561 Barrier Island Ecology
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|Plant adaptations to dune habitatsHuman use of dune and slack environments||
Salt aerosols in the atmosphere impact the vegetation on barrier island systems. Salt aerosols "prune" shrub and tree vegetation.
Plant and Animal Adaptations to Dune and Slack Habitats
Plants exhibit myriad adaptations to the environmental conditions that
exist in dunes. Unstable
substrates, fluctuating temperature and water regimes, and salt aerosols are
the most obvious of these conditions. Survival
strategies in plants involve adjustments in morphology and physiology that
substitute resources, minimize impact, or totally avoid periods when resources
are scarce. Ehrenfeld (1990)
reviewed studies on the life history of coastal plant species and found that
tolerance to one coastal environmental factor, even a major factor such as
salt aerosols, did not guarantee survival of the species.
To survive in the coastal environment, species must possess a suite of
adaptations to complexes of environmental factors. Also, according to Ehrenfeld
(1990:457), “. . . the wide
diversity of physiological adaptations observed in coastal plants is
suggestive of a diversity of microhabitats on a small spatial scale as well as
a diversity of possible mechanisms of adaptation."
Sea oats is an example of a species well adapted to the dune
environment. This dominant plant
of the dune system has morphological expressions and physiological responses
for surviving the extreme environmental conditions found in dunes.
Sea oats dominate in areas where the level of salt aerosols, the rate
of water evaporation, and burial by shifting sands eliminate competing
The typical life cycle of plants of mesic environments is altered under
conditions characteristic of dune systems. Plants are highly vulnerable during germination and their
early stages of growth, but once plants reach full size, the impact of the
environment is reduced.
Annual dune grasses and forbs typically maintain high rates of growth
for short periods, a strategy that gives them a competitive advantage in
spring before drought conditions set in (Boorman
1982). Blue grass (Poa annua)
and Canada horseweed exemplify this growth pattern in dune systems.
Compared with perennials, annual plants are poor competitors in the
dunes; their small size, limited lateral spread, and negligible litter
accumulation are disadvantages. However, even dominant dune perennials, such as American
beachgrass, show reduced vigor with increased competition from other species (Huiskes
Three distinct life cycles are exhibited by the minor species of dune
plants near Cape Hatteras (van der Valk
1974). Fall-germinating annuals, represented by Canada horseweed and
cudweed (Gnaphalium obtusifolium),
typically overwinter as small rosettes and begin extensive vegetative growth
the following spring. The plants
flower and set seed in August and thereafter.
Trailing wildbean (Strophostyles
helvola) and seaside broomspurge are spring-germinating plants.
These species grow slowly during the summer, but in the fall have a
growth spurt and then flower and fruit. For
perennial species, exemplified by sea elder, ground cherry, and evening
primrose, reproduction is generally by vegetative means.
These species produce many seeds; however, van der Valk
observed few seedlings of these species on the foredunes.
For seedlings of many plants adapted to the xeric conditions of the
dunes, survival is not guaranteed. The
seedling stage is the most vulnerable period in the life cycle of most plants;
dune species are not exceptions.
der Valk (1974) presents a model for seed germination and seedling survival
in dune forbs (Fig. 6.1). He
considered soil moisture, soil temperature, and burial depth of seeds as major
influences on the germination of dune plants.
Scarification and stratification processes determine if a particular
seed can germinate. Seeds of sea
elder require stratification; trailing wildbean seeds require scarification;
and beach hogwort seeds require both stratification and scarification (van der
vary considerably across a dune; therefore, species composition on the front,
top and back of dunes is distinct, and the probability of a seedling's
survival is often determined by where the seed germinates (van der Valk
1974). For example, seeds of
Canada horseweed germinating on the seaward side of the primary dune are
likely to perish in the seedling stage because survival at that stage is
controlled by soil moisture conditions, soil and air temperatures, sand
movement, and salt aerosols.
It seems that the concept of ecologically related species exhibiting
similar germination requirements is not supported in the slack environment.
Schat (1983), in a study of slack plants in northwestern Europe,
reported considerable variation in respect to germination strategies.
Schat found that seeds of Samolus
valerandi and Parnassia palustris
exhibit an innate dormancy that can be rapidly broken by cold stratification,
whereas Centaurium littorale shows
explosive germination over a very narrow temperature range and germination in Plantago
coronopus is spread over several months.
Cold treatment, soil salinity, and soil moisture are important
germination factors. Seneca
(1969) reported that holding seeds at 6.1/ C for 30 days enhances
germination in the four dune species he studied: American beachgrass, saltmeadow cordgrass, sea oats, and
panic grass (Panicum amarum,
identified as P. amarulum; Table
6.1). Sea oats and American
beachgrass do not germinate if soil salinity is greater than about 1.0 percent
NaCl (Seneca 1969). This
requirement restricts sea oats and American beachgrass to the dunes; they
generally do not occur on sand flats or other low areas occasionally flooded
with salt water. Huiskes (1977)
and Colosi and McCormick (1978) found that seedlings of dune plants that
germinated in moist substrates had greater survival rates than those that
germinated in drier substrates.
The ecological conditions under which the major grasses grow are
matched by the salinity tolerance of their seedlings. Saltmeadow cordgrass seedlings tolerate the highest salinity,
whereas panic grass and sea oats are less tolerant. American beachgrass seedlings are the least tolerant to
salinity (Seneca 1972).
Most dominant dune species reproduce successfully by vegetative means
and not by seeds. Rhizomes, the
propagating organs of seaside goldenrod, ground cherry, cat greenbriars, and
coastal plain pennywort, overwinter underground where they are protected from
salt aerosols yet remain capable of surviving sand burial.
Successful species often have large taproots (e.g., evening primrose
and beach heather).
Sand movement, either accumulation or erosion, is a distinctive factor
in dune environments. The most
distinguishing ecological characteristic of plants that colonize dunes is
their ability to survive burial, which sometimes may be quite rapid.
On primary dunes, Ranwell (1972) measured sand deposition of 0.3 to 1.0
m/year. However, there are limits
to foredune plants' ability to survive rapid burial.
Rates of burial of more than 30 cm/year inhibit plant growth on dunes.
Gemmell et al. (1953) described the leafy shoot formed by marram grass,
the European species similar to American beachgrass, as "vertical
rhizomes." These shoots
sustain marram grass under conditions of rapid burial and are responsible for
the sand accumulating potential of the species.
Van der Valk (1974) concluded that sand movement is the major factor
preventing the establishment of most forbs on the front (seaward) or other
areas of primary dunes (Fig. 6.2). Huiskes
(1977) presented a similar argument attributing the success of American
beachgrass to the suppression of other plants' growth by sand burial, which,
in turn, allows beachgrass to capture scarce nutrients.
Burial is considered a necessary stimulus for the production of new
roots in American beachgrass (Art 1976).
Physiological senescence of beachgrass roots is a rapid process;
without the stimulus of additional sand, the plant usually shows a rapid
decline in vigor (Laing 1958). On
foredunes, American beachgrass grows rapidly as sand buries the culms.
These plants produce new rhizomes that grow most vigorously toward the
ocean. As a result, American
beachgrass dunes grow seaward. Sea oats also require sand accumulation to survive.
Once established, growth of sea oats is enhanced by burial.
In fact, when the rate of burial is reduced or stopped, sea oats is
usually replaced by other species (Wagner 1964).
