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A step back in maritime: Taking a closer look at Sullivan’s accreted land

By Billy McCord; ecologist, naturalist and wildlife biologist; SCDNR retired*

As a naturalist and ecologist, I was instantly struck by several thoughts during a recent trip to Station 25 on Sullivan’s Island. First and foremost, I was reminded of the incredible diversity of the “maritime” ecosystem, the transitional upland habitats that front the Atlantic Ocean and that specifically evolved to tolerate – and even thrive – under the harsh impacts of wind-blown salt and sand. These specialized habitats and associated plant communities lie in near proximity to the “marine” ecosystem, an area which is under the direct influence of saltwater and the forces of tides, currents and wind. Relatively few plants and other living organisms have evolved to prosper in this highly dynamic and transitional interface which forms the barrier between the mainland and the forces of wind and sea. Not coincidentally, this barrier of “maritime” ecosystem most typically occurs on barrier islands (such as Sullivan’s Island) off the coast of the Southeast.

The “maritime” ecosystem is perhaps the most dynamic type of upland other than shifting sandbars in and along water-courses. As many of us observed following Hurricane Hugo in 1989, a major tropical storm event has the potential to quickly and drastically re-sculpt barrier islands and the mainland oceanfront, including all maritime habitats.

The more stable portions of most barrier islands are several thousand years of age with the older, most stable areas most distantly removed from the beachfront. Barrier islands are typically a mosaic of linear habitat types which are generally oriented north to south and descending from alternate zones of dunes and inter-dune swales or depressions. The influence of wind-born sand and salt also diminishes with distance from the oceanfront. Accordingly, the inland, or more westerly portions of barrier islands, support plants and other wildlife that are generally intolerant of more oceanfront climes.

The most mature upland portion of the “maritime” ecosystem is colonized by “maritime forest.” Several hundred years may be required for a maritime forest to reach climax stage with a high, nearly closed canopy dominated by live oak and Darlington oak. There is no true maritime forest development yet along the Station 25 path; however, maritime forest likely covered much of the interior of Sullivan’s Island and other area barrier islands when the first Europeans arrived. Near Fort Moultrie is a developing maritime forest, but this area is most likely less than a century old and may require another century before a climax forest is attained. The majority of the Accreted Land is much younger, and the primary “forest” coverage is by relatively young or developing maritime forest and “maritime shrub thicket”, the latter dominated by shrubs such as wax myrtle and yaupon. After several decades without extreme impacts from storms, erosion or human activities, maritime shrub thicket is destined to gradually evolve into maritime forest. The fact that much of the Accreted Land is in younger plant communities is because the Accreted Land is growing or accreting relatively quickly, unlike most of the Charleston County coast which is generally losing sand or eroding.

The Oceanfront: Primary and Secondary Dunes

The youngest and most dynamic uplands are the fore-dunes, or primary dunes, located very near the surf. If not for the stabilizing effects of sea-oats and other grasses, sand along the beachfront would nearly constantly move. The descendents of the primary dunes are secondary dunes which were once on the beachfront before additional dunes were formed to their east by rapid build-up of sand from storms, wind and tide. With age and erosion, secondary dunes flatten and broaden, and because they are removed from the more direct influence of wind-blown sand and salt, they support much higher plant diversity than primary dunes. Areas of the maritime ecosystem with high, sandy soils, including fore-dunes, are generally colonized by maritime grassland; so-named because grasses are the dominant inhabitants of these typically dry, sandy and exposed areas. However, herbs, shrubs and vines may be scattered throughout, particularly in maritime grasslands of secondary dune ridges. A native shrub or small tree that often occurs as individual plants or small colonies in maritime grassland or maritime shrub thicket is Hercules Club or toothache tree (so-called be because Native Americans chewed the bark to anesthetize their gums or toothaches). This plant has glossy leaves, spiny stems and is the caterpillar food-plant of our largest native butterfly, the Giant Swallowtail, which has a wingspread of about six inches. I saw two of these elegant butterflies seemingly floating just over shrub thickets and dune ridges near the Station 25 path. Open secondary dunes often support native wildflowers such as dune camphorweed (yellow aster-like flowers) and spotted beebalm (a native mint), both of which are used as nectar-sources by many insects, including migratory Monarch butterflies. Very old interior dunes (relic dune ridges) are often enshrouded within maritime forest.

