Knowledge of ecosystem processes and functions is an essential component of ecosystem management. This knowledge improves the ability of managers to balance the array of demands on natural systems while maintaining ecosystem productivity and integrity.  Several important ecosystem processes circulate, transform, and accumulate energy and matter. These processes include biogeochemical cycling, primary productivity, respiration, food-web interactions, and succession. At the heart of these ecological processes are individual species that serve to purify water, build soils, and recycle nutrients.  Natural disturbances continuously disrupt these ecosystem processes and maintain most systems in a constant state of biotic and environmental change.  This section reviews research knowledge on common disturbance types in the southern Appalachians, successional processes, biogeochemical cycling, aquatic ecology, and species ecology of selected species.

The following sections discuss the occurrence and effects of common disturbance types in the southern Appalachians and processes of succession that follow after these disturbances.

Noble and Slatyer (1980) suggested that species composition of stands that develop after disturbance could be predicted from three vital attributes of species: (Loftis 1993a)

1. The method of arrival or persistence of a species at a site during or after a disturbance.
2. The ability to grow to maturity in the developing stand.
3. The time needed for an individual of a species to reach critical life stages. (Loftis 1993a)

Several authors have addressed the first attribute by examining the primary reproduction source that is characteristic for various species (Beck 1988, Kelty 1988, Johnson 1989). Oaks, for example, are considered to be highly dependent on advance growth; i.e., they persist through a disturbance as advance reproduction and stump sprouts. Yellow-poplar is an example of a species that usually becomes established after disturbance as new seedlings from seed stored in the forest floor (Loftis 1993a).

The second attribute, the ability to grow to maturity in the developing stand, has been, and is being, addressed more fully for oaks than for any group of hardwood species. In general, the fairly strong relationship between preharvest size of advance growth and its post-harvest development are being used to predict the amount of oak expected in the next stand (Loftis 1993a).

Fire is one of the most importance agents of disturbance in the southern Appalachians. Unlike most other disturbance types, fire regimes have changed dramatically during recent history. Although fire has been largely excluded from most forests in the southern Appalachians in recent times, it was once a far-more pervasive influence on southern Appalachian forests. In fact, several forest types in the Appalachians appear to require fire to maintain their natural community structures. The following sections review the role of fire as a disturbance agent in the southern Appalachians.

• Types of Natural Fire defines crown, surface, and ground fires.
• Historical and Present-day Fire Regimes summarizes changes in natural and anthropogenic fire regimes in the southern Appalachians from prehistory to present day.
• Effects of Fire discusses the effects of natural and anthropogenic fires on abiotic and biotic ecosystem properties.

Surface fires

Surface fires burn the upper litter layer and small branches that lie on or near the ground. Surface fires usually move rapidly through an area, and do not consume all the organic layer. Moisture in the organic horizons often prevents ignition of the humus layer, and protects the soil and soil-inhabiting organisms from the heat. The heat pulse generated at the burning front of these fast-moving fires does not normally persist long enough to damage tissues underneath the thick bark of large trees. However, it will girdle the root collar of small trees and shrubs, and reduce small-diameter branches and other fine surface fuels (Nyland 1996).

Ground fires

Ground fires normally smolder or creep slowly through the litter and humus layers, consuming all or most of the organic cover, and exposing mineral soil or underlying rock (Davis et al. 1959; Kimmins 1987). These fires usually only occur during periods of protracted drought when the entire soil organic layer may dry sufficiently, but they may burn for weeks or months until precipitation and low temperatures extinguish the fire, or they run out of fuel.

Some of the effects of ground fires are deleterious. For example, they generally kill large and small trees because of the long and high temperature heat pulse generated. They release considerable amounts of nutrients from the burned fuels, destroy many small organisms and fungi that live in the humus and organic layers, consume seed stored in the litter, and kill roots in all but deep soil layers. They increase the chance of surface flow and erosion on slopes, and leave a baked and hardened seedbed that may prevent rapid revegetation. Increased surface runoff across the exposed surface may carry away ash and dissolved nutrients, making conditions even less favorable for plant growth. Foresters normally consider ground fires unsuited for site preparation, and restrict prescribed burning to periods when the humus remains too moist for complete combustion (Nyland 1996).

Crown fires

Crown fires occur when surface or ground fires ignite slash piles and dead or living lower branches of standing trees, tree crowns becomes engulfed in flames, and the fire spreads to nearby trees. Crown fires occur in forests during periods of drought and low relative humidity, particularly in areas with heavy accumulations of understory material called ladderfuels (e.g., fallen trees, logging slash, and combustible understory vegetation). Crown fires generate tremendous heat that rises in a strong convection column, drawing in brisk surface winds that fan the flames even more. Heated air blowing across the flames also warms and dries the fuels ahead of the fire, and releases volatile gases from vegetation ahead of the flaming front. Crown fires have environmental effects similar to those of ground fires, killing all trees and shrubs in their path and usually consuming the surface organic layers. Similar to ground fires, crown fires only stop by running out of fuel, or if precipitation cools the fire and wets the fuels sufficiently to inhibit burning. Overall, crown fires have little value for site preparation within forested ecosystems, and have not become common part of silviculture (Nyland 1996).

Historical fire regimes have played an important role in shaping the species rich landscapes of the southern Appalachians. Although the montane forests of the southern Appalachians experienced fires less frequently than pine-grasslands in the Piedmont and coastal plains, fires of both natural (lightning) and cultural origins were common before European contact in 1492. The first two sections describe natural and anthropogenic fire regimes in the southern Appalachians before European settlement.

• Frequency of Lightning-Caused Fires
• Anthropogenic Fire Regimes Before European Settlement

Fire regimes following European contact are characterized by distinct eras. Forest fires were frequent during the period of European settlement until an era of fire suppression was initiated in the early 1900s. Fire is now slowly being reintroduced to the southern Appalachian landscape through the use of prescribed fire. Present-day fire regimes also include arson- or lightning-caused wildfires, still relatively common occurrences in this region.

• Fire Regimes Following European Settlement
• Era Fire Suppression
• Re-Introduction of Prescribed Fire
• Present-Day Wildfires
  
  

Ecological and meteorological evidence suggests that lightning-caused fires were a major environmental force shaping the vegetation of the Southeast for millions of years before Indians arrived (Komarek 1965, 1974). Although forests of the southern Appalachians probably did not burn as frequently as the pine-grasslands of the adjacent Piedmont and coastal plains, there is evidence that they did burn periodically. Several authors assert that the frequency of lightening caused fires was sufficient to maintain oak-pine forests in the southern Appalachians, particularly forests composed of pine species dependent upon fire for most of their reproduction (Whittaker 1956, Zoebel 1969, Komarek 1974, Barden and Woods 1976, White and Lloyd 1997). Other authors point out that the mesic, broad-leaved forests of the southern Appalachians are not conducive to large catastrophic fires, and therefore lightning strikes in these forests result in mostly small, low-intensity, down-slope burns (Barden and Woods 1974; Buckner and Turrill 1999).