Woody plants show adaptations to burial as well; however, these
responses may not be expressed because woody species are sensitive to salt
aerosols and do not occur on those primary dunes that receive high quantities
of salt aerosols. The production
of adventitious roots is common in woody species such as sea elder.
Plants growing on dunes, especially on the primary dune, exhibit
several adaptations to sand burial. Senescence
in American beachgrass occurs because of a decreased ability of old roots to
absorb nutrients, primarily nitrogen, and a decrease in the ability of the
plant to develop new roots (Marshall
Changes in the moisture regime of the soil and increased competition in
older dunes contribute to senescence in dune species.
Biomass, leaf area, number of tillers, number of buds per tiller,
internode length of vertical tillers, number of new plants from rhizomes,
plant height, and plant cover are all positively related to the depth of sand
burial (Disraeli 1984; Table
length of horizontal rhizomes and number of horizontal rhizomes were
negatively affected by burial.
In contrast to the response of the dominant dune grasses, seedlings of
Canada horseweed, trailing wildbean, and cudweed are unable to survive burial
of 20 to 30 cm. This depth of
burial is not uncommon, especially on the seaward face of the primary dune.
Species with buoyant seeds (e.g., sea rocket) have a selective
advantage; their seeds remain at the surface after shedding, even if flooding
water reaches the site of deposition. This
advantage is greatest in the strandline community (Tyndall et al.
Seed burial in sea oats and saltmeadow cordgrass, however, is an
important factor in determining survival, but seeds of these species can be
buried too deep for seedlings to reach the surface (Tyndall et al.
Dune plants show adaptations similar to those exhibited by plants
characteristic of deserts. Xeromorphic
features--succulence, thick cuticles and epidermal layers, and leaf surfaces
with dense hairs and depressed stomata--are present in many species (Harshberger
1908, 1909; Wells and Shunk
1938; Purer 1936).
Dusty miller, evening primrose, beach hogwort, and other dune
inhabitants exhibit dense hairs on one or both leaf surfaces; these pubescent
surfaces were once considered important in reducing the temperature of leaf
surfaces and transpiration. Johnson
(1975) concluded that hairs are not adaptive to arid environments but
developed under a variety of selection pressures.
Thick, leathery, highly cutinized leaves are found in coastal plain
pennywort and seaside goldenrod (Fig. 6.3).
In these plants, the epidermal cells generate hydrophobic cuticular
layers that not only reduce the impact of salt aerosols but also decrease
Succulence in the plants of dune environments indicates a mechanism for
water storage. Sea elder, prickly
pear cactus, Russian thistle, and sea rocket are dune plants with succulent
leaves or stems (Bellis 1980). These
species develop widespreading root systems that can absorb water quickly over
a large area.
In marram grass, bulbous cells near the vascular system become flaccid
when the total water potential decreases in the leaves; after that, each leaf
rolls tightly along its length, forming a tube-like structure resembling a
straw. A humid atmosphere
develops within the tube as stomata release water vapor. The water potential gradient from leaf to atmosphere is less
steep and evapotranspiration is reduced.
The presence of a cuticle on the epidermis of the adaxial surface
further reduces water loss (Milburn 1979).
The mechanism of carbon assimilation has been modified in some plants
to conserve water. Most mesic
plants carry out C3 photosynthesis (Calvin cycle) where ribulose
bisphosphate is an acceptor molecule for CO2.
This process is very inefficient at high temperatures.
Many warm temperate or tropical plants carry out C4
photosynthesis, a process in which the initial CO2 acceptor
molecule is phosphoenolpyruvate. C4
plants can carry out photosynthesis despite the greater stomatal resistance
that enhances water conservation. Many
desert plants use the C4 pathway for photosynthesis, but this
feature has been determined for only a few species living in coastal dunes (Table
6.3). Several genera
exhibiting the C4 pathway in desert plants also occur in coastal
environments, including Amaranthus, Atriplex, Spartina, Uniola,
and Panicum (Fry et al.
1978; Pearcy and Ehleringer
Intense solar radiation typical of coastal dune systems limits plant
distribution because the temperature of plants increases when they absorb
sunlight energy. Plants respond
to sunlight by avoiding radiation, by solar tracking, and by increasing their
surface area (Ehleringer and Forseth
The latter is most often expressed by the production of hairs and other
surface protuberances. On coastal
plain pennywort, the heat load is reduced when these plants orient their
leaves vertically (Fig. 6.4). In
shade, coastal plain pennywort leaves are larger than in the sun and they
appear to track the sun.
Evans (1989) recently showed that extensive rhizomatous growth is a
strategy that maximizes the development of plants in environments with patchy
resources, especially nutrients. The
rhizomatous growth in coastal plain pennywort, for example, not only provides
a mechanism for responding to sand burial, but also effectively translocates
resources among its several ramets. Thus,
when a colonizing plant encounters a microhabitat with a dearth of nutrients,
crucial resources are supplied through other rhizomes growing where sufficient
nutrients and water are available.
Soil Water Content
Plants colonizing dunes exhibit various strategies for surviving low
water content in the soils. Dune
annuals such as blue grass survive the driest periods as seeds and germinate
only during wet periods. For blue
grass, early spring is the peak period of germination; for other species, fall
is the best germination time. Dune
annuals are highly dependent upon rapid absorption of surface moisture from
The root systems of most plants within the dunes are between 7.5 and 38
cm deep (Table 6.4). Only a few
plants have roots that penetrate beyond 60 cm (Stalter
Perennials may produce lengthy root systems and draw water from deep
within the dunes. Sea elder,
seaside goldenrod, and bear grass (Yucca filamentosa) are examples of species producing spectacularly
extensive root or rhizome systems. Other
species produce widespreading but shallow root systems that allow the plants
to exploit light rainfall in the dune.
Although a variety of seed-dispersal mechanisms are observed in
coastal plants, no one dispersal mechanism seems favored.
Wind-dispersed plumed seeds are typical of many plants in the
family Asteraceae; Canada horseweed, seaside goldenrod, and cudweed are
examples. Seeds forcibly ejected
from a fruiting structure are found in beach hogwort, seaside broomspurge, and
trailing wildbean. Seeds of sea
rocket disperse throughout the dunes by rolling across the ground surface.
Sea rocket seeds also may be dispersed over considerable distances by
water, either alongshore or cross-beach.
Similarly, wind and flood waters help disperse the seeds of sea elder (Colosi
and McCormick 1978). Seeds of
dune sandburs (Fig. 6.5) and pads of prickly pear cactus (Opuntia
spp.) are distributed by animals.
The most important modes of reproduction for the two dominant dune
plants, sea oats and American beachgrass, differ significantly.
In sea oats, reproduction resulting from fragmented rhizomes is rare.
Although seed production is low compared to its potential (Wagner
1964), sea oats produce large quantities of seeds; spikelets that become
buried in moist sand germinate and quickly extend roots into the substrate.
Conversely, in beachgrass, rhizome fragments are stripped from intact
plants by wind and water. These
fragments root readily and expand rapidly.
Seed production is a less important mechanism of dispersal in this
species. Although many seeds are
produced by beachgrass plants, Laing (1958) measured a seed viability of only
Seeds of camphorweed (Heterotheca
subaxillaris var. subaxillaris) exhibit
germination dimorphism (Baskin and Baskin
1976). Ray achenes are dormant at the time of dispersal and must
undergo after-ripening (a period of dormancy after dispersal during which
seeds undergo physiological changes) before they can germinate. After-ripening is inhibited by low temperatures and promoted
by high temperatures. Germination,
therefore, is delayed for at least 1 year after seed dispersal in autumn.
