Disruptions To The Coastal Plain

The Mississippi Delta—A physiographic feature that breaks into the general pattern of the Gulf Coastal Plain is the lower alluvial valley of the Mississippi River, called the Delta locally and in forestry literature. It interrupts the prevailing east-west orientation of the Coastal Plain with a north-south strip of alluvial soils bearing distinctive bottomland hardwood forests.

Pioneering settlers seemed to have had an understanding of world geography, especially the similarity of the lower Mississippi Valley to the mouth of the rich Nile River of Egypt. Cairo, IL;

Loessal Mississippi
Figure 1.14 A Coastal Plain longleaf pine pole stand that regenerated following a blow-down 50 years earlier. Periodically burned, the stand with its "rough" (the ground cover) provides good habitat for quail. (USDA Forest Service photo)

Memphis, TN; and Alexandria, LA, remind one of the cities by the same names not far from where the Nile pours its silt-laden waters into the Mediterranean Sea. The influence upon the land of the Nile and Mississippi rivers is not dissimilar.

The Mississippi, an aggrading river, built up this alluvial plain more than 500 miles long and 25 to 125 miles wide in the large structural trough between the Appalachian and the Ozark mountains. Because of modification by silt deposition, the average valley gradient is less than 8 inches per mile; land sometimes slopes downward away from the river to lower-elevation basins within the flood plain. These flood basins range in size from a few hundred acres to major regions embracing numerous subbasins, such as those drained by the Yazoo River in Mississippi, the St. Francis and Black rivers in Arkansas, and the Tensas and Atchafalaya rivers in Louisiana. Except between Memphis and Vicksburg, the Mississippi River channel currently lies near the eastern margin of the flood plain at the foot of the Bluff Hills, which rise immediately to the east of the Delta. Over the centuries, the River has meandered throughout the plain, producing a landscape of old terraces that represent stages of valley fill. Many low ridges, and cutoff meanders forming oxbow lakes, mark the location of old natural levees along the stream channel.9

Average annual precipitation ranges from about 45 inches in the north to 60 inches in the Delta's south. However, dry periods have little effect on the growth of forests in the Delta's poorly drained soils.

Alluvial silts, often originating in the mountains a thousand miles or more from their present locale, fill the streams and overflow the rivers of both the Atlantic and Gulf coasts. The Mississippi provides a classic example. While continuing to build, its delta got its start from glacial silt eroded from northern precincts and deposited during the Pleistocene Period of geologic history. These soils are especially important in the Great Mississippi Delta, extending from southern Illinois to the Gulf shore below New Orleans. The fine particles making up this sediment may have traveled

Figure 1.15 Post oaks of the so-called tension zone in Texas and Oklahoma. The relatively pure stands lie just west of the pine-hardwood forests. Rainfall too low to sustain pine seedlings through a second year precludes their establishment. As a rule of thumb, two consecutive years of adequate summer and early autumn rainfall assures a new stand of loblolly and/or shortleaf pines, provided a seed source is available and the site has been prepared by fire or disturbance.

Figure 1.15 Post oaks of the so-called tension zone in Texas and Oklahoma. The relatively pure stands lie just west of the pine-hardwood forests. Rainfall too low to sustain pine seedlings through a second year precludes their establishment. As a rule of thumb, two consecutive years of adequate summer and early autumn rainfall assures a new stand of loblolly and/or shortleaf pines, provided a seed source is available and the site has been prepared by fire or disturbance.

from what is now the glaciated Allegheny Mountains via the river of the same name in northwestern Pennsylvania to the Ohio River, moving with its flow to the Mississippi at Cairo, Illinois, and thence southward.

Other silts and clays have been transported to the Mississippi from as far away as the upper reaches of the Southern Appalachians, above 5000 feet, just to the west of the summit of the subcontinental divide in Tennessee. First through trickling springs, then infant streams, and then cascading down creeks to the Little Tennessee River, these suspended sediments move. Then they flow with the current to the Tennessee River, from there to race from corner to corner through northern Alabama, turning north at the Mississippi line and pushing through Tennessee and Kentucky. That water and its silts, along with the flow of the Cumberland River that drains the mid-section of the Appalachians from West Virginia and Tennessee, then funnel into the Ohio River. A hundred miles of river course and the Ohio joins the Mighty Mississippi.

