Coastal Plain

Physiography—The Coastal Plain, a 320,000-square-mile zone that extends east from the western limits of the southern forest in Texas to the Atlantic shore, supports the most extensive and productive pine forests of the South. A happy combination of the four factors of site makes this so. Geologically young, the underlying sediments of the region are mainly sands, gravels, clays, and marls, in strata that drop toward the coast. Older sediments are exposed in the interior, younger ones seaward. Elevations rarely reach 1000 feet, and local relief is seldom more than 500 feet.

Along the eastern sector of the Gulf of Mexico, the Plain is characterized by a series of terraces oriented almost parallel to the present coastline. In contrast, the western Gulf Coastal Plain consists of alternating low ridges and valleys, again approximately parallel to the shore, that give rise to a belted topography. The Florida peninsula and the Great Alluvial Valley of the lower Mississippi River interrupt these patterns.

From central North Carolina northward, the Atlantic Coastal Plain is indented by bays and estuaries penetrating halfway or more to the Piedmont section of the Appalachians, thus dividing the land surface into a series of peninsula-like extensions. Long barrier beaches and islands separated from the mainland by sleepy lagoons are an attraction for wildlife and saltwater enthusiasts.

Sand dunes and salt-spray sites commonly occur at the fringes of these barrier islands as well as on strips of mainland up to a mile inland along either coastline. (Where winds constantly move beach sands, few woody plants become established until grasses and sedges first capture the site.) Though deep, the soils are infertile and frequently flooded by storm-blown salt spray or seawater. Some pines seem to resist frequent salt-spray damage; such especially persist on the barrier islands off the Mississippi Coast. Some salt-tolerant trees, like live oak, wax myrtle, and yaupon, endure the harsh environment of saltwater coasts.

Figure 1.12 Mature stand of sand pine in the Big Scrub of central peninsular Florida. Trees of this serotinous species of poor form mature at about 35 years of age. They persist for another 25 years, gradually falling apart. The stand shown is 50 years old.

From the Delaware River southward to the Georgia shoreline, the Atlantic Coastal Plain is characterized by a series of broad marine and fluvial step-like terraces roughly paralleling the ocean. These zones are separated by low escarpments, each of slightly greater seaward slope. So distinct are the terraces in the southern section that travelers may observe tide-washed boulders on either side of a road as it dips toward the sea, perhaps a hundred miles from where waves now buffet the shore. Deep sands, too, tell that far inland, where great forests now stand, was once the ocean's edge.

These terraces, laid down at different times, exhibit variation otherwise attributed to length of exposure for weathering and the degree of uplift above the present sea level in ages past. The innermost terraces are older, lifted higher, and exhibit more pronounced relief. Here, leaching and oxidation of nutrients and organic matter over a long period have formed thicker, weathered zones in the soil profile. Plow furrows expose a brilliant red color in the soils, thus early settlers called them redlands. Names like that persist for titles of towns in the upper Coastal Plain.

Flatwoods are important components of the forest of the lower Coastal Plain. Often they are on the low side of a terrace, immediately above where the land gives way to a swamp.

The uppermost terrace on the Atlantic Ocean side of the Coastal Plain is called the Fall Line. This sandy zone also forms the outer limit of the Piedmont province—the foot of the mountains—that lies to the northwest. The outcrops of sand extend in a belt roughly paralleling the coast, in places more than 20 miles wide, while in some zones the Fall Line appears only as a narrow strip of imperceptible width. Elevations of the hills of the Fall Line approach a thousand feet, occasionally rising higher than the adjacent Piedmont province. Longleaf pines, with an unusual resistance to brown spot disease (a seedling needle blight) and scrub oaks capture these xeric sites.

Figure 1.13 Typical scrub oak site in the Fall Line sandhills. Such stands tally 3400 stems per acre, ranging from 1/2 to 6 inches in diameter at breast height (DBH). (USDA Forest Service photo by R. Shipman, 1954)

Soils—Typically, the podzolic soils of the Coastal Plains have deep, well-developed profiles that are slightly acid and low in organic matter and soluble nutrients. Occasionally, they are saline, from salt-water intrusion; in other places alkaline, where limestone solution has played a role in their genesis. Texture of the yellowish to reddish surface materials varies from clay through silt to sand, although sandy loam and silty loam classes are common. Some of these soils contain fragments of rock.

Mottled red, yellow, and bluish-gray subsoils occur throughout the Coastal Plain. Oxidation, reduction, and hydration of iron coatings on the surfaces of the grains of soil give the particles hues of various shades. These chemical reactions also result in the development of hardpans or clay pans several feet below the surface. Spades, picks, and soil augers are impeded by this tough zone, unable to penetrate it. Sometimes subsoilers, towed by tractors, cut through this zone to improve drainage; sometimes dynamite is necessary to fracture the subsoil.

Atop the parent material or lying on former soil surfaces might be an overburden of peat (with more than 50% organic matter) or muck (between 20 and 50%). Soil scientists also classify wet mineral soils within 100 miles of the coasts of both the Atlantic Ocean and the Gulf of Mexico as wiesenboden. Associated with these are the half-bogs, a rich mixture of organic matter and silt and clay. In this broad area, too, one charts the bogs; they may occur in small patches or in large swamps, like the Dismal, the Okefenokee, and the Everglades. Atlantic white-cedar trees predominate in the former, baldcypress and many broadleaf species in the Okefenokee, while grasses and shrubby woody plants cover most of South Florida's Everglades.

Forests—Upland Coastal Plain forest types include mixtures of loblolly and shortleaf pines, amounting to about one-fourth of the total acreage of the zone. Longleaf and slash pines together compose 17% of the province, and the composite oak-pine forests some 33%. Pond pine, mostly along the Atlantic seaboard, accounts for 2% of the total, while sand pine, eastern redcedar, and Virginia pine amount to less than 0.3%. These values change as hardwoods encroach in pine forests with fire suppression and pine cutting, and, conversely, as abandoned farmland reverts to old-field pine forests.

The longleaf pine-slash pine mixture predominates on sandy soils within the natural ranges of these two species. This is so even where hardpans underlie the surface soil. (Longleaf pine is a deep tap-rooted tree.) Tall, straight stems, prized for poles, piling, and plywood bolts, suggest the favorable moisture-holding capacity of better soils. Turkey oak and other scrub oaks are associates of longleaf pine on more droughty soils, while blackgum and red maple accompany slash pine.

Slash pine is not found naturally west of the Mississippi River, but foresters introduced planted stands in East Texas and western Louisiana as early as the 1930s. The South Florida variety of slash pine occurs in the subtropical zone of that state along with some tropical hardwoods.

Although often considered a moist-site species, loblolly pine has wide site tolerance. It does well from the edges of wet savannas in Virginia to the xeric rocky outcrops of Central Texas.8 It can also be found associated with longleaf pine. Indeed, where wildfire is excluded, as it has been since the 1940s throughout much of its range, loblolly pine replaces longleaf pine. Consequently, the acreage of longleaf pine rapidly declined until the 1980s. With the use of prescribed fire for stand regeneration, the area covered with longleaf pine has since increased and continues to do so. Indeed, foresters aggressively work to restore this highest-quality southern pine to much of its original range. Growing straight and tall, the wood is dense.

Upland hardwoods, including those in the Post Oak Belt in Oklahoma and Texas, constitute a fifth of the forests of the Coastal Plain. Apart from the Belt, the occurrence of broadleaf species in the region largely relates to past use of the land. Narrow bands of broadleaf trees occur along major drainages and minor streams where dampness excludes fire.

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