The disk achenes are non-dormant and can germinate immediately; this
dimorphism helps spread germination over time and thus may be of survival
value to the species.
The juxtaposition of coastal dunes and slacks allows clonal plants to
employ a unique reproductive strategy. Evans
(1989) shows that recruitment via seeds in coastal plain pennywort occurs
exclusively within slacks; no evidence of seed germination in the dunes was
observed. Through rhizomatous
growth, adult plants grow out of the slacks into the surrounding dunes.
Sexual reproduction at least maintains or possibly increases genetic
diversity in coastal plain pennywort. Saltmeadow
cordgrass may exhibit the same reproductive strategy.
Salt aerosols have long been considered an important factor in
determining the distribution of plant species in dune systems (Wells and Shunk
1938; Oosting and Billings
Oosting's experimental conditions, Canada horseweed, trailing wildbean, and
camphorweed showed 100 percent mortality after being sprayed with salt water
once per day for 9 days. Sea
oats, saltmeadow cordgrass, and seabeach orach (Atriplex
pentandra, identified as
A. arenaria) exhibited 100
percent survival after salt water was applied 4 times per day for 9 days (Oosting
1945). The observed distribution
of species, however, cannot be explained completely by the effects of salt
aerosols. Van der Valk
proposes that salt aerosol is a secondary factor in eliminating forbs from the
foredune. Many species not found
on the foredunes can survive the effects of salt aerosols.
In contrast, dune species with a low tolerance to salt aerosols may
inhabit areas of considerable spray. Blue
grass, an annual plant, completes its life cycle within a short time, often
between major salt storms (dry windy periods when salt-containing aerosol
deposition is high). A low
profile, as seen in seabeach amaranth and seaside broomspurge, is a growth
form that reduces the effects of salt aerosols.
Canada horseweed may grow to heights of 1-1.5 m in old fields, but
mature flowering specimens of Canada horseweed are less than 5 cm high where
they are exposed to salt aerosols. Survival
under low-light conditions is adaptive for some dune plants because dominating
shrubs intercept damaging salt aerosols, thus protecting those understory
species that can tolerate the shade of the canopy.
Barbour and DeJong (1977) suggested that tolerance to salt spray alone
does not explain zonation in nature for all beach taxa.
They found that Ambrosia chamissonis was less tolerant of salt spray than they
expected based on the species’ position near the shore, while Lathyrus
japonicus and L. littoralis
were more tolerant to salt aerosols than they hypothesized by their positions
on the beach.
Salt aerosols act as selective agents for species, especially plants,
by eliminating those species that are sensitive to elevated atmospheric salts.
Dune plants that survive this selective force exhibit adaptations that
enable them to tolerate, reduce, eliminate, or avoid the effects of salt
aerosols in coastal areas. Seneca
(1972) observed a significant increase in the biomass of sea oats seedlings
treated with salt aerosols, suggesting positive responses to this factor.
He suggested that species such as sea oats and American beachgrass not
only tolerated the spray (salt aerosol), but also received some nutrition from
it, a hypothesis later refuted by van der Valk
Hairs and other protuberances on leaf and stem surfaces reduce the
effects of salt aerosols and enhance the survival of those dune plants in the
spray zone. Hairs intercept salt
droplets, thus preventing salt from reaching leaf or stem surfaces.
Thickened cuticles concurrently reduce the penetration of salt aerosols
and prevent the desiccation of plant tissues.
Foliar uptake of nutrients has been demonstrated in many plants.
According to Boyce (1954), dune grasses are less susceptible to the
effects of salt aerosols where nutrient levels, especially nitrogen, are
suboptimal. Thus, foliar uptake
of nutrients in this species would not necessarily be a selective advantage.
It is not surprising that foliar uptake of nutrients by sea oats and
American beachgrass is negligible (van der Valk
Aulacomnium palustre and Polytrichum commune, two
species of mosses, are examples of plants intolerant to salt aerosols that
avoid aerosols through habitat selection.
These species occur only in swales behind large secondary dunes where
standing fresh water may dilute or wash off salts deposited on their leaves by
wind turbulence or rainfall (Boerner and Forman
On the mid-Atlantic beaches, Ceratodon
purpurea often grows under beach plum.
Ceratodon purpurea exhibits a slight tolerance to salt aerosols, but
only when it rains frequently. Its
survival under beach plum is probably the result of the limited salt impact
The death of the leading apices on the seaward side of a plant creates
conditions where lateral branches are released from dominance and grow
rapidly. This response by woody
plants results in an espaliered (flattened and spread) appearance.
The smooth surface characteristic of aerosol‑influenced plants
has a lower deposition efficiency for salt aerosols than an irregular,
perforated surface (Boyce 1954). Young
shoots that grow above the canopy are efficient collectors of salt aerosols
and are quickly killed. As this
process proceeds, a smooth, leaf-covered, and espaliered surface forms on
arborescent plants. Boyce (1954)
further showed that the canopy angle of the shrub is correlated with the
intensity of salt aerosols. Where
salt-aerosol intensities are highest, the canopy angle is lowest. In many dune slacks, shrubs such as yaupon, northern
bayberry, and waxmyrtle show little salt-aerosol pruning, while similar plants
on open dunes may be severely pruned.
Salt aerosols may have an impact on local populations.
Cartica and Quinn (1980) determined that succulence in seaside
goldenrod is not genetically fixed but is a response of the phenotype to the
local environment. Seaside
goldenrod plants receiving greater concentrations of salt aerosols were more
succulent than those receiving less spray.
Other coastal plant species are likely to respond similarly.
Grazing greatly affects the composition of plant communities on dunes.
Reduction of rabbit populations in dune slacks in Wales led to major
changes in the vegetation; reduced grazing increased the growth and flowering
of grasses followed by a decrease in the presence and diversity of annual
plants (Ranwell 1960b).
et al. (1975) proposed that grazing by hogs, cows, and horses produced live
dunes (unvegetated and migrating dunes) on Cumberland Island, Georgia, a
process that may have begun in the colonial period when livestock were
introduced. Au (1974) supported
this argument, suggesting that grazing animals maintained the instability of
the dune system on Shackleford Banks, North Carolina.
Grazing animals can profoundly influence the composition of
dune-and-slack systems. On
Shackleford Banks, broomsedge is uncommon in dune environments, whereas this
species is abundant on nearby Bogue Banks and Bear Island where no feral
animals graze. Horses, goats, and
sheep selectively graze this plant. Where
trampling from animals or humans is common, dune sandbur and coastal plain
pennywort are abundant. Constant
disturbance of the habitat apparently favors these species.
Where habitats are chronically disturbed and the plant is avoided by
grazers, Russian thistle is abundant. Coile
and Jones (1988) attribute the depauperate flora on St. Catherines Island,
Georgia, to the fact that excessively large populations of grazing herbivores
(deer and cattle) have eaten almost every available digestible plant.
Although no measure of the carrying capacity for grazers has been
established for dune-and-slack communities, Turner (1988) has developed a
simulation model of the salt marsh to examine the effects of grazing animals.
Horses also graze in the dune-and-slack communities on many islands.
Despite the considerable damage caused by feral animals, people enjoy
seeing grazing animals in the coastal setting.
