Chapter 7 - Forest Soils and Landscapes
Contents - Class Homepage
Sections:
Introduction - Forest Sites - Forest Soils

Back to topINTRODUCTION

Soils are critical to the survival of civilization. The earliest major civilizations, such as those along the Nile or Tigris-Euphrates Rivers, were founded in part due to their fertile soils. The wealth of the United States is largely based on our abundant natural resources, many of which are directly linked to the soil resource. The soil resource affects the quantity and quality of the water that we drink, the costs and availability of the lumber for our homes, the cotton and wool in our clothes, and the foods that we eat. Therefore, it is the duty of land stewards, such as foresters, to protect the soil resource so that it can benefit future generations.

Forest managers must be able to recognize and understand of forest site-soil types. The soil and site will control what species can survive, how fast they will grow, and how susceptible a site will be to erosion, landslides, and flooding. Forest sites influence sunlight, rainfall, temperature, wind speed, humidity, and growing season length. Forest soils affect the tree species on a site by controlling how much water is available to tree roots, whether or not tree roots become waterlogged, and if required nutrients are available. Soil and site conditions are not mere abstractions, rather they are fundamental properties that must be considered for applied, real world activities as important and diverse as:

Therefore, foresters need to have a wide appreciation and basic understanding of geology, hydrology, climatology, soil chemistry, and soil physics in order to the best stewards of the forest resource.

Forest soil scientists commonly use soil augers to extract soil so that soil classifications and interpretations can be conducted.

Forest soil scientists use a Munsell soil color book to classify the soil color. Soil color is useful for understanding soil aeration, drainage, and organic matter content. As a generalization for Virginia, black soils will have very high levels of organic matter, brown soils have some organic matter red soils are well aerated, and grey soils are poorly aerated.

Back to top FOREST SITES - Physiographic Regions
Physiographic Regions are areas having similar combinations of topography and soils. Virginia has five major physiographic regions; from east to west they are the Physiographic regions of Virginia. (http://www.wm.edu/geology/virginia/)

FOREST SITES - Watersheds
Watersheds are areas that have a similar hydrologic output, or more simply, a watershed is the area that provides water to a particular river, perennial stream, intermittent stream, or ephemeral stream.

A watershed on the Virginia Tech Fishburn Forest is outlined in blue (left). This watershed is small (106 acres), and drains into an perrenial stream. The intermittant stream is part of the Slate Branch watershed, which drains approximately 10 square miles. In turn, the Slate Branch Watershed is part of the New River (above) Watershed, which covers parts of western North Carolina, western Virginia, and West Virginia.

FOREST SITES - Watersheds
Watersheds of small ephemeral streams may only contain a few acres or watersheds of major river systems, such as the James River, can be vast and contain hundreds of thousands of acres.

FOREST SITES - Watersheds
Watersheds are an important concept for land managers, because the activities within the watershed can have a direct effect on the quality of downstream water.

FOREST SITES - Watersheds
The United States Forest Service was originally formed specifically for the protection of watershed and water quality because forests have historically provided humans with clean water. Currently watersheds are used as a basis for development of soil and water conservation districts and for a variety of management activities such as culvert size requirements and stream crossing permit acquisition.

Culvert size should be based on watershed size, topography, and land use.

FOREST SITES - Landscape
Landscape is a layperson's term that simply refers to appearance of the land features in a region. The term is quite nebulous and may refer to the appearance of a large region such as the Appalachian Mountains or a smaller area such as a stream bottom. Instead of the term landscape, land managers often use terms such as landforms, slope position, or micro-topography to describe the appearance and ecological parameters of an area.

