Silviculturists generally classify silvicultural systems into two broad groups: "high-forest" methods that rely on reproduction from seed and "low-forest" or coppice methods that rely on sprout reproduction (Smith 1986). Within high-forest methods, there are three subdivisions:

• Even-aged systems use regeneration harvests to create even-aged stands. By convention, the spread of ages in an even-aged stand does not differ by more than 20 percent of the intended rotation. Even-age methods include the clearcuttting, shelterwood, and seed-tree methods.
• Uneven-aged systems use regeneration harvests to create uneven-aged stands. Commonly, at least three distinct age classes are required for an uneven-aged stand. Uneven-age methods include single-tree selection and group selection.
• Two-aged systems provide for regeneration of shade-intolerant species while carrying a sparse overstory of mature trees to offset the negative aesthetic impact of clearcutting.

Either natural or artificial means can be used before or after regeneration harvests to establish new age classes. These concepts are discussed in the following section.

• Natural vs. Artificial Regeneration Methods

An even-aged silvicultural system is a planned sequence of treatments designed to maintain and regenerate a stand with one age class (see figure below). The range of tree ages is usually less than 20 percent of the rotation length. There are three regeneration harvest methods used in even-aged systems: the clearcutting, the shelterwood, and the seed-tree methods. These systems vary in the amount of residual stand left after harvesting and the purpose of these residual trees.

Clearcutting regenerates an even-aged stand in which a new age class develops in a fully exposed micro-environment after removal of all trees in the previous stand in a single cutting. Regeneration can be from natural seeding, sprouting, direct seeding, planted seedlings, and/or advance reproduction. In the shelterwood method, one or more cuttings are made to begin the development of a new age class before the old stand is completely removed. Regeneration in shelterwoods is primarily from advance regeneration fostered by opening of the canopy to increase light levels and allow seedlings and stump sprouts to grow. The seed-tree method is an even-aged management system in which only a few widely spaced residual trees are maintained on site as seed sources. The seed-tree method is very similar to the shelterwood method, differing only in the amount of residual stocking left during seed cutting and the purpose of these trees. In the seed-tree method, fewer trees are left on site and these residual trees serve only as a seed source (seed-trees). Coppice silviculture is also considered an even-aged system, but is discussed in a separate section.

Cycle of an Even-Aged
Silvicultural System

The choice of an even-aged regeneration method will depend on both landowner objectives, which may include wildlife, water, and aesthetic objectives in addition to timber, and the species and forest type under management. Clearcutting is less costly than other methods, due to fewer stand entries. It therefore is the preferred method for reproducing appropriate species, such as loblolly, shortleaf, and white pines, in individual forests. Clearcutting has historically been the most widely applied regeneration method in the management of oak forests. However, the clearcutting method has not always regenerated oaks, particularly on productive sites. Also, clearcutting produces the most drastic changes in microclimate, wildlife habitat, and aesthetics, and therefore may not be an attractive choice for landowners concerned with these nontimber forest benefits (Hicks 1998).

Historically, foresters have used shelterwood and seed-tree methods as alternatives to clearcutting. Foresters normally use the seed-tree method with light-seeded, wind-disseminated, shade-intolerant species. The seed-tree method has been widely used with loblolly and shortleaf pines. The seed-tree method, however, is less successful than the shelterwood method for hardwood management if the objective is to maintain or enhance the oak component of the forest. Like clearcutting, the seed-tree method opens up the stand to exploitation by pioneer species such as yellow-poplar, which may or may not be desirable. The shelterwood method can be very successful in hardwood regeneration when the number, intensity and sequence of cuts, intervals between cuts, and supplementary treatments applied are carefully tailored to species and stand conditions in which it is used (Hicks 1998).

Most even-aged regeneration systems in hardwood management rely on natural regeneration, but in some cases artificial regeneration (planting or direct-seeding) is used as a primary or supplemental source of regeneration. A situation where artificial regeneration is especially applicable in the central hardwood region is planting of abandoned fields with pines (eastern white pines, southern pines, or hybrid pines). Supplemental underplanting with oaks has been recommended in conjunction with tree shelters in areas with high deer impact (Hicks 1998).

The clearcutting method is an even-aged regeneration system that removes the entire overstory (the mature trees) in one operation. Landowners may use either natural or artificial means to establish new trees. Foresters differentiate between the clearcutting method or clearcutting system, which is designed to enhance the regeneration of a new even-aged community, and clearcutting, which simply defines the harvesting method of clean felling (Helms 1998, Soc. Am. For. 1989). In contrast with the clearcutting method, commercial clearcutting or economic clearcutting is a type of exploitative cutting in which there is little regard for regeneration; all merchantable trees are removed and the unsalable trees are left standing (Helms 1998, Wenger 1984, Soc. Am. For. 1989, Nyland 1996).

There are several variations of the clearcutting method. The alternate-strip or -patch method is a clearcutting technique in which strips or patches covering one-half the stand are cut during the first entry and the remainder are cut at an appropriately timed second cutting. The progressive-strip or -patch method is a technique in which strips or patches are cut in progressive series over three or more entries, covering an equal area on each occasion (Nyland 1996).

The primary advantage of the clearcutting method is that it provides the sunlight required for the development and growth of moderately to highly shade-intolerant species such as oaks (Kellison and others 1988, Clatterbuck and Meadows 1993). The basic ecological premise behind the clearcutting method is to redistribute the resources of the site to a new crop by removing the existing stand. This type of cutting is designed to mimic natural disturbances, such as fire, windstorms, and catastrophic insect and disease outbreaks, which promote regeneration of species that have evolved to exploit these conditions. Such disturbances are relatively common in the central hardwood region. They occurred historically at intervals of one to a few hundred years (Abrams 1992). Clearcutting may also improve food resources for wild animals or increase water yields from a site (Hicks 1998).