Since they are likely to be maintained in some public areas (e.g.,
state and national parks), the development of grazing models like the one by
Turner 1988) would help set acceptable population levels for dune-and-slack
Saturated and Waterlogged Soils
Saturated soils are characteristic environmental features of slacks and
plants that are periodically subjected to prolonged flooding.
Periodic flooding or continuous saturation of the soils affect plant
development. The root system of
slack plants experimentally grown in half-waterlogged soils are confined
to the non-waterlogged part of the soil (Jones and Etherington
Tiller production in grasses is reduced by waterlogging (Fig. 6.6),
whereas Jones and Etherington (1971) reported no change in shoot production in
carices (Carex spp.).
This difference may influence the competitive ability of grasses and
explain why sedges are often dominant in wet environments.
Redox potential (Eh), a measure of the oxidation state of the soil, is
sensitive to waterlogging; Jones and Etherington (1971) measured a reduction
of Eh as close to the surface as -1.0 cm in waterlogged soils compared
with controls (Table 6.5).
Waterlogging causes a physiological change in plants that results in
morphological changes. When
waterlogged, bulrushes growing in slacks are shorter and the amount of
aerenchyma tissue increases (Seliskar 1988).
Seliskar suggested that the morphological changes result from an
increased level of ethylene produced in the tissues of waterlogged bulrush.
Orchard grass (Dactylis glomerata)
is adapted to the water stresses occurring in slack habitats.
Plants of orchard grass inhabiting the driest dune sites exhibit a
lower transpiration rate than those in the wettest sites (Table
Plants from the wettest sites average three times as many stomata per
mm2 as plants from the driest environments.
In the same species, significant differences in cuticular waxes on the
leaves occur between plants inhabiting wet and dry areas.
These phenotypic differences are present when seeds from dry and wet
environments are grown in a common environment (Ashenden
Saltmeadow cordgrass responds to waterlogged conditions by producing
roots with large amounts of aerenchyma tissue.
With increased waterlogging, Burdick and Mendelssohn (1987) found that
saltmeadow cordgrass increasingly relies on anaerobic root respiration.
Slack plants are susceptible to drought as well as waterlogging.
Although two months is the average duration of waterlogging in slack
soils, a seasonally high water table may persist in some slacks for up to 6
months. Conversely, in the dry
months, during which low rainfall and high evapotranspiration lower the water
table, plants may experience extended drought conditions.
Willis et al. (1959) established a dividing line between dunes and
slacks based on species presence and duration of flooding.
Where slacks flood 2 months or less per year, populations of species
characteristic of dunes are established.
Water Table Fluctuation
A fluctuating water level is common in slacks.
Plants such as water-hyssop and ludwigia (Ludwigia
sp.) survive flooding for considerable periods without dying, often forming
adventitious roots. In the
spring, small spike-rush grows in rings around temporary pools in the
dunegrass community on Assateague Island.
As the pool dries, small spike-rush
follows the water's edge inward until the pool is completely dry.
This pattern of development creates a solid mat of vegetation from edge
to center. If the site is not
flooded again, toad rush (Juncus
bufonius) and saltmarsh sandspurry (Spergularia
salina, identified as S. marina)
may appear in association with small spike-rush (Higgins et al.
In coastal dune environments, water table fluctuations of 0.3-0.6
m are enough to cause major perturbations within wetland ecosystems.
During wet periods, water levels may be high enough to kill emergent
wetland plants; during droughts, floating vegetation is stranded and killed (Hillestad
et al. 1975). During warm, dry
periods, accumulated organic matter undergoes aerobic decomposition.
These occasional perturbations may maintain open-water environments
within the dune systems of some barrier beaches.
Thus, the surface freshwater systems on barrier islands exhibit pulse
stability (Odum 1971).
Almost regular, but acute, perturbations in the water table maintain
and perpetuate the wetland systems. Without
these stochastic changes in water level, shallow, open‑water systems
would fill rapidly with organic matter, and succession would proceed toward
shrub or swamp forest (Hillestad et al.
1975). Since pulse-stable systems are typically very
productive, the rise and fall of the water table should be considered a
natural process for these ecosystems and necessary for maintenance of the
slack and other wetland systems.
Adaptations of dune animals parallel those of desert organisms.
Nighttime foraging strategies, subsurface dens, and protective
coloration are examples of animal adaptations to life in the dunes.
Unlike the desert, however, environments within and surrounding the
dunes possess sufficient water resources to effectively reduce animal stresses
associated with water conservation and heat loading.
Fowler's toad (Bufo woodhousei
fowleri), one of the few common amphibians in dunes and slacks, exhibits a
distinctive coloration that reflects light, thereby reducing its heat load and
serving as protective coloration in the gray-to-tan dune sands. During the heat of the day, Fowler's toads burrow in moist
sand, although Engels (1952) frequently encountered the toads in dunes and
slacks during the daytime.
Wolf spiders (Geolycosa spp.)
spend daylight hours in burrows that may be as deep as 1.0 m.
At night, these animals forage for insects.
The light color of the wolf spider serves as protective coloration.
Velvet ants (family Mutilidae) are common in dune environments; the
velvet hairs on this ant's abdomen allow it to withstand all but the highest
temperatures on dunes.
Ants (family Formicidae) are ever-present in dunes and slacks along the
entire Atlantic Coast. Their
intensive burrowing behavior establishes these animals as well adapted to the
dune environment. A predator of
ants, the doodlebug or ant lion (Myrmeleon
immaculatus) constructs a conical pit in dry sand and lies at the bottom.
When a hapless ant or other crawling insect falls into the pit, the
doodlebug throws sand into the air causing miniature landslides.
The ant lion then seizes the insect as it tumbles to the bottom of the
pit, paralyzes it, and devours it.
To avoid high daytime temperatures, reptiles and mammals maintain dens
below the surface or in dense tussocks of plants. Six-lined racerunners (Cnemidophorus
sexlineatus) occupy dens in moist, cool soils. When they are discovered, racerunners may escape by entering
a burrow or by hiding underneath tussocks.
Few dune animals are active when temperatures exceed 35oC. Mammals generally feed
in the early morning or late evening when temperatures are more moderate than
at midday. Small mammals such as
house mice (Mus musculus) and rice
rats (Oryzomys sp.) construct nests
of grasses and forbs in dense patches of vegetation.
The plant cover provides shade from the heat of mid-day.
Coastal communities are not static, but are continuously changing.
The coastal dune-and-slack systems are especially dynamic.
Natural disturbances--hurricanes, extratropical storms, and variations
in climate--result in observable changes in the vegetation and fauna of the
dune and slack environments.
Successional patterns and processes on barrier beaches have received
considerable study since the early 1900s.
At mid-century, Tansley (1949) and
Salisbury (1952) published treatises
that summarized plant succession on coastal dunes. Generally, they found that mobile dunes are first stabilized
by several colonizing species; second, a stable turf forms; and last, a dense
scrub or heath forms over the dunes.
While salt aerosols may be of secondary importance in determining the
species presence on the foredunes (van der Valk
1974), salt aerosols in the
atmosphere are considered by some researchers as the primary factor
determining the direction and rate of change in species composition over time.
When prevailing conditions reduce salt aerosols, shrubs and trees
invade herbaceous communities; however, herbaceous communities replace
arborescent vegetation when salt aerosols increase.
The distribution pattern along the coast mimics a time sequence; older
dunes are farther inland and have fewer effects from salt aerosols.
In contrast, younger dunes are closer to the source of salt aerosol.
