North Coast Chapter - CNPS
Dunes of the Humboldt Bay Region
by Linda Miller (1997)
|photo by Andrea Pickart|
The dune system of Humboldt Bay is an important part of the greater North Coast eco-region, and provides a unique experience for hikers, botanists, bird-watchers and explorers. There are a number of ways to visit these dunes. The non-profit organization Friends of the Dunes offers interpretive nature walks to the Lanphere-Christensen Dunes Preserve on the first and third Saturday of every month beginning at 10:00 a.m. Meet at the Pacific Union School parking lot to car pool (parking is very limited at the preserve). On the second and fourth Saturdays, meet at the Manila Community Center at 10:00 a.m. for a tour of the Manila Beach and Dunes. Call Friends of the Dunes at (707) 444-1397 for more information. Other access to the dunes include Mad River County Park and the Samoa Dunes Recreation Area (Bureau of Land Management).
Opportunities to volunteer with Friends of the Dunes and to participate in dune restoration work days are also encouraged. Regular restoration work days are scheduled at the Manila Beach and Dunes on the third Saturday and Sunday of each month. Meet at the Manila Community Center at 10 a.m. On the first Saturday of each month, work days are co-sponsored by Friends of the Dunes and the Center for Natural Lands Management at the Eureka Dunes Protected Area. Meet at the Samoa boat ramp camping facility at 10:00 a.m.
Along the rugged, wind-swept north coast lies an extraordinary ecosystem that begins far inland at the upper reaches of Northern California's major watersheds. Sand dunes are formed from sediments washed away from the erosive soils of the Franciscan Assemblage by plentiful and intense north coast rain. These sediments are carried to the ocean by the many rivers characteristic of the area, and are dumped at river mouths. Two of these rivers, the Mad and Eel, feed sediments into the dunes of the Humboldt Bay region, via longshore transport, summer ocean swells and predominant northwesterly winds. In winter, large storm waves continue the dune building cycle by scouring the beach and washing sediments back out to sea. The net effect is the continual building and movement of dunes.
Many of the morphological features of Humboldt Bay sand dunes have been stabilized by native vegetation highly adapted to the dynamic and sometimes harsh environment of the dunes. Many habitat types occur within this system, including semi-stabilized foredunes, dune hollows, open moving dunes, beach-pine forest, freshwater swamps and marshes and riparian. On the bay side, freshwater draining from the dunes mixes with bay waters to support brackish marsh, and areas that receive periodic inundation by tidal waters from the bay are vegetated by salt marsh.
The semi-stabilized foredunes support a high diversity of plant life, including the Native dunegrass series, considered to be "globally endangered" by the California Department of Fish and Game Natural Diversity Database. This series, or plant community, is dominated by native dunegrass (Leymus mollis). Other co-dominants in this low-cover, low-growing plant community include beach bursage (Ambrosia chamissonis), beach morning glory (Calystegia soldanella), seashore bluegrass (Poa douglasii), yellow sand-verbena (Abronia latifolia) and sea-rocket (Cakile spp.). This plant community is endangered primarily due to the encroachment of European beachgrass (Ammophila arenaria), introduced here at the turn of the century. Extensive efforts have been made to eradicate this species from protected areas on the North Spit of Humboldt Bay.
The Sand-verbena - beach bursage series also occurs on the foredunes, but is not limited to the primary foredune as is the Native dunegrass series. The Sand-verbena - beach bursage series is also one of low cover, and is dominated primarily by low-growing semi-shrubby perennials including beach bursage (Ambrosia chamissonis), yellow sand-verbena (Abronia latifolia), beach morning glory (Calystegia soldanella), beach strawberry (Fragaria chiloensis), beach pea (Lathyrus littoralis), dune buckwheat (Eriogonum latifolium), beach sagewort (Artemisia pycnocephala) and seashore bluegrass (Poa douglasii). These species are well adapted to the harsh conditions of the open dunes which include salt spray, erosion caused by waves and high-velocity winds, and the drying effects of winds in summer and dry periods. Some plant species have evolved tiny "hairs" on their leaves that help reflect intense sun rays and provide a fine layer of insulation from drying winds. Other plants have developed large tap-roots that reach down to provide water and provide a source of stored food energy.
The foredunes and the Sand-verbena - beach bursage series are also home to three CNPS List 1B species, Humboldt Bay wallflower (Erysimum menziesii ssp. eurekense), beach layia (Layia carnosa), both federally-listed as endangered, and pink sand-verbena (Abronia umbellata ssp. breviflora).
Dune hollows are formed when prevailing northwest winds erode the sand down to the summer water table, and the resulting low area becomes inundated with water when the water table rises in winter. These areas are then colonized by water-loving plants such as rushes (Juncus spp.) and sedges (Carex spp.). One can observe ecological succession in dune hollows over a period of just a few years, as the plant community develops and changes. Hooker's willow (Salix hookeriana) colonizes the hollow next, with red alder (Alnus rubra), beach-pine (Pinus contorta ssp. contorta), wax-myrtle (Myrica californica) and sometimes Sitka spruce (Picea sitchensis) to follow. Mature dune hollows can provide a haven for wildlife such as song-birds, nesting harriers, small mammals and amphibians.
The forested dunes of the North Spit are unique among all north coast forests, with their own assemblage of plant species, fungi, lichens, birds and mammals. These forests have been likened to biological islands due to their isolation and relative small size. The forest supports a high diversity of tree and shrub species, giving it a high structural diversity. In addition to this structural diversity, the existence of a variety of adjacent habitats, including riparian and other wetland habitats with high productivity of invertebrate prey source, produces a high diversity of bird species within the forest complex. Common species include yellow-rumped warblers, chestnut-backed chickadee, winter wren, mourning dove, red crossbill, wrentit, song sparrow, common flicker, Bewick's wren and black phoebe.
Meandering through the enchanted beach-pine forest, one encounters a variety of tree species including beach pine (Pinus contorta ssp. contorta), Sitka spruce, Douglas-fir (Pseudotsuga menziesii), grand fir (Abies grandis), wax-myrtle (Myrica californica), and Pacific madrone (Arbutus menziesii). Attached to branches and trunks as well as the forest floor are a wide variety of lichens and mosses, adding to the charm of the forest. The association of bear-berry and reindeer lichen is a unique and interesting feature of these forests.