FOREST SITES - Geomorphology
Geomorphology specifically refers to the study of landforms and geomorphic surfaces are area of the earth's surface that have a common age and were formed by a common set of processes. Specific geomorphic components commonly found in Virginia include mountains, hills, stream terraces, floodplains, and stream channels. Mountains and hills may be formed by a variety of uplifting, aggrading, or degrading processes. Uplifting activities, such as plate tectonics, earthquakes, or volcano activity build mountains while erosional activities due to water, wind, gravity, and ice shape them. For example, the mountains of the ridge and valley physiographic province were deposited as sediments, uplifted and faulted by continental drift, and subsequently eroded over millions of years.

FOREST SITES - Terraces
Terraces are generally considered to be upland positions, but terraces were originally formed by either riverine (river or stream), marine, or lacustrine (lake) processes of erosion and deposition. Active terraces are those that are still being flooded periodically while older or ancient terraces may not have been flooded by the original marine or river for literally millions of years. Perhaps the most complex example of terraces is found in the coastal plain. The entire coastal plain was formed on marine sediments that have subsequently been further acted on by stream actions that superimposed stream terraces on marine terraces.

Terraces in the Atlantic and Gulf Coastal Plains.
A.) C.W. Cooke (1930)
B.) Colquhoun et al. (1969)
Figure Reproduced from:
Buol, S. W. (ed.) 1973. Soils of the Southern States and Puerto Rico. USDA Southern Cooperative Series Bulletin 174.

FOREST SITES - Landforms, Micro-relief and Microtopography
Geomorphic positions have been subdivided into a variety of classifications by numerous researchers and land managers across the United States. In the eastern United States upland geomorphologic positions are commonly subdivided into subdivisions that indicate climatic differences (landforms) or slope and erosional (micro-relief) differences. Landforms such as spur ridges, nose slopes, coves, and backslopes are indicative of surface drainage patterns and microclimate. Spur ridges and nose slopes tend to drain fairly rapidly. Coves tend to collect water from the surrounding area and backslopes are intermediate with regard to drainage.

FOREST SITES - Landforms, Micro-relief and Microtopography
Micro-relief positions are indicative of erosional patterns, soil depth, and site productivity. In upland positions the relatively stable and level to convex area at the peak or top of the ridge is referred to as the summit. Adjacent to the summit is the shoulder, a distinctly convex area where the topography begins to curve downslope. The backslope is the area below where the convex shoulder becomes a uniform, or straight slope. Below the backslope, the area where the slope becomes concave is the footslope. Toeslopes, or less concave areas may occur below the footslope. If the area is flat as opposed to slightly concave, then the micro-relief is also referred to as a terrace.

FOREST SITES - Landforms, Micro-relief and Microtopography
Geomorphologic positions within floodplains are also classified with regard to microtopography which is indicative of soil texture and soil aeration/flooding. Common microtopographic positions within larger floodplains include sandbars, berms, ridge and swales, sloughs, and flats.

FOREST SITES - Landforms, Micro-relief and Microtopography
Streams having higher gradients (greater slope) tend to have straighter channels than do streams of low gradient. This is why streams in the mountains and piedmont tend to have greater water velocities and less stream sinuosity or meander than do streams in the coastal plain. Over time, stream channels meander or shift across floodplains and these shifts create microtopography. Near the main stream channel the outside of stream bends are often referred to as cut banks or erosional areas while coarser materials such as sand tend to be deposited on the inside of the stream bends, resulting in sandbars. Pioneer species such as river birch, cottonwood, sycamore, and black willow may form almost pure stands on these sandbars.

Cut bank near Roanoke Rapids, North Carolina where over 20 feet of soil was eroded by one major flood.

Sycamore stand on sandbar.

FOREST SITES - Landforms, Micro-relief and Microtopography
Streams that transport large volumes of sediment and that have overbank flooding often have microtopography known variously as berms, natural levees, fronts, or ridges. As the sediment laden floodwaters move from the channel into the floodplain the velocity of the water is reduced so that the coarser sediments (sands) fall out of the water column. These sandy and moderately well drained sites can have a wide range of species such as sycamore, sweetgum, water oak, cherrybark oak, loblolly pine, and river birch.

This lush forest is located on a berm position of the Tensaw River in Southeastern Alabama.

