Tidal elevation and land use impact early development in a restored Pacific Northwest tidal wetland

Over 80% of historical tidal wetlands along the west coast of the United States have been lost to development and agriculture, but habitat restoration can help reverse loss of valued ecosystem functions such as flood attenuation, habitat provision for wildlife, and carbon sequestration. We investigated early ecosystem recovery of soils and vegetation in the Southern Flow Corridor (SFC) project in Tillamook Bay, one of the largest coastal restoration projects in the Pacific Northwest. We hypothesized that gradients of elevation and pre-restoration land use (cropped versus grazed versus several distinct unmanaged seasonal freshwater wetlands) would impact recovery of tidal wetland structure, with greater dissimilarity between intensely managed areas and reference conditions, and greater post-restoration change in soils and vegetation at lower tidal elevations. Elevations of land-use zones within SFC varied prior to restoration, with the zone used for cropping comparable to low reference marsh while one unmanaged zone and the grazed area occurred at intermediate elevation. Soils in all SFC zones were relatively fresh and low in pH prior to restoration, and all had high plant cover. Post-restoration vegetation change was greatest in the cropped zone at SFC which experienced a large decline in non-native species; the greatest increase in pH was in one of the unmanaged zones at the site. Consistent with our hypothesis, post-restoration change of some parameters was also linked to tidal elevation. Sediment accretion, loss of non-native species cover, and increases in soil pH and salinity were highest at lower elevations. Our results demonstrate that spatial heterogeneity within a tidal wetland restoration site can impact early recovery relative to reference conditions. Pre-restoration assessments of such heterogeneity may help establish more spatially-explicit management goals relative to restoration trajectories.