Lightning strikes cause forest fires by igniting a mixture of volatile extractives and finely divided bark, wood, and needle particles to an intense, short-lived ball of fire which in turn ignites flash fuels in the tree crown or on the forest floor (Taylor 1973). Lightning strikes and subsequent fires are more common on dry, exposed ridges and south facing slopes at higher elevations. Frost (1995) estimated presettlement fire frequency on ridges and upper slopes in the Appalachians was 7-12 years in the lower mountains and >12 years in higher mountains (>3,000 ft). (Buckner and Turrill 1999)

Today, only 12% of wildland fires in the southern Appalachians are caused by lightning; approximately 88% are caused by man. Annually, an average of six lightning fires per one million acres occurs in the Southern Appalachians. This frequency is greater than that recorded for the Great Plains, Mississippi Basin, or northeastern United States, but less than portions of the western and southeastern United States (Schroeder and Buck 1970; SAMAB 1996e).

Historical fire regimes throughout North America were greatly influenced by aboriginal man. Homo sapiens sapiens migrated to North America across the Bering Strait about 20,000 to 35,000 years ago (Aschmann 1978; Komarek 1974). Ancestors of these original peoples arrived in the Southern Appalachians about 10,000 years ago (Keel 1976). Natural fire regimes were dramatically altered with the arrival of paleo-Indians. In the southern Appalachians, as elsewhere, paleo-Indians increasedfire frequencies from the natural fire regimes of mostly lightning ignited fires. (Van Lear and Waldrop 1988)

When paleo-Indians first arrived in the southern Appalachian region, the landscape was dominated by boreal forests (tundra or taiga). Gradual global warming shifted the dominant forest type in the southern Appalachians to upland hardwood forest (Delcourt and Delcourt 1991). It is believed that paleo-Indians initiated widespread burning to encourage grazing habitat in these deciduous forests, as hunting was their primary means of survival for most of the millennia that they occupied the Southern Appalachians (Buckner and Turrill 1999).

It is estimated that paleo-Indians in the Southeast developed agricultural techniques around 800 to 1000 A.D. (Hudson 1982). Fire was used by these early agriculturalists to clear fertile floodplains for cultivation (Delcourt and Delcourt 1997, Chapman 1985). Fires set intentionally by paleo-Indians in agricultural plots likely escaped to surrounding uplands. But paleo-Indians also intentionally used fire outside of cultivated lands for other benefits including improving grazing habitat for wildlife, exposing nuts, and encouraging fruit production (Williams 1989; Buckner and Turrill 1999).

The relatively high human population densities in prehistoric America, although still a subject of much debate, indicate that most regions were likely subject to frequent anthropogenic fires. In 1492, the time of European contact, an estimated 18 to 20 million native Americans inhabited the North American landscape (Dobyns 1983). Evidence from paleo-ecological studies also indicate that during most of the last 4000 years, paleo-Indians played an important role in determining the composition of the southern Appalachian vegetation through their selective use of fire (Delcourt and Delcourt 1997). Perhaps the best, and most objective, evidence about the composition of forests before European settlement are pollen records from pond and bog sediments that have accumulated for thousands of years. These studies indicate that anthropogenic fires increased populations of fire-tolerant oaks, chestnut, and pines in upland forests of the southern Appalachians (Delcourt et al.1986, Delcourt et al. 1998, Delcourt and Delcourt 1997). Landscapes likely contained open pine and oak forests with widely spaced trees and herbaceous understories when European settlers arrived. For example, the Shenandoah Valley of Virginia was reported to be a vast prairie between the Blue Ridge Mountains and the Alleghenies in the mid-1700s (Leyburn 1962). (Van Lear and Waldrop 1988, Buckner and Turrill 1999)

Fire regimes that characterized the southern Appalachian landscape began to change with the settlement of Europeans in the 16th century. Europeans settling in the southern Appalachians found a landscape very different from wilderness areas today. Indian use of fire in hunting, gathering, and agriculture produced a "shifting mosaic of open grasslands, woodlands, and closed forests with widely scattered Indian villages" (Buckner 1989). Early accounts by Europeans in eastern North America emphasize the open character of the forest (Guffey 1977), sometimes alluding to vast treeless areas ( e.g., the French Broad Valley in western North Carolina, DeVivo 1991; the Shennandoah Valley of Virginia, Maxwell 1910).The fact that Spanish explorers with their armies of men and herds of livestock traversed the southern Appalachians in the 16th century can only be explained by open forest understories (Van Lear and Waldrop 1988).The greatest impact of European settlers on the pre-Columbian fire regime was not the further addition of a cultural influence, but rather the removal of one, namely fire (Buckner and Turrill 1999).

European settlement had devastating impacts on the native American population, and therefore indirectly affected the influence of anthropogenic fires. Pandemics caused by introduced diseases killed an estimated 90% of the native population (Buckner and Turrill 1999). This drastic decrease in the native American population slowly resulted in the development of closed forests over vast areas that had been kept open by frequent anthropogenic fires. Historical accounts of the Appalachian region dating to the Revolutionary War era (1770s) often describe it as a "wilderness" - a wilderness likely composed of 200-250 year-old forests that established after the decline of the native American population (Buckner and Turrill 1999). Native Americans of the l6th-18th centuries continued to influence on landscape conditions, albeit on a much smaller scale. The Cherokee, for example, still claimed a portion of their hunting grounds as late as 1836, before they were forced off their lands by the U.S. government (Van Lear and Waldrop 1988).

European settlers began moving into the Appalachian mountains in significant numbers in the late 1700s and early 1800s. These early settlers adopted the firing practices of the Indians (Dyne 1982) to both clear land for agriculture and improve forage for livestock (Buckner and Turrill 1999). As European populations increased, the rich bottomlands along major streams were quickly settled, and new settlers moved agriculture, and associated burning practices, up-slope into the mountains (Van Lear and Waldrop 1989). This settlement activity in the Appalachian uplands increased markedly after the Civil War when land grants were given for military service (Buckner and Turrill 1999).

Around 1880, railroads were constructed along both sides of the Appalachian uplands making their rich timber resources accessible to national and world markets. Timber companies began buying large tracts of land in the more remote sections of the southern Appalachians, resulting in rapid exploitation of the forests. Utilization standards of the time were such that most of each tree was left in the woods, therefore logging was often followed by slash-burning. (Buckner and Turrill 1999)

The use of prescribed fire in the southern Appalachians tends to differ according to land ownership. State agencies use prescribed fire primarily for silvicultural objectives. The Forest Service uses prescribed fire in ecosystem management to achieve certain desired ecosystem conditions. In contrast, the Park Service takes a more protective approach to land management and invasive actions such as prescribed fires have been until recently, rarely used. (Buckner and Turrill 1999).

See: Prescribed burns

Today, wildfires are a relatively common occurrence in the southern Appalachians. The majority of wildfires on federal lands (88%) are human-caused, due to either carelessness or arson (SAMAB 1996e). A small proportion of wildfires (12%) are started by lightning, although most of these are small and restricted to ridge tops.