FOREST SITES - Landforms, Micro-relief and Microtopography
In some floodplains, the area behind the berm is an area of turbulent water where a series of small ridges and swales (depressions) form. These ridges and swales allow species such as baldcypress and sweetgum to regenerate within the same general area. As the stream meanders back and forth across the floodplain it abandons some channels and creates new ones. The abandoned channels are called ox-bow or cut-off lakes if they are large and are called sloughs if they are small. These areas may contain standing water for much of the year and, since they are in less turbulent waters, they tend to trap finer silt and clay materials. These oxbows or sloughs often have species that are very tolerant of flooding such as water tupelo or baldcypress.

Water tupelo invading a slough on the Blackwater River near Franklin, Virginia.

Back to top FOREST SOILS - Introduction
Soils are defined as the naturally occurring bodies that are capable of supporting plant growth. Forest soils are those that are currently supporting forest vegetation. Prior to European settlement, the majority of Virginia was forested but many of these forests were converted to agricultural or urban land uses.

FOREST SOILS - Introduction
Forest soils are distinctly different from agricultural or urban soils for several reasons:

FOREST SOILS - Formative Elements
Jenny (1941) recognized the importance and interactions of five major soil formative elements and proposed that soils are the result of climatic factors and living organisms acting on geologic (parent) material over periods of time as affected by relief (topography). The chemical and physical nature of the parent material will affect the soil that is eventually formed. For example, some materials such as hornblende result in darker soil colors while others, such as quartz are light colored. Granite is composed of coarser materials while basalt has finer materials. Shale is an extremely acidic parent material, while dolerite is basic.

Coastal plain parent materials may be unconsolidated muds or sands or may be consolidated materials such as this sedimentary rock These sediments tend to be softer than metamorphic or igneous rocks.

Resistant sandstone boulders on a ridge in southwest Virginia.

FOREST SOILS - Formative Elements
Geological classification of the parent materials, which is based on the placement, also affects soil properties. Soils formed from parent materials that were not transported from the original site are called residual soils.

This rocky knob in the Ridge and Valley shows sandstone parent material and residual soils.

FOREST SOILS - Formative Elements
Soils that were formed from transported parent materials include colluvial soils (transported by gravity), alluvial soils (transported by rivers or streams), marine soils (transported by marine waters), lacustrine (transported by lake waters), glacial soils (transported by ice), and eolian soils (transported by wind). These geologic classifications are not restricted to any particular physiographic provinces and mosaics of geologic classifications may occur across one mountain.

FOREST SOILS - Formative Elements
Climatic factors act upon parent material by a variety of mechanisms, including freezing and thawing which can crack rocks, precipitation which can erode materials or leach chemicals, and wind which can erode rocks over long periods of time. Living organisms act to create and modify soils in a variety of ways. Lichens slowly dissolve parent materials by acid secretion and tree roots penetrate cracks in soils. Burrowing organisms such as worms create channels within the soil. Plants deposit materials, such as leaves and stems, to the soil surface and dead roots and root exudates within the soil and this organic matter affects the chemical and physical nature of the soil. In some areas, where decomposition processes are slower than plant productivity, plant materials may actually form the entire soil. Peats are well known examples of an organic soil.

These lichens slowly break down parent material.

FOREST SOILS - Formative Elements
A typical site following Hurricane Hugo near Monck's Corner, South Carolina. This type of climatic disturbance periodically deposits huge quantities of litter from broken stems, twigs, and leaves

Climatic factors, such as periodic droughts, control the frequency and severity of wildfires. This wildfire in Yellowstone National Park in Wyoming removed large quantities of litter and killed many stands.

Ice storms can also affect soil formation by adding organic debris and creating mounds caused by trees tipping over and uprooting the soil. These mounds are known as wind-throw mounds, tip over mounds, root wad mounds, and root mounds.

