Biology 561 Barrier Island Ecology Fall Semester

Dune-and-Slack Formation, Morphology, and Environment
Dune formation and Morphology		Dunes and dune slack environments at Cape Henlopen, DE. Slacks form in the moist swales between dune ridges in prograding coastal environments and on islands with significant ungulate populations.
	Dune formation
	Dune morphology
Physical and chemical Environment in dunes and slacks
	Climate
	Atmosphere
	Temperature
	Soils
Slack formation and environment
	Definition
	General features
	Formation
	Soil factors
	Slack Hydrology
	Other Wetland Environments
References

Dunes form where there is a large supply of sand, the wind to move it, and a place for the sand to accumulate (Fig.1). Waves erode beaches and dunes along one area of the shoreline and the longshore current sweeps along the shore, depositing the sand elsewhere; constructive waves then move the sand onto the intertidal zone of the beach. During at least part of the year, onshore winds move sand from the intertidal zone landward. Sand moved to the backshore accumulates around nearly anything--including flotsam and plants--almost imperceptibly at first, then more rapidly as seedlings or clumps of vegetation expand, eventually forming hummocks and hollows of various sizes and shapes.

The size and form of dunes found along the Atlantic Coast are highly variable and dependent upon many factors: strength and direction of prevailing winds, vegetation cover, nature of the backshore environment, and the rate at which sand is removed from the beach. For example, if the strongest and prevailing winds along a particular shore coincide, the highest dunes are typically located inland from the beach and foredune zone. If these winds are opposing, the highest dunes become established closer to the beachfront (Boaden and Seed 1985).

Plants act as geomorphic agents. Sand collects around plants, creating the characteristic patchwork of hillocks and swales. Differential efficiency in sand trapping among plant species results in distinctly different dune morphologies. Beaches with wide, flat intertidal zones expose a large sand source for building dunes.

The intertidal beach is the major source of sand for building dunes. On beaches where conditions are favorable for dune formation (i.e., strong offshore winds, abundant sand supply, and stabilizing vegetation), sand dries rapidly when the intertidal beach is exposed. The dried sand can then be moved by two processes: saltation and surface creep.

Saltation is the process in which friction from air moving over the surface lifts individual sand grains into the air. When these grains fall back to the surface, the collisions cause other grains to bounce into the air (Bagnold 1941). Thus, like a chain reaction, more grains are forced into the air by the increased number of collisions (Fig. 2). The outcome of saltation is a net movement of the lightest grains in the direction of the wind. Wind speeds of 5.4 m/sec (i.e., very light winds) can move fine sand.

Surface creep is a by-product of saltation that involves the movement of larger and heavier grains of sand. Because of their mass, the larger grains do not bounce into the air by saltation, but instead are pushed across the surface by collisions with saltating grains (Fig. 2).

The transport rate of sand varies with wind speed, which, in turn, is influenced by the local topography. On the smooth intertidal beach zone, sand may be moved great distances by winds unimpeded by an irregular surface. As the winds reach the backshore, however, strandline debris, vegetation, and existing dune topography alter wind patterns, creating microsites with increased or decreased wind speed (Olson 1958a). Where the wind speed is increased, saltation and surface creep are increased and large quantities of sand are moved. Conversely, sand accumulates in microsites where wind speed drops, such as the zone of still air at the base of vegetation (Fig. 3).

Dune sands along the Atlantic Coast are derived from Pleistocene and Holocene sediments. Dunes along northeastern barrier islands and barrier beaches, including Cape Cod, Massachusetts, the southern shore of Long Island, New York, and parts of northern New Jersey, are derived from headlands, primarily Pleistocene-aged glacial moraines. Myrtle Beach, South Carolina, is a mainland beach partly composed of sand originating from Pleistocene-aged beach ridges.

The interaction of coastal geological and biological processes creates a diversity of barrier dune systems. When abundant sand is delivered to the beach, barrier beaches prograde (build seaward) and parallel--or nearly parallel--dune ridges form. Dune ridges formed by beach progradation can be altered. Blowouts, created by strong winds or storm waters, alter existing topographic patterns as sand is blown away from devegetated sites.

Complex linear ridges typically form on barrier systems near capes or on shorelines undergoing a change in orientation. Inlet migration and closure also facilitate the formation of new, topographically complex dune systems. Understanding the processes that create coastal dunes has enabled the efficient and effective creation of artificial dunes.

The shape and pattern of dunes vary along the Atlantic Coast. Dune morphology ranges from distinct sand ridges running parallel to the shoreline and separated from each other by slacks (e.g., Bogue Banks, North Carolina), to ridges stretching obliquely, or occasionally perpendicular, to the shore (e.g., St. Catherine's Island, Georgia). Other areas are characterized by massive dune fields (e.g., Shackleford Banks, North Carolina) or low-relief hummocks forming a weakly developed ridge (e.g., Wolf Island, Georgia).

Coastal dune systems have been variously divided into regions that reflect their location along the shore, vegetation cover and type, age and height. A dune system can be divided into three zones: (1) strandline and embryo dune zone, (2) primary dunes (also called foredunes or yellow dunes), and (3) secondary dunes (also called hind-dunes or grey dunes). Characteristics of these zones are described later (see Dune Zonation).

An abundant supply of suitably sized sand is necessary to build large dune systems. In a classic study of dune systems, Cooper (1958) determined that the largest dune complexes were closely associated with a ready supply of sand. The relationship between the volume of available sand and the size of dunes is evident along the Atlantic Coast. For example, the Provinceland dunes on Cape Cod, Massachusetts, have formed under conditions of abundant sand derived from erosion of dunes along the outer shore.

Sediment bypassing, the process by which shoals become detached from the ebb tidal delta and migrate onshore, delivers sand in pulses to shorelines next to inlets (Hubbard 1977). Kana (1989) described the role of sediment bypassing at Captain Sam's Inlet located between Seabrook and Kiawah islands, South Carolina. In this process, the intertidal beach is widened and the sand is moved landward to form new dunes or it is added to existing dunes. Conversely, dune scarping and washovers are common along shoreline segments where sand supplied to the beach becomes inadequate for dune construction. However, dunes may continue to form from sediments remobilized by erosion or overwash.

The orientation of dunes is related to the direction of the highest windspeeds. Landsberg (1956) determined that the most significant correlating factor for dune orientation was the wind direction for windspeeds greater than 4.47 m/sec. Godfrey (1977) observed a similar relationship between wind direction and dune orientation and hypothesized that varying orientation of barrier-beach features to the prevailing winds leads to morphological differences in dune formation and development. According to Godfrey (1976), extensive dune growth along the East Coast of the United States is greatest where beaches lie perpendicular to the prevailing fair-weather winds. He also noted that most sand movement results from the influence of dry winds common to high pressure systems, and not from winds associated with wet storms. Along the northern Atlantic seaboard, the best-developed dunes are on beaches with northwest or south and southwest exposures. On the southern Atlantic Coast, the largest dunes are found on coasts with northeast exposures; however, extensive dune systems also occur on south-facing beaches (Cleary and Hosier 1979).

Godfrey (1977) suggested that the rate of sea-level rise, the tidal range, the sources of sediment, and the frequency of coastal storms and their direction of approach to the coast all modify the effects of orientation. Further, natural vegetation and its response to the regional environment can modify the physical processes that influence the major physiographic features of the coast.

Tidal range affects the distribution and size of dunes. Beaches with large tidal ranges have a larger width of intertidal sand exposed to wind action. Therefore, greater movements of sand toward the dunes can be expected. Because many other factors also influence dune size, tidal range influences may be localized. Regions with the greatest tidal range along the Atlantic Coast are Cape Cod, Massachusetts, and vicinity; southern South Carolina; and the Georgia embayment.

Oceanic overwash occurs on barrier beaches with low or discontinuous dunes. Where weakly expressed foredune systems are found, storm surges may penetrate the dunes. Washover passes are cut perpendicular to the beach as sand and water are carried landward. This sediment is deposited in a fan-shaped area landward of the washover pass through the dunes (Fig. 4). Washover passes typically become wider as moving water causes slumping along the margins.

During a storm, oceanic overwash can remove the foredunes along large stretches of the shoreline within a few hours. Where slacks occur to the lee of these dunes, overwashing sand may bury the vegetation. Often, when saltwater flows into low-lying slacks, plants intolerant of extended flooding or brackish conditions are killed. Waxmyrtle (Myrica cerifera) is one of the species least tolerant to flooding and is quickly damaged or killed by saltwater. Roman and Nordstrom (1988) confirmed a relationship between overwash and plant communities. On Assateague Island, Virginia, overwash-tolerant vegetation dominates shorelines with erosion greater than 4.5 m/year. Conversely, dense shrub thickets dominate those shorelines where erosion is less than 0.5 m/year and along shorelines that are accreting.