Numerous advantages and limitations of the clearcutting methods have been noted.

See also: Application of the clearcutting method for oak management

Nyland (1996) listed the following general advantages and limitations of the clearcutting method:

Advantages of the clearcutting method

1. High yields per unit of area potentially lower the harvesting costs.
2. Setup and control require few technical skills, except for the skid trail system.
3. Brightness of the area will sustain even the most shade-intolerant species, and promote the rapid growth of most species.
4. Cutting all trees facilitates site preparation to control pests and competing vegetation, improve seedbeds, and ameliorate soil deficiencies (for example, by cultivation, drainage, or fertilization).
5. Easy access by machines simplifies artificial regeneration.
6. Clearcutting controls pests that damage older trees left by partial cutting.
7. Clearcutting facilitates natural regeneration of species with serotinous cones.
8. Clearcutting precludes blowdown of residual trees, and removes decadent trees from a site.
9. High density of the new community promotes early lower branch mortality, formation of long clear boles, and less taper on surviving trees.
10. Herbaceous vegetation and woody shoots close to the ground provide abundant food and excellent cover for many birds and small mammals.
11. Limiting the regeneration period to a small part of the rotation facilitates later uses, such as grazing or recreation.
12. When applied systematically across a forest ownership, clearcutting creates well-defined age classes in distinct stands, simplifying the management for evenflow sustained yield from a forest.

Limitations of the clearcutting methods

1. Landowners must depend on stored seeds, and those dispersed into the site from adjacent sources.
2. Any shortage of seed on site limits regeneration to light-seeded species, barring an unusual dispersal mechanism.
3. The abundance and uniformity of any particular species, and the spacing and species composition of a new stand depend on an uncontrollable seed supply.
4. Dependence on seed trees in adjacent stands and seeds already stored on site reduces the chance to control the seed source for genetic improvement of the new community.
5. Cutting during poor seed years may lead to regeneration failure or irregular stocking, and particularly with species that have a distinct periodicity for seed production.
6. The open environment may inhibit some species, and will favor many herbaceous plants that impede the regeneration of desirable trees.
7. Dense competing vegetation or harsh soil conditions may require costly site preparation.
8. Soils with a shallow depth to the water table may become saturated or waterlogged due to reduced transpiration, inhibiting seed germination and reducing seedling survival.
9. Reduced transpiration increases percolation and subsurface flow, and accelerates nutrient leaching until a new vegetation cover develops It also increases the chances of mass soil movement on steep slopes.
10. In flattened or concave topography, the lack of overstory protection may increase the chance of freezing temperatures early in a growing season killing or damaging all but frost-resistant species.
11. On dry sites, the unshaded surface may become unsuitable for many species.
12. Disturbance of the surface litter during logging displaces stored seeds and increases chances for surface erosion on hillsides, at least until new plants colonize the site.
13. Overstory removal precludes a second chance for regeneration if unusual conditions cause failures immediately after clearcutting.
14. Prolonged litter decomposition in areas that do not regenerate promptly may change moisture balance and nutritional status of the soil.
15. Removing all the mature trees, leaving abundant logging slash and fresh stumps, and exposing soil across the area degrades the visual quality for many forest users.
16. Abundant dry logging slash increases the fire danger during dry periods, and provides ideal habitat for some harmful insects and small mammals.
17. Resulting even-aged communities have less resistance than uneven-aged stands to snow and wind damage.
18. Removing all the large trees eliminates essential habitat for some wildlife.

The seed-tree method is an even-age management system in which only a few widely spaced residual trees are maintained on site as seed sources. Foresters use the seed-tree method where an inadequate seed supply might preclude success after clearcutting. Similar to the shelterwood method, the seed-tree method involves two or three cutting treatments (Nyland 1996):

1. Preparatory cutting: a cutting designed to remove poor quality trees and to increase vigor and seed production among the residuals.
2. Seed cutting: a cutting performed to open the stand sufficiently to encourage the development of regeneration.
3. Removal cutting: a cutting that is done after regeneration is established to remove the seed trees and to allow the new stand to grow.

The partial cuttings in the seed-tree method share similar objectives with the shelterwood method. Retention of the seed trees ensures a seed source for a long period in case there is difficulty in obtaining regeneration immediately after cutting (see selecting seed trees). Opening the canopy is expected to stimulate seed production of the residuals and to provide additional light to the forest floor that should promote growth of the new regeneration (Marquis 1979). The two approaches differ only in the amount of residual stocking left during seed cutting and the purpose of these trees. In the seed-tree method, fewer trees remain after the seed cutting and these residual trees serve only as a seed source (Nyland 1996).

See also: Precautions for harvesting seed and reserve trees

Some advantages and disadvantages of the seed-tree method have been noted.

See also: Application of seed-tree method for oak management

The primary advantage of the seed-tree method is that trees of the best phenotypes can be retained as seed sources. Compared to the clearcutting method, the seed-tree method has fewer constraints over the size, shape, and orientation of a regeneration area. However, compared to the shelterwood method, the seed-tree method leaves insufficient residual stocking to mitigate changes in environmental conditions. In fact, changes in levels of light, temperature, soil moisture, and other physical site attributes are similar to those after clearcutting (Nyland 1996).

Seed trees may deteriorate in quality or vigor due to their extra-wide spacing. In contrast, the shelterwood method leaves a higher residual stocking (closer spacing between the seed trees) and thus provides more protection. Whether the change in exposure enhances or harms a tree depends upon its silvical characteristics, and its health and condition at the time of seed cutting. For certain species, vigor will improve and the diameter increment will increase. However, for species requiring protection from exposure, the seed cutting should leave at least 40 percent canopy cover (Klinka and Carter 1991), or the shelterwood method should be used. The seed-tree method is not a good option for stands with a high risk of blow-down, such as along ridge tops with shallow soil, or on poorly drained flats along the base of slopes and in the valleys (Nyland 1996). 