Climatic factors such as rainfall have pronounced effects on forest soils and vegetation. This rainforest, located on the Olympic Peninsula of Washington receives nearly 200 inches of rainfall per year. Litter inputs are tremendous, decomposition rates are fast, and soil weathering and leaching due to rainfall is accelerated.

Peat soil In areas where litter accumulation rates exceed litter decomposition rates, organic soils may be formed. These organic soils or peats are usually formed in very cool and/or wet areas and again emphasize the effects of climate on soil formation.

Microclimate measurement station used to measure wind speed, wind direction, solar radiation, relative humidity, temperature, and soil moisture.

FOREST SOILS - Formative Elements
Topography or relief can affect soil formation in numerous ways. Topography, or relief, influences the stability of the parent material or soil, the volume and rate or water infiltration into the soil profile or surface runoff, the microclimate on different aspects, and the form and quantity of precipitation.

FOREST SOILS - Formative Elements
The time during which soil formative processes have acted affect the relative "age" and characteristics of a particular soil. Some residual soils may be millions of years old, and if they are in a stable, warm, and humid area they may be very deep and highly leached of nutrients. Some alluvial soils may be as young as the last flood event and they may be very fertile and lacking in distinctive features.

This alluvial soil is considered to be "young" because it does not have strong indications of weathering. The total age of the sediments above the water table is actually less than 80 years.

This residual soil is considered to be an "old" soil because it has been weathered as indicated by its horizons.

FOREST SOILS - Formative Elements
The Soil Horizon The soil profile is a vertical exposure of a particular soil that allows classification of distinct horizontal layers referred to as soil horizons. Five master horizons (major horizons) are recognized and symbolized as O, A, E, B, and C.

FOREST SOILS - Formative Elements
The O horizon (or litter layer) is the organic horizon on the soil surface. The organic horizon, also known as the litter layer, can be subdivided into three subhorizons. The Oi subhorizon (also known as the Litter or L layer) is the layer comprised of only slightly decomposed organic material, where leaf shape is still obvious. The Oe subhorizon (also known as the fragmented or F layer) layer is made of moderately decomposed plant materials where portions of the original organic material are still recognizable as plants, but the leaf shape is obliterated. The Oa subhorizon (also known as the humus or H layer) is the layer comprised of highly decomposed organic material where the original form of the organic material is lost. The O horizon can be almost non-existent on agricultural soils to several inches deep in forests of Virginia.

This Histosol (organic soil) located near Washington North Carolina has a very deep O horizon that basically extends from the top of the soil tray to just below the lower hand. The very thick O horizon for this soil was due to the very high water table combined with vegetative production of litter.

FOREST SOILS - Formative Elements
The A horizon is the uppermost mineral horizon that contains a significant amount of organic matter. The organic matter causes this horizon to be somewhat darker colored than deeper soil horizons. The A- horizon is roughly analogous to the term topsoil.

FOREST SOILS - Formative Elements
The E horizon is the mineral horizon where the process of eluviation has caused materials such as clay, iron, or aluminum oxides to migrate downward). The E horizon is composed of the more erosion resistant materials such as sand.

This spodosol from near Zuni Virginia has very distinctive soil horizons. The uppermost horizon is an A, which is a mineral horizon with an accumulation of organic matter. The organic matter causes it to be darker in color. Underneath the A is a very light colored E that has less color because it has been leached or eluviated. Underneath the E horizon is a B horizon that has accumulated the material that was leached from the E.

FOREST SOILS - Formative Elements
The B horizon is the mineral horizon where the process of illuviation (the zone of accumulation or the inverse of eluviation) has occurred. With illuviation materials such as clay, iron, and aluminum oxides collect or have moved into the horizon. (Hint: I remember the difference between Illuviation and Eluviation as I = into and E = exit).

This soil profile was is located in the piedmont near Danville, Virginia. It was eroded by agriculture and has an almost indiscernible A horizon. Most of the soil profile is a B horizon, an illuvial horizon that has accumulated clays that were eluviated from above.

