The Geography of Invasive Plant Species on the Coastal Dunes: Northern California
Derek H. Kanwischer
Montana State University
The coastal dunes of northern California have been impacted by the introduction of invasive weeds to stabilize the environment for development purposes. Preservation and restoration of this unique environment can be enhanced with a geographical understanding of the multiple processes which affect this area. Storm waves frequently remove all established vegetation leaving an area exposed to establishment by alternative species transported by wind or littoral drift. Poor soil conditions, salt spray, high amounts of radiation, and human impact all contribute to creating an area susceptible to biological invasion. The degradation of the coastal dune environment has become a concern as the preservation of natural environments has become more important to the human community and as native species are becoming more scarce due to the spread of invasive species.
The coastal dunes of Humboldt County, California, have been extensively altered by invasive plant species (Buell and others, 1995; Wiedemann and Pickart, 1996; Walter, 2003; Wear, 2003). The objective of this study was to observe and photograph the geography of these plants, in an effort to determine where they were and why. It was hypothesized that the plants would be in areas susceptible to the natural deposition of seeds and rhizomes, and also in areas that have been impacted by humans. Specific areas of investigation are lee-side slopes, hollows, and areas of established vegetation. These areas would presumably enable vegetative establishment due to wind protection. The two weed species of specific interest that have the greatest impact on the coastal dunes are European beachgrass (Ammophila arenaria) and yellow bush lupine (Lupinus arboreus) (Wiedemann, 1987; Wiedemann and Pickart, 1996).
The west coast of the United States is 42%
coastal dunes. The dunes are formed from the
accumulation of local river sediments being
washed ashore by ocean and wave currents.
The Mad and Eel rivers are the source for the
formation of dunes in northern California.
The expansive Humboldt Bay dunes encompass
a continuous area of 34 miles along the
northern California coast (fig. 1). Figure 1. Coastal sand dunes (Wiedemann, 1984).
Observations of species distribution were conducted at two locations: the Manila Dunes and the Lanphere-Christiansen Dune Preserve. The Manila Dunes are located on the fringes of the small coastal town of Manila which has undergone periods of coastal development that has introduced and proliferated the spread of European beachgrass (Wiedemann, 1984; Walter, 2003). Friends of the Dunes, a local non-profit community based organization, is based out of a Manila. The organization facilitates community efforts to educate the public and eradicate the invasive species in order to restore the dune habitat to natural conditions.
The Lanphere Dunes are the largest remaining stand of pristine coastal dunes in the Pacific Northwest (Martin, 2001). In 1992, an extensive effort was initiated to eradicate the existing European beach grass. The effort was a success and the Lanphere Dunes now exist as a model an inspiration to other conservation organizations (Wear, 2003). To facilitate restoration and preservation activities, public access to these dunes is restricted to guided educational tours put on by Friends of the Dunes.
The exotic European beachgrass (Ammophila arenaria) (figs. 5-9) was first introduced to the west coast of the United States at Golden Gate Park in San Francisco in the late 1800’s (Buell et al., 1995). During westward expansion and development in the early 20th century, European beachgrass was used extensively to stabilize the drifting sands of the dunes (Walter, 2003). Over time, the plant has spread to occupy a large percentage of
the coastal dune environment displacing native plant communities and disrupting the natural processes of dune geomorphology (Wiedemann and Pickart, 1996). Natural establishment of this perennial is from seeds and vegetative reproduction that often occurs as pieces of rhizome are washed along shore by high tidal currents (Baye 1990, Buell et al., 1995). The northern California dunes receive the highest amount of impact from storm waves in the winter months. Baye (1990) demonstrated that European beachgrass can survive and potentially recolonize after 8 tidal cycles in the cold seawater.
Once established, European beachgrass develops a vigorous rhizome system, both horizontal and vertical (Baye, 1990). Active sand burial stimulates the production of new shoots. Roots growing in stable sand dunes are sometimes infected by harmful micro-organisms that reduce the formation of new tillers (Wiedemann and Pickart, 1996). Fresh sand accumulation prevents the build up of these pathogens. In the absence of moving sand European beach grass declines in vigor, a phenomenon that has been attributed to a number of causes including physiological ‘ageing’ expressed as reduced root production and competition (Wiedemann and Pickart, 1996).
Yellow bush lupine (Lupinus arboreus) (figs. 14-19) was introduced into northern California in the early 1900’s (Wiedemann, 1984). Prior to introduction, the coastal dune environment was deficient in nitrogen and shrub species (Pickart et al., 1998). Yellow bush lupine has dramatically altered the native coastal landscape by altering the chemistry of the dune mat. Nitrogen fixing nodules accumulate nitrogen and transport it to the leaves, which eventually drop to the ground. Once this occurs, the leaves enrich the duff and litter causing soil enrichment and allowing invasion of other exotic colonists such as weedy grasses and forbs, namely brom (Bromus spp.), velvet grass (Holcus lanatus), and European hairgrass (Aira spp.) (Pickart et al., 1998; Walter, 2003).
