Andrea Pickart, CA Native Plant Society
For the past seven years, a major collaborative research project has been carried out on Humboldt County’s dunes. Participants include US Fish and Wildlife Service, Arizona State University, Friends of the Dunes, the Wiyot Tribe, Humboldt Bay Municipal Water District, Bureau of Land Management, California Department of Fish and Wildlife, National Park Service, and others. Two consecutive four-year grants were funded by the State Coastal Conservancy, Ocean Protection Council, and Bureau of Land Management. The overarching purpose of the research was to better understand how dunes will respond to climate change and how we can intervene in order to adapt to those changes. There are numerous components to the grant, including measuring changes in topography and vegetation on 72 transects distributed along the coast from Little River to Centerville Beach, calculating historic changes in the position of the shoreline since the first air photos were taken in 1939, assessing vulnerability of natural and cultural resources and infrastructure, and documenting the role of native plants in coastal resilience in a demonstration adaptation site. This last study wrapped up recently with the completion and publication of a dissertation by PhD student Zach Hilgendorf of Arizona State University.
The Lanphere Adaptation Site was established in 2015 to test the hypothesis that foredunes vegetated by native plants are more resilient than those vegetated by invasive European beachgrass. A quarter-mile stretch of dunes vegetated by dense beachgrass was sectioned into four areas. In three of these areas European beachgrass was removed and native plants introduced. The fourth area remained as a control on the experiment (no removal). Over the following seven years measurement of change in morphology of the foredune was carried out by researchers from the Arizona State University (ASU) under the direction of Dr. Ian Walker (now with UC Santa Barbara), while changes in vegetation were monitored by Humboldt Bay National Wildlife Refuge staff. ASU researchers used a state-of-the-art laser scanner to create topographic surfaces that could be used to detect changes in elevation over time. During the study there were two “extreme events,” storms during which tides were extraordinarily high combined with strong wave energy. This resulted in erosion of the foredune and enabled us to measure recovery time. The native foredune recovered almost two years faster than the control. This is significant given the increasing frequency and intensity of storms due to climate change. If foredunes are not able to recover between erosive events, shoreline retreat occurs.
Foredunes are a sand-sharing system between the beach and the dunes. During storms when water levels and wave energy are high, the foredune is scarped (cliffed). To recover, a sand “ramp” slowly builds up against the scarp during calmer weather until it reaches the top, when sand can again be transported on to the foredune and, if native plants are present, beyond. We observed that invasive foredunes responded with intact block slumping, retaining the vertical scarp. In contrast, native foredunes were characterized by “avalanching,” contributing to more rapid ramp formation.
The study also showed that the restored foredunes increased in elevation and shifted the crest of the foredune back (landward) while maintaining the toe of the foredune, creating a broader foredune with greater volume, a more resilient condition. Invasive foredunes are narrow, steep and remain static. No sand reaches beyond the seaward face of the foredune. The native foredune is therefore more adapted to maintaining a buffering foredune feature during sea level rise.
All of this underscores the important role native plants play in dune resiliency. Humboldt Bay National Wildlife Refuge added 300 acres of highly degraded dunes to the refuge over the past decade, known as the Wadulh Unit (Wadulh is the Wiyot term for dune). The management goal at this site is to restore native dunes to increase resiliency while promoting biodiversity, using methods tested in the adaptation site.