The classic study by Oosting and Billings (1942) showed that biotic
succession cannot be used successfully to explain zonation in coastal dune
Topographic position, and the environmental variables associated with
it, played a significant role in determining the direction and rate of change
in communities. The driest
communities change most slowly; low, moist sites change rapidly.
Martin (1959) stated
. . . it seems reasonable to conclude that plant succession on Island
Beach, and perhaps on barrier beaches in general, is largely an intra-zonal
phenomenon. The herbaceous,
shrubby, and arborescent zones do not necessarily represent seral stages in
biotic (autogenic) succession. The
herbaceous zones… are structurally and compositionally stable under the
environmental conditions which characterize them."
Simple models of succession, however, are inadequate to explain the
diversity of plant patterns observed on the barrier beaches.
Art (1976) agrees with other investigators (e.g.,
Oosting 1954; Martin
1959) that the pattern of plant communities from ocean to sound does not
necessarily indicate a successional sequence.
In fact, high dunes do not appear to undergo successional change (Willis et al.
direction and rate of succession are closely related to soil development (Olson
Art (1976) found that American beachgrass is a pioneer plant where sand
is being deposited in dune-and-slack areas. Except for the seaward faces of the primary dunes, beach plum
and northern bayberry eventually replace American beachgrass.
In blowouts, Art (1976) recognized beach heather as the pioneer
species. Bearberry spreads into,
and then replaces, the stands of beach heather.
In stable, barren dune-and-slack sites, pitch pine (Pinus
rigida) and eastern red cedar (Juniperus
virginiana) may act as pioneers. Art's
successional model is based on a simple unidirectional change from initial
conditions of deposition, erosion, or stability (Fig. 6.7).
According to Art, the terms “pioneer” and “climax” species are
meaningless in dune environments that experience continuous disturbance.
Martin (1959) views succession as capable of occurring on the primary
dunes; however, it is inhibited by high salt-aerosol concentrations.
Several thicket associations are persistent subterminal stages of
succession. In the mesic zones
away from the salt-laden winds and where soils are well drained with fresh
water, succession is not inhibited. Martin
speculated that the red cedar woodland and pine woodland communities on Island
Beach, New Jersey, which are not duplicated on the mainland, represent true
salt-spray climax communities as defined by Wells and Shunk (1938) and
Au (1974) proposes three pathways leading to a maritime forest (Fig.
6.8). Succession from bare sand
to grassy flats or grassy dunes leads to scrub flats or dry thickets.
Succession of scrub flats or thickets may lead to the formation of
maritime forests. The blowout
cycle results in the establishment of what Au referred to as a freshwater
marsh, then to a wet thicket and, finally, maritime forest.
Ranwell (1960a) promoted a cyclic dune-and-slack successional model
(Fig. 6.9). At Anglesey, United
Kingdom, Ranwell determined that temporal and spatial successions were not
necessarily coincident. Where
dunes migrate, the most landward dunes may be young and do not necessarily
exhibit vegetative cover representing the ultimate stage in succession.
During an accretionary phase, blowouts that expose damp sand undergo
changes in plant cover until low-growing willow shrubs (Salix
repens) colonize the slacks. These
plants accumulate sand rapidly and form large hummocks.
Willow plants are replaced by dune grasses.
Erosion of the hummocks starts the cycle again.
The entire cycle was completed in approximately 80 years (Ranwell
Van der Laan (1979) attributed temporal and spatial differences in
vegetation to the dynamic nature of the water table in slack environments;
seasonal fluctuations and differences in the level of the water table between
one year and another results in the sorting of plant species within individual
slacks and among slacks along an elevational gradient (Fig. 6.10).
Long-term studies that relate vegetation changes to fluctuations in the
environment are needed to differentiate among competing succession models or
hypotheses of vegetation change.
Most descriptions of coastal succession are site-specific, but some
generalizations are possible. Dunes
undergo succession to a point where grasses and forbs dominate.
Succession to a dominance of shrubs and trees occurs only when the
effects of salt aerosols are reduced. This
may occur as the barrier beach grows seaward, thereby reducing the effect of
salts in the atmosphere, or as salt-aerosol shadows are formed on the lee side
of high dunes. Many factors, including erosion, overgrazing, drought, and
human activities can disrupt succession.
Temporal vegetation changes appear to occur episodically in response to
changes in physiography or other disturbances (Ehrenfeld
A successful succession model for the coastal zone must consider the
significant environmental factors influencing coastal species and synthesize
accumulated data, including both temporal and spatial differences in
Dune Trophic System
Unlike estuaries and tidal marshes, whose trophic relationships and
nutrient cycling have been intensively studied, dune systems have received
considerably less attention. Many
ecological generalizations have been proposed, but the ways in which these
operate in dune ecosystems have not been fully explored.
Facies of the dune-and-slack ecosystem vary in physical structure from
low diversity, simple systems in the strandline and foredune area, to complex,
highly diverse, stratified systems at the ecotone between the predominantly
herbaceous dunes and the inland forested environments.
Across this continuum, vegetation cover, height, woodiness, and
diversity increase. Also,
organisms exert increasing control over the dune environment from the foredune
area to the maritime forest.
Primary productivity in dunes and slacks is limited by water and
nutrient supplies, salt aerosols, and sand burial and erosion.
In a South African dune slack, the belowground plant biomass is more
than twice the aboveground plant biomass (McLachlan et al.
1987; Fig. 6.11).
Belowground detritus biomass is more than 30 times the aboveground
detritus biomass. The large
amounts of belowground biomass are typical of both dunes and slacks.
In their study area, McLachlan et al. (1987) found a large excess of
phytomass compared with consumers, suggesting that other factors are operating
to reduce the success of consumers. Decomposers
were responsible for processing approximately 40 percent of the net primary
production in marram grass in Great Britain (Deshmukh
Measures of primary production and decomposition rates in Atlantic
Coast dune-and-slack communities are lacking.
Similar to the South African coastal systems, Atlantic dune-and-slack
environments support three food chains-- macroscopic grazers, macroscopic
detritivores and interstitial decomposers
(Fig. 6.12). The macroscopic
grazers range from insects feeding on flower nectar and plant leaves to
ungulates grazing on grasses and forbs. Macroscopic detritivores feed on litter from aboveground
plant parts while the interstitial detritivores (microfauna) feed on the pool
of belowground detritus (McLachlan and van der Merwe
Energy Transfers Between
Energy flows into the dune-and-slack ecosystem from the beach as
macroscopic detritus, namely culms of cordgrass, marine algae, seagrasses and,
occasionally, a stranded animal. This
material, originally deposited on the beach on high tides, is carried into the
dunes and slacks by onshore winds. Energy
leaves the dune-and-slack system as food consumed by transient animals and
when wind removes both living (e.g., insects) and dead organic matter.
The predatory ghost crab and omnivorous raccoon introduce energy into
dune systems when they forage for prey along the intertidal beach.
The paucity of data
concerning energy flux within dunes and slacks is evident; few studies have
taken an integrative approach to the study of these communities.
The resources of dunes and slacks are intimately connected; the
groundwater regime links the communities and many animals move freely between
environments. Investigation of
the magnitude and direction of the fluxes between dunes and slacks would add
to our basic knowledge of ecosystem dynamics and uncover the functional
interrelationships of these communities.
