Elevating resilience: a multi-site synthesis of sediment placement as a lifeline for saltmarsh sparrows (Ammospiza caudacuta)

Many New England salt marshes are rapidly degrading due to the combined effects of accelerating sea-level rise (SLR) and other anthropogenic stressors, resulting in loss of high-marsh vegetation and threatening marsh-dependent wildlife such as the Saltmarsh Sparrow ( Ammospiza caudacuta ). Sediment placement (SP) is widely used to increase marsh elevation and resilience, yet long-term ecological outcomes, particularly for wildlife, remain uncertain. To evaluate SP efficacy, we synthesized biological and physical responses from four large-scale salt marsh restoration projects in Rhode Island monitored for up to 8 years following SP using a Before–After–Control–Impact (BACI) design. Vegetation recovery trajectories varied widely among sites. Spartina alterniflora cover declined consistently after SP, but robust high-marsh plant communities reestablished at only two sites, indicating site-specific restoration outcomes. Plant community complexity increased overall, with gains in species richness and diversity as S. alterniflora monocultures transitioned toward heterogeneous high-marsh meadows. Vegetation recovery was strongly linked to hydrology, with greater cover near drainage features (<35 m) and in areas of lower porewater salinity (<25 ppt). Avian use shifted through time. Recently restored, sparsely vegetated marshes were used by Piping Plover ( Charadrius melodus ) and other beach-nesting species, while Saltmarsh Sparrows nested within 4–5 years post-restoration. Male sparrow abundance was modestly higher at SP sites than controls, and nests were concentrated in areas recovering with Spartina patens , though limited vegetation structure resulted in lower nest placement and shallower thatch than reference areas. Marsh surface elevations increased 7–33 cm relative to mean high water following SP, improving hydrology by reducing flooding and enhancing drainage, and extending estimated marsh lifespans by 24–111 years under current SLR rates. Our multi-site synthesis demonstrates that SP can effectively restore marsh elevation, but ecological outcomes are strongly mediated by site-specific physical conditions, particularly hydrology. These results highlight the importance of long-term, multi-metric monitoring and adaptive management to maximize the conservation benefits of SP salt marsh restoration.