Twenty years out and counting: Creation and monitoring of dune swale wetlands in coastal Central California

Background/Question/Methods

Mitigation for a military construction project in a central California coastal dune area required creation of eight acres of dune swale wetlands.  The goal was creation of self-sustaining wetlands exhibiting attributes, functions, values, and permanence equivalent to natural dune swale wetlands. Recognition of wetland life cycles—that depressional wetlands ultimately fill in--and knowledge of local groundwater fluctuations contributed to the successful design. We excavated to groundwater at two sites, applied soil and seedbank salvaged from impacted wetlands and created mixes of locally-collected seed for wetland bottoms, wetland margins, and upland slopes. Based on monitoring of the perched water table plus observations made in natural dune swales, we anticipated groundwater fluctuations and designed accordingly. We included gradual slopes allowing movement of vegetation up and down in response to year-to-year changes in water levels, and excavated two areas, each about 1/8 acre, 2 feet deeper than the rest to provide a moist refuge during drought.  From 1990-1996 we concurrently monitored six nearby sites, representing a range of natural dune swale wetlands, for comparison with the created wetlands. The senior author took advantage of a rare opportunity to observe the created sites 22 years after project completion.

Results/Conclusions

The wetlands developed rapidly at both created sites during the five-year monitoring period and had dense cover of emergent marsh on the bottoms, with open water and submerged macrophytes in the deepest portions.  The margins of one site were colonized by a dense growth of willows (Salix spp.) and wax myrtles (Myrica californica) whereas the other site had only patchy growth of trees, with the emergent marsh transitioning abruptly to upland coastal dune scrub vegetation.  At this latter site the water rose rapidly after the initial establishment of willows, submerging and eventually killing them. Dense herbaceous vegetation established on the wetted soil of the margins and precluded later establishment of willows, which require bare, moist soil. From the outset, the created sites were used extensively by wildlife. Species abundances, numbers, and diversities in the created wetlands surpassed or fit within the range found at the reference wetlands.  In 2012, the created wetlands continue to flourish.  Open water habitat still exists at both sites but is concealed by dense native vegetation surrounding the wetlands. A key element of this project’s success stemmed from knowledge of ecological history, in particular studies by ESA founding fathers Cowles and Shelford on Lake Michigan dune slack wetlands in the early 1900s.