As an alternative for some community types, foresters may leave scattered groups or strips of seed trees to provide mutual protection within these clusters (Cheyney 1942, Smith 1986). However, this technique does not ensure good growth of seed trees. In fact, only shelterwood and seed-tree cuttings that leave uniformly dispersed seed trees ensure adequate crown release to effectively stimulate diameter increment (Nyland 1996).

Snags that ultimately develop from crown die-back and deterioration of seed trees can provide valuable habitat for cavity nesting birds and standing dead wood essential to preserving biodiversity (Franklin 1989, Hansen and others 1991). Snags can be created by girdling trees in the original stand that have little or no value for seed production or other purposes (Johnson 1993).

In the shelterwood and seed-tree methods, seed or reserve trees are removed when the regeneration of desirable species has reached an adequate size and density. This removal inevitably damages or destroys a portion of the new cohort. In some cases, a poorly stocked new stand is the result. Many foresters consider this problem the most important shortcoming of the shelterwood method and the major deterrent to its more widespread use (Nyland 1996).

Foresters can minimize damage to regeneration from removal of seed and reserve trees by utilizing careful logging practices and by appropriately timing the removal cutting. The harvesting plan should include provisions to:

• Keep landings outside the regeneration areas.
• Limit ground skidding to a minimum number of well-marked trails, and preferably those established for the seed cutting.
• Carefully clear skid trails of old logging debris and windfalls without pushing them into the adjacent regeneration.
• Place skid trails away from sensitive microsites such as wet or thin soils, or highly erodible areas.
• Require directional felling to minimize turning and twisting of the logs during skidding.
• Keep tractors on the trails to the degree possible, and use winches to move the logs from stumps to skidding corridors.
• Restrict skidding to fully limbed pieces with no large side branches.
• Remove unmerchantable logs by the same procedures required for the original slash disposal, or leave them in place to decay (Nyland 1996).

By using the same trails for both the seed and removal cuttings, foresters can protect much of the new age class from skidding damage. After logging, waterbars should be installed and skid trails graded to minimize surface runoff. Colonization of skid trails by herbaceous plants stabilize the soil until a new litter layer forms and provide food for many wild animals (Nyland 1996).

Foresters can also reduce damage through the timing of removal of seed and reserve trees. For example, trees can be removed while the seedlings are still flexible. With hardwoods, small and flexible seedlings bend over or break off close to the ground. These bent or broken seedlings will resprout from the root collar and the new shoots often reach the prebreakage height within 2 to 3 years. On the other hand, larger seedlings tend to be uprooted or to break well above the ground. In the southern Appalachians, however, oak seedlings should be at 4.5 feet tall before reserve trees are removed. Thus, the timing of this harvest may not be as flexible as in other community types (Nyland 1996).

The shelterwood method usually is an even-aged management system, but can also be a two-age system. In it, one or more cuttings are made to begin the development of the new age class before the old stand is completely removed. Foresters use the shelterwood method where an inadequate seed supply or a sharp change of environmental conditions might preclude sufficient regeneration after clearcutting. The shelterwood method involves two or three cutting treatments extended over a 15 to 30 year period (Nyland 1996):

1. Preparatory cutting: a cutting designed to remove poor quality trees and to increase vigor and seed production among the residuals.
2. Seed cutting: a cutting performed to open the stand sufficiently to encourage the development of regeneration.
3. Removal cutting: a cutting that is done after regeneration is established to remove the overstory and to allow the new stand to grow.

The partial cuttings in the shelterwood method share similar objectives with the seed-tree method. The two approaches differ only in the amount of residual stocking left during seed cutting and the purpose of these trees. In the seed-tree method, fewer trees remain after the seed cutting and these residual trees serve only as a seed source (Nyland 1996).

Partial cuttings in the shelterwood method accomplish several goals. First, good phenotypes are selected as reserve trees with the expectation that they will contribute good genes to the next generation. Retention of the overwood ensures a seed source for a long period in case there is difficulty in obtaining regeneration immediately after cutting. Second, the opening of the canopy stimulates seed production of the residuals and provides additional light to the forest floor, promoting growth of the new regeneration (Marquis 1979). Third, shelterwood cuttings can be tailored to favor particular species by providing enough light to encourage some species but not others; regeneration under such cutting is usually composed of shade intolerant species, similar to those that regenerate after clearcutting. Finally, shelterwoods mitigate some of the adverse aesthetic effects of clearcutting while maintaining some of the qualities of mature forests, such as hard mast production. Shelterwoods are commercially attractive because a high percentage of the stand basal area is removed in the initial cut and the high-quality trees that are left have the capacity to increase in value after they are released (Hicks 1998).

Variations of the Shelterwood Method

There are several variations of the shelterwood method; some methods vary by the number of cuttings. The one-cut shelterwood method is used when adequate advance regeneration exists and preparatory or seed cuttings are not needed. This method resembles clearcutting, except that most of the new age class already is present prior to overstory removal. The two-cut shelterwood method includes only a seed cutting and a removal cutting, and the three-cut shelterwood method includes all three cuttings (Nyland 1996).

