Soils and Vegetation in North Carolina

 

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     In this section we take a regional approach and describe the major features of the biosphere - the soils and vegetation combined - in each region.  A short introduction covers the main points in the processes acting to develop and change the biosphere. 

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Page Index

 

Soils and Vegetation together

 

Mountains

 

Piedmont

 

Coastal Plain

 

Sandhills

 

Tidewater

 

The Coast

 

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All diagrams are taken from:

 

Daniels, R.B., S.W. Buol, H.J. Kleiss and C.A. Ditzler, 1999: Soil Systems in North Carolina, North Carolina State University, Soil Science Department, Raleigh, NC.

 

 

SOILS AND VEGETATION TOGETHER

     The techniques used to study and describe soils and vegetation are very different - there are two separate sciences, pedology and ecology, to deal with them.  Nevertheless, vegetation grows in soil and soil develops partly as a result of the vegetation on it.  Consequently they are very closely allied, and we talk about them together here, first with a brief overview of how they develop, then a regional view of soils and vegetation.. 

 

 

     It is convenient to think of soil and  vegetation development starting on a solid rock surface.  On the longest timescale, this may be the case when land is newly uplifted out of the ocean as a result of earth movement.  On a smaller scale, a rockslide may expose equally solid rock. 

     Weathering breaks down the solid rock and often chemically alters it.  Micro-organisms move, or are blown, onto the rock and begin to colonize.  During their life-cycle they produce acids which assist in rock breakdown, while their remains, after death, provide a new, wider range of nutrients for other colonizers.

     Over time the soil will get deeper.  This in turn means that bigger plants can thrive, since the soil can accommodate the extensive root system which the plant needs. The roots must collect sufficient water and nutrients from the soil to support the plants life, and must also provide the stability needed to stay upright.  At the same time, the roots grow into the soil, helping to deepen in and breaking down new rock.

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A bare rock surface exposed by road work along the Blue Ridge Parkway.  Soil is developing and vegetation is colonizing the region.

 

 

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     The type of soil that develops depends on the type of underlying rock, the kind of vegetation available, and the climate.  Taking our state as a whole, the climate plays a key role in determining soil type.

     Our abundant rainfall tends to soak into the soil and the broken, weathered rock below it.  It reacts chemically with the soil, dissolving some of the nutrients and carrying them away through underground flow, eventually putting them into the flowing rivers. Not all nutrients are lost, of course, since roots intercept the downward moving water, while evaporation brings the dissolved material back to the surface layers of the soil.  But on balance we have much more rain than evaporation, so there is a net downward movement.

     The rain also physically pushes downwards - in infinitesimally small amounts each time, but an incredibly large number of times - the clay particles.  These are the smallest particles in the soil, the only ones the water percolating through the soil has sufficient energy to move.

     The result is that the soil develops a series of layers, called horizons.  Taken together, these horizons constitute the soil profile.  We commonly have - as all farmers and every gardener knows - a rather thin layer of rather easily worked soil on top of a much more solid clay horizon.

Soil profile in central Orange County - the light layer is exceptionally rich in clay.

 

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     Our rocks tend to contain considerable amounts of  iron oxides.  With our wet, warm climate these easily 'rust', and give the red color typical of our upper soil horizons.  Not all soils, of course, have this.  Some area of rock are deficient in iron, and may give a much lighter color, often appearing brown, or even almost white.          Casw_201.jpg (84402 bytes)
A much lighter soil in Orange County - about  5 miles from that shown above, and about the same from the Caswell County soil shown at right. A typical red Piedmont soil in Caswell County

 

     

     One consequence of the clay horizon in many of our soils is that water percolates only slowly through the clay layer.  That can provide great problems for septic systems of sewage disposal as well as for drainage in both rural and urban areas.  The clay, however, tends to be relatively fertile, so that plowing to bring it towards the surface not only enhances drainage but increases fertility.

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Agricultural soils on the Coastal Plain are often poorly drained.  Tile drains in the fields are needed if they are not to remain waterlogged to much of the year.