Table: Wildfire incidence in the southern Appalachians: 1988-1993 (SAMAB 1996e)

Table: Causes of wildfires in the southern Appalachians: 1988-1993 (SAMAB 1996e)

US Forest Service policy on wildfires is to "suppress all wildfires in a timely, energetic, and thorough manner" (USDA Forest Service 1989, p. 49). This policy is implemented, in part, through fuels management: reducing fuel loads through prescribed fire, salvage harvest, and piling and burning of debris (Thompson et al. 1988). Due to effective suppression efforts, very few wildfires exceed 100 acres in the southern Appalachians (Buckner and Turril 1999).

Seventy to ninety years of highly effective <fire prevention and suppression> programs on public lands has resulted in changes in forest structure and composition in many areas of the southern Appalachians. Forest communities that established during times of frequent burning, such as pine and oak forests, have declined throughout the southern Appalachian region. For example, once vast areas of open oak-pine forests in the Great Smoky Mountains National Park (GSMNP) are essentially non-existent in today.

In the absence of periodic fire, forest succession has allowed hardwood species to invade areas previously dominated by yellow pines on upper elevation, southwest facing slopes. This process has affected two rare forest communities in particular, mountain longleaf pine woodlands and Table Mountain pine/pitch pine woodlands (SAMAB 1996e). Due in part to fire suppression, most pine-hardwood ecosystems in the southern Appalachians are characterized by high overstory mortality and slow growth rates, inhibited regeneration of overstory species, increased density and biomass of mountain laurel in the shrub layer, heavy fuel loads (i.e., large nutrient and carbon pools) in the forest floor and shrub layer, decreased herbaceous abundance and diversity, and increased susceptibility to insect infestations (Vose 2000). Without the reintroduction of periodic fires, oak dominance may also continue to shift toward shade tolerant and fire intolerant species such as soft maples, white pine, and sourwood (SAMAB 1996e).

Buckner and Turrill (1999) state that failure to restore some of these fire-dependent communities will likely result in their permanent loss as ecosystem components. This situation is made more urgent when considering the fact that extensive areas that once supported these communities are without a seed source needed to re-establish their fire-dependent components. Overall, the decline in fire dependent communities has resulted in decreased regional biodiversity and landscape heterogeneity in the southern Appalachians. (Buckner and Turrill 1999)

See: Negative effects of fire suppression on oaks

Fire suppression also effects other ecosystem components and attributes. For example, one consequence of fire exclusion is a change in the amount, distribution, and availability of ecosystem carbon and nutrient pools. Fire exclusion increases the amount of N in unavailable forms (i.e., greater aboveground organic matter) and reduces the abundance of N-fixers (Vose 2000).

These combined effect of fire exclusion serve to reduce both ecosystem resistance and resilience. For example, heavy fuel accumulation may result in fire intensity and severity levels that exceed the lethal threshold in thick-barked species. Therefore, fire exclusion has increased the likelihood of catastrophic fires in forest ecosystems more adapted to low-severity and -intensity fire. (Vose 2000)

Forest regeneration originates from several sources: from dispersed seeds or dormant seeds (seed bank); from seedling banks (advance regeneration); or from sprouts (vegetative reproduction). Species are often categorized into regeneration guilds or groups according to the relative importance of a given reproduction source.

There are two sources of regeneration from seed: seedlings germinating from dormant seeds stored in the seed bank or seedlings colonizing from recently dispersed seeds. A number of Appalachian species, such as yellow-poplar and sweet birch, colonize harvested or disturbed areas from seed banks. These species produce seeds that remain dormant in the soil or litter, and then germinate when moisture, temperature, and light conditions become favorable, such as following a cutting (Kelty 1988). On the other hand, very few Appalachian hardwood species regenerate by seeds dispersed into harvested or disturbed areas (Kelty 1988). Seed production is too variable for this to be a reliable means of regeneration, plus newly dispersed seeds are already at a competitive disadvantage once they arrive at a site due to the amount of colonizing herbs and shrubs.

Factors affecting regeneration from seed

The success of regeneration from seed depends on a convergence of suitable conditions for flowering, pollination, and seed maturation; abundant seed production; adequate seed dispersal; favorable conditions at the forest floor, including suitable seedbeds; limited losses to pathogens, insects, birds, and mammals before and after seed dispersal; adequate light, moisture, temperature, and nutrients for seedling establishment and survival; and appropriate conditions for the growth and long-term survival of preferred species. (Nyland 1996).

Both external agents and intrinsic site factors may threaten this sequence. External agents include seed predation, browsing, disease, insect attack, fire, flooding, drought. Intrinsic site factors, such as temperature, moisture, nutrients, and light near the forest floor, also affect germination, survival, and growth. In some cases, the necessary conditions converge almost annually, and abundant regeneration occurs rather readily (e.g., northern hardwoods). In other cases, many years may pass before all conditions became favorable, and regeneration succeeds (e.g., the oaks). (Nyland 1996)

Vegetative reproduction, also called vegetative regeneration, is an important source of regeneration among hardwood species. There three types of vegetative reproduction: stump sprouts, seedling sprouts, and root suckers.

Stump Sprouts

Sprouts, i.e., sprouts originating from cut stumps of trees at least 2 inches in dbh., can arise from several different origins. Some shoots arise from dormant buds that originate at the pith, and grow just under the bark. Sprouts of this origin may appear at the root collar at the side or base of the stool. Others develop from adventitious buds formed in the cambium. These emerge from callus that forms along the cut surface around the top of the stump, just inside the bark.(Nyland 1996)

Stump sprouts are an important source of regeneration in Appalachian hardwoods. It is not uncommon for stump sprouts to account for half the stems in young stands (Johnson 1976; Wendel and Trimble 1968). This mode of regeneration has long been considered to produce poor quality stems compared to seedlings of the same species, mainly because of a high incidence of decay and breakage in stump sprouts.Decay organisms can enter stump sprouts through heartwood connections with either the decaying parent stump or dead companion stems. However, it is now clear that all stump sprouts do not suffer equally from this problem, and many of the fine trees in second-growth stands originated as sprouts. It has been shown that stump sprouts can produce sawlogs as high in quality as those from seedlings--and usually in a shorter time (Johnson and Rogers 1983). The incidence of decay and breakage increases with the size of stump and the height on the stump from which the sprout originates, with the latter being the more important factor. Sprouts that originate at or near the ground line have less decay and are likely to survive to maturity with the potential of producing a high quality stem (Wendel 1975; Lamson 1976). Sprouts are a more reliable source of reproduction in upland oak stands on sites of medium or lower quality because of the relatively large number of small trees present. Stump sprouts, however, play a less important role in reproducing most mature cove hardwood stands. (Kelty 1988)

