and abundance of some finfish species. Other parameters and processes may take more time to become similar to reference marshes. In terms of support for native plant, invertebrate, and finfish species, our monitoring data suggest the project is enhancing tidal wetland functions in Tillamook Bay. The heterogenous nature of SFC prior to restoration allowed us to examine the role of land use/land cover in post-restoration change. We found that early rates of recovery in soils and vegetation at SFC were linked to pre-restoration gradients of elevation and land-use/land-cover differences. As development of the site proceeds, we anticipate on-going changes such as widening of channels, sediment accretion that raises wetland elevations, succession of plant composition, and potentially establishment (or persistence) of tidal forested or scrub-shrub wetlands in portions of the SFC site that have sufficiently high elevation and low salinities. To further characterize rates of change, and to collect data necessary for possible adaptive management in the future, we recommend continued periodic measurement of key ecosystem parameters at the SFC site and in reference wetlands in the coming decades. We suggest that additional data on wetland processes (such as carbon dynamics, soil accretion, fish use, and food web structure) would be a powerful complement to the parameters that have ben monitored to date. Finally, in terms of monitoring design we note that this project highlighted the value of including a variety of reference wetlands (at both low and high elevation), since their inclusion allows a more robust picture of restoration site development in comparison to the diversity of least-disturbed wetlands within an estuary.
Read the full study here: Early post-restoration recovery of tidal wetland structure and function at the Southern Flow Corridor project, Tillamook Bay, Oregon