Large-scale field studies inform adaptive management of California wetland restoration

Salt marsh vegetation provides the structure that supports key wetland functions and the success of wetland restoration efforts depend on the establishment of vegetation, which can take decades. Field experiments aimed at identifying the factors that limit the colonization rate of marsh vegetation can be useful for guiding restoration efforts. We used field experiments and additional measurements to determine the causes for persistently low vegetation cover in a restored salt marsh in southern California. First, we characterized the soil properties of the marsh and found that high salinity and low soil moisture increased with elevation and improved with vegetation. Using this information, we conducted two field experiments-one in the high marsh and the other in the mid marsh to determine the effectiveness of different remedial actions (e.g. irrigation, soil decompaction, soil amendments, and planting and seeding different species) on increasing vegetation cover; the mid marsh experiment evaluated planting and seeding only. In the high marsh, plantings increased cover, and irrigation in combination with soil decompaction facilitated natural plant establishment. In the mid marsh, increases in vegetation cover varied by species planted. In both experiments, the marsh dominant (Salicornia pacifica) naturally recruited, increasing cover whereas seeding did not increase cover. In general, the magnitude and pace of vegetation growth was greater in the mid marsh relative to the high marsh that received less tidal inundation. Collectively, our results show how information gained from experiments conducted during wetland restoration can inform the processes underlying the establishment of vegetation.