Seedling Sprouts

"Seedling sprouts are another form of vegetative reproduction; they are biologically identical to stump sprouts, with the only distinction being that they originate from small stumps (less than 2 inches in diameter). While this exact size is arbitrary, the distinction is quite important, because the small stumps are quickly covered with callus tissue, such that no open wound is present for infection. Because they often develop after the seedling stem dies or is broken off at ground level, there is no problem of sprouts originating high on the stump. Thus, seedling sprouts tend to be free of the bad qualities that can occur with sprouts from large stumps. In fact, they are best considered in conjunction with advance-regeneration seedlings, since they usually occur in abundance when seedlings are damaged during logging operations. They tend to be even more desirable than seedlings, because seedling sprouts have established root systems and grow faster than the original seedling shoots, so the stems are often straighter. This characteristic has led to trials of mowing advance growth just before overstory removal in order to produce sprouts, but enough are usually created during logging that this generally does not appear to be necessary." (Kelty 1988)

Root Suckers

A few species sprout from adventitious buds produced on roots; these sprouts are generally referred to as root suckers. Suckers may also arise from shallow roots, and generally as single stems at multiple points of origin (loci) along the root system. Root suckers are of minor importance in Appalachian hardwood stands, limited to two commercial species--beech and black locust. Beech is a minor commercial species in Appalachian hardwood stands. Black locust normally is short-lived and is valuable for local post products and fuelwood. (Lamson 1988)

Factors Affecting sprouting

Lamson (1988) summarized several characteristics that affect sprouting frequency, such as the species, age, and diameter of the parent tree as well the season of cutting, site quality, and residual density of the stand:

Seedlings of many species germinate beneath a partial or full overstory and exist as suppressed advance regeneration, responding to release when the overstory is removed. For most advance-growth dependent species, seedling-sprouts are the primary reproduction growth form; they arise from the recurrent dieback of shoots of reproduction that originate as seedlings (Johnson 1989).

Advance regeneration is the primary source of regeneration for oak species in the southern Appalachians, particularly red oaks. Other upland oaks typically occur on somewhat poorer quality sites, where mature stands contain a relatively large number of smaller diameter oak stems that sprout vigorously from stumps after a harvest cut. In mature stands on high-quality sites, however, red oak usually occurs as scattered, large individuals that have a low probability of sprouting after cutting. On these sites, therefore, advance red oak reproduction is the primary source of regeneration. Other species that also rely on advance regeneration include: white oak, chestnut oak, black oak, scarlet oak, white ash, cucumbertree, white pine, hickories, basswood, red maple, sugar maple, beech, buckeye, and hemlock. Even more intolerant species, such as yellow-poplar and black cherry, depend to some extent on advance regeneration.

Where advance reproduction is the primary source of reproduction, the number, size, and distribution of stems of advance reproduction collectively determine regeneration potential. The ability of advance-growth seedlings to respond to release depends on their size: larger stems have a greater likelihood of capturing growing space after canopy opening than smaller ones. However, the minimum size necessary varies considerably among species. Black cherry is the best adapted to respond when small (2 to 6 inches); at the other extreme are the oak species, which must be at least 4.5 feet tall to respond well to an overstory release. Therefore, manipulating mature stands to enhance the survival and growth of advance reproduction prior to overstory removal is crucial to the success of oak regeneration (Loftis 1990). This concept is integral to oak <shelterwood methods> (Kelty 1988)

The relative ability to survive in the understory and respond to release also varies among species. Advance seedlings of most species are present at some time, even in stands with relatively dense overstories. Intolerants such as yellow-poplar and cherry germinate, live for several years, then die. Intermediates such as the oaks, hickories, and ash may survive for a decade or more but grow very little beneath fully stocked stands (Beck 1970, Loftis 1983). Only the most tolerant species are likely to grow to any size in the understory without substantial overstory disturbance. (Kelty 1988)

Because advance regeneration plays such an important role in hardwood forests, some suggest thinking of regeneration guilds based upon their need to develop as advance growth and their ability as advance-growth seedlings to respond to release from overstory shading. This continuum would range from progression/pioneer species to very tolerant species

1. Pioneer species that can become established in open conditions, not requiring development as advance growth at all (e.g., yellow-poplar).
2. Species that can respond to release as fairly small seedlings, or on some sites become established in open conditions (e.g. black cherry, white ash).
3. Species that rely almost entirely on developing as advance regeneration and must achieve a large seedling size to respond to release (e.g., oak species).
4. Very tolerant species that must reach sapling size or larger before release, if they are to reach the overstory of the new stand (e.g., sugar maple, hemlock).

This scheme is only slightly different from Keltys division of species into three regeneration quilds. Placing species along this progression can be helpful in organizing thoughts about species responses to various silvicultural treatments; however, there are enough complexities in reproductive characteristics that it is important to know the details for each species and not depend too heavily upon such generalizations.

It is often useful to categorize species by their predominant strategy of regeneration. These categories are often called regeneration guilds or regeneration groups. Although placement of species into categories will be somewhat artificial (most species exhibit many combinations of reproductive strategies), these categories help silviculturalists design efficient regeneration treatments.

Spanning the range from pioneer to extremely shade-tolerant, Appalachian hardwoods represent most of the reproductive strategies possible in trees. Two systems of categorization are summarized in the following sections:

Kelty (1988) reviewed the reproductive characteristics of the more important Appalachian hardwood species and divided them into three basic regeneration guilds:

1. Pioneer Species
   Yellow-Poplar, Sweet Birch, Black Locust
2. Advance-Growth-Dependent Species: Moderate Shade Tolerance
   Black Cherry, White Ash, Basswood, Red Maple, Sweet Birch, Oaks, Hickories, Black Walnut
3. Advance-Growth-Dependent Species: Extreme Shade Tolerance
   Sugar Maple, Beech, Hemlock

Sutherland and others (2000) classified 62 tree species in the central hardwoods region with respect 16 regeneration attributes that describe flowering, seed production and dispersal, dormancy, germination environment, seedling characteristics, and vegetative reproduction using multivariate techniques. Their analysis resulted in nine guilds arranged into three regeneration strategies:

1. Pioneer
   Spring-dispersed, moist site intolerants; spring-dispersed, moist site tolerants; dry site intolerants
2. Opportunistic
   Long-lived intolerants and intermediates; fast-growing understory tolerant; dispersal-limited (large-seeded or sprout dependent)
3. Persistent
   Large-seeded, advance growth dependent; slow-growing, understory tolerant

Yellow-Poplar, Sweet Birch, Black Locust

The only species of the Appalachian hardwoods that truly fits this description is yellow-poplar. Both yellow-poplar and sweet birch have a number of the traits common to pioneer species that enable them to invade open sites. They both produce seed abundantly, with good seed years occurring frequently. Heavy birch seed crops are produced every 1 to 2 years (BROKEN-LINK Fowells 1965), with heavy crops of yellow-poplar being described variably as nearly annual (Beck and Sims 1983) to somewhat irregular (BROKEN-LINK Fowells 1965). Seed is dispersed by wind, with yellow-poplar being blown for 4 to 5 tree heights; the small seeds of sweet birch can probably be blown even further. In the case of yellow-poplar, seed remains dormant in the forest floor for at least 3 years in abundance, and some remains viable up to 8 years (Clark and Boyce 1964). The dormancy of sweet birch seed is not known precisely; most germinates after the first winter, while some may overwinter perhaps for several years (BROKEN-LINK Fowells 1965; Beck and Hooper 1986). Exposure of mineral soil aids the germination of both species, especially for birch because of the small seed size. Both have very fast juvenile height growth rates and can outgrow most competitors when growing in full sunlight. However, the ability of yellow-poplar to compete with other vegetation is affected by the timing of germination. If cutting occurs by mid-spring at the latest, yellow-poplar will germinate early enough to compete successfully with the flush of weeds that also invades the site. If a cutting takes place in the late spring or summer, yellow-poplar germination is delayed and on some sites the seedlings will not be able to grow fast enough to compete successfully (Trimble and Tryon 1969).