Some sediments, too, have come from the east slope of the Continental Divide, high in the Rocky Mountains. They move, via the Arkansas and Missouri rivers and the tributaries that feed them, into the "Father of Waters" en route to the Gulf of Mexico.

Not all of these materials are consigned to salt the sea as the Mississippi empties its sediment loads into the Gulf. Historically, in times of flood and overflows, for 50 or more miles from the main channel, the water-carrying sediments become trapped when the River's high water recedes behind both natural and man-made levees. These soil grains reach the delta in suspension, as sugar is held in a supersaturated glass of tea, and not in solution as salt dissolves in a bowl of hot soup. While flood waters recede, moisture moves to the air by evaporation or percolates by gravity through the ground below, in both cases leaving behind particles of silt and clay.

Figure 1.16 Cottonwood trees thrive on batture lands of the Mississippi River Delta. Plowing during the first year following planting is necessary to control vegetative competition. Later, low-level plants provide habitat for small wildlife. (USDA Forest Service photo)

Little of the sands dislodged by the erosive forces that sent the silt and clay downstream travel as far as the delta. Each sand grain is relatively large and heavy. Because it is not in suspension, its movement is by water force. After the raindrops' impact that releases sand sediments either from a river's bank, high above and far away, or a river's nearby thrust, sand particles are pushed to the side of the channel to build new land. On this new land, cottonwood and willow trees promptly seed in to hold the soil in place. The suspended finer materials rush on below, either to precipitate in the river's overflow or to build a new strata of sediment along the coast beneath the sea.

The soils of this alluvial plain are well supplied with nutrients other than nitrogen. Texture varies with the velocity of the flood waters by which the soils were deposited. The coarser materials, those fine sands and sandy loams that have reached the delta, are found on natural levees adjacent to present or former river channels where flood waters flow most rapidly. Progressively, the coarser sediments tend to drop out as flood waters spread from the main channel, leaving little but clay to be deposited in the black backwater swamps. Differences in aeration due to these texture gradients and to variability in surface drainage have marked effects on soil productiveness for forest tree growth. Soil patterns may be extremely complicated in areas where remnants of natural levees from old meanders are unrelated to the present channel.

Artificial levees and other engineering protection works have greatly reduced flooding in much of the Mississippi River's delta. Few major overflows of the main river have occurred since 1927, the latest occurring in 1993. This protection and, with it, improved local drainage have lowered water tables and encouraged widespread land clearing for agriculture. With less area available for forestry in protected basins, there is increased silvical interest in the unprotected lands, called

Figure 1.17 A ragged, unmanaged cottonwood stand on Mississippi River batture land. A recent sawtimber harvest had removed 1/3 of the merchantable volume of 36,000 board feet per acre. (authors' collection)

battures, that lie between the levees and the river channel. Light-textured and fertile, these sites, like new land, often support excellent stands of cottonwood and willow.

Forest types of the Delta are related to the elevation of the land; slight differences cause significant changes in species composition. Flats, fronts, new land, ridges, swales, and swamps are often characterized by certain tree groups. About 70 species of commercial importance occur, usually in mixed stands. Along a river front are eastern cottonwood and willow, while farther back, pure stands of sweetgum, water oak, white oak, or ash may be found. Mixed stands on the ridges—slight rises of a few inches to a few feet above a surrounding flat—may be predominantly of white, red, and water oaks, hickories, sweetgum, baldcypress, blackgum, and water tupelo.

Fire, tornado, logging, agriculture, flooding, sedimentation, and erosion have each played a role in determining the present mixture of species and age classes, ranging from seedlings to mature trees. Many species intolerant of shade become established in the openings; others develop in the understories of older stands. Generally, seedbeds must be moist, but not flooded, and for many species like the willow and cottonwood that seed-in on new land recently built up alongside a river during a flood, the mineral soil must be exposed.