Other shelterwood methods differ in the amount of time between cuttings. In the reserve shelterwood system, the overstory is maintained for more than 20 percent of the length of the rotation. In deferment cutting, harvest of the overstory is deferred through a complete rotation of the regeneration (Miller and Schuler 1995). In the two-age silvicultural system, the overstory and new regeneration are tended as a two-aged stand (Sims 1992). It appears that the differences between these methods have more to do with their objectives than with the way they are applied or how the new stand develops (Hicks 1998). Though attractive in many respects, these systems have several operational and financial disadvantages (Nyland 1996):

• A greater chance of damaging reserve trees during cuttings
• Higher risk of reserve tree loss due to exposure, blowdown, or old age
• Difficulties in maintaining an open overstory
• Heavy losses of the new cohort during eventual reserve tree removal, and
• Higher logging costs due to multiple entries and precautions taken to minimize damage to both cohorts

Alternative approaches include the group- or strip-shelterwood systems, which vary from other methods in the shape and arrangement of cuttings. In the strip-shelterwood method, which resembles strip clearcutting, the mature age class is removed over a series of entries in narrow parallel strips not exceeding the height of adjacent standing trees. The adjacent residual strips provide seeds and partially shade the openings. To maximize seed dispersal and reduce chances of blowdown, strips can be oriented at right angles to the prevailing winds. Where side shading is more important than seed dispersal, strips can be aligned in an east-west direction to minimize direct insolation. In the group-shelterwood method, the seed cutting creates well-dispersed openings, each with a diameter not exceeding the height of adjacent trees. Once seedlings are established, a band or ring of residual trees around all or part of each opening is removed. Additional patches can be cut at each entry to create new pockets of regeneration throughout the stand (Nyland 1996).

See also: Application of the Shelterwood Method for Oak Management

In the seed-tree method, trees selected as seed trees serve as both the gene source of new cohorts and as a source of timber in removal cuttings. Reserve trees in the shelterwood method are also used as a timber source. Therefore, both seed trees and reserves trees must be chosen carefully based upon both economic and biological factors. Trees selected as seed or reserve trees should be:

1. Phenotypically superior, to insure good growth and form in the new community, as well as resistance to local insect, disease, and other pest problems
2. Prolific in seeding and flowering to sustain a high level of seed production (both sexes of dioecious species must be included)
3. Sturdy and healthy enough to withstand wind and sun exposure and remain alive until the removal cutting
4. Free of logging injuries that might serve as entry courts for disease organisms, or root damage that might make trees more susceptible to blowdown
5. Of good growth form: single, dominant stem, no major forks; dominant or codominant crown position; no more than 10 degree lean from vertical; no dead or dying major branches; no signs of developing epicormic branches
6. Species not prone to dieback or decline after heavy cutting (Nyland 1996, Hicks 1998).

In some cases, trees with actively used upper-stem cavities may be selected to maintain essential nesting and denning sites for a variety of birds and animals. Yet even these trees should have good vigor, and generally desirable characteristics (Nyland 1996).

Susceptibility to wind depends upon physical site factors such as soil depth, texture, and moisture, aspect and slope, as well as biological attributes of species and individual trees, such as rooting depth, branching patterns, crown size and density, and height-diameter ratio. Foresters cannot alter topographic and edaphic conditions, but they can consider the hazard in choosing among shelterwood, seed-tree, and other methods. For the most part, stands along ridgetops with shallow soil, and on poorly drained flats along the base of slopes and in the valleys, have the highest risk of blowdown. In such places, the seed-tree method often is not a good option. With the shelterwood method, foresters can leave a higher residual stocking (closer spacing between the seed trees) providing a reasonable degree of protection from windthrow. Other options include progressive strip or patch clearcutting and artificial reforestation (Nyland 1996).

Residual stocking is the stand density resulting from cutting to select reserve trees in the shelterwood method or seed trees in the seed-tree method. The residual stocking for a cutting varies by method (shelterwood vs. seed-tree method) and can vary appreciably from stand to stand, even for a single method. One must take into account the minimum number of trees needed for a reliable seed supply and the minimum needed for sufficient canopy cover. At the same time, shade from the overstory canopy must not preclude germination or inhibit seedling development. Additional factors to be considered include (Nyland 1996):

• The minimum volume to eventually sustain a commercial removal cutting
• The volume of wood that will accumulate on the seed trees between the seed and removal cuttings
• How rapidly crowns of the residual trees close and light near the ground drops below acceptable levels
• The ease and cost of logging during the seed and removal cuttings
• The extent that logging will damage the regeneration during removal cutting

Advance Regenerating Systems

In southern Appalachian oak forests, some of the larger trees are left primarily to inhibit competition from shade-intolerant species. In these systems, the success of shelterwoods depends in a large part on the presence of abundant advance oak regeneration. In stands having these advance seedlings, foresters can reduce the overstory to about 60 to 70 percent of the stocking found in unmanaged stands, removing trees in lower crown positions and keeping the overstory largely intact. On site indexes of 70, 80, and 90 feet, recommended residual basal areas are 60, 65, and 70 percent of initial basal area, respectively. Once the oak seedlings reach about 3 feet (0.9 m) tall, foresters should reduce the stocking to about 50 percent of that in unmanaged stands (Loftis 1990, Loftis 1990).

Non-advance Regenerating Systems

In community types where seed trees are left to serve as a seed source for a new cohort, as in true seed-tree methods, foresters must consider all the factors that affect natural seeding from these residual trees when determining residual stocking. These factors include (Nyland 1996):

• Durability of seed trees to exposure
• Quantities of seed produced per tree based upon its health, vigor, age, and species
• Frequency of anticipated good seed crops, and amounts produced during subsequent years
• Direction and velocity of prevailing winds
• Heights of seed trees
• Duration of seed viability after dispersal
• Seedbed characteristics
• Expected germination rates, and subsequent survival of new germinants
• Density of seedlings for effective site utilization

The following formula can be used to determine the average numbers of seed trees to leave in the shelterwood or seed-tree methods (Nyland 1996):

seed trees/unit area = (c / F) * D / (G * N * Y)

where:

c = land unit conversion (= 43,560 ft²/acre, = 10,000 for m²/hectare)
F = area of ground space (ft² or m²) covered by seeds falling from a single tree
D = the density of seedlings needed per unit of area (ft² or m²) to occupy the site
G = percent seed viability
N = number of seeds cast per tree
Y = seedling survival factor