Although black locust lacks some of the characteristics of seed production and dispersal common to pioneer species, it fits best in this category. This is because it generally reproduces by root suckering from a cut or injured tree (BROKEN-LINK Fowells 1965). Locust root suckers are very intolerant and grow rapidly in height, so they compete well in full sunlight. Thus, black locust acts as a pioneer, except that it cannot readily be dispersed onto a new site if it does not already exist there.

The pioneer status of these three species is most apparent following clearcutting. Seedlings of most species decline in numbers following clearcutting, as smaller advance-growth seedlings die from exposure, but yellow-poplar, sweet birch, and black locust increase in numbers and dominate in height development (McGee and Hooper 1970; Beck and Hooper 1986). Only stump sprouts of other species can match the height growth of these species when growing in full sunlight.

Yellow-poplar and sweet birch both have alternate modes of reproduction. Yellow-poplar can germinate beneath a partial overstory and exist for a short period as an advance-growth seedling, although this process is not crucial to its establishment as it is for many other species. Sweet birch is unusual for a species exhibiting many pioneer characteristics, in that it is tolerant of shade as a seedling, and can regenerate beneath a fairly dense overstory canopy. Both species can also sprout from seedlings or stumps.

Excerpted from: Kelty 1988
Last updated: 10/3/01

Black Cherry, White Ash, Basswood, Red Maple, Sweet Birch, Oaks, Hickories, Black Walnut

The majority of Appalachian hardwood species fall into this category. The ecological term "gap-phase" that can be applied to these species refers to the usual mode of reproduction in unmanaged forests where seedlings are established beneath small gaps in the canopy caused by windthrow or other natural disturbance. They survive in the understory as advance regeneration until another disturbance enlarges the gap and releases them. The overstory shade is necessary with these species to moderate the temperature extremes and droughty conditions that occur in the upper soil layers in exposed conditions. These kinds of disturbances can be duplicated in shelterwood cuttings, but may occur following thinnings or other partial cuttings as well. These species can be thought of as advance-growth-dependent (even though they do not actually rely completely upon this pathway) to emphasize the fact that abundant regeneration is dependably obtained only from vigorous advance growth. The species in this group range from intolerant to tolerant, with only the extremes (very intolerant and very tolerant) being excluded. There is considerable variation in details of regeneration mechanisms within this category, and three subgroups can be recognized.

Black Cherry

The first subgroup consists only of black cherry, which is the closest of the advance-growth-dependent species to having pioneer characteristics. Frequent cherry seed crops occur every 3 to 4 years (BROKEN-LINK Fowells 1965), and the seed can remain dormant in the litter for 3 years or more (Trimble 1975). Seed dispersal is by birds, so it is more limited than wind-dispersed pioneer species. This species also differs from a true pioneer in that germination is greatly enhanced by partial shade. Unlike yellow-poplar, black cherry seedlings become established in much greater numbers beneath a canopy as compared to open conditions.

In Allegheny cherry-maple stands, the greatest numbers of cherry seedlings become established beneath a fairly heavy overstory (approximately 2/3 of full canopy) (Marquis 1979). These seedlings are intolerant of shade and can survive only a few years beneath a dense overstory, but they are able to respond to overstory removal when fairly small. Seedlings smaller than 2 inches in height do not survive well after overstory removal, but those that have reached 6 inches in height and larger have good survivorship and grow very rapidly in height upon release (Marquis 1982). In contrast, black cherry is sometimes able to become successfully established following clearcutting; this was found to be the case on Appalachian mixed hardwood sites in West Virginia (Trimble et al. 1986). In these cases, most germination is probably from dormant seed stored in the forest floor.

White Ash, Basswood, Red Maple, Sweet Birch

The next subgroup includes white ash, basswood, and red maple. Seeds of ash and maple are wind-dispersed for a moderate distance, but basswood seed dispersal is very limited. Scarification is beneficial for the moderate-sized seeds of these species, but is not critical for germination. Basswood produces good seed crops every 1 to 2 years, with white ash being more irregular. Seed of both can remain dormant up to 3 years in the litter (Trimble 1975). Nearly all red maple germinates the first season, but this is not especially limiting to reproduction because large crops are produced almost every spring. These species can sometimes invade open sites under the right set of conditions of site quality and seed source, but this generally does not provide dependable regeneration. Dense reproduction is only obtained beneath the shelter of a partial overstory. In general, small advance-growth seedlings of these species do not respond as well to release as black cherry. Marquis (1982) found that survival was good for red maple and white ash seedlings at least 6 inches tall, but recommendations based on research with many species in northern hardwood forests are that seedlings should be a minimum of 2 feet tall in order to dependably show good survival and response (Tubbs 1977).

In addition to behaving as a pioneer species, sweet birch can be included with these species because it is tolerant of shade (similar in degree to red maple and basswood) and can become established under a fairly dense overstory. It often germinates on decaying stumps and logs, and thus is not limited by the lack of a mineral soil seedbed on relatively undisturbed sites.

Oaks, Hickories, Black Walnut

The third subgroup consists of species with heavy, animal-dispersed seed--the oaks, hickories, and black walnut. Regeneration of these species is more irregular and difficult than of any other group. Seed production of black walnut and hickory is somewhat irregular, with good seed crops occurring every 2 to 3 years. Good acorn crops are produced very sporadically in all the important oak species, occurring at 2 to 10 year intervals (Watt 1979). During years with poor seed production, predation by insects and mammals may eliminate the entire crop (Watt 1979). The seed of these species is dispersed only as far as small mammals carry them, and best germination occurs when seeds are buried beneath the litter, bringing them in contact with mineral soil.

Of the "gap-phase" species, the oaks are the most dependent upon the advance regeneration pathway. Oaks have slow initial height growth even with full sunlight, because much of the early growth goes into establishing the root system. Thus, they do not compete successfully with many intolerant species. Because they are only moderately tolerant of shade, the seedling shoots cannot survive for long periods beneath an overstory. However, the oaks have the ability to produce sprouts repeatedly when the shoot dies back or is damaged, so they can persist for extended periods in the understory as seedling sprouts (Watt 1979). As with other species, oak advance-growth seedlings or seedling sprouts respond better to release from overstory shading if they are large and vigorous. The minimum size is greater for the oaks than for other species, although; only those that have reached a height of 4.5 feet or greater have a high probability of surviving and competing well with other vegetation upon release (Sander 1972). Furthermore, seedlings with flat-topped crowns do not show rapid growth compared with more vigorous stems having definite recognizable leaders (Carvell 1979). Obtaining this large, vigorous, oak advance growth is the major stumbling block in oak regeneration. Increasing the light levels reaching the understory is the major way of stimulating height growth of oaks, but this usually produces conditions that are also favorable to many faster growing competing species.