Bluff Hills—Formed from the wind-blown silts of the Mississippi Delta, the Bluff Hills lie east of the river in the states of Mississippi and Tennessee. The bluffs stand 125 to 250 feet above the river flood plain. While the wind-deposited loess thins rapidly eastward, brown loam soils indicate the presence of aeolian deposits as far as 100 miles east of the western edge of the bluffs.

Exposed loess rapidly erodes. Yet these sites are among the most fertile in the South. They were cleared before 1860 for agriculture, but by 1930 erosion was so severe that cultivation was hopeless. In 1906, the agriculturist E.W. Hilgard noted that both industrial communities and agrarian

Classic Gully Erosion

Figure 1.18 Gully erosion in loess (silty) soil. Congress appropriated money for the USDA Forest Service to plant trees on these privately owned, devastated and abandoned cotton fields and hay meadows in western Tennessee and northern Mississippi in the post-World War II years. Grasses had not halted these "washes;" hopefully trees would, as these sites produce quality timber. (Soil Conservation Service photo)

Figure 1.18 Gully erosion in loess (silty) soil. Congress appropriated money for the USDA Forest Service to plant trees on these privately owned, devastated and abandoned cotton fields and hay meadows in western Tennessee and northern Mississippi in the post-World War II years. Grasses had not halted these "washes;" hopefully trees would, as these sites produce quality timber. (Soil Conservation Service photo)

lands had washed down the hills.10 Annual losses of over 100 tons per acre from cultivated land were not unusual. Here, the old southern term "gullywasher" expresses a tragic truth as it describes intense summer thunderstorms common to the area.

Travelers crossing the Mississippi River from the west at Vicksburg have an excellent view of a classical loessal bluff. Here, 100 feet above the water, Confederate troops watched the adversary on the river, readily fired upon his battle boats, and for almost 3 months stayed the federal offensive. The river has changed its course since 1863, moving slightly west, thus hiding the proximity of the guns mounted on the Bluff Hills from the vessels in the channel below.

The Mississippi had changed its course often before those eventful days, creating new alluvium and leaving the old strata to dry out and seed-in to trees. Unknown are the frequencies during the Pleistocene Period of the powerful dust blows that moved the soft silt from the dry bottomlands lying to the west. Mounded in hills, horizon upon horizon, winds gently deposited nutrient-rich sediments.11 Sediments still blow into the Bluff Hills from plowed fields of eastern Arkansas and Louisiana. These deposits of yellowish-brown silt differ little from that of the buff-colored prairies of Iowa and the plains of Nebraska.

Bogs—We turn now to the peat and muck in the bog sites along both the Atlantic and Gulf coasts. Often these organic soils are met by sand dunes at one extreme, and by marshes at the other. The sands are usually coarse, largely coming from the mountain-forming granites and associated bedrock materials. The sands of quartz, the fourth-hardest natural mineral (after diamond, corundum, and topaz) were released in some past geologic millennia from granite rocks, also containing feldspar and mica. In contrast to the silicon dioxide quartz (like flint and sandstone), the feldspar and mica are relatively soft. These minerals are thus more readily decomposed and dissolved, their nutrient elements rather promptly released for plant growth. (The slowly soluble silicaceous mineral is not essential for plant growth.) The abundance of sand at the edge and in the bottom of the sea is related to the mineral's insolubility. It is released from the rocks of the land when the rocks are fractured by freezing and thawing and by water and acidic solution of other more-soluble components of the earth's mantle. Conversely, the sea is saline because of the solubility of the mineral-containing salts in the softer materials of the earth's rocky crust.

The marshes have an abundance of these salts, storing them for a season in the grasses and herbaceous plants that grow in savannas along the coasts from New Jersey to Texas. Like all of the sediments of the Coastal Plain except for the Bluffs, the saline soils of these wet sites are water-laid deposits. The rock beneath these soils, often at depths of hundreds of feet, has had little to do with the genesis of the soils in which trees and lesser plants grow. (In contrast, soils in areas above the Coastal Plain, as in the Piedmont and the several mountain ranges, have been formed in place. Such in situ development depends upon the weather, the action of plants and animals, and the nature of the parent material and time.)