The Wetlands

Organic material from dead plants and nutrients from animal droppings move with rain-water, wind and gravity into lower swales or depressions. Increased organic matter and moisture in soils provide habitat for additional plants and animals. As evident along the Station 25 beach path, shallower swales may become thickly colonized by shrubs such as Wax Myrtle, and vines including greenbriers, pepper-vine, and poison ivy. Deeper swales collect more organic materials and retain more moisture in the sediments. Accordingly, such depressions typically become colonized by wetland plants, with younger depression wetlands usually dominated by grasses and other herbs while older, more inland depressions may support trees such as coastal plain willow. A relatively “young” open inter-dune depression wetland occurs near the beach end of the Station 25 beach path. An obvious member of the plant community in this wetland is southern cat-tail, as well as one of our specialized wildflowers; the Seashore Mallow. Seashore Mallow has pink, hibiscus-like flowers on which hummingbirds often feed. This member of the hibiscus family is native to the coast of the Southeast and has adapted to tolerate and sometimes prefer brackish soils. Another wildflower I noticed along the Station 25 path is Marsh Morning Glory, which has large hot-pink blooms that open in the morning and typically close as the sun gets high. This plant is also fond of damp, brackish soils.

Another interesting wildflower that thrives in open, slightly damp brackish soils is fogfruit, a native groundcover with small, pale lavender, clustered flowers that are generally present spring through fall. This plant occurs on the open edges of the Station 25 depression. It may also be found in slight swales or depressions within maritime grassland. This interesting plant in related to verbenas and is an excellent nectar plant for butterflies. It is also the caterpillar host-plant for a localized small butterfly, the phaon crescent, which is rarely seen other than on barrier islands and similar coastal uplands. This small butterfly can be common on the Accreted Land near colonies of fogfruit, and I saw one along the path as I walked by.

Wetlands similar to the Station 25 depression are classified as globally rare by Nature Serve, a website maintained by The Nature Conservancy that uses data assimilated from various sources to track the population status of plants and animals and plant communities, and they can be found scattered throughout the Accreted Land. In a similar open depression wetland adjacent to the beach path at Station 16, a South Carolina-listed and regionally-listed rare plant known as the Carolina Grasswort was documented during a plant survey conducted in summer 2009 by both myself and botanists from the University of North Carolina. The oldest interdune depressions may have canopy closure by willows, particularly in the deeper portion of wetlands where surface water is retained for relatively long periods of time, and vegetation may be sparse along the ground. The four species of amphibians (Southern Toad, Eastern Narrowmouth Toad, Green Treefrog, and Squirrel Treefrog) that I have documented on the Accreted Land are dependent upon these seasonally-flooded depression wetlands since the larval stage (tadpoles) are aquatic.

Pruned vs. Natural Growth of Maritime Forest

Something else which I noticed while I was strolling through the Station 25 beach path was the extremely unnatural, pruned and hedge-like shaping of shrubs within maritime shrub thickets and interdune depressions. If one accesses the beach via the Station 28 path, there is a striking difference in the structure of plant communities on the northeast and southwest sides of the path. Though the age and topography of habitats on either side of the path are very similar, plant communities adjacent to the northeast side of the path are natural and have not been pruned. A willow-dominated wetland on the northeast side shortly after entering the path is an example of a globally-imperiled plant community only found along the southeast coast of the United States. Several examples of this wetland type have been identified on the Accreted Land, including the willow-dominated wetland beside the Station 28 path. The wetland on the southwest side is an altered version of the same rare habitat.