Less information on natural regeneration characteristics is available for hickory and black walnut, because hickory is not a highly desired species, and black walnut is so desirable that most research has dealt with plantation management. In general, these species have many of the same root and shoot growth characteristics as oak. Black walnut is even less tolerant, so it is necessary to provide full or nearly full sunlight when seedlings are young. However, walnut also suffers from competition with many faster growing species when growing in high light conditions, especially on poor sites. Also, mature stands usually contain only scattered walnut trees, so the seed source for natural regeneration is often poor. Natural walnut regeneration is thus very unpredictable, requiring just the right combination of seed source, moderate-sized canopy gaps, and sufficiently rich sites.

Sugar Maple, Beech, Hemlock

Two of the Appalachian hardwood species--sugar maple and beech--and one important conifer associate--hemlock--are tolerant enough to become established and survive for considerable periods beneath intact overstories. They achieve this by different mechanisms. Overstory sugar maples are good seed producers, with heavy crops occurring at 2- to 5-year intervals (Fowells 1965), and the large seeds can become established on undisturbed leaf litter. Seedlings cannot exist indefinitely under a completely closed canopy, but new establishment is frequent enough where a seed source exists that there are generally abundant seedlings in the understory. Beech often reproduces by root suckering, sometimes producing dense understory thickets. They can persist for many years at low light levels and still respond vigorously to release.

The small seeds of hemlock require either exposed mineral soil or the moist conditions of decaying logs or stumps for germination. Hemlock is so tolerant that it can persist for decades with very little growth under low light conditions. The adequacy of seed source can be deceptive for these species because they often occur primarily in the lower crown classes of mature stands, with faster growing species in the upper canopy. Trees in the lower canopy positions are generally not vigorous enough to produce seed, so the mere presence of these species in a mature stand does not assure adequate seed production (Marquis 1979).

Advance-growth seedlings of sugar maple, beech, and hemlock can respond to release from overstory shading if they meet the 2-foot minimum height applicable to other species, but their height growth is so slow that they will be surpassed in height by many other species. They will still survive because of their great tolerance of shade, but will be relegated to the lower crown classes of the new stand. If they are to reach the overstory at heights comparable to competing intolerant and midtolerant species, they must be of sapling size or greater as advance growth, so that they have a considerable headstart on the other species (Marquis 1981, Kelty 1986).

Pioneer guilds contain species that regenerate where disturbances are either frequent or severe. Seasonally flooded areas, abandoned fields or areas subjected to high-intensity fires are most likely to be colonized by pioneer guilds. Seeds come from outside the disturbed site or are stored on site as buried seed or in an above-ground seed bank. (Sutherland and others 2000)

Spring-dispersed, moist site intolerants

Betula nigra, Populus deltoides var. deltoides, Populus grandidentata, Populus tremuloides, Salix nigra

Short-lived pioneers that colonize sites rapidly and are too shade intolerant to replace themselves These species are closest to the traditional definition of pioneers: they mature early, have rapid growth rates and are generally short-lived. Seed crops are copious and widely dispersed by wind; regeneration potential is limited mainly by the conditions necessary for germination and establishment. Full sunlight and mineral soil seedbeds are essential for successful germination and establishment. (Sutherland and others 2000)

Spring-dispersed, moist site tolerants

Acer rubrum, Acer saccharinum, Ulmus americana, Ulmus rubra, Ulmus thomasii

These species have many pioneer characteristics but they are neither as short-lived nor as intolerant as those intolerant. These trees are common early successional species that establish in a variety of situations as long as sufficient light is available to maintain growth. persistent pioneers that colonize frequently disturbed sites more slowly and are shade tolerant. (Sutherland and others 2000)

Dry site intolerants

Juniperus virginiana, Pinus echinata, Pinus rigida, Pinus virginiana

This group includes highly competitive intolerants that colonize dry uplands and abandoned fields. Species begin seeding early, but unlike trees in the other pioneer guilds, seed crops are infrequent and dispersal is limited to nearby areas. Although these species may establish and survive under pine or hardwood canopies on dry or nutrient poor sites, regeneration is most favored where disturbances create large openings. (Sutherland and others 2000)

Opportunistic guilds contain species that use several reproductive mechanisms and thus are capable of regenerating following a range of disturbance types. Frequent seeding and extended seed viability ensure that seeds are continually available; sprouts from damaged individuals may also provide an additional source of propagules. Shade tolerant opportunists maintain seedling or sprout banks that readily respond to canopy openings. (Sutherland and others 2000)

Long-lived intolerants and intermediates 

Betula alleghaniensis, Betula lenta, Juglans cinerea, Juglans nigra, Liquidambar styraciflua, Liriodendron tulipifera, Pinus strobus,
Magnolia acuminata, Platanus occidentalis

This group includes some of the fastest growing species in the central hardwood forest. They are commonly found on mesic sites and are long-lived and highly competitive. Once they are established they are capable of occupying a site for a long time. Large canopy openings and disturbed seed beds favor germination and all but Pinus strobus are vigorous sprouters. Platanus occidentalis, Juglans spp. and Betula spp. reproduction depends on continued high light levels for successful recruitment into the canopy. Seedlings and sprouts of the other species are more tolerant of overhead shade and new cohorts may survive for several years in the understory. (Sutherland and others 2000)

Fast-growing understory tolerant 

Acer negundo, Celtis occidentalis, Cornus florida, Fraxinus americana, Fraxinus nigra, Fraxinus pennsylvanica, Morus rubra, Prunus serotina

This guild contains the more shade tolerant opportunistic species. They have abundant seed available for germination because good seed crops are produced frequently and seeds remain viable for several years. Understory light levels are sufficient for germination and reproduction quickly becomes sapling-size. Although saplings may persist for several years in the shaded understory they soon must be released by some form of canopy opening in order to survive. Some of these species may form a subcanopy layer that either persists or gradually dies out depending on overstory composition.  (Sutherland and others 2000)

Dispersal-limited (large-seeded or sprout dependent) 

Aesculus glabra, Aesculus octandra, Cercis canadensis, Diospyros virginiana, Gleditsia triacanthos, Nyssa sylvatica var. sylvatica, Robinia pseudoacacia, Sassafras albidum