See: Stocking

Stocking Charts for Oak History Forests

Stocking equations, and charts derived from them, are commonly used as standards for defining and controlling the density of oak stands. Currently available standards express stand density as "stocking percent," a measure of relative stand density (Stout and Larson 1988). Related stocking charts specify the upper and lower limits of absolute measures of stand density (i.e., basal area and numbers of trees per acre) that define the normal range of residual stocking for thinning and other silvicultural operations. The upper and lower levels of this range are called A and B levels, respectively. A level, also called 100 percent stocking, represents stands at average maximum density. B level represents the minimum stand density at which trees use all the growing space, assuming trees are well distributed. There are stocking charts for the oak-hickory forests of the Central States (Gingrich 1967), and northern red oak forests of New England (Sampson and others 1983) and Wisconsin (see figure) (McGill and others 1991, Isebrands and Dickson 1994).

Stocking percent provides a better measure of the degree of tree crowding than basal area alone because large trees require less space, proportionate to their basal area, than small trees. Thus, for a given basal area per acre, stands comprised of large trees are less crowded than stands comprised of small trees. The application, derivation, advantages, and limitations of stocking concepts are discussed in more detail by Ernst and Knapp 1985, Gingrich 1967, Leak 1981, Leary and Stansfield 1986, Stout and Larson 1988, Isebrands and Dickson 1994.

Uneven-aged silvicultural systems, also called selection systems, are planned sequences of treatments designed to maintain and regenerate uneven-aged stands. Uneven-aged stands are defined by several developmental and structural features: they contain multiple cohorts of trees, typically with three or more age classes, and they have a reverse-J diameter distribution. In theory, selection systems provide a sustained yield through the harvesting of mature and excess immature trees while creating a new age class with each entry and maintaining a predetermined diameter distribution among immature age classes (Nyland 1996).

Uneven-aged systems may use several variants for the selection method, which is defined as the regeneration harvest or reproduction method used to remove mature trees, improve the stand by removing undesirable or lower quality individuals, and provide space and seedbed conditions for the regeneration of new trees (Mills and others 1987). The single-tree selection method is used to create new age classes in uneven-aged stands in which individual trees of all size classes are removed more or less uniformly throughout the stand. Single-tree selection methods are most appropriate for stands in which primarily shade-tolerant species are desired. Therefore, the single-tree selection method generally is considered inappropriate for regenerating oak forests and this method is very restricted in its application in the southern Appalachians (Nyland 1996).

In the group selection method, trees are removed and new age-classes established, in small groups. Group selection is similar to single-tree selection in that it involves periodic cuts that create a balanced uneven-aged stand. The distinctive feature of group selection is that these cuttings are concentrated into fewer gaps of larger sizes. As a result, intermediate and shade-intolerant species can be regenerated (Roach 1974). Variations of the group selection method include the patch selection method, the strip selection method, the group shelterwood method, and group selection with reserves (Hicks 1998).

The basic steps of how to apply selection systems as well as their general advantages and disadvantages apply for both single-tree and group selection.

Selection systems should not be confused with selective cutting. While selection systems apply planned silvicultural treatments to individual stands to control all age classes between cutting cycles, selective cutting removes trees of high value for short-term benefit and largely disregards regeneration and other sustained yield principles. There are several important differences between selection systems and selective cutting. By carefully regulating diameter distributions and residual stocking, selection systems stabilize stand structure and optimize volume production over repeated cutting cycles.  Theoretically, each age class in a well-balanced stand has just the number of trees needed to fully capture a proportionate share of the incoming solar energy. By contrast, exploitive cuttings that remove only large trees do not maintain an appropriate diameter distribution, do not regulate the spacing, and do not control stocking at optimal levels. Generally, they lead to reduced production, structural instability, and an irregular age distribution. Due to the continued lack of tending of the small pole and large sapling classes, large numbers of slow growing trees accumulate just below the cutting diameter limit. Selective cutting also leads to dysgenic selection, which is the premature removal of the best genotypes (Smith 1986, Zobel and Talbert 1984). Dysgenic selection can be averted only if the defined age classes are narrow, the oldest and largest trees within each age class are cut, outstanding phenotypes in younger age classes are retained, and poor phenotypes in each age class are removed in each cutting cycle (Howe 1989, Johnson 1993, Nyland 1996).

Diameter-limit cutting, high-grading, creaming, and culling are all terms that fall under the broad definition of selective cutting. Even though partial cuttings, such as diameter-limit cutting, may occasionally produce good results, they are not management. Hicks (1998) argues that management involves planning, goal setting, and implementation. Unfortunately, diameter-limit cutting as it is applied on NIPF ownerships can lead to impoverished stands with no good options for future management (Hicks 1998).

Uneven-Aged Stand Structure

Uneven-aged stands have several structural and developmental features. They contain multiple cohorts of trees that develop either as a consequence of natural mortality and canopy gap formation or as a result of cuttings made during multiple entries into the stand at relatively short intervals (10-30 years). Uneven-aged stands usually possess a reverse J-shaped diameter distribution, with large numbers of small trees and relatively few large-diameter trees (figure a). In reality, each cohort has its own diameter distribution, and the overall stand distribution is a composite of these (figure b). The logarithmic diameter distribution of uneven-aged upland hardwood stands resembles a rotated-S, with a changing rate in the difference of trees between progressive diameter classes. The three different sections of the rotated-S distribution are due to an understory class with high mortality due to shading, a vigorous upper canopy stratum, and a senescent overstory class (Nyland 1996).

The reverse-J curve (figure a) that depicts the diameter distribution of a balanced uneven-aged stand represents a composite of the component age class distributions (figure b), where each age class occupies an equivalent amount of space (figure c) with the numbers of trees needed to fill that space dependent on the average crown spread for trees in each different age class (figure d).