The impact of sea-level changes on shorelines and wetlands has been extensively studied (Barth and Titus 1984; Titus 1988). The level of the sea is generally rising along the Atlantic Coast. The apparent rate of rise and the effect of rise varies regionally because of the effects of Pleistocene glaciation, different subsidence rates, and variable shoreline slope. Along ocean shorelines, sea-level rise is usually translated into a general landward retreat of the beach and dunes. Slack environments nearest the ocean shoreline are likely to be destroyed as the dunes erode. Sea-level rise models for wetlands show a landward shift of wetlands zonation when sea-level rise exceeds sedimentation there.

Based on our understanding of the influence of the ocean on freshwater reserves, and by projecting a continued rise of sea-level, it is probable that the areal extent of slacks will increase as the freshwater table is elevated by sea-level rise. Intermittent ponds may become permanent and slacks may be flooded more frequently and for longer periods. These changes may lead to subtle changes in slack environments and in species composition of slacks. Ehrenfeld (1990) recognizes that sea-level rise will affect dunes and slacks and, because there is little information that specifically addresses the effect of sea-level rise on slack environments, this should be an important direction for future research.

Height and spacing of parallel dune ridges along the shore depend on the amount of sand supplied to the backshore, the history of erosion and accretion, and the effectiveness of vegetation in binding the sand (Bird 1969). Low, closely spaced, parallel dune ridges are formed where the sand supply is abundant and the shore is prograding. Periodically, storm-induced erosion partly or completely destroys dune ridges. Erosion of the seaward edge of an existing dune ridge results in the steepening of the seaward side of the dune. In fair weather, if new dune ridges form seaward of the truncated dune, a high, steep-sided dune that marks the position of the former shoreline or storm surge line is preserved.

According to Goldsmith (1977), consecutive, parallel dune ridges represent different periods in dune development; this is similar to growth rings of trees. The slacks between successive dune ridges form a similar series of progressively-aged environments. Burk et al. (1981), after studying Portsmouth Island, North Carolina, determined that each dune ridge is a temporal stage in development of the entire dune system (Fig. 5). The age difference between successive ridges varies considerably; it may be as short as 1 or 2 years or as long as several hundred years.

Dunes increase in height until equilibrium is reached among sand grain size, wind speed and direction, the biological limits of the vegetation, and storm frequency (Art 1976). Small, light grains can be carried farther and higher; thus, larger dunes are characteristic of shorelines possessing a predominance of fine sands. Onshore winds are most effective in building large, high dunes. Frequent storms, especially if the storms cause oceanic overwash, tend to reduce the size of dunes and often create discontinuous dune ridges along the backbeach area.

Dunes may be several kilometers inland from the present-day shoreline, but these dunes are invariably relics that delimit the region of historic shoreline positions. Once the influence of salt-laden winds is reduced by progradation of the shoreline, these older dunes are colonized by vegetation. At Cape Henry, Virginia, the Great Dune apparently remained barren except a few pioneer grasses for over 100 years; however, since the 1940’s, the dune, except for a few minor blowouts, has gradually become forested (Wright, et al. 1990).

Godfrey (1977) examined differences in the dune systems of the East Coast of the United States from Maine to North Carolina. He observed that southern dunes are often low and scattered behind a wide beach berm that is frequently overwashed, whereas northern systems are characterized by a continuous dune line with higher elevations and a narrow berm that is rarely overwashed.

Based on a series of profiles across dune systems, Godfrey found a distinct difference in physiography between barrier beaches of New England and the mid-Atlantic states compared with those of southeastern Virginia and North Carolina. The northern systems feature extensive dune ridges close to the beach with dune topography dominating the entire island. Topography begins to change from the northern to the southern type along the Delaware-Maryland coast. Here the islands are low, wide and flat; dunes are present along the beach, but extend farther landward than the dunes on the northern coast. The southeastern dune systems are low, irregular, and discontinuous.

From central South Carolina to southern Georgia, dune systems are regarded as remnants of erosion derived from a Pleistocene-aged core of upland, often with a mantle of Holocene sand forming the present-day shoreline. Cumberland and Sapelo islands are examples of erosion-remnant islands.

Dunes are found along most coasts outside the humid tropics. In contrast to the high, nearly unbroken, and well-vegetated dunes of temperate zones, dunes in the humid tropics are low, scattered, and sparsely covered with vegetation. The dearth of dunes in humid tropics has been variously attributed to intense chemical weathering, the high shear stress required to move almost continuously wet sand, and the dense vegetation growth next to the beach (Davies 1977).

The change in growth form of dunes between the temperate and tropical climate zones reflects the preponderance of plant species in tropical areas that expose new buds above ground in warm, frost-free environments. In contrast, temperate species typically have buds that form below the sand surface. Also, less wind, and therefore lower rates of sand movement in tropical regions compared with temperate areas, allows plants with surficial or underground stems to dominate beach and foredune environments in temperate areas (Barbour et al. 1985).

The highest active dune system in the eastern United States is Jockey's Ridge, located near Nags Head on the Outer Banks of North Carolina. This highly mobile dune, which changes in elevation as it moves, measures up to 33 m in height (Fig. 6). Other large dunes on the Outer Banks reach 20 m in height (e.g., Jones Hill and Penny's Hill). Mount Ararat, on Cape Cod, Massachusetts, is about 30 m in height, whereas the highest dune elevations on Cumberland Island, Georgia, are 12 m.

Differences in coastal dunes often are related to the position of the dune in respect to the current shoreline. Dunes nearest the ocean exhibit the least vegetation cover and the lowest species diversity, whereas more landward dunes feature greater vegetation cover and higher species diversity. This relationship can be generalized throughout the ecosystem components, including the mycoflora (Yocum and Wicklow 1980).

Permanent dunes first form along the strandline, also called the driftline, wrackline, or tidemark (Doing 1985). The strandline forms when beach debris is carried by spring tides to the backbeach environment. There, algae, plant stems and leaves, and flotsam and jetsam collect and are buried by blowing sand (Fig. 7). This organic material decomposes rapidly, contributing to the nitrate and phosphate content of the sand (Pearse et al. 1942). The accumulated organic material also promotes water retention in the underlying sediment, and temperature fluctuation under the drift material may be damped (Boaden and Seed 1985).

Along the barrier beaches of the southern United States, strandlines are spread over a wide area of the backbeach. The size and position of the strandline may influence dune structure and pattern. On New England beaches, where the barrier beach is prograding, long, continuous strandlines accumulate along the highest tide or storm-surge line. These strandlines contain many fragments of beachgrass roots, which are quickly buried, produce new culms and usually form continuous dune ridges (Godfrey 1977). According to Godfrey, dunes frequently form by deflation of overwash sands, the process of winnowing fine-grained sand from the overwash surface to dunes; these dunes may form around scattered beachgrass fragments.

Dunes forming at the upper edge of the beach along the line of debris are termed embryo dunes. They are frequently less than 2.0 m high and are regarded as the first stage of dune development (Fig. 8). Over time, embryo dunes enlarge both laterally and vertically, coalesce, and form a continuous ridge parallel to the shoreline.

The strandline community is transient. In one year, the strandline may be well developed, with many seedling plants; in another year, the strandline may be nonexistent. Existence of the strandline may be regionally influenced by the rate of sea-level rise, the rate of erosion along the shore, and the climate (Doing 1985). Climate is important because it influences the life form of the dominant plants. Many annual plants colonize strandlines; however, in cold climates and in areas with hot, dry summers, conditions are unfavorable for the growth of annuals, and strandlines are poorly developed.

Strandline vegetation changes as environmental conditions change. Strandline plants often need large amounts of nitrogen, a requirement met by bacterial decomposition of the strandline debris. As this nutrient source is depleted, however, the vegetation cover changes. Soils become drier and less fertile (Ranwell 1972). Dune pioneers invade the strandline; annuals are replaced by perennials such as sea elder (Iva imbricata), sea oats (Uniola paniculata) and American beachgrass (Ammophila breviligulata); and, over time, cover changes from sparse and intermittent to continuous.

The primary dune is the frontal dune or foredune, which is a typical feature of most Atlantic Coast beaches. Doing (1985) defined the foredune complex as the complete range of zones that may be present as a direct result of recent transportation of sand and organic material perpendicular to the shoreline (Fig. 9). The dune usually develops parallel to the shoreline because the strandline, which forms along the high-tide line, creates a series of vegetated dune nuclei, not unlike a string of pearls, along the backshore. As plants trap sand, the nuclei expand and a primary dune line is formed. As described previously, storm waves may periodically trim the front edge of the growing dune, creating a straight line of dunes parallel or nearly parallel to the shore.