The most striking difference in plant communities or habitats along the Station 28 path is the condition of shrub thickets on either side. Wax Myrtle is the dominant plant of both sides, but those on the northeast side are old-growth Wax Myrtles, apparently un-pruned for a decade or more. The canopy of this thicket is nearly 25 feet high, with a relatively open forest floor developing. This type of maritime shrub thicket is a precursor to the early stages of maritime forest development. As the old-growth shrubs naturally thin and the forest floor opens, trees begin to grow within and above the shrubs, eventually shading some shrubs and forming a more mature forest canopy. On the southwest side of the path, as along both sides of the Station 25 path, shrub thickets, and undoubtedly many sapling trees within, have been repeatedly pruned. This practice has not only created an unnatural look, but has also virtually suspended the development or evolution of the ecosystem. Wax Myrtles and other shrubs, as well as tangles of vines, have been forced to remain low and thick since higher canopies that shade-out the “forest” floor have not developed. I also quickly noticed that fruit production was much more evident on the old-growth wax myrtles than on plants that have been periodically pruned. This is easily understood since Wax Myrtles only produce fruit on branches that are at least in their second growing season. Periodic pruning, particularly every few years, severely limits potential fruit production. Pruning can also expose Wax Myrtles to fungal diseases. Fruits of wax myrtle, pepper-vine, greenbriers, and even those of poison ivy, generally mature in fall and winter and provide extremely valuable food for migratory and winter birds. Many migratory birds such as thrushes and gray catbirds, as well as winter birds such as Yellow-Rumped Warblers (once called myrtle warblers because of their dependency on wax myrtle fruit) and Gray Catbirds, rely upon such fruits for much of their diets. In fact, the maritime ecosystem, and particularly maritime shrub thicket and maritime forest, have been identified as critical habitat for migratory songbirds that have evolved to migrate down and very near the Atlantic coast to the southern hemisphere during winter after breeding in North America. About 75 species of birds are included in this migratory group, and diverse habitats and associated plants within the natural maritime ecosystem provide cover for resting and predator avoidance, and food for the full diversity of birds as they partition into preferred habitats, thereby minimizing competition.

Invasive Species

Sadly, another observation that I quickly made along both the Station 25 and Station 28 paths was the presence of several invasive plant species. Invasive plants are non-native plants, most of which were imported for landscaping, that reproduce by seeds or fruit and invade natural habitats. Invasive plants displace native plants by taking up space and either overgrowing or outcompeting native plants. Chinese Tallow tree is a particularly aggressive invasive plant that is spread primarily by bird droppings. Chinese Tallow tree, often called “popcorn tree,” is known for its full shape, beautiful fall color, and interesting popcorn-like fall fruit. It prefers damp, low-salinity soils of depressions and wetland edges. A single twenty-foot specimen of Chinese Tallow tree is located on the beach end and just to the north of the Station 25 path. Over 100,000 seeds may be produced annually by a single tree, and the seeds can survive for many years in the soil until sufficient sun exposure and moisture sponsor germination. Because of its seed productivity, Chinese Tallow tree frequently forms colonies in shallow wetlands and along the upland edges of deeper wetlands. A number of large Chinese Tallow trees occur near the outer edge of the forested depression wetland near the Station 28 path. Pampus grass has also become invasive on the Accreted Land, and specimens of this species can be seen near both Station 25 and Station 28 paths. Pampus grass prefers abundant sunlight and soils that are slightly to moderately moist. Pruning of Wax Myrtles and other shrubs and small trees has likely benefited the spread of this invasive plant on the Accreted Land, and scattered specimens of Pampus grass can be seen nearly throughout the interior of the Accreted Land from the highest point of the Station 25 path near the primary dunes.

Transition to Uplands

On the outer edges of the Accreted Land as it transitions to uplands, as well as scattered throughout the wetlands, are numerous specimens of Eastern Groundsel or Groundsel tree. This native shrub prefers such habitats with abundant sun exposure and damp, well-drained soil. Groundsel tree blooms in mid- to late- October and coincides with the peak of the southerly monarch butterfly migration along our coast. Millions of monarchs migrate along our coastline each fall, and the butterflies are very dependent upon native wildflowers and shrubs, particularly groundsel tree, for nectar, which they use for fuel and for laying down fat reserves for winter. Monarch researchers have deemed areas such as thickets of Groundsel tree as “monarch way stations”, and such areas are thought to be essential for the long-term survival of monarchs and their spectacular fall migration. Groundsel tree also attracts other butterflies, both migratory and non-migratory, as well as many other insects such as flies and bees. Not surprisingly, many insect-eating birds, including the state-listed prairie warbler, glean small insects from these plants.

From my perspective, the Accreted Land is a tremendous ecological treasure and a rare example of an accreting or growing maritime ecosystem on the central South Carolina coast where the vast majority of the Atlantic coastline is erosional. The Accreted Land is also rather unique in that it presents an excellent opportunity for the study and observation of an evolving maritime ecosystem with many unusual ecological assets. As a long-time professional ecologist and naturalist, I have no doubt that the best management of the Accreted Land is the establishment of an aggressive program to control invasive species and otherwise allow nature to take its course while providing us, as stewards, a grand opportunity to observe, learn and evolve with the evolution of the Accreted Land.

* Comments in this article are not necessarily reflective of opinions of the South Carolina Department of Natural Resources.

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