Human Impacts and Management Implications
coastal region is now the center of population, commerce, national defense,
energy development, and recreation, a result of the richness of the natural
resources of this environment. Unfortunately,
the coastal region is extremely vulnerable to development impacts:
building construction, alteration of dunes, beach stabilization,
maintenance of navigation channels, ground-water extraction and contamination,
and recreation (Clark 1991). Dunes
and slacks are among the most vulnerable environments in the coastal region
primarily because of their small size and the lack of scientific knowledge
concerning their ecologic role.
With few exceptions, barrier islands and barrier beaches of the
Atlantic Coast were not highly prized as sites for villages, towns, or other
enclaves during our early post-Columbian history.
With unstable substrates, agriculturally poor soils, inhospitable
climate, unpredictable weather, and remoteness from major arteries of
commerce, barrier islands were ignored. As
agriculture and industry grew, however, inhabitants of coastal areas turned to
the barrier beaches for resources. Lumbering
and grazing became important activities on barrier beaches; both activities
accelerated the effects of wind erosion and sand movement on barrier beaches.
the late 17th century, farmers found that the coastal islands in Virginia and
the Carolinas were excellent areas to graze their cattle, and for the next 100
years, they used them. As the
dune systems were overgrazed, they underwent major changes, primarily
expansion (Brown 1959). Development
began in the late 19th and early 20th centuries, and lavish hunting clubs and
private reserves were established on many islands.
The hunting clubs and reserves were not extensively developed to
preserve hunting opportunities, and the dune-and-slack communities were little
disturbed. Excellent examples of dune‑and‑slack communities
preserved in this manner are found on Currituck Banks, North Carolina;
Cumberland and Jekyll islands, Georgia; and North Island, South Carolina. Today, these areas have some of the most extensive and least
disturbed dune‑and‑slack systems along the Atlantic Coast.
series of severe Atlantic hurricanes beginning in the late 1800s followed by
the stock market crash in 1929 ended the era of ownership of barrier islands
and barrier beaches by wealthy individuals; instead, the islands were opened
to development by inland speculators. Coastal
cities grew rapidly as developers dredged, filled, and flattened the coastal
landscape to make way for the expanding population.
Dunes, forests, and marshes were obliterated to create human
habitations. During this period,
few controls were placed on development practices; coastal dunes and other
coastal environments were replaced by houses with manicured lawns, multilane
highways, and the usual commercial interests that follow initial development.
the 1960s, growing numbers of people championed the importance of natural
landscapes. Restraints were placed on coastal development, and citizens
lobbied to preserve those parts of the coast that had not already succumbed to
development. State and national
parks and wildlife refuges were created.
Cape Lookout National Seashore, Gateway National Recreation Area,
Cumberland Island National Seashore, and Cape Cod National Seashore preserved
coastal areas that also provided recreational outlets for millions of
visitors. This wave of
preservation in areas that included extensive natural dune systems resulted in
the protection of thousands of hectares of this community type from human
exploitation. It was not long,
however, before researchers recognized that coastal environments were highly
dynamic, both morphologically and ecologically, and it became necessary to
reassess management policies and strategies (Gaskin and Stottlemeyer
Godfrey and Godfrey 1974; Hayden and Dolan
dune-and-slack ecosystem is especially sensitive to human
influence. The natural stresses
of the environment (i.e., storms, periodic drought, salt aerosols, and low
nutrient content of the soils) on the plant and animal inhabitants are
exaggerated when humans pursue commercial and recreational activities in these
areas. These ecological units,
with their natural stresses recover slowly from natural and
human‑induced perturbations (Leatherman
all coastal developments are dependent on wells tapping into a freshwater
lens. The volume of water in the lens, expressed as the groundwater
table level, will decrease if the rate of withdrawal exceeds recharge from
precipitation. Dune slacks are
especially sensitive to changes in the water table. Because the environment of slacks is controlled by the depth
to the water table and the period of surface inundation or soil saturation,
slacks can quickly disappear when fresh water is removed faster than it is
naturally replenished. The
existence and character of many semi-permanent and permanent
wetlands--sloughs, fresh marshes, and ponds--are
controlled by the water table level on barrier beaches and barrier islands (List and List
enrichment in dune systems results in major changes in plant composition.
Following the addition of all necessary nutrients, broad-leaved
plants are initially favored, but are soon outcompeted and suppressed by
grasses. Most low-growing
plants are quickly eliminated (Willis 1963).
In slacks, species diversity is decreased by added nutrients. Septic-tank fields placed in dunes and slacks may allow human
wastes to enrich the soils; once these effluents reach the water table, the
shallow groundwater upon which slacks are dependent may become polluted (Godfrey
soils along the Atlantic Coast are composed of quartz sands and have little
buffering capacity. Along the
mid-Atlantic Coast, the die out of American beachgrass over large areas has
led to a hypothesis that acid rain is responsible for this die out (Seliskar
1992). However, virtually no
damage was evident in American beachgrass plants treated with acid rain.
Treated plants were shorter, but shoot density, live shoot biomass, and
rhizome biomass were not adversely affected by acid rain.
Seliskar (1992) determined that root biomass in American beachgrass was
greatest in plants treated with acid rain.
Continued investigation of the effects of acid rain is necessary to
better understand the response of dune-and-slack communities to this
animals have considerable impacts on dune-and-slack systems.
On Shackleford Banks, North Carolina, feral animals graze and trample
the vegetation, enrich the soils with feces, and, to a considerable extent,
control plant succession in slacks (Fig. 7.1).
On the Outer Banks, feral cattle opened the forest canopy, resulting in
the remobilization of dune sands (Brown 1959;
Tyndall and Levy (1978) found that rooting hogs created
micro-environments (e.g., potholes and mounds) that altered the natural
distribution patterns of plants in the dune slacks of the Virginia section of
Currituck Spit. Feral cats hunt
in dunes and slacks, competing with the native animals on many islands
including Cape Hatteras and Cape Lookout national seashores.
Pet dogs and cats are particularly disturbing to ground-nesting
of Shoreline Overwash Barriers
and northeasters are a part of the climate of the Atlantic Coast.
Shorelines, including the dune-and-slack environments, are continually
changing in response to storm activity. Constructive
forces facilitate building of dunes and slacks in one location; these
formations are removed at another location.
However, construction of homes, businesses, and other services on
barrier beaches has been accompanied by modifying or removing the dunes and
slacks along the shore and constructing seawalls, jetties, and groins (Fig.
7.2). These structures result in
"drawing a line in the sand"; as natural sand transport occurs, the
height and width of the protective barrier of dunes and slacks diminish.
Humans view this sand transport as erosion and often take actions such
as raising seawall heights, replenishing sand, or expanding groins and
jetties. Unfortunately, we have yet to meet this challenge with an
understanding of the necessity for us to respond with human flexibility near
can be expected to change as sea-level rises.
Along undeveloped shorelines, the consensus is to "let nature to
take its course." This
action would allow dunes to be inundated by salt water and eroded by waves as
sea level rises; the shoreline would respond by retreating (Titus
1990). Titus (1990) outlined four potential responses to sea level rise along
developed shorelines: 1) removing structures as the shoreline retreats; 2) an
engineering a retreat where threatened structures on the oceanfront are
abandoned and the lagoon side of the island is opened to development by
elevating it with sand pumped from the lagoon; 3) maintaining the existing
land and water relationships as sea level rises by adding sand equally over
the entire island; and 4) surrounding islands with a levee to prevent
intrusion of the sea. Whichever
alternative is implemented at a particular site, dune-and-slack environments
will probably not be viewed as important enough to be preserved, conserved, or
and slacks are affected by the global sea level rise.