 

     In the same way the soil develops, so does vegetation.  Again, our climate, rocks and soils support - as a statewide generalization - vegetation of trees.  Humans - from the native Americans onwards - have used, and therefore modified, the vegetation. Still trees predominate, although most have developed during the last century or so.

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Two Carolina Bays - naturally occurring elliptical depressions on the surface of the Coastal Plain - at different stages of development.  Jones Lake in Bladen County, on the left, still has water at the center, held in as a lake by the raised sand rim. The Great Desert in Robeson County has a similar sand rim, but the original lake has been filled in by sediment. A soil has formed and vegetation has been established.

 

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MOUNTAIN SOILS AND VEGETATION

 

Soils-NC mountains map.jpg (25708 bytes) Topography and altitude plays a major role in the soil and vegetation development in the Mountains. Soils-NC mountains topography.jpg (18988 bytes)
Soils-NC Mountains topography exposure & bedrock.jpg (32285 bytes) Polk_604.jpg (46737 bytes) The valleys are often areas of easily eroded weak rock, with the results of mass wasting filling much of the flat valley floor.  As a result they are frequently now used for agriculture - often intensive agriculture - while the slopes remain forested.

The intensive agriculture demands that the plants have water at the exact times - and in the exact quantities - that they need it.  Although water in the soil is commonly adequate in this valley-bottom area, and the soil is thick enough to provide an adequate supply the irrigation ensures a high-quality crop. 

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The road on the valley edge picks out the flood plain - a clear indication that this area has floods with some frequency.

 

Soils-NC Mountain topography veg & soil.jpg (45544 bytes) This diagram summarizes much about the relationship between the forces creating the mountain environment (even if the quality needs much improving)

 

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One of the major unexplained features of  North Carolina's natural vegetation is the presence of 'balds' on mountain tops.  The one at right has dense rhododendron vegetation - others have mainly grasses.

Trees are more normal for mountain vegetation, in this case poplars which are 'relics' from much colder climatic conditions associated with the last ice age.  They only survive in sheltered, shady spots, which are often frost hollows (and were sufficiently isolated to avoid removal by forestry in the 19th century

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Jack_200.jpg (32479 bytes) The pattern of agriculture in the valleys, forestry on the higher land (with exceptions such as that at the right!), continues throughout the mountains, with various tree species present.  Soil tends to be rather thin, and gray-black in color. Swai_801.jpg (45450 bytes)

 

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THE PIEDMONT

 

Soils-NC Piedmont map.jpg (19610 bytes) Although the soils of much of the Piedmont have been derived from similar parent material to those of the mountains, they have been exposed to higher temperatures and more severe chemical weathering. Soils-NC Piedmont Georgeville Herndon landscape.jpg (33071 bytes)

 

 

 

Almost all the Piedmont soils are Ultisols, with light upper layers and a reddish sub-soil

    The most common local soil (and the "State Soil") is the Cecil soil, seen here in profile, uprooted, and with a distribution map

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The red soil shows slightly through this grass field.  Many ultisols can be very productive although they need frequent attention to ensure that the clays do not become dominant.

 

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Not all Piedmont soil is red. This forest is on a clay soil which developed in the Triassic basin near Durham.  This grayish, very compact clay soil is extremely difficult to work, and has been little used for agriculture

 

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The Upper Piedmont has rolling terrains, and the distribution of soil types is frequently akin to that in the mountains.  The soils, however, are warmer and, generally, redder.  The vegetation also characteristically shows the 3 layers typical of the southern forests.

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THE SANDHILLS

 

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A long-leaf pine woodland near Southern Pines

The Sandhills sit astride the Piedmont-Coastal Plain border, with a landscape dominated by the sandy soils derived from sediments produced by an ancient river delta. Hoke_204.jpg (34055 bytes)

A real 'sandy' soil - this one in the middle of the Fort Bragg Military Reservation - with a pine vegetation dominating.