With the availability of abundant sand and colonizing vegetation, the foredunes often form a high, continuous ridge. With the opposite conditions, the result is a poorly developed, highly breached primary dune zone or an elevated back-barrier platform.

The term "yellow dune" is sometimes applied to primary dunes. Yellow refers to the color of the young soils found in the primary dune zone. Significant leaching of carbonates and iron has not yet occurred in these areas and the soils are therefore yellow (Salisbury 1952). Atlantic Coast dunes rich in shell fragments are often yellow in appearance.

Terminology concerning secondary dunes is confusing in the literature. In geologic terms, a secondary dune is one that is derived from a primary dune, often after a series of primary dunes has developed along a shoreline. In ecological terms, however, any dune landward of the primary or foredune is considered a secondary dune. Secondary dunes are remote from the foreshore source of sand. In secondary dunes, established vegetation reduces saltation; only the most intense wind storms move sand grains across the primary dune to the secondary dunes. Because the more landward dunes are sand-starved, the vigor of the original colonizing plants declines and the straight ridges may break down as blowouts occur. All evidence of a series of parallel dune ridges may be erased by this process. For dunes remote from the effects of wind and salt aerosols, the seaward advance of trees of the maritime forest is rapid; distinct primary, secondary, and sometimes tertiary dune ridges are maintained under the forest cover. Blowouts are rare in these forested environments, occasioned only by severe storms, disease or human intervention (e.g., logging).

The large parabolic dunes of Currituck Spit on the North Carolina-Virginia border (Goldsmith 1977; Hosier and Cleary 1987), Cumberland Island, Georgia (Hillestad et al. 1975), and Cape Cod, Massachusetts, are examples of secondary dunes reshaped after blowouts occurred on primary dunes (Fig. 10). On Cumberland Island, Hillestad et al. (1975) attributed the existence of the extensive secondary dune system along most of the island to deflation of the foredunes followed by entrapment of mobilized sand along the seaward edge of the forest directly landward. Secondary dunes on Currituck Spit developed similarly (Hosier and Cleary 1987). Historic records show that Currituck Banks was an open range for cattle and pigs during the 1920’s and 1930’s. Overgrazing probably started the breakup of primary dunes.

Organic material deposited by wave swash on the beach above the spring tide line traps saltating and creeping sand. These lines of debris along the shoreline are precursors of new dunes. In New England, fragments of dune plants such as American beachgrass and dusty miller (Artemisia stelleriana) may break away from the parent plants during stormy winter months and lodge in rafts above the tide line (Godfrey 1976). Along southern Atlantic beaches, rafts of smooth cordgrass (Spartina alterniflora) stems carried out to sea from the expansive tidal marshes are deposited along the upper beach. Fruits and seeds of sea oats, sea rocket (Cakile edentula; identified as C. harperi) and sea elder often accumulate along the edge of the swash line. The seeds germinate during the spring, and sand is trapped as the seedlings grow.

Two dune grasses--American beachgrass, north of Cape Hatteras, and sea oats, south of the Cape--are the most important sand‑trapping plants (Godfrey 1976). These plants tolerate salt spray and burial by wind-blown sand. American beachgrass and sea oats grow upward and laterally as sand accumulates around them, thereby fixing moving sand and anchoring the incipient dune. Both species stabilize the dunes by preventing sand from blowing unimpeded across the island and into the lagoon or far inland.

The growth habit of plant colonizers may determine dune form. In the southern, semitropical section of the Atlantic Coast, sand colonizers such as beach morning glory (Ipomoea imperata) produce long, trailing shoots and cover the ground with a sheet of vegetation. Few ridges or hummocks are produced in areas dominated by this species. Similarly, saltmeadow cordgrass (Spartina patens), although tufted in appearance, produces extensive rhizomes; each internode on a rhizome can produce one to several culms. Dunes dominated by this species are often low and flat.

In contrast, dunes colonized by American beachgrass are often ridged. Strong, compact tufts of this grass encourage sand accumulation. Dunes dominated by sea oats are similar to dunes colonized by American beachgrass, although dunes with sea oats may be more hummocky than those with beachgrass (Moreno-Casasola 1988). Sea oats rarely colonize strandlines by rhizome fragments; instead, they germinate their seeds, which requires considerably more time to establish new dunes. Sea oats accumulate sand vertically but do not spread outward; thus, dunes dominated by sea oats are typically scattered and hummocky.

Factors that affect the rate and pattern of dune migration are the distance of the dune from the sand source (e.g., the beach), the grain size of the dune sands, and the direction, intensity and frequency of winds capable of moving sand. Dunes remote from a sand source (typically the intertidal beach) move at a slower rate than those that are next to a sand source. Fine-grained sands travel farther and are more easily moved than coarse sediments. Under the same conditions, dunes composed of fine sands can move farther than dunes with coarse sands.

Vegetated dunes can migrate, but at a slower rate than unvegetated dunes. The typical crested shape of vegetated dunes forces wind upward from the base, creating a wind speed that peaks at the crest. This funneling effect moves sand upward along the front of the dune. On the lee side of the dune, wind speed drops precipitously, and sand accumulates in this part of the dune. Herbs and grasses continue to grow until sand deposition exceeds their rate of growth.

Where vegetation cover is sparse or has been destroyed, sand can no longer be held in place; blowouts, or localized sites of sand removal, often result (Fig. 11). Once a blowout starts, it often erodes downward to a surface close to the water table. Blowouts usually form along either the upwind or the seaward crest of dunes. At these sensitive sites on a natural dune, desiccation is increased because the drying effects of the wind are concentrated. Vegetation destruction, once started, is often rapid, and the affected area expands as more sand is displaced. The naturally uneven cover of plant life on the dune crests results in an uneven erosion of the same areas, which typically produces irregular crests along the ridge and a sinuous dune system along the shore. Environmental conditions at the new site are different from the formerly existing dune; this is usually reflected by a change in colonizing vegetation.

Where erosion causes slumping at the base of a dune, blowouts may occur along the seaward margin of a dune ridge. Onshore winds remobilize this unstable sand, causing vegetation landward of the dune to be overwhelmed, thereby increasing the size of the unvegetated part of a dune. According to Ranwell (1959), many slacks form when dune ridges are devegetated and blowouts develop. The slacks may or may not reach the water table depending upon season and the rate of plant establishment. Along shorelines with dominant onshore winds, parabolic dunes usually form and migrate landward. As a dune migrates, it is typically lowered in elevation and exhibits a planiform shape.

Recolonization of barren sand in low, wet areas follows as the dune migrates downwind from the sand source. At this point, plants can survive modest rates of erosion or burial. During the process of parabolic dune formation and migration, sand is moved upslope from the center, leaving a flat, moist surface near the level of the water table. During dry weather, slacks in the arms of parabolic dunes may be excavated by deflation, resulting in the formation of either temporary or permanent ponds (Willis et al. 1959).

Blowouts form quickly, but revegetation is a slower process, especially if the central depression of the blowout does not reach the water table. On Fire Island, beach heather (Hudsonia tomentosa) and seaside goldenrod (Solidago sempervirens) are frequently the only species found on these shallow blowout areas (Art, 1976). Pin-weed (Lechea maritima), joint-weed (Polygonella articulata), wormwood (Artemisia campestris) and panic grasses (Panicum spp.) may be found infrequently. American beachgrass, however, is notably absent. If blowouts remove sand to the surface of the water table, conditions suitable for more hydric species may result in the rapid development of slack communities. Dense, stabilizing vegetation cover may be established within one or two seasons.

Climate is an important environmental variable, providing a framework within which dune ecosystem processes operate. Prevailing climatic conditions influence the size, shape, and vegetation cover of dune systems. Doing (1985) determined that two types of dunes exist: one typical of tropical climates and the other of temperate climates. In the humid tropics, south of the region considered in this report, dunes are low, scarce and dominated by grasses on the front ridge with woody species landward. The warm, tropical growing season favors the development of dense woody vegetation across the dunes right up to the intertidal beach. Dune formation is less pronounced (Fig. 13) because sand transport by wind is reduced. The difference between high and low tides, a factor also influencing dune formation, is less in tropical than in temperate regions.

In temperate regions, dune building is more extensive and the dunes are larger and more abundant than in tropical or polar regions. In the polar regions, the shorter growing season leads to poor sand-binding capabilities; ice and snow reduce the period of time the sand can blow. Doing (1985:73) summarized the influence of climate by stating that the most impressive foredunes and parabolic dunes occur where "the struggle between wind and vegetation is undecisive [sic] during longer periods."

