Multi-site salt marsh restoration can recover key natural functions despite long-term structural deviations and site-level differences

Restoration studies often focus on short-term structural recovery, while delivering consistent, long-term functional results across multiple locations remains a challenge, limiting their scalability. We sampled multiple restored salt marshes, at least 10 years old, at mid and low shore levels in the Venice Lagoon to evaluate the long-term structural and functional effectiveness (average match to reference natural sites) and outcome consistency (dispersion around the average) of two restoration methods: RC which favours tidal creek development at the low shore, and RB, which does not. At the mid shore, neither method fully replicated the natural sediment characteristics, and site-level differences often exceeded natural levels. Nevertheless, on average, restored vegetation matched the biotic structur and function of natural sites, including biomass, soil carbon and macrofaunal diversity. In contrast, at the low shore, neither method supported the native cordgrass Spartina maritima, despite more natural-like sediment conditions. Only the non-native S. anglica established at the low shore — thriving at RC sites, with biotic and functional traits similar to natural sites, but failing to establish at RB sites. These results suggest that methods restoring natural foreshore dynamics, like RC, better approximate low-shore natural conditions and support vegetation establishment—albeit non-native S. anglica—while RB sites supported none. Moreover, restoring multiple diffuse sites may be more effective than targeting individual locations, as a network of restored sites can, on average, better replicate key natural structural and functional conditions, even in the presence of persistent structural deviations and elevated site-level differences.