Slack Formation and Environment

 

Chapter 6.  Plant and Animal Adaptations to Dune and Slack Habitats

 

Plant Adaptations to Dune 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 species. 

Life-cycle Patterns

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 1977).

Three distinct life cycles are exhibited by the minor species of dune plants near Cape Hatteras (van der Valk 1974a).  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 (1974a) 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.  Van der Valk (1974a) 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 Valk 1974a). 

  Environmental factors 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 1974a).  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 1972b).

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).

Burial

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 (1974a) 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 1965).  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 6.2).  Internode 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. 1986).  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. 1986).

Water Conservation

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 evapotranspiration losses.

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 1984).

Solar Radiation

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 1980).  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.

Soil Nutrients

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 rainfall. 

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 1974).  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.

Seed Dispersal

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.

Reproductive Strategies

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 5 percent.

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

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 1942).  Under 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 (1974a) 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 (1972b) 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 (1977). 

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 1977).

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 1975).  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 during summer. 

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.

Responses to Disturbance and Grazing

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).  Hillestad 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 communities.

Plant Adaptations to the Slack Habitat

 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 1971).  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 6.6).  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 1978).  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. 1971).

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. 

Animal Adaptations to the Dune and Slack Habitats

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 35/ C.  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.

Succession in Dune-and-Slack Environments

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.

Controlling Factors

While salt aerosols may be of secondary importance in determining the species presence on the foredunes (van der Valk 1974a), 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 vegetation.

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. 

Succession Models

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. 1959).  The direction and rate of succession are closely related to soil development (Olson 1958b).

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 Wells (1939).

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 1960a).

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 1990).  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 communities. 

Structure of Dune-and-Slack Ecosystems

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 1979).  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 1991).

Energy Transfers Between Systems

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

 Human Use of Dune-And-Slack Environments

The 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.

In 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.

A 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.

In 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 1974; Godfrey and Godfrey 1974; Hayden and Dolan 1974).

Sensitivity of Dune-And-Slack Environments to Perturbation

The 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 1979).

Freshwater Recharge

Nearly 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 1988).

Nutrient Enrichment

Nutrient 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 1976).

Acid Rain

Dune 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 environmental factor.

Feral Animals

Feral 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; Au 1974).  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 shorebirds.

Destruction of Shoreline Overwash Barriers

Hurricanes 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 the shore.

Sea-level Rise

Dunes 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 mitigated. 

Dunes 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 1988). 

Recreational Uses

Following 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 1981).

The 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 shorebirds.

The 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 brooding.

Large 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 1980).  Dunes activated by ORV and foot traffic may quickly modify the natural patterns in dune-and-slack communities.

Light 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.

The 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).    

The 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 1979).  Vehicles 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. 

The 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 1979).  Vehicle tracks that run parallel to the prevailing winds on dunes are likely to become blowouts and increase erosion. 

Vegetation 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. 1985).

In 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 1974).  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 1979).

Recreational 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).

Extensive 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 1976).  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).

Barrier beaches and islands support ecosystems with only limited potential for direct restoration (Cairns et al. 1977).  Except 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. 1981).

Extensive 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 1974).

Construction 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.

Preservation and Management of Dune-And-Slack Systems

The 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.       

Some 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.

Conservation 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.

The 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.

The  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.

Nearly all states on the U.S. Atlantic Coast manage parks that include significant dune-and‑slack resources.  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.

The 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.

The 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 1991).  The 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 1991). 

Besides 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.

Outlook for the Future  

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.