Climatic conditions also influence soil formation and maturation. Along the Atlantic Coast of the United States, abundant rainfall and moderate temperatures enhance the process of mineralization. High temperatures promote oxidation of organic matter; generally, organic matter accumulation in the soil decreases southward from Cape Cod, Massachusetts.

Atmospheric factors influencing dune-and-slack environments are rainfall and wind. Environmental factors related to wind include desiccation, salt aerosol and, to a lesser extent, mineral input from dust in the atmosphere. The sandy substrate of dunes and slacks magnifies the effect of these factors as compared with their importance in many other environments.

Rainfall is the source of fresh water for both dunes and slacks. Water not immediately evaporated from the dune surface or absorbed by roots percolates rapidly through the coarse sand to the water table. Willis et al. (1959) determined that 25-35 percent of the rain falling on a vegetated dune system reached the water table; the balance (65-75 percent) was lost to transpiration and evaporation. Vegetation cover influences the amount of water intercepted at the surface. For example, in densely vegetated dunes, not as much water reaches the water table as in less densely vegetated dunes because more rain is intercepted by plant cover.

Rainfall may be a source of nutrients and pollutants in coastal dune systems (Table1). Salts derived from seawater were detected in rainwater near the coast and small, but significant, amounts of nitrogen and phosphorus have been measured in coastal rainfall (Willey et al. 1988). Offshore winds may carry pollutants (usually sulfates) from inland sources to the coastal regions.

Chronic wind, characteristic of nearly all coastal environments, is a major factor contributing to stressed conditions in the dune ecosystem. Willis et al. (1959) argued that wind and sand movement are the most important environmental factors influencing plant communities on high dunes (15-20 m). Stress from wind and blowing sand have particularly severe impacts on immature plants or young parts of mature plants. Seedlings of dune plants such as sea oats can be sand blasted, buried, or blown out by wind (Woodhouse et al. 1968). Tissue wounds caused by abrasive sand blast intensify the toxic effects of salt aerosols on plants. In addition, strong winds may cause mechanical injury to plants; hurricane-force winds can defoliate entire plants and can break branches. Wind-driven waves may also erode dunes or cause flooding in slacks.

During periods of drought or low humidity, the effects of wind are exaggerated. Coupled with the high air temperatures often associated with dunes, wind can cause plant desiccation. Au (1974) measured the vapor-pressure deficit as high as 15 mm Hg in the atmosphere above a dune system.

Vegetation greatly affects wind speed near the ground. The shelter of plants such as tussocks of beachgrass produces a wind shadow on the lee side of a dune. The reduction of windspeed across a dune persists as far as 10 m behind a beachgrass tussock (Willis, et al. 1959; Fig. 14). Many plant species otherwise intolerant of salt aerosols or desiccation can survive in the microclimate created by this wind shadow.

Indisputably, salt aerosols are the most important influence on the biota of coastal dune-and-slack ecosystems. Many studies have revealed that species presence and distribution, morphology, vigor and life history are influenced, if not controlled, by salt aerosols. Van der Valk (1974b) conservatively estimated that the annual meteorological input of sodium to the dune system at Ocracoke, North Carolina, was 1,300 kg/ha (Table 2). For comparison, measurements at an inland site (Coweeta, North Carolina) show the meteorological input is only 1.0 kg/ha (Johnson and Swank 1973).

Effervescence of seawater caused by breaking waves is the source of salt aerosols. Bursting air bubbles eject droplets of seawater into the air. These saltwater droplets are concentrated by evaporation, transported inland by wind, and deposited on the windward side of plants. The formation of salt-spray aerosols depends on wind speed. At wind speeds less than 7.0 m/sec, few whitecaps are formed on the ocean and the production of aerosols is minimal (Boyce 1951a, 1954). At windspeeds greater than 11.0 m/sec, interception of aerosols by the forest canopy is four to six times greater than at windspeeds of 7.0 to 9.0 m/sec (Table 3).

The deposition of salt aerosols is variable and site-specific. For example, Martin (1959) measured daily salt deposition of 15.35 mg^.dm^-2.day^-1, 30 cm above the ground and 25.15 mg^.dm^-2^.day^-1, 91 cm above the soil on the secondary dunes. Oosting and Billings (1942) measured salt deposition of 23.3 mg^.dm^-2^.day^-1 on the secondary dunes on Bogue Banks, North Carolina. Deposition rates are influenced by wind speed and direction, and by the intensity of wave action.

Salt aerosol input of micro-nutrients (cationic metals) is considerably higher than input from rainfall (see Tables 1 and 2). Despite the higher input from salt aerosols, direct nutrient uptake by leaves or stems is unimportant. Coastal dune-and-slack plants do not typically absorb cations directly from foliar deposition of salt (van der Valk 1977). Salts deposited on plant and soil surfaces are washed into the soil by rainfall; however, dune soils do not retain these salts. Dune soils consistently show low concentrations of chlorides in the root zone. Chlorides are leached downward by rainfall events and are diluted in the groundwater. Consequently, chloride uptake is not a limiting factor for plants colonizing dune environments.

Low cation exchange capacity, high permeability, and rapid leaching are important factors that prevent retention of nutrients in the soil. Because dune soils lack the buffering effects of a large nutrient pool, the concentration of cations varies considerably over time and space (van der Valk 1974b). The evergreen nature of dune-and-slack vegetation in the southern part of the Atlantic Coast may provide some nutrient-buffering capacity. Evergreen plants such as yaupon (Ilex vomitoria) and waxmyrtle slowly lose their leaves and gradually release nutrients throughout the year. Leaf litter from these plants is moderately resistant to decay and releases nutrients gradually to the environment.

Fine, sandy soils exhibit higher cation concentrations because finer sands have a greater surface area for the adsorption of water and cations. Van der Valk (1974b), in his study of the annual input of the cations K⁺, Na⁺, Ca⁺⁺, and Mg⁺⁺ in salt aerosols and rainfall, determined that turnover times for sodium, potassium and magnesium ranged from 11 to 37 days. The calcium turnover time was longer, ranging from 32 to 106 days. The high turnover rate, coupled with the lack of an internal reservoir for cations, makes mineral cycles in the dunes and slacks less stable than other terrestrial ecosystems, and van der Valk (1974b) cautioned that interference with the natural meteorological inputs to the dune ecosystem could have severe negative effects.

Coastal rains originating from offshore are less acidic than thunderstorms or continental frontal storms. While coastal storms deliver some sulfates from seawater to the coast near Wilmington, North Carolina, the largest percentage of sulfate in rainwater reaching this city is from atmospheric pollutants carried by storms beginning inland (Willey, et al. 1988).

The deposition of high concentrations of salt as salt aerosols results in necrosis and death of leaves, twigs and sometimes entire plants. Death is the result of high chloride concentration in tissues (Boyce 1954). Chlorides enter the plant through small traumata, injuries or open wounds that result from the mechanical abrasion of leaves and twigs beating against each other (Boyce 1954).

After entry into the plant, chloride ions are translocated to the growing parts--the leaf and twig apices. Here, chlorides reach injurious concentrations, however, the internal concentration levels have not been quantified. The twig apices on the windward side (closest to the source of salt aerosols) are influenced by chlorides more than those on the lee side of plants; this differential death of the apices and leaves creates the asymmetrical growth of coastal shrubs and trees. The apparent bending of trees is associated with branch apex and leaf death rather than bending due to the force of coastal winds (Boyce 1954; Fig. 15).

Dune flora and fauna are exposed to wide variations in air and soil temperatures. In studies conducted in North Carolina, surface temperatures of dune soils range up to 37^oC during summer with the average between 29^o and 31^o C (Oosting and Billings 1942; van der Valk 1974a). In these areas, the average air temperature during summer is 25^o to 30^o C; temperature fluctuations of 20^o C can be experienced in the dunes during a single day. Au (1974) measured a dune soil-surface temperature of 52^o C. Even within a reflective shelter, the temperature reached 35^o C for 8 weeks and 30^o C for 13 weeks during the summer. In winter, with average air temperatures of 4^o to 9^o C, the soil temperature at 5 to 10 cm below the surface varied between 4^o and 11^o C. At 20 to 25 cm below the surface, soil temperatures ranged from 4^o to 12^o C.

Figures 16 and 17 show the temporal and depth variations in dune temperature. Daily temperature extremes are reached progressively later as depth increases. A similar relationship is evident for seasonal changes in temperature as soil depth increases. Despite the temperature extremes occurring in dunes and slacks, most investigators have concluded that species distribution in the dune system is not caused solely by temperature differences.