During the summer, the perennial lupine produces bright yellow flowers (Kittelson and Maron, 2000). Various types of bees (honey and bumble) have been observed to be the main agents responsible for pollination. Reproduction is achieved through both self and outcrossed pollinations (Kittelson and Maron, 2000). Wear (2003) stated that when lupine density passes a threshold, it is common to see a fungal infection develop that may destroy the local lupine community. When dead lupines decompose they release high amounts of nitrogen into the soil (Pickart et al., 1998). Eradicating yellow bush lupine from the coastal dune community requires manual removal of the entire plant.
There have been several studies conducted to determine the most effective manner of eradicating the yellow bush lupine and European beachgrass to reverse negative ecosystem effects, thereby increasing the chance of long-term restoration success (Pickart et al., 1998; Wiedemann and Pickart, 1996). Biological, mechanical, and various degrees of manual techniques have been utilized. Pickart (1998) suggests that the most appropriate treatments are variable and site-specific. Restoration efforts burn the collected invasive plants upon removal to eliminate seeds, rhizomes, and any remaining nitrogen fixing remnants from the beach environment (Walter, 2003; Wear, 2003).
One of the primary purposes for the efforts to eradicate invasive species is to preserve and restore the environment of native species. Native pioneering plant species are highly adapted and require a semi-stable dune environment with shifting sands and a suitable climate type. American dunegrass (Elymus mollis) along with silver bursage (Ambrosia chamisonis), beach morning glory (Convovulus soldanella), and dune tansy (Tanacetum douglasii) form the low hummock complexes (foredunes) that used to be extensive along the coast before the spread of invasive species (Wiedemann, 1984). The Humboldt Bay wallflower (Erysimum menziesii) and the beach layia (Layia arenia) are of particular interest now that they have been listed as State and Federally endangered species (Walter, 2003).
The dune environment also provides a unique niche for several species of animals and birds to inhabit. The Federally endangered snowy plover (Charadrius alexandrinus) is a unique beach nesting bird that inhabits the foredune environment. Unique tracks are commonly seen in the beach sand that indicate the presence of porcupine, snowshoe hare, deer mouse, and Douglas squirrel. The Douglas squirrel is considered especially significant because they eat the seeds of the European beachgrass (Walter, 2003).
The vegetative species composition of the dunes is determined by sand stability and the availability of moisture (Wiedemann and Pickart, 1996). The northern California coast experiences primary winds and storm systems from the Northwest (Wiedemann, 1984). Orientation of the dunes at Manila and Lanphere are consistent with the formation of parabolic dunes. Vegetation in places ‘pins’ the edges and sand movement is concentrated in between those points to form quickly moving sand sheets (Wiedemann, 1984). In this environment, it is difficult for vegetation to stabilize or reproduce in an area because they are completely buried by moving sand.
The invasion of Ammophila changes dune morphology significantly. Foredunes that are inhabited by native vegetation allows sand movement that sustains the existing dune morphology (Wear, 2003). As Ammophila grows more dense it traps more sand than native species (Wiedemann, 1984, 1987). As a result, the dunes become stabilized and the foredune becomes steeper than those formed under native conditions consequently starving the rest of the dunes of essential moving sand. Once yellow bush lupine becomes established it changes the soil chemistry enabling other invasive species to become established, consequently stabilizing the dunes and crowding out native species (Pickart et al., 1998).
I observed and photographed the geographic characteristics of the dunes and plant species at two separate locations (Manila and Lanphere) for 4 days over a 1 week period. The temporal scope of this study was provided by records from ‘Friends of the Dunes’. My primary photographic documentary compared hypothesized locations of weed presence and absence. Areas susceptible to wind and water deposition of seeds and rhizomes were suspected to exhibit high weed densities. Other locations that were investigated were those that were currently influenced or had been human impacted.
The geographic distribution and density of plant species and dune morphology were observed and photo-documented in two separate locations: Manila dunes, and Lanphere dunes.
The Manila Dunes are areas of common fog and persistent winds. These factors, along with vegetation effect the biotic species composition and morphology of the dunes.
Note the native red fescue (beach goldenrod) in foreground hollow. Individual sand hummocks are formed by native dune grasses.
Standing on top of the more stabilized rear dunes we can observe the foredune environment. Behind the foredune, dense vegetation grows in the hollows with a wet deflation plain in the sandy plain before us. Steeper stabilized foredunes would truncate sand migration and enable complete vegetative cover of the dunes.