FOREST SOILS - Formative Elements
The C horizon is the mineral layer where the soil properties are still dominated by the parent material and evidence of biological activity is minimal. The C horizon is considered to be outside of the soil solum, the areas of major biological activities that include the Oa through B horizons.

Marine deposited sand dunes on the eastern shore of Virginia are primarily C horizons because the parent materials have been little affected by biological activity.

FOREST SOILS - Formative Elements
The regolith (R layer) is the bedrock material that has not been weathered to any great extent. The regolith of various soils are commonly exposed by road construction.

FOREST SOILS - Formative Elements
The Soil Profile

A vertical cross section of the various soil horizons as you might see in a road through cut or in a stream bank is referred to as the soil profile. The characteristics of the soil horizons and soil profile form the basic unit for classifying soils to the level at which plant growth interpretations can be made. The arrangement, depth, physical and chemical properties of the soil horizons are used as criteria that allow soils to be classified for the purposes of taxonomy and intended uses. Soils have been classified by several different taxonomic methods, but the soil taxonomy used in the United States classifies soils into soil 12 Orders, 47 Suborders, 250 Great Groups, 1250 Subgroups, 6600 Families, and approximately 17,000 Series. For this discussion, only the 12 soil orders will be discussed.

FOREST SOILS - Formative Elements
Soil Orders

Entisols are generally considered to be newly formed or young soils because they have little profile development. A typical entisol profile might simply consist of an O horizon, an A horizon, and then C horizons. They are often light colored and have low organic matter content. A typical example of the type of site and forest for an entisol are the bottomland hardwoods of river bottoms.

A Tupelo Cypress stand in the Four Holes Swamp is representative of vegetation on Entisols.

Vertisols are soils that are dominated by clays that shrink when dry and swell when wet. The expansion and cracking of these clays can literally buckle interstate highways and crack cement and brick structures. The forces are also sufficient to break the fine roots of trees, causing some vertisols to be less productive for forestlands. Vertisols occur in the coastal plain and piedmont of Virginia where shrink-swell clays were deposited). Due to the self-churning nature of these soils, horizon differentiation tends to be minimized as the soils mix themselves. These soils are also noted for their extreme stickiness when wet, if you walk across a wet plowed vertisol you may carry off ten pounds of mud on your feet.

Vertisol soils crack when dried.

The cracks in this building are caused by the shrinking and swelling of Vertisols as they become wet and dry.

Inceptisols are also considered to young soils of minimal weathering, but they are slightly more weathered than entisols. A typical inceptisol might be found on a river terrace site of on a steep sideslope in the mountains that naturally erodes at about the same rate as it form. A fairly common sequence of horizons for an inceptisol might be an O horizon, followed by an A horizon, then a B horizon, and finally a C horizon.

Aridisols are soils formed in dry areas such as deserts. Aridisols are often characterized by a "desert pavement" or layer of stones on the soil surface caused by the wind erosion of finer particles and accumulation of stones. Aridisols may actually be productive if irrigated as evidenced by the success of the pioneer Mormon farms near Salt Lake City, Utah. Aridisols are not found in Virginia, but are common to the southwestern United States.

Pinion Juniper on an Aridisol in Kings Canyon National Park in California.

Mollisols are the deep, brown soils that were commonly formed under grasslands or grasslike vegetation such as reeds or cattails. The dense and easily decomposed grass roots have allowed deep, dark colored A-horizons to form. These grassland or prairie soils can be very productive for agriculture, given sufficient moisture, and the "bread-basket" soils of the American Midwest are often mollisols. Mollisols tend to form in areas where tree growth is limited by insufficient rainfall and/or common wildfire, where grasses will dominate. The famous wheat fields of Kansas and Nebraska are typical of prairie mollisols, while the bur oak stands of Missouri are typical of the savanna woodlands.

Osage orange woodland located on Mollisols in the Black Prairie Region of Alabama.