On most Atlantic Coast dunes, soil formation leads to podzolization, a condition where upper soil layers are leached by percolating rainwater resulting in podzol, an acidic, infertile soil. Yellow iron oxides are removed from the upper horizons of the substrate and white or grey mineral sands remain. The lower horizons are enriched by iron and manganese leached from above. Precipitation of these minerals creates a slightly cemented orange to red-brown horizon. As dune soils mature, soil moisture increases, probably due to an increase in organic matter content.

Many authors describe the unique moisture conditions of dune soils, but all agree that variation in soil moisture on the dunes is not the factor that solely controls the distribution of plant species (e.g., Oosting and Billings 1942; Salisbury 1952; Au 1974; van der Valk 1974b). The moisture level of dune soils is low but remarkably constant below about 20 cm. The moisture content of the near-surface dune sands ranges from 0-4 percent. Only one day following rainfall, gravitational water moves to depths greater than 15 cm (Fig. 18). The dry surface layer of dune soils acts as mulch and reduces evaporation from subsurface layers (Au 1974). The extremely porous structure of the soil limits the capillary rise of water beneath dunes to only 30-50 cm above the free water level. Little capillary moisture moves from the subsurface to the drier surface layers (Fig. 19). On most large dunes, plants can exploit neither the free water table nor the capillary moisture.

Despite the relative dryness of dune soils, plants experience drought conditions infrequently. Salisbury (1952) suggested that much of the water available to plants may come from condensation from the cool soil atmosphere at night, a phenomenon termed “internal dew formation.” However, Willis et al. (1959) noted that temperature gradients preclude the possibility that water can condense from the soil atmosphere. Plants remove a significant amount of water from the soil through the transpiration stream. The soil water content under well-vegetated dunes is lower than that of sparsely vegetated dunes. Willis et al. (1959) attributed this to the rapid absorption and exploitation of the surface moisture by plants. Despite the extensive water relations research on dune plants, additional study in this area is warranted.

Soil organic matter contrasts sharply in dunes and slacks. Organic matter in young foredunes may be as low as 0.01 percent (dry wt), whereas cranberry bogs on Long Island have an organic layer of 2-15 cm thick overlying the mineral sand (Johnson 1981). Soil moisture as a percentage of dry weight changes dramatically at the margin of the dune and slack, probably because of the difference in soil organic matter between dune and slack soils.

Sand movement, whether from accretion or erosion, is an important environmental factor for dune organisms (Oosting and Billings 1942; Salisbury 1952; Willis et al. 1959; and Wagner 1964). Mobile dune fields such as Jockey's Ridge, North Carolina, may be dotted with seedlings in the spring. By early summer, however, nearly all of the seedlings have been eliminated by burial or erosion. Near a blowout, burial of 20-30 cm is enough to eliminate forb seedlings (van der Valk 1974a). Burrowing animals typically avoid areas subject to rapid erosion or accretion.

The dominant wind direction, dune size, and vegetation cover affect the growth and movement of foredunes. Along shorelines that parallel the prevailing winds, sand blows along the island rather than onshore or offshore. Less sand is added to these dunes than to dunes on shorelines with winds blowing across the beach. Woodhouse and Hanes (1966), and Savage and Woodhouse (1969) measured sand movement on constructed dunes on Ocracoke Island, North Carolina. On dunes less than 2.5 m in height, sand accumulates on the front, top, and back of the foredune. In contrast, on dunes over 2.5 m, sand accumulates primarily on the seaward side. A greater vegetation cover resulted in higher surface roughness and decreased wind speed over the dunes. Where vegetation cover is sparse, the entire dune system may advance landward at a rate that may reach 7.0 m per year (Boorman 1977).

Rainfall and salt aerosols are the primary sources of nutrients in dune-and-slack soils. Except for calcium derived from shell material, virtually no salts are leached from the parent material in the dunes. The low level of nutrients in the dune and, to a lesser extent, slack soils is cited as the major factor creating the open, sparse plant growth typical of coastal dune systems. Exposure to salt aerosols and the depth to the water table are secondary in importance to nitrogen and phosphorus deficiencies in the soils.

Low availability of nitrogen operates as a conditioning factor. Species subjected to low concentrations of nitrogen have an increased tolerance to salt aerosols (Boyce 1954). The species that Boyce noted were salt aerosol-conditioned by low nitrogen were sea oats, bitter panic grass (Panicum amarum, identified as P. amarulum), purple sand grass (Triplasis purpurea), beach hogwort (Croton punctatus), ground cherry (Physalis walteri identified as P. maritima), evening primrose (Oenothera humifusa), sea elder, and sea rocket (Cakile edentula). Field studies show that soils are potassium-deficient, but not to the same extent as nitrogen and phosphorus (Willis et al. 1959). When potassium is added without nitrogen, vegetation growth remains sparse. Calcium, however, is usually abundant in dune soils because of the presence of shell material (Table 4). Despite the constant input of salts as salt aerosols, precipitation prevents chloride concentrations from reaching toxic levels in the soil.

Nutrient levels vary dramatically on the foredunes. Daily and seasonal differences in wind speed, wind direction, and rainfall contribute to the nutrient changes. These factors influence the quantity of salts deposited on or leached from the soil. The constant leaching of salts by rainwater is considered responsible for the low concentration of cations, nitrogen and phosphorus in the soil compared to most non-dune soils (Boyce 1954; Willis and Yemm 1961; Willis 1963). After 300-400 years, leaching removes calcium-rich shell material and the top 1 cm in dune soil becomes base deficient (Boaden and Seed 1985). Nitrogen concentration (ammonium and nitrate-nitrogen) in soils increases from younger to older dunes (Lakshmi and Day 1993); however, concentrations were lower in dunes compared to adjacent slacks on Hog Island, Virginia.

Soluble nutrients leach rapidly from the secondary dunes; because they are older than primary dunes, secondary dunes are more deficient in potassium and magnesium (Willis et al. 1959). Usually, dune soils contain enough minor mineral nutrients (i.e., boron, copper, iron, manganese, molybdenum, and zinc) for unrestricted plant growth.

Slack soils are enriched from dune leachates. Willis et al. (1959) noted that the vegetation was especially thick and tall in a zone where dune slopes merge into the floor of slacks. He suggests this is a region of seepage from surrounding dunes and may be richer in nutrients than elsewhere. Field experiments show a less dramatic response by plants to added nitrogen, phosphorus and potassium, suggesting that these elements are not as limiting in slacks as they are in dunes.

Additional studies synthesizing our knowledge of mineral nutrition in dune and slack plants are necessary. Individual studies of mineral nutrition of dune-and-slack systems have provided a view of the levels of nutrients available to organisms; however, no studies have considered nutrient status over time (nutrient flux) for these environments.

The natural pattern of dunes--hummocks or hillocks of sand alternating with low-lying flats--creates communities with contrasting environmental conditions only steps apart (Fig. 20). The term “slack” is derived from the Norse word "slakki," meaning swale or hollow in a dune system. Tansley (1949:861) defines slacks as "damp or wet hollows left between dune ridges, where the groundwater reaches or approaches the surface of the sand." According to Ranwell (1972), a British ecologist noted for his pioneering work on coastal environments, swales or hollows between dunes are characteristic features of large dune systems, especially those underlain by impervious deposits in the more humid temperate zones.

The most thorough studies of dune slacks were conducted by Ranwell (1959) at Newborough Warren, Anglesey, United Kingdom, and by Crawford and Wishart (1966) at Tentsmuir, Fife, United Kingdom. Slacks are common in many Atlantic Coast dune systems, and the most prominent slacks occur where grazing pressure by domesticated or feral ungulates retards succession to shrub or forest cover. The most comprehensive study of slacks in the United States was conducted by Tyndall and Levy (1978) on the southern Virginia and northern North Carolina barrier beaches.

The natural formation of dune ridges results in the creation of slacks that divide successive ridges. The low-lying slack environments between ridges usually have slope, soil, soil water, and atmospheric conditions very different from the ridges (Jones and Etherington 1971). Therefore, slacks have a flora and fauna different from the dune communities. Unlike dunes, where moisture conditions are considered variable because of low water-holding capacity and high evaporation rates, slacks exhibit an environment that produces fewer extremes in water content. The moderation of extremes is due to the comparative nearness of the water table to the surface and the consistency with which this level is maintained throughout the year.

Because of their topographic position, slacks are sheltered from the wind, often seasonally flooded, and usually have a water table within 1.0 m of the surface. Dunes surrounding a slack have an important effect on slack development because organic and inorganic materials move down and into the slacks from these adjacent dunes. Wind erosion also moves organic matter into the slack.

