Atlantic Coastal Plain Carolina Bay Wetland

This system consists of wetlands associated with ovoid, shallow depressions with nearly flat bottoms in parts of the Atlantic Coastal Plain. Often called Carolina bays, these areas are most numerous and extensive in South Carolina but are also present in adjacent Georgia and the Inner Coastal Plain of North Carolina. These flat-bottomed depressions have mineral soils with clay hardpans, fragipans, or some other drainage-impeding mechanism that traps and retains water from a combination of rainfall and exposure of a high regional water table. Some examples are essentially permanently flooded, while others support water levels that vary substantially from year to year and over longer climatic cycles. Vegetation includes a series of primarily herbaceous and woodland associations. The wettest sites have open water and floating-leaved aquatic vegetation, or marsh vegetation of tall graminoids. Drier sites often have an open canopy of Taxodium ascendens, with a dense, often fairly species-rich herbaceous layer beneath. In a very few cases, Taxodium ascendens is replaced by Taxodium distichum. A few occurrences are shrubby, but none contain the dense shrub layers of characteristic pocosin species that occur in the bays with organic soils. Vegetational composition often varies substantially from year to year, in response to differences in water levels and drawdown times. Variation in hydroperiod is the most important dynamic, causing rapid major changes in the herbaceous vegetation. Unlike the steeper-sided solution depressions, where many different hydroperiods are present within a short distance and vegetation zones simply shift, the flat-bottomed Carolina bays experience drastic yearly changes in hydroperiod over most of their extent. Fire periodically spreads into the bays from adjacent uplands when conditions are dry, helps prevents invasion by less water-tolerant trees during dry periods, and interacts with flooding to affect vegetational composition. Where fire is removed, Pinus taeda often invades the bays. Fire may also be important in preventing buildup of organic matter on the soil surface.

Vegetation includes a series of primarily herbaceous and woodland associations. The wettest sites have open water and floating-leaved aquatic vegetation, or marsh vegetation of tall graminoids. Drier sites often have an open canopy of Taxodium ascendens, with a dense, often fairly species-rich herbaceous layer beneath. In a very few cases, Taxodium ascendens is replaced by Taxodium distichum (Bennett and Nelson. 1991). A large number of annual species are present. Showy, characteristic plants include species of Symphyotrichum, Boltonia, Xyris, Ludwigia, and Solidago (Bennett and Nelson 1991). Some sites have similar herbaceous vegetation without trees. A few occurrences are shrubby, but none contain the dense shrub layers of characteristic pocosin species that occur in the bays with organic soils. Vegetational composition often varies substantially from year to year, in response to differences in water levels and drawdown times. Seed banking plays an important role in component communities. The system is also important as amphibian breeding habitat and may support a distinctive aquatic invertebrate community.

Examples of this system occur in Carolina bays with mineral soils and with seasonal to permanent standing water. Carolina bays are oriented, oval, shallow depressions with nearly flat bottoms, which range from North Carolina through South Carolina, and into adjacent Georgia. The general thought has been that most of the Carolina bays in the Outer Coastal Plain occur in sandy sediments and are filled with peat, while most Carolina bays in the Inner Coastal Plain occur in loamy sediments and have mineral soils with clay hardpans, but the situation may be more complex than this. These depressions hold water, due to a combination of rainfall and exposure of a high regional water table. Some are essentially permanently flooded. Others contain water well into the growing season in most years, but water levels vary substantially from year to year and over longer climatic cycles. Fire is an important natural influence in dry times. The McColl soil series (a fine, kaolinitic, thermic Typic Fragiaquult) is the soil most consistently associated with Carolina bays which are not dominated by "pocosin-like" vegetation (M. Schafale pers. comm.). Its depth to fragic soil properties is 30-90 cm (12-36 inches); the depth to a fragipan is 38-100 cm (15-40 inches). Some pedons have few to common concretions of ironstone.

Variation in hydroperiod is the most important dynamic, causing rapid major changes in the herbaceous vegetation. Unlike the steeper-sided solution depressions, where many different hydroperiods are present within a short distance and vegetation zones simply shift, the flat-bottomed Carolina bays experience drastic yearly changes in hydroperiod over most of their extent. Many plants persist in seed banks for periods of years when conditions are not suitable. Fire is also an important process, spreading into the bays from adjacent uplands when conditions are dry. Fire prevents invasion by less water-tolerant trees during dry periods, and interacts with flooding to affect vegetational composition. Where there is a lack of fire, Pinus taeda often invades the bays. Fire may also be important in preventing buildup of organic matter on the soil surface.

Threats include logging of the Taxodium spp. canopy, lack of fire, alteration to the hydrology, and damage to the herbaceous ground cover from vehicles, feral hog (Sus scrofa) rooting, firebreak plowlines, and ditching. Many of these habitats that were forested with Taxodium ascendens have been cleared of trees which are used for cypress mulch. Invasion by trees or shrubs due to lack of fire has become a widespread threat. Mainly sites which are within an area managed for conservation (in conjunction with other resource management goals) have prescribed fires frequently enough to conserve the biological diversity of this open wetland habitat. The lack of fire can lead to shrub and tree encroachment, especially invasion by Pinus taeda, accompanied by increased shading and evapotranspiration, accumulation of leaf litter, and a drying out of the Carolina bay depression wetland during drier times of year. Since many of the herbaceous plants which grow in these predominantly herbaceous wetlands have corms, or starchy root structures, feral hogs are a real threat. Feral hogs will turn up the soil and eat the below-ground plant parts and amphibians and invertebrates that live in the wet soil. In doing this they disturb the soil and degrade the habitat. The areas with disturbed soil where feral hogs have rooted (or vehicles have rutted the wet soil) can provide habitat for weedy or invasive exotic plants. On lands managed as pine plantations, sometimes the depression pond habitat is bedded and planted in Pinus elliottii or Pinus taeda. Many Carolina bays have been ditched, resulting in lowered water levels and shortened hydroperiod. These wetlands are also potentially subject to eutrophication by nutrient input in runoff from surrounding developed or agricultural lands. In the 1950s, the number of Carolina bays was estimated at 500,000 (Prouty 1952). Only 10,000-20,000 remained by the early 1990s (Richardson and Gibbons 1993). In South Carolina, 97% of Carolina bays larger than 0.8 ha (2 acres) had been disturbed by agriculture or logging (Bennett and Nelson 1991).

This system is found in the Inner to Middle Coastal Plain, from southern North Carolina, through South Carolina, and into adjacent Georgia. It is most numerous and extensive in South Carolina.