 

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THE COASTAL PLAIN

 

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The map indicates that the Coastal Plain extends from the Piedmont edge right to the coast.  There are a series of terraces - steps down - created by old shorelines, which give changing landscape patterns as we move coastward.  So we have divided this section into 2:
 - Coastal Plain

 - Tidewater

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The Coastal Plain itself is commonly regarded as an exceptionally rich agriculture land.  The soils can vary tremendously, particularly in texture, which depends on exactly how the parent material was deposited when this region was ocean at the edge of the continent.

Nash_600.jpg (43148 bytes) The illustration at right shows the gently rolling Coastal Plain not too far east of Raleigh, with a rather sandy soil.  At left, somewhat farther east, is a flatter region and a lighter soil (the road through the cotton is too light, but gives a good indication of the light gray topsoil).  Nort_006.jpg (29960 bytes)

 

Nort_002.jpg (24339 bytes) Throughout the coastal plain, but particularly as the lower, poorer drained tidewater is reached, there is a distinct land-use difference which reflects soil difference. Often agriculture on the well-drained level 'uplands', with the swampier river channels left in forest Soils-NC Coastal Plain middle landscape and soils.jpg (20190 bytes)

 

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The rivers of the southern portion of the Coastal Plain - from the Neuse southwards - have carved asymmetric valleys in response to the general dip of the rocks towards the south.  The southern walls are often well marked and several feet - or even tens of feet high, while the northern ones may be only a few feet in height, and often very gradual. 

 

A view from the top of one of the southern 'valley walls'  In this case the feature (the 'Cliffs of the Neuse') is very well developed and has been designated a State Park  

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A 'reverse' view - looking from the river towards the valley wall.  Although this is not far from the illustration at left - being about 5 miles upstream- the cliffs are by no means as spectacular.

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Another feature - combining soil, vegetation and landform - which is unique to the coastal plain of North and South Carolina (with a slight extension into Georgia), is the Carolina Bay.  The origin of these oval shaped depressions is obscure. None of the obvious explanations - meteor shower, glaciation, out gassing from the earth's crust - fits.  They remain a mystery.  The key feature is the bay trees which fill the interior once the lakes have been infilled as a result of ecological succession. 

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TIDEWATER

 

Much of tidewater is very close to sea level and rather poorly drained.  The land is generally flat. The various old cliff-line 'scarps' are the major physiographic features, and they influence the soils and vegetation.  Often, below the scarp, is a pocosin - often a swampy feature, sometimes one with marshy grassland.  The soil, if drained, is fertile.  Much of the area is now used for extensive agriculture.

 

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On the horizon is the Surry scarp, seen here west of Elizabeth City.  The feature is less than 10 feet high, but is made more impressive (and visible) in this area because trees have been left on the slope.  All the flat land in front has been cleared for cultivation.
Soils-NC Coastal Plain pocosin cross section.jpg (26599 bytes) Prior to drainage, much of the area was in pocosin - 'swamp on a hill' - vegetation.  The swamp interior was commonly  composed of organic soils, which gave rather little strucutral support for large plants.  The result was rather low-growing scrubland (as seen to the right, where the force of the gale suggests some of the stresses likely on tall vegetation in this area).  The edge of the region may have had larger trees on the more 'normal' soils with a high inorganic matter content. 

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Perq_200.jpg (73123 bytes) Where land is not drained the true swamp survives - as suggested by the conditions at left.  Where drainage has taken place, the rich agricultural land has tended to compact.  Thus the field at right is below the level of the road and the (managed) forest seen in the left of the picture. Wash_600.jpg (16562 bytes)

 

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THE COAST

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Finally - the coast itself.

In some places the swamp seems to grade imperceptibly from freshwater to salt water.

 
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Dare_202.jpg (55024 bytes) In other areas there is clearly a dune system, which itself supports a highly distinct vegetation, adapted to salty and alternate wet and dry conditions. Dare_204.jpg (54921 bytes)

 

 

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