Slacks do not occur at the same elevation throughout the dune system; their height varies considerably, depending upon where and when they are formed (Willis et al. 1959). Slacks created when successive dunes grow and coalesce into ridges on the open beach may have elevations roughly equivalent to the elevation of the backshore which existed during their formation. Dunes formed secondarily from blowouts in the winter are shallower than those formed during spare summer, a time when blowouts can be formed with a greater depth. As a result, slacks formed during summer are often flooded during winter.

Dune slacks are rich in plant species. At Newborough Warren, Anglesey, United Kingdom, for example, Ranwell (1959) counted more than 100 species of vascular plants in the slacks. The flora has few halophytic (salt tolerant) species; mesophytic hemicryptophytes (plants with overwintering buds at, or just below, the surface of the ground) capable of withstanding temporary flooding are common. The ameliorated habitat of the slacks is suitable for the more sensitive seedling stage of species; as a result, slacks may influence the species composition of surrounding dunes. For example, clonal species such as coastal plain pennywort (Hydrocotyle bonariensis) may germinate in the slack soils and grow outward into the dunes.

Species presence in a slack is controlled by soil moisture. Extended periods of waterlogging in the soil are sufficient to exclude some plants (Ranwell 1959). On the other hand, species adapted to mesic conditions may be limited by the extended dry periods experienced in dune slacks.

Dune slacks form in swales created between parallel dune ridges, among dune hummocks, or within blowouts that alter existing dune topography. Shoreline progradation, inlet migration, blowouts, and other alterations are examples of processes that form extensive, low-lying, moist environments within dune systems. Conversely, these same processes can destroy these wetland environments within dune systems.

Dune-and-slack environments form along the back beach where sand is trapped along progressively younger strandlines. Slack environments develop between these ridges (Fig. 21). Because the water table is convex to the land surface (elevated toward the center of the dune field), the elevation of the water table within the slacks of successive dune ridges is higher in the older, more landward slacks. As sand is moved from the intertidal beach to the backshore and dune ridges are added to the seaward edge of the shore, the earliest-formed slacks become steadily more moist. Slacks experience a natural rise in the level of the water table as the distance between the slack and current shoreline increases.

Dune-and-slack topography created by shoreline progradation is evident at many locations along the Atlantic Coast. For example, at Cape Hatteras, North Carolina, alternating dunes and slacks, some now forested, are interpreted as having formed as the cape prograded to the south. Portions of Blackbeard, Sapelo, and Cumberland islands exhibit this topography and were similarly formed.

Burk et al. (1981) provided an example of accretionary dune-and-slack topography on Portsmouth Island, North Carolina. A series of dune ridges was formed after the removal of grazing animals from the island. This dune-and-slack topography, nearly 300 m wide, had developed within 30 years. The extensive back-beach area provided a source of sand for dune building and vegetative growth rebounded.

Dune-and-slack topography may form on the updrift side of artificial structures, including jetties and groins. At the south end of Cumberland Island, Georgia, near St. Mary's Inlet, a dune-and-slack environment was created by impoundment of sand along the inlet jetty. Following emplacement of the jetty, sand entrained in the longshore current accumulated against the jetty and the beach prograded, forming an extensive dune-and-slack community.

Dune-and-slack environments commonly form along shorelines influenced by migrating inlets. As an inlet migrates, a recurved spit forms. Along inlets moving predominantly in one direction, a series of recurved dunes develops. Dune-and-slack topography is found updrift of inlets such as Fire Island Inlet, New York; Barnegat Inlet, New Jersey; Oregon and Drum inlets, North Carolina; and Captain Sam's Inlet, South Carolina (Fig. 22). Extensive slack environments occur near these inlets. Fisherman's Island near Cape Charles, Virginia, possesses extensive slacks and is a product of channel migration and complex spit development (BoulJ 1979).

Willis et al. (1959) noted that slacks in the center of a massive dune system were derived secondarily as blowouts. They described conditions in several slacks located as far as 100 m landward of the foredunes. Because the water table is convex-shaped, the most landward slacks are up to 10 m above sea-level. Additional moisture in these slacks is derived from groundwater that moves laterally from beneath the surrounding massive dunes. Large permanent ponds occur near the bases of large dunes near Provincetown, Massachusetts, and Corolla, North Carolina. Intermittent ponds are found in dune slacks near the base of Jockey's Ridge on the Outer Banks, North Carolina, and Cumberland Island, Georgia (Fig. 23).

Brown (1959) noted extensive slacks occupied by a dense cover of vegetation landward of an artificial dune ridge built between 1936 and 1940 along the Outer Banks of North Carolina. Artificially built dunes are often high, but narrow. Two or more rows of dunes were constructed along the shoreline leading to the creation of slack environments between the ridges. Besides the narrow slacks between constructed dunes on Ocracoke Island, North Carolina, extensive barrier flats were formed after the constructed dunes practically eliminated oceanic overwash.

Parabolic dunes form along an exposed coast where sand is abundant and the prevailing wind is from one direction. The center of the dune migrates more rapidly than the sides, creating an unvegetated dune with a steep windward side, a gently sloping lee side, and semi-fixed "arms" trailing upwind. The arms are created by the partial stabilization of the dune sides by plants. The migration rate of these dunes is highly variable; dunes have been reported to have moved from less than 1.0 m/year to as much as 15.0 m/year (Willis et al. 1959; Goldsmith 1977; Hennigar 1979).

Slacks form between the arms of migrating parabolic dunes (Fig. 24). The flat area between the arms is typically within 1.0 to 2.0 m of the water table and therefore supports a dense cover of grasses or shrubs. According to Willis et al. (1959), the depression between the arms of a parabolic dune usually exhibits a lag deposit of coarse sand and shell because the fine sands are moved upslope and over the crest of the dune to the slipface.

Mobile dunes secondarily derived from formerly stabilized dunes are responsible for the existence of a series of slacks in Back Bay National Wildlife Refuge and False Cape State Park, Virginia (Tyndall and Levy 1978). The slacks in this region are typically found in the central depressions of large parabolic dunes that migrate southwest (Goldsmith 1977). These depressions vary from circular to irregular in outline, and saucer- to bowl-shaped. Tyndall and Levy (1978) measured slacks from 10 to 60 m wide and 10 to 225 m long. The depths of the slacks range from 0.5 m below the maximum water table to approximately 1.0 m above it.

On Shackleford Banks, North Carolina, heavy grazing by horses, goats, and sheep is responsible for the mobilization of dune sands once covered with trees. Dozens of slacks were formed on Shackleford Banks after the vegetation cover was removed. Early bankers, the inhabitants of the Outer Banks, burned forests and cut trees which opened the dunes and created slacks. Deforestation due to the effects of hurricanes and northeasters on the barriers also created mobile dunes that, in turn, created slacks as the dunes migrated across the islands. Ranwell (1959) suggested that grazing by horses, cattle, sheep, and rabbits was responsible for maintaining dunes in an unstable condition and promoted slack formation.

Slacks commonly form from blowouts. Blowouts usually begin near the crest of dunes and lower the elevation of the dune ridge. Once the vegetation cover is disturbed, blowouts may expand and deepen as sand is blown downwind from the original break in the vegetation cover. In mobile parabolic dunes, deflation of the central section of a blowout often occurs, and slacks may form in this depression. If the dunes of an area are unstable, slacks may form and later be buried as the dune sands migrate across the island or beach.

Slack soils are usually derived from quartz sands that are highly resistant to decomposition by chemical and physical weathering. Development of soil horizons is inhibited and glei-type soils predominate. The moist conditions in slacks result in soils that are high in organic matter and nutrients compared to adjacent dunes. Humus decomposition is lower with the presence of the water table near the surface; therefore, wet slacks accumulate more organic matter, thereby increasing the water-holding capacity of the soil. In turn, increased water-holding capacity accentuates differences between dune and slack soils (Fig. 25). Soil development from parent material occurs less rapidly in the poorly-drained slacks than on well-drained dunes.

Ranwell (1959) showed, on both a weight and a volume basis, that soil organic matter increases as slacks mature, but even the youngest slacks often have twice as much organic matter as the surrounding dunes. Leaf fall is the primary source of organic material; however, as slacks mature, roots increase in importance as a source of organic material. Slacks with a constantly high water table show limited accumulation of organic matter.

Slacks have a higher carbon:nitrogen ratio than adjacent dunes. While the soil surface may undergo leaching during the driest months, bases are returned to the upper soil during the wet season when the water table is nearer the surface. Overall conditions, however, lead to slack soils being low in phosphate and potassium.