In balanced even-aged stands, diameter and age classes represent roughly the same cohorts. In these stands, each age class occupies an equivalent amount of horizontal crown space. Thus, in a stand containing four age classes, each age class occupies 25 percent of the space (figure c). Generally, young (small) trees each take up a fairly limited amount of horizontal space, while older trees occupy a large horizontal area (figure d). Selection system stands should have at least three age classes (Smith 1986) to provide a consistent yield of commodities (Nyland 1996).

Uneven-aged stands are an aggregation of small even-aged patches, which, depending on the management intensity or perspective, may be seen as individual small "stands" (Smith 1986). Larger gaps apparently regenerate so as to resemble small even-aged stands. In naturally occurring canopy gaps in the southern Appalachians, Clinton and others (1994) noted that both density and diversity of regeneration were positively correlated with gap size. For purposes of defining uneven-age stands, Smith (1986) limits the opening size to that which is smaller than twice as wide as the height of mature trees, but large enough that its center is not under the microclimatic influence of adjacent mature trees. Thus, the range of stand sizes considered as minimum for even-aged and maximum for uneven-aged management appears to lie between 0.5 and 2.0 acres (Hicks 1998).

Selection cutting can improve individual tree survival and development in uneven-aged stands. It has been shown that for northern hardwoods (Eyre and Zillgitt 1953):

• Radial increment generally increases in proportion to the intensity of cutting, but light cutting has little benefit
• Intermediate and heavy cuttings better stimulate the growth of trees less than 15 inches (38 cm) dbh, and especially poles and saplings growing in the shade of taller trees
• Appropriate tending insures good rates of volume growth per tree, especially within the small sawtimber classes
• Matching the residual density to the cutting cycle length ensures peak levels of production
• Balanced stands remain structurally stable over repeated cutting cycles

Appropriate control of density and spacing across the diameter classes elevates light levels throughout the canopy and understory of selection stands. Due to the uniform spacing, both the short (young) and tall (older) trees receive more direct and indirect sunlight. Crown vigor and volume improve, increasing the rate of diameter growth (Nyland 1996).

Uneven-aged stands managed by single-tree methods ultimately become dominated by shade tolerant species, because stand openings usually are too small to permit regeneration of intolerant species. Species that become established, persist, and grow reasonably well as advance regeneration, often fill the lower part of a diameter distribution. With regular tending, these cohorts move up through successive diameter classes to eventually become the mature crop trees. This cycle requires tree species with the following characteristics: (1) seeds must be able to germinate in undisturbed litter and a partly shaded environment; (2) regeneration must be shade-tolerant; and (3) species must be long-lived and have the ability to grow well and produce regular seed crops at advanced ages (Nyland 1996).

Yields from selection system stands are from cuttings aimed at harvesting the mature age class and tending immature classes. These cuttings reduce the standing volume or basal area to a specified level. After each cutting, the enhanced growth fosters recruitment of trees into larger size classes and standing volume increases until it approaches its original level. This process is repeated after each cutting cycle (Nyland 1996).

Single-tree selection is used in uneven-aged silvicultural systems in which individual trees of all size classes are removed more or less uniformly throughout the stand to achieve desired stand structural characteristics. The primary advantage of single tree selection is that it maintains tree cover and moderates environmental conditions. As a result, it is well suited for many nonmarket objectives and is ideal for protection forests. There are many other advantages and disadvantages of single-tree selection (Nyland 1996).

Single-tree selection methods are most appropriate for stands in which shade-tolerant species are desired. Therefore, the single-tree selection method generally is considered inappropriate for regenerating oak forests and this method is very restricted in its application in the southern Appalachians (Hicks 1998).

Differences Between Managed and Unmanaged Uneven-Aged Stands

Single-tree selection, in theory, simulates the natural gap-phase dynamics that occur in mature unmanaged natural stands (Bormann and Likens 1979). According to the gap-phase hypothesis, the death of a mature tree creates a canopy gap, and a new cohort develops in the patch of light that reaches the understory. The single-tree selection method differs from gap-phase regeneration in several respects (Nyland 1996):

• Selection cutting creates more gaps per unit area and with a more regular distribution than normally occur in a single year by natural gap-phase replacement.
• Foresters apply the selection method at regular intervals, creating a more uniform age class distribution over time.
• The concurrent thinning of immature age classes between the mature-tree openings creates additional small gaps that generally close before the end of a cutting cycle.

Therefore, compared with unmanaged uneven-aged stands, single-tree selection stands have: (1) greater numbers of seedlings and saplings per unit of area, (2) less distance and more regular spacing between the regeneration openings, and (3) added understory brightening due to periodic thinning and regularly scheduled cutting to recruit new age classes across fixed proportions of the stand area at predictable intervals. Furthermore, single-tree selection theoretically results in a "balanced" uneven-aged stand. In unmanaged stands, a weak correlation between dbh and age results in an irregular distribution of age classes. Such as stand is said to be unbalanced (Nyland 1996).

See also: Application of Single-Tree Selection for Oak Management

Advantages

Uneven-aged selection methods provide the following benefits (Nyland 1996):

1. An interspersion and balance of age classes occurs in perpetuity within each stand.
2. A well-distributed tree cover continually moderates environmental conditions within the soil and near the ground surface, making selection system ideal for protection forests.
3. Spatially uniform and vertically dispersed foliage slows wind movement and reduces blowdown on soils with good rooting depth.
4. Well-tended residual trees with large and well-developed crowns and good rates of radial increment exist in all diameter classes.
5. An abundance of reproductively mature trees ensures an adequate source for regeneration, even with heavy-seeded species that serve as mast for wildlife.
6. Partial shading protects seedlings from extremes of light and temperature, and against wide variations of soil moisture.
7. Moist and cool understory conditions and the dispersed nature of logging slash limit the risks of forest fire, except in years of drought.
8. Age classes are continually upgraded in quality so that trees have high value at maturity.
9. Intermixing of size classes makes stands picturesque to many viewers, and well suited to many nonmarket objectives.
10. Even-flow sustained yield is had in each stand, offering the opportunity for income at frequent intervals.
11. Large-diameter trees are always present to ensure high levels of sawtimber volume growth, and a steady supply of accessible sawtimber.
12. An interspersion of size classes and uniform distribution of canopy provide cover and good habitat for many plants and animals (gray squirrels, scarlet tanagers, etc.), including several adapted to old-growth stands.