This guild contains species with very different successional roles. For example, Sassafras albidum and Robinia pseudoacacia are old-field pioneers whereas Aesculus octandra and Diospyros virginiana are understory tolerants. Although some species fit the profiles of other guild types, they were considered opportunistic because they produce seeds frequently and germination occurs on a wide range of seed beds. Established individuals have fast growth rates in open conditions (Diospyros virginiana is an exception), and all but Sassafras albidum and Gleditsia triacanthos are tolerant of shade. Seeds are large and have a limited dispersal range, but colonization of new areas is possible because birds, small rodents or livestock often act as dispersal agents. Large seeds also provide food reserves that allow new seedlings to establish on a broad range of sites. Survival rates are usually high because tap roots or wide-spreading root systems support vigorous growth. Root suckering is also common in this guild and many new cohorts of Sassafras albidum, Robinia pseudoacacia and Gleditsia trzacanthos arise on or adjacent to previously occupied sites. The buckeyes (Aesculus spp.) have the most limited regeneration potential in this guild. Their large seeds quickly lose viability if they are not kept moist so germination is restricted to mesic, litter-covered sites.  (Sutherland and others 2000)

Persistent guilds contain species that develop and maintain advance regeneration. Many persistent species have large seed reserves that enable germinants to establish in shade or on dry sites. Large cohorts often develop as a result of mast seeding or other occasional events (e.g., wet years). Species with moderate understory tolerance may survive with a cycle of die back and resprouting, whereas more tolerant species maintain seedling or sapling banks. Small canopy gaps maintain the regeneration pool for these species, but individuals do not enter the overstory until more significant canopy disturbances occur. (Sutherland and others 2000)

Large-seeded, advance growth dependent 

Carya cordiformis, Carya glabra, Carya laciniosa, Carya ovata, Carya tomentosa, Quercus coccinea, Quercus palustris, Quercus rubra, Quercus velutina, Quercus alba, Quercus bicolor, Quercus macrocarpa, Quercus prinus, Quercus stellata

Oaks and hickories develop and maintain "advance growth" regeneration. New cohorts arise from established seedlings or sprouts that are released by canopy openings: Seeds germinate well in the understory, but seedlings have intermediate shade tolerance so survival under a closed canopy is limited to a few years. Seedlings actually persist as "seedling sprouts" with repeated episodes of dieback and resprouting. In spite of the large, gravity-dispersed seeds, this guild often colonizes new areas with seeds cached and forgotten by birds and rodents. Vigorous sprouts also develop from cut or damaged stems. Seedling sprouts or other sprout origin stems: exhibit fast height growth which makes this group more competitive in canopy openings than the other persistent species. Hickories and white oaks are slow-growing and more shade tolerant than red oaks. Hickory seeds germinate in the spring following a period of cold stratification unlike white oak. (Sutherland and others 2000)

Slow-growing, understory tolerant

Acer nigrum, Acer saccharum, Carpinus caroliniana, Fagus grandifolia, Ostrya virgimana, Oxydendrum arboreum, Tilia americana, Tilia heterophylla, Tsuga canadensis

This is a highly versatile group. Good seed crops are produced at regular intervals and a variety of seedbeds are suitable for germination. Seedlings grow slowly and survive in the shaded understory for many years. As small canopy gaps form, individuals are gradually recruited into the overstory. Seedling or sapling banks are common and Ostrya virginiana and Tilia spp. also maintain a seed bank. (Sutherland and others 2000)

relay floristics model

There are two concepts posed as alternative views of vegetation development (Egler 1954). Relay floristics presents a very familiar pattern of development, one that many people would associate with Clements (1916). Well-defined seral stages, beginning with pioneer species, successively occupy a site, culminating in a relatively stable community of climax species. We also usually tend to think of pioneer species as shade intolerant and of climax species as shade tolerant (Loftis, 1993a).

In using this concept to predict the outcome of the application of various regeneration methods, we have associated species we classify as pioneer and shade intolerant with clearcutting, climax and shade tolerant species with selection methods, and species that are intermediate in their successional status and shade tolerance with shelterwood. And one frequently hears that "cutting sets succession back to an earlier stage." (Loftis, 1993a)

However, reconciling this neat scheme with observed outcomes of regeneration methods is problematic, particularly in hardwood forests. For example, clearcutting a stand of oaks on a relatively xeric site frequently results in a new stand dominated by oaks, but oaks are not usually considered pioneer species. Regeneration cuts of any kind in some hardwood forests result in excellent representation of species we regard as climax and shade tolerant in the new stand. Further, the application of different regeneration methods in some forest types frequently results in the same species composition. In short, categorization of species by presumed successional status lends little to our ability to predict species composition resulting from regeneration cutting. (Loftis, 1993a)

Initial floristics model

As an alternative to relay floristics, Egler proposed a concept he called initial floristic composition.

This concept suggests that species composition of vegetation following disturbance is determined by the propagules that exist on the site at the time of the disturbance and those which arrive early in the process of stand development. Changing species dominance over time results from differential growth and development of species present. Interestingly, it was Clements (1916) who first made this observation. According to Clements, logging does not initiate a successional sequence because propagules of a variety of species are left after logging. Rather, it creates a dysclimax. (Loftis, 1993a)

For forest trees, then, species composition following disturbance is determined by the initial load of propagules: stump- and root-sprouts and advance reproduction of the various species that occur in the stand, and by new seedlings that become established soon after the disturbance from buried seed or from seed blown or carried into the stand. The essential feature of the initial floristic composition concept that makes it useful for prediction modeling is that the individuals of species that form the dominant tree canopy 50 to 100 years after disturbance come from the initial load of propagules. A corollary to this concept is that variations in species composition among stands 50 to 100 years after similar disturbances results from variations among stands in the initial loads of propagules. (Loftis, 1993a)

A number of investigators have recognized the general applicability of the initial floristic composition concept to hardwood stand development following disturbance (e.g. Oliver 1981, Leopold and others 1985, Shugart 1984, Drury and Nisbet 1973). From the standpoint of developing prediction models, we can model the post-disturbance development of existing advance growth both for advance reproduction and stump or root-sprout potential and we can deal in a probabilistic sense with input and development of new seedlings. (Loftis, 1993a)

Initial floristic composition, vital attributes of species, and other concepts from population biology are the ecological bases for the oak regeneration potential concept that provides the framework for developing models to predict how much oak we might expect in a new stand following a regeneration cut.

This section will discuss concepts of nutrient cycling and carbon cycling relevant to forest management in the southern Appalachians. 

The southern Appalachians are a center of endemism-- a region where enormous concentrations of life forms occur, reflecting special local conditions. In contrast, most of the world is inhabited by widely distributed and very adaptive species.

The uniqueness of the aquatic systems in the southern Appalachians can be attributed primarily to the geologic history and climatic conditions that have influenced the evolution of species and development of biological communities. Since their rise in the late Paleozoic Era (around 250 million years ago), the southern Appalachian mountains have been above sea level. Precipitation falling on these highlands has drained into thousands of kilometers of freshwater streams and rivers. During the Pleistocene Epoch (1.8 million to 11,000 years ago), glaciers advanced over North America but did not reach southern Appalachia. Consequently, evolution proceeded in this region without the catastrophic disruption that occurred further north during that time. The great time scales over which the flora and fauna developed, a relatively mild climate, and the great heterogeneity of landscapes and habitats all combined to produce unique ecological conditions within a temperate aquatic setting. The region forms a global center of biological diversity for many groups of organisms, including fishes, mussels, crayfishes, and aquatic insects.