Ranwell (1959) reported that reducing conditions occur as near as 3 cm from the surface in waterlogged, but not flooded, dune slacks. In mobile dune systems, the soil of slacks is constantly enriched with carbonate materials and becomes base-enriched. Once the dry slacks become fixed (i.e., permanent features), pH declines. In contrast, soil pH remains high in almost continuously moist slacks because soil water leaches bases from surrounding dunes. Conn-Thomas and Day (1993) measured lower oxidation-reduction potentials in slack environments compared to dunes. Slack oxidation-reduction potentials varied from -35 to 573 mV; dune ridge measurements ranged from 423 to 573 mV.

The water table lens under a dune system on a barrier island is convex shape; the seaward and lagoonward edges are inclined steeply (Fig. 26). For example, in Braunton Burrows, United Kingdom, Willis et al. (1959) found that the water table in the center of the dune system was 6.6 m higher than at the edges. They attributed this difference in elevation of the water table to the differential rate of subsurface water movement; water moved slower downward and laterally in the center than at the landward and seaward edges. Across areas with extensive dune-and-slack environments, the water table rises beneath the surrounding dunes; thus, the water table surface reflects the land surface contours.

The presence of groundwater near the surface is the most important factor in slack formation, vegetation establishment, and maintenance. The adjacent high dunes have a water table many meters below the surface. Because the water table is so deep, it does not influence the vegetation, even during winter months when the water table is at its highest (Ranwell 1959). In the slacks or swales, however, the presence of the water table at or near the surface is a very important environmental factor; the soil immediately above the free water table is saturated and soil aeration is reduced.

The level of the water table varies throughout the year, depending upon rainfall and evapotranspiration. The water table usually exhibits a seasonal pattern: high in winter, falling in spring, and rising during the fall (van der Laan 1979). Single measurements of the water table cannot be considered representative of the water table regime. In a long-term study at Braunton Burrows, United Kingdom, Willis et al. (1959) found the water table correlated well with rainfall for the previous three months, supporting the hypothesis that rainfall is the primary source of water for the dune system (Fig. 27). Slacks are typically subject to flooding from 2 to 6 months during the year; thus these slacks are dominated by wetland plants, although other species may occur. In those slacks flooded less than 2 months per year, many species characteristic of dunes become established. Ranwell (1959) measured annual water table fluctuations of 70-100 cm at Newborough Warren, Anglesey, United Kingdom (Fig. 28). The water table level in slacks also varies on a daily basis. High evapotranspiration during the day causes the water table to drop, but after sunset, the water table rises rapidly. Studies quantifying evaporation, transpiration and water movement in slack environments are lacking. It is important to assess the role of slacks in the water balance of the coastal dune-and-slack environments.

Slacks with a large surrounding dune catchment system appear to have a greater water table rise following rainfall than slacks with few surrounding dunes. Rainwater appears to percolate through the dunes and move laterally into slacks. Salts leached from adjacent dunes produce elevated soluble nutrient levels in the slack soils. Van der Laan (1979) found that several soil factors correlated with the depth to the water table, and that depth to the water table correlated with species composition of a slack.

The groundwater level in dune fields on barrier islands and beaches are not affected by tidal fluctuations. This is in marked contrast to shingle (gravel) beaches, where the rise and fall of the tide affects the level of the groundwater on a daily basis.

Dune slacks directly landward of the primary dune system are susceptible to flooding with salt water. The most seaward slacks experience oceanic overwash during storms and occasionally on spring tides. The chloride level of these slacks is elevated briefly following washover; chloride levels as much as 30 times that measured in the next landward slack have been observed in these seaward slacks. Crawford and Wishart (1966) report chloride concentrations averaging 708 mg/L in seaward slacks compared to levels ranging from 13 to 94 mg/L in landward slacks. Only the most salt-tolerant species can survive in these seaward slacks. In slacks on Virginia barrier islands, soil water salinity ranged from 1.0 to 6.4 ppt (Conn-Thomas and Day 1993). On Cumberland Island, Georgia, unusually high tides periodically introduce salt water to freshwater environments, including slacks; this pulse of seawater is enough to kill plants such as cattails (Typha spp.). Some freshwater ponds on Cumberland Island have euryhaline fish species, suggesting the occurrence of chronic saltwater flooding.

On barrier beaches, fresh water from rainfall percolates through the substrate to the free groundwater, carrying with it ions leached from the soil. The water and nutrients move laterally and discharge at the ocean edge. On barrier islands, lateral discharge occurs on both ocean and estuarine margins. Virtually no mixing of salt water and fresh water occurs within the root zone of the plants on the dunes and slacks (Crandall 1962). Similarly, the diffusion of nutrients from saltwater into the freshwater lens is probably negligible under normal conditions (Fig. 29; Art 1976).

The influence of groundwater on dune vegetation is open to speculation. According to Art (1976), the water table on Fire Island, New York, is influenced by ocean tides, barometric pressure, and evapotranspiration losses. Willis et al. (1959), however, state flatly that dune communities are beyond the direct influence of groundwater. Because the dune slacks exhibit seasonal flooding and typically moist soils throughout the year, plants on the adjacent dunes must have roots that approach, if not reach, the water table. Despite whether dune plants are rooted in the free water table or not, these plants are highly susceptible to drought. Additional research in this area of dune-and-slack water relations is warranted.

Besides the dune slacks that are the foci of this report, several other types of wetlands occur on barrier islands and barrier beaches along the Atlantic shore. Many small bodies of permanent fresh water dot the dune landscape, primarily on forested dunes landward of the herbaceous dunes. Hillestad et al. (1975) described a wide range of fresh to brackish water systems on Cumberland Island, Georgia. These environments included freshwater lakes and ponds, permanent saline ponds, temporary freshwater lakes and ponds, and brackish temporary ponds. The beach ridge system of large sea islands such as Cumberland Island creates conditions for diverse wetland systems.

On Assateague Island in Maryland and Virginia, and Pea Island, North Carolina, the U.S. Fish and Wildlife Service built impoundments to encourage food plants for waterfowl (Fig. 30). These areas are in shrub or dunegrass-shrub plant communities and vary from small watering holes, 3-4 m across, to one that covers more than 200 ha. These wetland environments are effective feeding, resting and loafing sites for many species of waterfowl.

On many barrier beaches between Cape Henlopen, Delaware, and Cape Fear, North Carolina, oceanic overwash is an important coastal process. On these beaches, a broad, flat, grassy plain forms landward of the foredunes (Brown 1959; Godfrey and Godfrey 1976). Brown (1959:34) characterized these areas as "open, wind-swept sand with a flat to undulating surface." These environments, termed “barrier flats,” are formed when oceanic overwash overtops or breaches the dune system and carries sand across the barrier island.

Barrier flats range from dry, sparsely vegetated flats to areas densely vegetated and having small, round dunes. Because of the nearness of the water table to the land surface on the flats, colonization by grasses and short-lived perennial forbs is rapid. On Core Banks, North Carolina, species such as saltmeadow cordgrass, seaside goldenrod, evening primrose, and coastal plain pennywort are characteristic dominants. Usually barrier flats grade into tidal marshes at the lower elevations or at the distal ends of the overwash fans where fimbristylis (Fimbristylis thermalis), Carolina sea lavender (Limonium carolinianum), and saltgrass (Distichlis spicata) dominate.

If barrier flats are infrequently or rarely inundated with washover sediments, they support woodland communities, either shrub thickets or forests. The flats are invaded by shrubs along the distal or landward edge of the flats and expand seaward. Eastern false willow (Baccharis halimifolia), waxmyrtle, and saltwater false willow (B. angustifolia) may be the initial invaders.

Between the towns of Wellfleet and Truro on Cape Cod, Massachusetts, kettle-hole depressions are distributed throughout the coastal dune system. Kettles are basin- or bowl-shaped holes in glacial-drift deposits, and they are formed by the melting of a large block of ice left behind by a retreating glacier that had been partly or wholly buried by glacial drift (Leatherman 1988). Some of these depressions are deep and extend well below the existing water table. Others are so shallow that they support upland vegetation. Several different types of freshwater wetlands are found in these areas, including those that exhibit bog succession. Kettles are particularly susceptible to human influences: because kettles have no natural drainage, materials entering the system from the surface or groundwater (e.g., sediments or human-generated pollutants) accumulate in the kettle. Groundwater pollution from septic tanks can lead to eutrophication; excessive pumping of groundwater may lower the water table.

Art, H. W. 1976. Ecological studies of the Sunken Forest, Fire Island National Seashore, New York. U. S. National Park Service Scientific Monograph Series 7. 67 pp.

