Fragmentation Drives Dominant Plant Encroachment on a Horizontal Wastewater Treatment Levee

Urban coastal wetlands protect humans from sea-level rise while providing valuable habitat for wildlife. Degradation and loss of these wetlands threaten urban infrastructure including wastewater treatment facilities. Nature-based adaptive solutions, with the combined purposes of bioremediation, coastal defense, and habitat creation, are being tested to make communities safer and more resilient. The current research examines an experimental horizontal levee installed in 2015 at the Oro Loma Sanitary District in San Lorenzo, California, 5 years after installation. Using quadrat sampling, we compare succession of two plant assemblages – a wet meadow and a riparian scrub community – on an ecotone slope. We use the wet meadow assemblage to document the effects of fragmentation and dominant plant species on plant diversity and abundance. Although most planted species survived from 2015 to 2021, plant diversity decreased over time in both communities. Fragmentation was associated with encroachment by a native dominant willow ( Salix lasiolepis) and an invasive nonnative jubatagrass ( Cortaderia jubata) in the wet meadow. Both fragmentation and the presence of the willow or cattails ( Typha ) correlated with reduced native species diversity and cover. In the absence of natural disturbance processes, created wetlands, especially fragmented wetlands with substantial edge, may progress to a successional state dominated by a few species. Future projects might benefit from specifying habitat creation goals in addition to wastewater treatment goals, selecting native plant assemblages that inhibit succession, planting larger patches, and incorporating natural or human disturbance to break dominance cycles.