Aquatic systems integrate the landscape through hydrologic processes. Aquatic ecosystems, therefore, are influenced by terrestrial characteristics of watersheds as well as submersed, in-channel features. Our description of aquatic ecology discusses: 

• Drainages and the Hydrologic Cycle-- Primary organization of aquatic ecosystems. The broad scale interactions of climate and geology that determine drainage systems.
• Ecology of Running Water-- Structure and function of streams and rivers (lotic).
• Ecology of Lakes and Reservoirs-- Structure and function of standing water (lentic).

Aquatic ecosystems are part of a larger earth process, the hydrologic cycle. The interaction of climate with geology through erosion has created large topographic features in the Appalachian Highlands. Most areas of high elevation and relief have been formed on resistant rocks such as those of the Blue Ridge, and the major drainage systems of the region have become closely adjusted to rock type (Wallace and others 1992). The process of infiltration and runoff of precipitation drives streamflow, and interaction with geology and landforms create drainages.

Water draining from the western side of the Appalachians eventually reaches the Gulf of Mexico, while drainages on the eastern side end up in the Atlantic Ocean. Major Gulf drainages of the region are the Ohio River (New-Kanawha, Tennessee, and Cumberland River drainages) or the Alabama-Apalachicola Rivers (Coosa-Tallapoosa, Chattahoochee River drainages). Atlantic slope drainages include the Chesapeake Bay (Potomac, Rapidan-Rappahannock, and James River drainages) and those in the Carolinas (Roanoke, PeeDee, Santee-Cooper, and Savannah River drainages). The U.S. Geological Survey has delineated drainages nationwide into hydrologic units that are commonly used to summarize information about watersheds.

High-gradient streams of the crystalline Appalachians are typically dendritic. Stream density is high in this region.  Reported densities of 12 to 16 feet per acre (1.5 to 1.9 miles per square mile) (SAMAB 1996), were determined from the EPA Reach File version 3.0 (RF3), which is based on a map scale of 1:100,000.  This map does not include many first- to third-order streams (SAMAB 1996) and stream length is underestimated at this scale (Hansen 2001).  In the Ridge and Valley, or sedimentary Appalachians, streams may be dendritic, but they tend to follow a trellised pattern downstream (Wallace and others 1992). Streams are often classified by their stream order.

Hydrologic Cycle

Water cycles from the atmosphere to the earth in precipitation, then moves across the landscape through runoff and groundwater to streams, rivers, and lakes and eventually to the ocean. Water also cycles from soil to plants and other biota. The hydrologic cycle is completed when water returns to the atmosphere via evaporation from water bodies and transpiration from plants (Allan 1995).

HUCs in the Southern Appalachians

During the 1970s the U.S. Geological Survey (USGS) developed a hierarchical hydrologic unit code (HUC) system that divides the United States into 21 regions, 222 subregions, 352 accounting units, and 2,149 cataloging units based on surface hydrologic features, or drainages. The smallest USGS unit, the 8-digit HUC (or 4th level HUC), averages about 448,000 acres, and is usually the level referred to as a "HUC".

Seventy-two HUCs are wholly or partly in the southern Appalachians.

Stream Order

Stream order refers to systems for classifying streams based on branching patterns, starting at the headwaters. Stream order is closely related to stream size -- the smallest streams are the lowest order and stream size increases with stream order. In the most commonly used scheme, the uppermost headwater streams that have no tributaries are first-order streams (Strahler 1952). When two first order streams join, they form a second-order stream. Likewise, when two second-order streams join, they form a third-order stream, and so forth.... When a lower order stream joins a higher order stream (for example, a first-order stream joins a second-order stream), the order of the receiving stream does not change. When a stream is assigned an order, the number refers to the lowermost section of the stream.

Some confusion, however, arises because scientists use different criteria for determining where first-order streams begin in the headwaters. Some headwater streams are ephemeral, with short-lived or transitory flow after precipitation or snowmelt and others are intermittent-- they flow at certain times of the year, cease flowing in dry years, or are reduced to a series of separated pools. Nevertheless, these streams are important for some invertebrates and for stream ecosystem processes. Strahler (1952) evidently intended to begin with ephemeral and intermittent streams. Hydrologists include all these streams when assigning stream order and tend to use crenulations in the contours on a 1:24,000 USGS topgraphic map to identify first order channels. For some fish biologists, only streams with perennial flow are of interest, and the uppermost perennial stream is designated as first order. For convenience, many people define first-order streams as the uppermost streams shown by blue lines on a 1:24,000 USGS topographic map, which generally includes most perennial streams, some intermittent streams, but rarely ephemeral streams.

Running water in streams and rivers is an important feature of the southern Appalachian landscape. It also plays an important role in the hydrologic cycle of the earth. Streams are organized on the landscape in drainages.

Key aspects of stream and river ecology are:

• Physical and Chemical Environment
• Biological Communities
• Ecosystem Processes

Streams and rivers are strongly influenced by the landscapes that they drain, and exhibit considerable variation from headwaters to larger rivers. The River Continuum Concept (Vannote and others 1980) is a useful framework for understanding changes in aquatic ecosystem processes, such as organic matter inputs, that correspond to changes in geophysical characteristics occurring longitudinally along stream courses. In turn, biological communities dependent on aquatic environments show similar longitudinal variation.

Streams and rivers have a number of abiotic characteristics that are determined by their physical and chemical environment. This environment, in turn, influences stream biota.

Geophysical characteristics of streams are determined by underlying geologic composition and geomorphic processes. Geomorphic processes occur at different spatial and temporal scales, resulting in a continuum of stream habitats.

Water, the moving medium of streams, exerts physical influence on streams through streamflow and water quantity and chemical influence through water quality

Sediment is an important physical component of streams that is of particular concern to biologists and managers. Sediment production is a natural process; but it is accelerated by human activities, often to the detriment of stream biota. Sediment entering streams is transported downstream and also forms the substrate of streams.

Geologic structure affects water chemistry, patterns of hydrology and drainage formation, local substrate, slope, and longitudinal profiles of the streambed. As such, the geomorphic processes that have been occurring over 200 million years strongly influence the structure and function of todays stream biota. The long period of mountain formation in the southern Appalachians produced extremely complex bedrock patterns (Hack 1969, Isphording and Fitzpatrick 1992). Variation in rock types and their resistance to weathering and erosion have important influences on local relief, landscape forms, and water chemistry (Wallace and others 1992):

• Granites and other light-colored, course-grained, crystalline rocks. Fairly resistant to erosion and water penetration, often form areas of steep slopes. These rocks erode primarily by mechanical chipping.
• Mica schists. Often occupy areas of low relief in the Blue Ridge. They are generally very susceptible to chemical and mechanical weathering.
• Sandstone and quartzites