Edges of opportunity: tidal creek edge processes drive pioneer marsh vegetation establishment

Tidal marshes provide many critical ecosystem services, yet extensive losses have occurred due to human land use change. This has created a demand for tidal marsh restoration. A key challenge in tidal marsh restoration projects is providing the conditions for early establishment of pioneer algae and plants, which are often limited by unfavorable environmental conditions. Observations reveal that pioneer species preferentially establish along tidal creek edges, yet the mechanisms causing this colonization pattern remain poorly understood. Here, we investigated the drivers of pioneer vegetation colonization along tidal creeks edges in a brackish marsh in the Scheldt estuary (Belgium). First, we characterized the spatio-temporal colonization pattern using geospatial analyses and seedling survival (via a field transplantation experiment). Then, we tested potential mechanisms underlying this pattern through field assessments of seed deposition and retention (via seed traps and seed bank sampling), and sediment characteristics. Higher seed retention (around 3 to 4 times higher near tidal creek edge compared to interior tidal flat) and improved seedling survival were found near tidal creeks. Furthermore, increased sediment shear strength (around 4 to 10 times higher near tidal creek edge compared to interior tidal flat) was measured in the superficial sediment layer near tidal creeks. In contrast, other sediment properties measured at greater depth (up to 5 cm), such as dry bulk density, did not consistently explain these spatial patterns. Instead, we hypothesize that enhanced surface drainage and evaporation-driven consolidation near tidal creeks increase sediment erosion resistance of the thin sediment surface layer, which enhances seed retention and seedling survival. Overall, our results show that the superficial sediment bed of creek edges forms a hotspot for pioneer algal and plant establishment, demonstrating the importance of tidal creek networks to promote tidal marsh development in restoration projects.