A Global Review of Salt Marsh Restoration: Why Simpler, Context-Specific Strategies Can Outperform Complex Approaches.

Improving our understanding of the drivers of restoration success is a global priority, as effective restoration efforts are essential for biodiversity conservation, climate resilience and regenerative development. We present a global meta-analysis of active salt marsh restoration to evaluate how different restoration approaches approximate natural reference conditions across physical, biotic, and functional attributes, how factors such as project age, project timing, spatial extent, and latitude influence outcomes, and whether combining interventions yields synergistic benefits over single methods. Most approaches, including tidal restoration, sediment addition, and fertilisation, approximated reference conditions on average, but outcomes varied widely among studies, with attributes often falling short of or exceeding those of natural sites. The diversity of metrics used to assess restoration outcomes further complicates the interpretation of this variability. Deviations from reference conditions did not diminish with increasing project age, spatial extent or timing, highlighting the strong context dependency of restoration outcomes and the need for improved evaluation frameworks. Contrary to prevailing assumptions, combining multiple restoration interventions did not consistently enhance restoration success and was also associated with lower similarity to reference conditions and greater variability in outcomes. However, this pattern appears to be driven by the performance of specific intervention combinations rather than reflecting a general disadvantage of multi-intervention approaches. These findings suggest that combinatorial restoration strategies are heterogeneous in effectiveness and may be less predictable across contexts than simpler interventions, emphasising the need for careful restoration planning and indicating that context-specific strategies may provide more scalable solutions under global change.