Under poorly drained conditions at low elevations, groundwater podzols occur. These tend to show an organic-rich layer at the surface. Underlying that strata is a thick organic and iron-rich hardpan mottled with an array of yellow and brown colors that suggest hydration in times past. Bog sites are common from the Atlantic white-cedar swamps of New Jersey to the pocosins of pond pine in the Carolinas.

Flatwoods—The term flatwoods is applied to a number of extensive areas of low relief within the Coastal Plain, usually limited to pine and pine-hardwood sites. Such areas occur on the lower terraces along the Atlantic and Gulf coasts. They also are found in interior belts where clays or other fine sediments result in the development of flat topography.

The coastal flatwoods, mostly at elevations below 25 feet, are poorly drained but not permanently inundated. Although soils are usually sandy, root development is often limited by high water tables or hardpans that underlie the surface horizon. Loblolly pine and numerous hardwoods are supported by these sites throughout the region. Pond pine and Atlantic white-cedar are common along the Atlantic coast, longleaf pine may be present south of Virginia, and slash pine east of the Mississippi River. The Big Thicket area, inland from the southeast Texas coast, is generally similar to the coastal flatwoods, though site quality may be higher due to lower, more favorable permanent water tables.

Interior flatwoods soils are typically fine-textured, with moderate to poor internal drainage, though often permeable for tree roots to depths of six feet or more. Plant nutrients are usually adequate, enabling good growth of a variety of species, including loblolly pine. These belts also support shortleaf pine and many hardwoods, notably red and post oaks.

Extending from central Alabama into northeastern Mississippi and terminating near the Tennessee line is another kind of flatwoods, a 6- to 12-mile-wide soil and physiographic region. At an elevation of only 200 to 300 feet, a smooth surface has developed on an outcrop of cold, gray, stiff, poorly drained clay. When wet, the soil is sticky; when dry, hard and cracked. The cracks in dry weather and the crayfish holes in wet weather enable conduction of oxygen to lower horizons. Locally this soil is called "crawfish gumbo."

Post Oak Belt—The Post Oak Belt of Texas and Oklahoma, from 50 to 100 miles wide, forms a fire-maintained transition between the pine and oak-pine types to the east and the tall-grass prairies (now mostly farmed or ranched) to the west. Stands of trees, dominated by post and blackjack oaks, may include several other oaks and hickories. Except in the most favorable sites, tree growth is slow and form is poor. Stand density declines from east to west, the border with the prairie often being a dry savanna. Rainfall diminishes about 1 inch for each 25 miles, from east to west, averaging about 30 inches for the Belt. Bluestems and prairie bunch grasses along with a rich variety of flowering annuals and perennials appear as understory plants. Generally, trees occur on the more broken terrain where soils are sandy, while flatter areas with clay soils originally covered with grass are now farmed or in pasture. Relict loblolly pine stands near the western edge of the Post Oak Belt, the results of studies with drought-hardy pine seedlings, and measurements showing adequate growth on more than 200 loblolly pine plantations within the Belt suggest that conversion to the more-valuable conifer may be an economically feasible option. But such may further diminish this endangered ecosystem.

The "Lost Pines"—Found in three "islands" totaling about 40,000 acres in the vicinity of Bastrop, TX, are loblolly pines. They occur about 75 miles west of the western edge of the East Texas pineries. Between these two coniferous forests lies the finger of post oaks here referred to.

Some consider the Bastrop pines as the nail of another finger, this one pointing southwestward from the pine-hardwood forest at the Oklahoma border. All along the finger, pines once grew with the post oaks. One now sees an occasional remnant pine tree along the roads that traverse the area. Except in the Bastrop vicinity, the conifers were harvested and the lands farmed and grazed. When abandoned by agriculture, a pine seed source was lacking, so too was adequate moisture for stand establishment which, under natural conditions, occurs infrequently. If once in a century the weather and seedfall collaborated, a new forest would develop. What of the Lost Pines at the fingernail? Those trees too were cut to supply wood for pioneer "peckerwood" mills in the 1800s, but the poor, rock-strewn soil precluded conversion of the land to other uses. The finger's nail is now an island of naturally regenerated pines in a sea of post oaks.