Top picture is a stabilized foredune affected by Ammophila. Small clumps of Ammophila in front of the foredune are broken and redistributed along the coast by winter storm waves.
Bottom picture is a natural dune mat with sand hummocks formed by native beach grasses.
Manila Dunes: Dunes appear to have been completely covered by Ammophila regardless of dune morphology or areas of transport or deposition. Human impacted trail represents an area that does not contain weeds.
Figure 4 (Wiedemann, 1984)
European beach grass (Ammophila arenia) Highly invasive species which stabilizes the natural dune morphology, outcompetes, and crowds out native species. Initially planted near developing areas and has spread to impact the entire dune envrironment including the foredune.
Ammophila is highly adapted to life on the sandy beach. Upon burial, roots grow accordingly to attain nutrients from the sandy soil. Eradication of the plant species requires up to 8 treatments. Picture shows a plant length of 6 ft. Longer roots are typically found higher up on the dune due to accumulated sand burial.
Ammophila has been manually eradicated from this area. Notice dense vegetation in swales and hollows. Observe large piles of removed beach grass along side of human trail from eradication efforts. These piles are burned to remove them from the beach.
Ammophila is not able to become established in areas susceptible to high winds in what are known as sand sheets. Areas on the periphery are often buried. Note the wind furrows on the open dune with stabilized dunes in the background covered with European beach grass.
Barchan dune: Area where vegetation has become established to interrupt the flow of the sand and develop a steep wind-ward slope.
Barchan dune: side view
Sand on lee slope rests at a natural angle of repose (34 degrees). Sand sheet is encroaching beach forest.
Note the dead and dying vegetation being overrun by moving sand.
Sand sheets naturally migrate into shrub and forest vegetation displacing the existing vegetation and re-establishing a community of dune vegetation.
The Yellow Bush Lupine in flower.
Eradication efforts were coordinated
to take place before the lupine developed flowers and seeds.
Rich organic soil found on dune surface enabling establishment of invasive grasses due to a high density of lupines in the area that have altered the biotic dune community.
Roots of the lupine contain nitrogen fixing nodules which change the chemistry of the soil creating an environment more suitable to other invasive plants. Note the nodules at and below the wrist of the glove.
Large, thick taproots had to be chopped in order to remove the larger lupines from the environment. Piles of collected lupines are periodically burned to remove all litter and seeds from the dunes.
Lupine distribution appeared random throughout the parabolic dune system at Lanphere Dunes. Lupine occurrence was apparent on windward and leeward slopes, open swales, hollows, on ridges, and flat terrain. Past eradication efforts made lupine free areas difficult to evaluate.
Exposed root system:
Lupines have an extensive vertical and horizontal root structure which draws nutrients out of the soil to fix nitrogen. Lupine is dying because eradicated Ammophila has enabled the sand to naturally migrate and expose the roots.
Predominantly native dune species. Ammophila has been eradicated. However, the presence of the lupine enables the introduction and stabilization of invasive species due to the change in soil chemistry. Note dune morphology, trees in hollows, shrub vegetation and grasses on higher slopes.
The Beach Layia habitat is limited to the coastal foredunes. It prefers open sandy areas.
Federally and State Endangered.
Efforts to eradicate invasive weed species has been motivated by a desire to preserve the dune environment for the endemic Humboldt Bay Wildflower. This flower exists in slightly vegetated, active, sandy meadows within the dunes.
Federally and State Endangered.
Manila Dunes: ATV tracks bisect the dune pictured. ATV use has been banned from many areas along the coast to prevent biological disturbance to flora and fauna.
Lanphere Dunes: native restored dune mat. Observe the parabolic dune shapes with native vegetation. Success story: Symbolic and native Humboldt Bay Wallflowers pictured in the foreground.
Emily stands next to Beach Layia that have become established on a remnant ATV track. Beach Layia prefers partially stabilized open sand.
Note the steep lee-ward slopes and emerging forest ecotone in the swell to the left. Landscape is covered in Ammophila with a lupine pictured in the bottom right.
Manila Dunes: European beach grass and Yellow bush lupine have been introduced into the inland dunes to stabilize these areas for local industrial, residential, and commercial development.
Development along the dune environment must contend with the winds and shifting sands. The Southern Pacific Railroad was one of the first industries to introduce Ammophila to stabilize the dunes in order to promote the great westward expansion of America.
The non-specific distribution of the invasive weeds at this site was greater than hypothesized. This distribution have been affected by eradication efforts. Dune areas that had not undergone any eradication efforts were completely covered by weeds. Areas that had more sparse cover were difficult to evaluate due to sporadic past efforts of eradication. The only observable change in species distribution was where more dense vegetation had become established. These areas were found in deep lee-side hollows where small Sitka spruce (Picea sitchensis) and western hemlock (Tsuga heterophylla) had become dominant. There was virtually no lupine or Ammophila found in these stands.