Spodosols are soils having a B horizon that has accumulated unusually high levels of iron or aluminum oxides or organic matter. Spodosols seem to form in situations that have a combination of periodically high and fluctuating water tables, coarse sediments, and vegetation having acid producing litter. Spodosols are common to the pine or pond cypress flatwoods areas of Virginia. The most striking features of spodosols are their very light colored E horizon underlain by a dark B, which combine to form a spodic horizon. The B horizon often has accumulated organic materials and actually appears to have been the result of a fire. Although these accumulations are the result of translocations, this burned appearance is why these soils are sometimes referred to as "wood ash soils" in older literature.

Typical flatwoods coniferous forest where spodosols are developed.

Alfisols are soils weathered soils having prominent horizons and medium to high base saturation. A typical alfisol might have an O horizon, a deep A horizon, an A/B transition horizon, two B horizons, and a C horizon. These very developed horizons are a trait of the alfisols and ultisols, but the alfisols have a higher base saturation. Lab analyses is often necessary in order to differentiate the two orders. The oak hickory forest of the Blue Ridge or Ridge and Valley Provinces are typical of the alfisols because hardwoods are often more nutrient demanding than conifers and benefit from the higher base saturation.

Mixed hardwood forests of the Blue Ridge and Ridge and Valley are common on alfisols because hardwoods are generally more nutrient demanding than conifers.

Ultisols are highly weathered or "old" soils that are similar to alfisols except they have low base saturation. Therefore, coniferous vegetation that does not require the same level of bases, tends to be more common on ultisols. The highly weathered and leached profiles of the Piedmont region and residual soils in the Coastal plain are typical of ultisols. Ultisols are common throughout much of the southeastern United States because the warm, humid, high-rainfall environment favors weathering and creation of horizons and leaching of bases.

Oxisols are highly weathered soils or "very old" soils with high aluminum and iron contents that are formed on non-sticky type clays. These non-sticky clays do not retain nutrients well and added nutrients are rapidly leached from such soils. Oxisols are most common to the tropics and are not found in Virginia. Slash and burn agricultural practices have occurred on many oxisols in tropical areas because the burning of the native vegetation provides enough available nutrients for short time farming operations to be conducted before the nutrients are depleted.

Histosols are the organic soils that are formed in areas where plant productivity is high and decomposition rates are low. Histosols are generally very dark due to the organic matter and may actually feel greasy or oily to the touch. These conditions often occur in wetter environments such as swamps or bogs. The pocosin areas of Virginia have slow internal drainage that has allowed histosols to form. Pond pine and baldcypress are two typical species found on histosols.

Histosols may form due to open water succession to bogs as in this Mountain Bog in the Canaan Valley of West Virginia.

Pocosins are bogs formed in the coastal plain from Virginia to Georgia due to poor soil drainage combined with litter inputs.

This organic soil actually burned during a wildfire. Notice the former root line.Fires in organic soils or peat fires are very difficult to control because the fire may actually be subterranean.

Andisols are soils that formed in volcanic materials, such as volcanic ash. Andisols are of recent origin and typically have minimal horizons, somewhat like inceptisols. Andisols are not found in Virginia, but form productive forests in the Pacific Northwest and in Hawaii. The soils forming from the eruption of Mount St. Helens are a good example of andisols. Many of the sugar cane fields of Hawaii are also located on andisols.

This magnificent stand of redwoods in Redwood National Park in California are growing on volcanic soils or andisols.

Gelisols are soils of cold areas that are dominated by a permanently frozen soil layer called permafrost. Gelisols are not found in Virginia, but are common in Alaska, Canada, Russia, and Scandinavia. Dwarf willows are an example of Gelisol forest vegetation.

Gelisols are formed in permanently cool areas of permafrost as in the Glacial area of the Grand Tetons during early August.

One possible mnemonic device for remembering the 12 orders is "Every Vertical Infant Ate More Spam And Ugly Old Ham And Grits".