As shorelines retreat, saltwater intrusion from oceanic overwash will
result. Dune slacks which now
have saturated soils will likely experience longer periods of standing water
in the future; slacks that exhibit soil saturation infrequently today will be
characterized by soils that are saturated with increasing frequency (Ehrenfeld
1990). For example, based on
accepted rates of sea level rise, the ponds at Nags Head Woods may have had
their beginning sometime between 1,500 and 3,000 years before present (Otte et
al. 1984) and became permanent about 400 years before present (List and List
development of Kiawah Island, South Carolina, as a resort community, the
population densities of the herpetofauna were considerably reduced (Gibbons
and Harrison 1981). A reduction
in frog and toad populations was the most noticeable change.
Only small changes in the herpetofauna of beaches and dunes occurred
because relatively few species of reptiles and amphibians inhabit beach and
dune environments (Gibbons and Harrison
beaches of barrier islands provide optimal nesting habitat for many
colony-nesting birds (Erwin 1980). Common
terns (Sterna hirundo), least terns (Sterna
albifons), black skimmers (Rynchops
niger), and herring gulls (Larus
argentatus) prefer natural barrier islands. Along the undeveloped shoreline of Virginia, 90 percent of
terns and skimmers and 50 percent of herring gulls select barrier beaches for
nesting; in New Jersey, developed barrier islands are used by only 9 percent
of these colony nesting species, implicating human intrusion as the cause (Erwin
1980). Additional studies
are essential to determine the impact of human intrusions on colony-nesting
population decline of piping plovers (Charadrius
melodus) along the Atlantic Coast is hypothesized to be caused by
alteration of chick behavior resulting from human activity around nesting
sites. Nearby human activity increases the exposure of chicks to
inclement weather and predators, thereby increasing mortality (Flemming, et
al. 1988). With the approach of
humans, chicks spend more time sitting vigilantly and less time feeding and
sections of the Atlantic shoreline, especially beach and dune systems in
national parks, are used by thousands of off-road vehicles each year (Primack,
1980). Off-road vehicle (ORV)
and pedestrian traffic greatly affect coastal ecosystems by crushing plants
and churning up roots, altering the water-holding capacity of dune-and-slack
soils, and opening extensive dune areas to the effects of wind (Liddle and
Greig-Smith 1975a, 1975b;
Godfrey 1976; Hosier and Eaton
Dunes activated by ORV and foot traffic may quickly modify the natural
patterns in dune-and-slack communities.
pedestrian traffic results in devegetation and dune instability (Leatherman
and Steiner 1979). At Fire Island
National Seashore, vehicles and pedestrians removed all traces of vegetation,
including dead plants on the dunes (Art 1976).
McDonnell (1981), studying Plum Island, Massachusetts, noted that
moderate, heavy and severe trampling (based on estimates of visitor use)
reduced species diversity. All
levels of trampling reduced the cover of beach heather, although trampling had
less impact on American beachgrass (Fig. 7.3).
The virtual elimination of beach heather and expansion of American
beachgrass suggests that trampling may lead to the enlargement of the foredune
community and the restriction of the interdune community to a narrow zone
behind the foredune.
strandline is an important part of the dune system, but this feature is
quickly disturbed by a single ORV pass. Scattering
of drift material by off-road vehicles affects the rate of organic decay and
may delay or prevent the formation of embryo dunes.
Vehicle traffic crushes and kills seedlings of annuals and small
perennials associated with the drift--often with a single pass.
Ultimately, the size, shape and distribution of the foredunes are
influenced by ORV disturbance (Leatherman and Godfrey
1979) (Fig. 7.4).
cryptic nests and young of beach-nesting birds render them particularly
vulnerable to disruption by ORV’s during the nesting season; however,
studies show that birds can acclimate to vehicles passing close to their nests
(Leatherman and Godfrey
are especially disruptive to beach-nesting birds where the beach is narrow at
high tide and vehicles are forced to pass directly through nest sites.
Unleashed pets, however, can cause greater disturbance to nesting
colonies than many indirect passes by an ORV.
continuously changing environment on the intertidal beach masks correlations
of vehicle use with meiofauna, interstitial algae, and bacterial populations.
Thus, no definite conclusions can be drawn regarding the long-term
impact of vehicle use on populations of these organisms (Leatherman and
Godfrey 1979). On the backshore,
passes by vehicles break the salt crust.
Although no data have been collected to support this hypothesis, it has
been proposed that once the salt crust is broken, sand movement increases.
Additional research on the intertidal beach and backshore is necessary
to uncover ORV impacts. Armed with this information, a coastal manager can develop
strategies to allow conflicting recreational uses to coexist without
significantly degrading the resources (Vogt 1979).
There is no minimal level
of disturbance for ORV traffic on dunes:
any ORV traffic in dune systems decreases the total number of species,
species diversity, and the area of slacks, while increasing the area without
vegetation cover (Hosier and Eaton
1980). Brodhead and Godfrey (1979) noted that rhizomes of American beachgrass
do not grow deep enough to avoid being churned up by ORV tires and the first
few passes are the most critical and damaging.
Fifty passes by an ORV can stop seaward growth of a foredune system
dominated by American beachgrass; this can set back dune expansion for as much
as 1 year (Brodhead and Godfrey
tracks that run parallel to the prevailing winds on dunes are likely to become
blowouts and increase erosion.
recovery on dunes varies with location and species composition.
On foredunes, recolonization begins immediately; tracks are nearly
obliterated within 4 years. On
secondary dunes, recovery may take 8 years or more (Leatherman and Godfrey
1979). The most stable dune sites
and those communities with natural stresses such as drought and low nutrient
levels take the longest time to recover.
More dynamic environments tend to recover rapidly.
At Race Point, Massachusetts, Leatherman and Godfrey (1979) ranked the
rate of recovery for dune communities from most rapid to least rapid:
American beachgrass foredune, American beachgrass backdune, bearberry
heath, hairgrass (Deschampsia cespitosa), lichen (Cladonia
sp.) grassland, and beach heather heath.
Page et al.
(1985), studying dunes and slacks in the Ynyslas Dunes,
Cardigan Bay, Wales, demonstrated a similar recovery pattern.
Grasses are resistant to pedestrian pressure and recover quickly when
impacts are removed. Dicotyledonous
plants, the dominant plants in older dunes and slacks, recover more slowly (Page et al.
dunes, soils within vehicle tracks have a greater bulk density and water
content than soils under natural vegetation (Fig. 7.5).
Dry soils exhibit a large increase in thermal capacity, a small
increase in thermal conductivity, and no change in thermal diffusivity under
ORV impacts. In slack soils,
thermal capacity is increased by ORV traffic, but thermal conductivity and
diffusivity are markedly reduced, probably due to lower water content in the
compressed soils (Liddle and Moore
The destruction of vegetation and litter decreases the thickness of the
surface boundary layer (Art 1976). Once
this occurs, winds tend to become channeled in the V-notch of the dune
line. This notch becomes a major
blowout with continued use by vehicles. When
breaches in the dunes are oriented parallel to the prevailing winds, sand
movement is accelerated (Leatherman and Steiner
visits to parks are at least partially responsible for changes in species
composition of dune-and-slack communities.