Au, S. F. 1974. Vegetation and ecological processes of Shackleford Bank, NC. U. S. National Park Service Scientific Monograph Series 6. 87 pp.

Bagnold, R. A. 1941. The physics of blown sand and desert dunes. Chapman and Hall, New York, NY. 265 pp.

Barbour, M. G., T. M. DeJong, and B. M. Pavlik. 1985. Marine beach and dune plant communities. Pages 296-322 in B. F. Chabot and H. A. Mooney, editors. Physiological ecology of North American plant communities. Chapman and Hall, New York, NY.

Bird, E. C. F. 1969. Coasts. Massachusetts Institute of Technology Press, Cambridge, Mass. 246 pp.

Boorman, L. A. 1977. Sand‑dunes. Pages 161-197 in R. S. K. Barnes, editor. The coastline. John Wiley and Sons, London, England.

BoulJ, M. E. 1979. The vegetation of Fisherman Island, Virginia, USA. Castanea 44:99‑108.

Boyce, S. G. 1951b. Salt hypertrophy in succulent dune plants. Science 114:544‑545.

Brown, C. A. 1959. Vegetation of the Outer Banks of North Carolina. Ph.D. dissertation, Louisiana State University, Baton Rouge. 179 pp.

Cartica, R. J., and J. A. Quinn. 1980. Responses of populations of Solidago sempervirens to salt spray across a barrier beach. American Journal of Botany 64:1236‑1242.

Cleary, W. J., and P. E. Hosier. 1979. Geomorphology, washover history, and inlet zonation: Cape Lookout, North Carolina to Bird Island, North Carolina. Pages 237-271 in S. P. Leatherman, editor. Barrier islands. Academic Press, New York, NY.

Conn-Thomas, C. E., and F. P. Day. 1993. Environmental influences on belowground decomposition rates along a barrier island chronosequence of ridge and swale formations. Association of Southeastern Biologists Bulletin 40:142.

Cooper, W. S. 1958. Coastal sand dunes of Oregon and Washington. Geological Society of America Memoir 72. 169 pp.

Crandall, H. C. 1962. Geology and ground-water resources of Plum Island, Suffolk County, New York. U. S. Geological Survey Water-Supply Paper 1539-X. 35 pp.

Crawford, R. M. M., and D. Wishart. 1966. A multivariate analysis of the development of dune slack vegetation in relation to coastal accretion at Tentsmuir, Fife. Journal of Ecology 54:729‑743.

Davies, J. L. 1977. Geographic variation in coastal development. Longman Press, New York, NY. 204 pp.

Doing, H. 1985. Coastal fore‑dune zonation and succession in various parts of the world. Vegetatio 61:65‑75.

Ehrenfeld, J. G. 1990. Dynamics and processes of barrier island vegetation. Reviews in Aquatic Sciences 2:437-480.

Godfrey, P. J. 1977. Climate, plant response, and development of dunes on barrier beaches along the U. S. east coast. International Journal of Biometeorology 21:203‑215.

Godfrey, P. J., and M. M. Godfrey. 1976. Barrier island ecology of Cape Lookout National Seashore and vicinity, North Carolina. U.S. National Park Service Scientific Monograph Series 9. 160 pp.

Goldsmith, V. 1977. Coastal processes and resulting forms of sediment accumulations, Currituck Spit, Virginia‑North Carolina: field trip guidebook. Virginia Institute of Marine Science, Glouster Point, VA. Special Report in Applied Marine Science and Ocean Engineering 143. 46 pp. + appendices.

Hennigar, H. F. 1979. Historical evolution of coastal sand dune in Currituck Spit. M. S. thesis, Virginia Institute of Marine Sciences, Gloucester Point, Virginia.

Hosier, P. E., and W. J. Cleary. 1987. Historic geomorphic changes on Currituck Spit, North Carolina. Pages 1745-1759 in N. Kraus, editor. Coastal Sediments, '87. Proceedings of the American Society of Civil Engineers, New York, NY.

Hubbard, D. K. 1977. Variations in tidal inlet processes and morphology in the Georgia embayment. Technical Report 14-CRD, Coastal Research Division, Department of Geology, University of South Carolina, Columbia. 79 pp.

Johnson, A. F. 1981. Plant communities of the Napeague Dunes. Bulletin of the Torrey Botanical Club 108:76‑84.

Johnson, P. L., and W. T. Swank. 1973. Studies of cation budgets in the southern Appalachians on four experimental watersheds with contrasting vegetation. Ecology 54:70‑80.

Jones, R., and J. R. Etherington. 1971. Comparative studies of plant growth and distribution in relation to water‑logging. IV. The growth of dune and slack plants. Journal of Ecology 59:793‑801.

Kana, T. W. 1989. Erosion and beach restoration at Seabrook Island, South Carolina. Shore and Beach 57:3‑18.

Lakshmi, B., and F. P. Day. 1993. Soil nitrogen levels and mineralization along a community chronosequence on a coastal barrier island. Association of Southeastern Biologists Bulletin 40:143.

Landsberg, S. Y. 1956. The orientation of dunes in relation to wind. Geographical Journal 122:176‑189.

Leatherman, S. P. 1988. Cape Cod field trips: from yesterday's glaciers to today's beaches. Laboratory for Coastal Research, University of Maryland, College Park, Coastal Publication Series. 132 pp.

Martin, W. E. 1959. The vegetation of Island Beach State Park, New Jersey. Ecological Monographs 29:1‑46.

Moreno‑Casasola, P. 1988. Patterns of plant species distribution on coastal dunes. Journal of Biogeography 15:787‑806.

Olson, J. S. 1958a. Lake Michigan dune development. I. Wind‑velocity profiles. Journal of Geology 66:254‑263.

Randall, R. E. 1970. Salt measurements on the coast of Barbados, West Indies. Oikos 21:65‑70.

Ranwell, D. S. 1959. Newborough Warren, Anglesey. I. The dune system and dune slack habitat. Journal of Ecology 47:571‑601.

Ranwell, D. S. 1972. Ecology of salt marshes and sand dunes. Chapman and Hall, London, England. 258 pp.

Roman, C. T., and K. F. Nordstrom. 1988. The effect of erosion rate on vegetation patterns of an east coast barrier island. Estuarine Coastal and Shelf Science 26:233‑242.

Salisbury, E. J. 1952. Downs and dunes: their plant life and its environment. Bell and Sons, Limited, London, England. 328 pp.

Savage, R. P., and W. W. Woodhouse, Jr. 1969. Creation and stabilization of coastal barrier dunes. Pages 672‑700 in Proceedings of the Eleventh Conference on Coastal Engineering, London, England. American Society of Civil Engineers, New York, NY.

Tansley, A. G. 1949. The British Isles and their vegetation. University Press, Cambridge, United Kingdom. 930 pp.

Titus, J. G. 1988. Greenhouse effect, sea level rise and coastal wetlands. U. S. Environmental Protection Agency, Office of Policy, Planning and Evaluation, Washington, DC. EPA-230-05-86-013. 152 pp.

Tyndall, R. W., and G. F. Levy. 1978. Plant distribution and succession within interdunal depressions on a Virginia barrier dune system. Journal of the Elisha Mitchell Scientific Society 94:1‑15.

van der Laan, D. 1979. Spatial and temporal variations in the vegetation of dune slacks in relation to the ground water regime. Vegetatio 39:43‑51.

van der Valk, A. G. 1974a. Environmental factors controlling the distribution of forbs on coastal foredunes in Cape Hatteras National Seashore. Canadian Journal of Botany 52:1057‑1073.

van der Valk, A. G. 1974b. Mineral cycling in coastal foredune plant communities in Cape Hatteras National Seashore. Ecology 55:1349‑1358.

van der Valk, A. G. 1977. The role of leaves in the uptake of nutrients by Uniola paniculata and Ammophila breviligulata. Chesapeake Science 18:77‑79.

Wagner, R. H. 1964. Ecology of Uniola paniculata in the dune‑strand habitat of North Carolina. Ecological Monographs 34:79‑96.

Willis, A. J. 1963. Braunton Burrows: the effects on the vegetation of the addition of mineral nutrients to the dune soils. Journal of Ecology 51:353‑374.

Woodhouse, W. W., Jr., and R. E. Hanes. 1966. Dune stabilization with vegetation on the Outer Banks of North Carolina. North Carolina State University, Department of Soil Science, Soils Information Series 8. 50 pp.

Yocum, D. H., and D. T. Wicklow. 1980. Community differentiation along a dune succession: an experimental approach with coprophilous fungi. Ecology 61:868‑880.