Swamps—Swamps occupy large areas of the Coastal Plain. Although coastal saltwater marshes and some adjacent swamplands such as the Everglades of southern Florida are essentially treeless, most inland swamps are timbered. These include the extensive Big Cypress in Florida, the Okefenokee in Florida and Georgia, and the Dismal in North Carolina and Virginia. Baldcypress occurs in nearly all deep swamps. Other species, with varying tolerance for inundation, include pond pine, blackgum, water tupelo, Atlantic white-cedar, green ash, sweetbay, and magnolia. Specially recognized types of swamps include the sharply defined elliptical depressions known as bays and pocosins. Most common on upper terraces in the Carolinas, bays vary from slightly wet depressions to deep swamps or lakes. Pocosins, waterlogged areas in lower terraces, however, range up to several thousand acres in extent. In them, soils to six-foot depths are black muck or brown peat, usually supporting stands of pond pine or Atlantic white-cedar. Prior to the 1980s' concern for maintaining wetlands, many pond pine sites had been converted by elaborate drainage systems to faster-growing and economically more valuable loblolly pine.

Certain waterlogged soils, usually adjacent to swamps, may support grassland vegetation. Such sites, called savannas in the Southeast and prairies west of the Mississippi River, become pine sites if local surface drainage is provided.

Figure 1.19 Into the 1960s, wetlands were drained for food crop production and to convert swampy sites to valuable tree species. Small ditches fed excess water into these larger canals that carried the water through locks to the sea. The locks prevented saltwater intrusion.

Areas of slow, subterranean drainage contain peat formed from hydrophytes. Better-drained areas have a more finely divided peat or muck formed from woody-plant residues, while thick beds of acid sandy loam produce half-bogs of organic surface soil and mineral subsoil. The low pH, less than 4.5, limits nutrient availability. Sometimes these swamps smell very unpleasant, the pH being so low (less than 3) and the soil so sour.

Baldcypress is found in swamps where water is too deep for competitive species. It occurs in pure, dense, even-aged stands. Where single stems are found mixed with hardwood forests, a change in the physiography of the site during the life of the stand has probably occurred. Perhaps man or nature has diked or drained the land or plowed the soil.

Florida Peninsula—Southward from southern Georgia lies an extensive flat-topped plateau rising abruptly from the sea floor. In the northern part of the exposed section of the zone, the land is a central upland that reaches elevations of about 300 feet and contains broad flats, hills, swamps, and many lakes. Underneath the sandy surface soil lie soluble calcareous materials on which has developed a surface of modified karst, with its limestone sinks and caverns. Terraced marine lowlands, normally less than 100 feet in elevation and containing old and new beach ridges with intervening swales, occur on either side of the central upland. These marine terraces, broader on the Atlantic than on the Gulf side, account for most of the surface area of peninsular Florida.

South of the latitude of Tampa Bay, the peninsula is a broad plain with an average elevation of less than 20 feet. Here, the surface is an almost flat marl and limestone shelf, generally overlain with a thin layer of peat and muck and a little sand. Swamps cover perhaps 80 to 85% of the surface—or did before lands were drained. The low, swampy surface is relieved by a rather continuous strip of slightly higher sandy land parallel to the east coast and a highly fragmented counterpart along the west coast. Pines cover the higher parts of these low ridges. Pure stands of short-lived and serotinous sand pine cover extensive areas near the center of the peninsula, while slash pine and its South Florida variety otherwise compose the principal forest type. Extensive forests of the peninsula are called the Big Scrub.

The Florida peninsula continues southward as a broad submerged ridge or bank surmounted by non-forested islands, called keys, composed entirely of calcareous materials ranging from solidified coral to clay-size particles. Maximum elevation in the southern extremity is about 15 feet. The everglades are lower.

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