The Geography of Invasive
Plant Species on the Coastal Dunes: Northern California
Derek H. Kanwischer
Montana State University
5-03-2003
Abstract:
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.
Introduction:
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).
Location:
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.
Invasive
Species:
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).
Native
Species:
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).
Dune
Morphology:
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).
Methods:
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.
Results:
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.
Figure 3
Figure 2
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.
Invasive Species:
Figure 5
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.
Figure 6
Figure 7
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.
Figure 8
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.
Figure
12
Note the dead and dying vegetation being overrun by
moving sand.
Figure 10
Figure 11
Sand sheets naturally migrate into shrub and forest
vegetation displacing the existing vegetation and re-establishing a community
of dune vegetation.
Figure 13
The Yellow Bush Lupine in flower.
Eradication efforts were coordinated
to take place before the lupine developed flowers and
seeds.
Figure 15
Lanphere dunes:
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.
Figure 14
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.
Figure 16
Figure 17
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.
Figure 19
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.
Figure 18
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.
Lanphere dunes:
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.
Native Species:
Figure 21
The Beach Layia habitat is limited to the coastal
foredunes. It prefers open sandy areas.
Federally and State Endangered.
Figure 20
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.
Figure 22
Human Impact:
Figure 23
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.
Figure 25
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.
Figure 24
Emily stands next to Beach Layia that have become
established on a remnant ATV track.
Beach Layia prefers partially stabilized open sand.
Manila Dunes:
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.
Figure 28
Figure 26
Figure 27
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.
Discussion:
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).
Climate:
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).
Restoration
Efforts:
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).
Conclusion:
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.
Acknowledgements:
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.
References:
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