Disadvantages

On the other hand, uneven-aged systems have some clear disadvantages, including (Nyland 1996):

1. The stand must be inventoried once in each cutting cycle to secure timely data for prescription making.
2. A skilled marking crew is needed to maintain a balance among the age classes.
3. Because of the interspersion of different age classes, at least some of the residual trees suffer logging damage, even with careful harvesting. Logging destroys many small trees and damages other residuals.
4. Shade-intolerant species commonly fail to reproduce unless foresters cut fairly large group openings or keep residual stocking low.
5. The interspersion of small and large residual trees precludes some methods of site preparation.
6. Failure to regenerate shade-intolerant species or those with special seedbed requirements eventually reduces the diversity of the plant community.
7. Overbrowsing by cattle or wildlife damages or destroys regeneration, leading to an imbalance of age classes.
8. Surface fires kill the younger age classes, reduce the habitat for many plants and animals, and alter other attributes with important nonmarket values.
9. For stands with an excess of small or otherwise unmerchantable trees, landowners must invest in tending, and this work reduces short-term profits.
10. Periodic tending enhances radial growth, but lower branches remain alive longer, reducing the quality of upper logs.
11. Frequent entry for selection cutting requires an elaborate network of carefully planned and permanent skid trails and access roads.
12. Contractors incur high logging costs to remove the widely dispersed sawtimber trees so revenues to landowners are reduced.
13. Short cutting cycles increase the frequency of site disturbance, particularly along main skid trails and at landings.
14. Selection system stands provide poor habitat for animals that depend upon early successional plant communities or high-density stands at early stages of development.
15. Several cutting cycles are needed to establish multiple age classes and create the balanced condition of uneven-aged stands.

In general, application of the single-tree selection system is complex and requires a great deal of information, skill, and effort. Hence, single-tree selection is difficult for small private landowners to apply.

Group selection is an uneven-aged silvicultural system in which trees are removed and new age-classes are established in small groups. Group selection is similar to single-tree selection in that it involves periodic cuts that: (1) establish and develop reproduction; (2) improve stand structure and quality; (3) create a balanced even-aged stand; and, (4) control residual stocking for an even flow of products. These cuttings open the same fixed proportion of stand area in both group and single-tree selection methods. The distinctive feature of group selection is that these cuttings are concentrated into fewer gaps of larger sizes; one advantage is that intermediate and shade-intolerant species can be regenerated (Roach 1974, Nyland 1996).

The basic steps of how to apply group and single-tree selection are similar, as are the advantages and disadvantages. Some studies have shown that the economics of timber harvesting using group selection compare favorably with those using clearcutting.

There are several variations of the group selection method, including patch selection, strip selection, group shelterwood, and group selection with reserves. These methods are explained below.

Patch Selection

Patch selection is a variation of group selection in which groups of trees covering 0.5 to 5 acres are removed (Marquis 1989). The cut is regulated through area control. Equal areas (total acres) are removed in each cutting cycle so that the entire stand is cut over in the equivalent of one even-aged rotation. The area (A) to regenerate at each entry can be determined by:

where r is the life span of an age class and cc is the cutting cycle length. The forest stand under patch cutting exhibits "uneven-aged characteristics even though it is managed as a collection of small, even-aged groups" (Marquis 1989).

See: How to Implement Patch Selection

There often is confusion about the difference between group selection and patch selection. Group selection and patch selection are both used to create uneven-aged stand structure by regulating regeneration, growth, and yield in a given stand. Group selection and patch selection differ in the methods used to determine the size, shape, and location of the openings, as well as the amount of volume removed and the frequency of periodic cuts: patch selection uses area control while group selection uses volume and stand density control (Miller and others 1995).

Strip Selection Method

The strip selection method is very similar in concept to the patch selection method except that instead of using dispersed circular or rectangular patches, the area of several patches is combined into a series of narrow strips that run across the entire width or length of a stand. The strip width and its orientation should fit the silvical attributes of the target shade-intolerant species. Strips are dispersed in a geometric pattern; in new cutting cycles, strips are moved progressively across a stand with each successive entry. There are several advantages of the patch and strip selection methods over the group selection method (Nyland 1996).

Group Shelterwood Method

The group shelterwood method is a variation of group selection in which groups are removed where adequate oak advance reproduction has accumulated. Because adequate oak reproduction occurs irregularly in time and space, the resulting groups are likely to occur irregularly. In some cases, these patches of reproduction can be enlarged by gradually removing the overstory around their perimeter. However, a disadvantage of this procedure is that it tends to follow rather than guide the development of reproduction (Smith 1986, Johnson 1993)

The group shelterwood method has potential application in sustaining timber production where noncommodity values, such as aesthetics, are important and an even flow of wood products is not important. The method can preserve diversity and aesthetics near sensitive recreation, scenic, and other areas that are not regulated for timber production. It also can be used to remove groups of dead or dying trees impacted by gypsy moth, oak decline, or other agents. However, such salvage cuttings may intensify crown dieback in trees that border group openings (Kessler 1992, Isebrands and Dickson 1994)

Group Selection with Reserves

Group selection with reserves is another variant of the group selection method in which some trees within the group are left standing to attain goals other than regeneration. The conditions created are identical to group selection, except for the effects of residual trees (Nyland 1996).