The wide-spread distribution of the examined weed species is related to the natural history of the area. Yellow bush lupine is native to southern California, but it’s spread into northern California and further north are considered invasive because they are detrimental to the native dune ecosystem. The lupine was desirable because of its bright yellow summer flowers (Walter, 2003). Ammophila was introduced to northern California to intentionally stabilize the dunes (Wiedemann, 1984). These coastal areas experienced significant development pressure with the construction of the railroad, roads, homes, and businesses throughout the 1900’s. Dune stabilization with Ammophila is considered convenient and economically efficient. This method of stabilization stops the moving sand from encroaching upon developing areas of residential, commercial, and industrial expansion. The spread of Ammophila has been documented to have spread in a satellite pattern over time, indicating the source for human introductions that are consistent with historical development patterns (Buell et al., 1995). Buell (1995) has
collaborated airphotos and ground truths to track the spread of Ammophila in Humboldt county from 1929, 1962, and 1989 (fig. 29) (table 1).
Table 1. Spread of Ammophila over time. Results indicate a natural spread
into areas not planted and an exponential rate of increase (Buell et al., 1995).
The natural distribution of invasive plant species into the coastal region of northern California is greatly influenced by the cool maritime climate. The maritime climate of this region is mild and remarkably uniform. The spread of Ammophila is proliferated by an average annual temperature range of 10°C to 11.2°C and distinct seasons with 80% of precipitation occurring in the winter months (Wiedemann and Pickart, 1996). Alternatively, a study by Hertling and Lubke (1999) of Ammophila distribution in South Africa found no evidence of any spread of the weed by natural processes. The introduced species is present only in the vicinity of developing coastal areas. The study asserts that the reason that Ammophila has not spread naturally is climatic and biological. It is too dry in areas where the temperature is conducive to weed growth and areas with sufficient rainfall experience excessive amounts of radiation and wind (Hertling and Lubke, 1999). The concern for the spread of invasive weed species in the northern California coastal dunes is attributed to its ability to spread naturally and alter the native dune environment. It is thought that the stabilization of the dunes will result in the loss of the natural scenic and recreational values found along the northern California coast (Wiedemann and Pickart, 1996).
Promising efforts are being carried on by various organizations along the coastal dunes in northern California to restore this environment to its natural state (figs. 29, 30, 31). Working in concert with Friends of the Dunes to manually remove weed species enlightened me to the magnitude of the problem and the procedures employed for restoration. Due to the abundance of observed Ammophila on the dunes, it became obvious that dune restoration was a long and arduous process dependent on an informed and dedicated community of volunteers. The foredune and subsequent ridges were the first priority due to the consequences experienced by inland dunes being cut off from coastal processes. Onshore winds move over and beyond the foredunes facilitating the movement of sand. It is important to destroy and remove a plant from the beach soil before it flowers and reproduces. Ammophila and yellow bush lupine both require repeated treatments. It takes as many as 8 manual treatment to fully restore a beach dune infested by Ammophila. Lupines produce seeds that can survive up to 8 years in dormancy, so repeated removal and observation is necessary. Ammophila is found along the entire west coast. Future research is needed to investigate the possible hybridization of the lupine that enable the northern spread of the species into neighboring climate regimes (Wear, 2003).
Figure 29. Efforts to eradicate European beachgrass from the Manila dunes (Wear, 2003).
Figure 30. 1992 photograph of European Figure 31. 1997 photograph of rehabilitated
beach grass and yellow bush lupine (Wear, 2003). dune mat with natural vegetation (Wear, 2003).
Despite the positive achievements and efforts to restore the dune community to a natural state, Ammophila continues to be used along the coast of Oregon and California by public agencies, private organizations, and individuals (Wiedemann, 1987; Wiedemann and Pickart, 1996; Hertling and Lubke, 1999). Continued deterioration of the coastal dunes will negatively influence aesthetic and recreational opportunities, loss of native species, and stabilize vital geomorphic processes. These phenomena warrant an increased need to encourage an understanding and awareness among the public about the importance of restoring the dune community to a natural state. A multi-disciplinary geographic understanding of the spread of invasive species in the northern California coastal dunes enhances the ability to promote conservation and restoration of the dunes.
I would like to thank the Undergraduate Scholars Program at Montana State University for the opportunity and funding to conduct this investigation. The Montana State University ‘Breaks Away’ program offered an excellent opportunity to work in the coastal environment of Humboldt County, California. Friends of the Dunes is an excellent community-based center providing education and opportunities for local citizens to make meaningful contributions to environmental preservation. I would also like to extend my appreciation to my advisor Dr. Kathy Hansen who has contributed to the organization and production of this report.
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