Designing microtopographic structures to facilitate seedling recruitment in degraded salt marshes

Seedling recruitment in many ecosystems depends upon seed dispersal to, and retention at, sites that are suitable for seed germination and seedling establishment. Seed retention has been well known to be facilitated by vegetation structures in ecosystems. In many degraded ecosystems (e.g., bare salt marsh areas), this facilitative effect of vegetation structures on seed retention is often restricted, limiting vegetation recovery. The role of vegetation structures as seed traps may be replaced by microtopographic structures, but the effectiveness of microtopographic structures in seed retention across temporal and spatial scales has been seldom tested, leaving its application value in restoration of degraded ecosystems unclear. We tested the facilitative effect of microtopographic structures on seedling recruitment in degraded, bare areas in a salt marsh ecosystem. Field experiments revealed that most (≥94.4%) microtopographic structures facilitated seedling recruitment via seed trapping and the number of recruited seedlings increased with the increasing size and depth treatment of microtopographic structure. However, when excluded the effect of size, recruited seedling density decreased with size. Moreover, this trend will be modified by variation rate in depth at smaller-sized structures, when the variation in depth was significant. The stability of those induced microtopographic structures would be influenced by tidal events, thus tidal frequency can modify the effect of structural characteristics on seed retention and seedling recruitment. Our results indicate that, the most efficient microtopographic structures in seedling recruitment will be different at different locations. These findings are valuable in designing suitable microtopographic structures to facilitate seedling recruitment in microtopography-lacking degraded or restored wetlands.