Since 1934, 85 species of plants have been extirpated and 104 species
have been introduced to Orient Beach State Park, New York (Lamont and Stalter
1991). The most human-altered communities possess populations of
exotic species found nowhere else on the island (Stalter and Lamont 1990).
and explosive development on the barrier beaches has caused far greater harm
to the ecosystem than the minor damage done by centuries of grazing and
localized land-use activities (Godfrey
Today, bulldozers can alter a landscape that 100 years ago took
extensive cutting or overgrazing to change even in a minor way (Fig. 7.6).
beaches and islands support ecosystems with only limited potential for direct
restoration (Cairns et al.
for foredunes and tidal marshes, most coastal environments cannot be recreated
quickly. Burk et al. (1981) have
documented natural dune recovery on Portsmouth Island, North Carolina. They concluded that, although Portsmouth Island has the
appearance of fragility, it has shown a greater natural recovery than
previously thought. This sense of
fragility may be a misleading artifact of intense disturbance related to
overgrazing and human occupation (Burk et al.
research designed to enhance dune construction by using native species has
been conducted by many scientists. This
applied research continually provides data on the ecology of plants and
animals inhabiting the dune-and-slack community and aids communities in
responding to damage caused by coastal storms (Fig. 7.7).
The products of the research range from technical reports (Seneca et
al. 1977; Knutson 1980) to popular conservation-oriented information
(Graetz 1973; Craig
of buildings can have significant impacts on dunes by altering wind patterns
and disturbing natural vegetation. Nordstrom
and McCluskey (1985) argued for a comprehensive research program to identify
the effects of structures on shoreline processes.
Additional research is necessary to determine construction criteria for
houses built within the dunes and slacks that will minimize human interference
with the natural system.
acreage of dune-and-slack habitat has dwindled during the 20th
century; however, no studies document the total loss of dunes and slacks.
Development has usurped many dunes and drained countless slacks and
other wetlands in dune environments. The
destruction of slacks parallels the loss of tidal marsh experienced before
rules and regulations limiting certain development practices were imposed. Development interests saw little value in preserving tidal
marshes; today the same is true of slack communities. Only when the public demands conservation or preservation of
dune-and-slack environments will the rate of destruction be reduced.
geologically unique dune systems have been preserved, however.
The Province Lands of Cape Cod, Massachusetts; the dunes at Sandy Hook,
New Jersey; Jockey's Ridge, North Carolina; the massive dune spine of
Cumberland Island, Georgia; and the intricate dune ridges of Cape Canaveral,
Florida, are now protected. The
classic dune slacks of Shackleford Banks and Currituck Spit, North Carolina;
some kettle-hole ponds of Cape Cod, Massachusetts; and the flats at the
southern end of Cumberland Island, Georgia, also have been preserved for the
enjoyment of future generations.
organizations have purchased coastal lands with dune-and-slack systems.
The Nature Conservancy purchased the Virginia Coast Reserve, an
outstanding 81-km barrier-island chain from Metompkin Island to Smith Island
on the Delmarva Peninsula. In
Massachusetts, the Trustees of Reservations bought Chappaquidick Island,
Crane's Beach, and areas on Nantucket and Martha's Vineyard.
National Park Service manages many barrier beaches that have exemplary
dune-and-slack environments, among them Cape Cod National Seashore
in Massachusetts. Fire Island
National Seashore and Gateway National Recreation Area on the shores of Long
Island and northern New Jersey are unique barrier island ecosystems located
within minutes of metropolitan New York.
Assateague Island National Seashore in Maryland and Virginia, as well
as Cape Lookout and Cape Hatteras National Seashores in North Carolina have
outstanding dune-and-slack systems.
Besides extensive dune environments, Cumberland Island National
Seashore has excellent examples of slacks.
At Cape Canaveral National Seashore, a nearly continuous, natural,
subtropical dune system offers a remarkable view of predeveloped Florida.
U.S. Fish and Wildlife Service manages several barrier beach systems
with dune-and-slack environments.
These managed areas include the Parker River Wildlife Refuge on Plum
Island and the Monomoy Island Wilderness Area in Massachusetts; Chincoteague,
Fisherman's Island, and Back Bay National Wildlife refuges in Virginia; Pea
Island in North Carolina; Cape Romain in South Carolina; Wolf, Blackbeard, and
Wassaw islands in Georgia; and Merritt Island in Florida.
all states on the U.S. Atlantic Coast manage parks that include significant
Island Beach, New Jersey; Delaware State Park, Delaware; Seashore and
False Cape, Virginia; Hammocks Beach, North Carolina; Huntington Beach, South
Carolina; Jekyll Island, Georgia; and Flagler Beach, Florida, are outstanding
examples of coastal state parks.
Historic sites such as Fort Clinch, Florida, and Fort Macon, North
Carolina, have considerable dune resources.
national Coastal Zone Management program, which funds land-use projects in the
coastal states, has required all coastal states to develop a management plan
to protect important resources, including dune and wetland systems.
Although most states have made significant progress toward managing
their dwindling coastal environments, many unique dune‑and‑slack
areas have yet to be preserved.
Cranberry bogs, which dot the dunes from Cape Cod, Massachusetts, to
Cape May, New Jersey; medaZos,
high, nearly circular, unvegetated dunes, that dominate portions of Currituck
Spit, North Carolina, and beach-ridge islands along the low country of South
Carolina are not secure.
These and other locally important dune-and-slack areas are in need of
preservation and careful management.
U.S. Man and the Biosphere Programme (US MAB) is ideally suited for
application to coastal areas where human use and ecosystem processes often
conflict (Ray and Gregg
US MAB program nominated representative sites from the Virginian and
Carolinian provinces (see Fig. 1.5) for biosphere reserve status.
Designated or proposed biosphere reserves include Narragansett Bay and
the Rhode Island barriers; the New Jersey pinelands; Delmarva barriers; the
Virginia Coast Reserve; and the Carolinian-South Atlantic biosphere reserve
composed of three units: Outer Banks (North Carolina), Santee Delta-Cape
Romain (South Carolina), and Sea Islands (Georgia).
The US MAB is designed to encourage regional cooperation and
management, to develop ecosystem-based management areas, and to promote
monitoring, research, and public education focused on marine and coastal
ecosystems (Ray and Gregg
designation as a biosphere reserve, the Virginia Coast Reserve has also been
designated as a Long-Term Ecological Research (LTER) site (Hayden et al.
1991). Research at the Virginia Coast Reserve focuses on the
processes of succession, disturbance, and system-state change. These long-term studies are certain to add to our knowledge
of coastal processes and environments, including dunes and slacks.
Carter et al. (1990) proposed that dunes should be viewed in a broad context ". . . as integral parts of larger coastal systems exchanging mass, energy, biota and information . . . with abutting environments." Carter continues ". . . the dune-beach-nearshore system operates homostatically, with numerous checks and balances controlling its behaviour. Thus, the alteration or removal of one element of the system from the remainder is inadvisable and can lead to major adjustment." Dunes have been, and continue to be, a focal point for human activities. Dunes and associated slack environments have been destroyed, altered, and otherwise subverted from their natural condition (Fig. 7.8). Coastal land, including dunes, has increased steeply in value since World War II, and the inevitable conflict among competing users has also increased. Resource use associated with leisure activities has prevailed along much of the Atlantic Coast. A renewed awareness of the important ecological role of dunes has led to an increased desire to preserve remaining undisturbed dune environments (Fig. 7.9). Continued research in dune and slack environments will expand our knowledge of the composition, function, and interrelationships of dunes and their associated wetlands.
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