See also: Application of Group Selection for Oak Management

The patch selection method usually combines single-tree selection across most of the stand area, with some small, fixed-size patches at widely scattered locations. In implementing this hybrid system, the total area covered by cutting mature individual trees and the fixed-area patches should not exceed the total space allocated to an age class. Therefore, many of the same steps for implementing single-tree selection also apply to the patch selection method:

1. Select an appropriate residual stand structure, and prepare a marking guide for single-tree selection cutting.
2. Determine the numbers of trees (per unit of area) larger than the diameter threshold that defines financial maturity.
3. Decide how many patches to cut per unit of area, and the size for each one.
4. Determine what proportion of the regeneration area to allocate to these patches.
5. Ascertain the number of single mature trees to retain per unit of area to compensate for the space in patch openings.

When laying out patches, the marking crew should select locations for a predetermined number of patches and mark the intervening area by single-tree selection procedures. Patches can either be arranged at a preset spacing or located according to specific features of the stand. Features sought include: (1) two or more adjacent mature or large-diameter trees, (2) existing advance regeneration of desirable species, (3) clusters of low-vigor trees or trees of less desirable species, (4) pockets of high-risk trees likely to die or deteriorate, (5) excess numbers of trees with similar diameters, and (6) places where cutting would enhance wildlife habitat or other nonmarket values (Law and Lorimer 1989, Nyland 1996).

Group selection methods require a combination of logging activities in a given stand. Logging in some parts of the stand is similar to that in a clearcut, where all merchantable trees are felled and skidded, and nonmerchantable tress are simply felled and left on the site. Logging in other parts of the stand is similar to a thinning or partial cutting in which only individual marked trees are felled and skidded. Overall logging productivity depends on the efficiencies associated with each activity. However, with group selection, the majority of cut volume is removed from openings, where logging efficiency is relatively high.

Miller and others (1995) compared logging productivity for clearcutting and small openings of 13 logging operations (200 work days) on the Fernow Experimental Forest in West Virginia from 1984 to 1993. Stands were second-growth, mixed hardwoods. Site index was 70 to 80 feet for northern red oak. In each operation, the logging crew consisted of three people using a rubber-tired skidder, a crawler tractor equipped with a rubber-tired arch, and a truck-mounted crane. One crew member was responsible for felling, topping, and limbing in the woods. The others were responsible for skidding, bucking, and decking logs. The same crew and equipment were used to complete all logging jobs throughout the study period. Average skidding distance was approximately equal in both treatments, averaging 589 feet. For the group selection method, 0.4 acre openings were located and marked prior to logging. The number of openings in each stand ranged from 8 to 25, depending on the size of the stand. Stands logged by clearcutting ranged from 4.5 to 8.9 acres. In both practices, trees 1 inch d.b.h. and larger were felled. Trees 11 inches d.b.h. and larger were skidded to the landing.

Observed productivity was 9.3 MBF/day for group selection and 8.0 MBF/day for clearcutting. Volume per acre and volume per tree, which both were higher in the group selection cuttings, are factors that may account for higher observed productivity associated with harvesting small openings in group selection. Openings were located in part to harvest clumps of large, mature trees. This technique has the effect of increasing the average harvest volume per acre in parts of the stand where the crew actually works to clear small openings. Therefore, productivity was enhanced by both larger average product size and greater concentration of volume per unit of area in the small openings. In larger clearcuts, volume is dispersed through the stand and average volume per acre is much lower than in small openings.

Miller concludes (1995) that group selection cutting does not reduce stumpage prices compared to clearcutting. However, the added costs of inventories, marking, and sale preparation incurred with each periodic cut must be considered when group selection practices are used.

1. Decide on an idealized diameter distribution, defined by:

   1. Determining the maximum diameter of trees to grow,

   2. Selecting a residual stocking level, and 

   3. Selecting a diameter distribution (Q-structure).

2. Select cutting cycle length.

3. Conduct an inventory to determine how the existing stand diameter compares with the idealized distribution.

4. Construct a marking guide to remove "surplus trees" from diameter classes.

5. Mark and cut the stand to move toward the idealized distribution.

One of the first steps in uneven-aged silviculture is selection of the maximum tree size to be grown. For a given species and site, selection of an upper diameter limit is made by considering value growth rates, diameter growth rates, and ownership objectives (Mills and others 1987). Traditionally, landowners have set the maximum diameter by determining the time it takes a tree to grow from one diameter class to another, estimating the expected increase in average volume and value over a cutting cycle, and calculating the compound rate of return. The diameter for financial maturity (maximum diameter) is the diameter where expected returns from holding a tree for another cutting cycle will no longer equal the required rate of return (Duerr and others 1956, Mills and Callahan 1981, Murphy and Guldin 1987, Nyland 1996).

Since the required rate of return and site conditions differ among ownerships, so will the diameter for financial maturity. Generally, enterprises demanding high rates of return such as a commercial properties will use a smaller diameter for financial maturity, and tracts having lower financial requirements such as public land will grow trees to larger sizes. Where landowners seek primarily nonmarket values and services such as recreation opportunities or wildlife habitat enhancement, they usually make subjective judgments rather than using strict financial criteria (Nyland 1996).

Hardwood Stocking Guide

One of the steps in uneven-aged silviculture is selection of the desired residual stocking level, another component of an idealized diameter distribution. Residual stocking level is normally expressed in basal area of all trees above a certain minimum diameter. Residual stocking varies among and within regions, by site quality and stand composition. For example, for northern hardwoods in New England (Leak and Filip 1977), residual stocking ranges from 65-80 ft²/acre for trees 6 inches d.b.h. and larger. There are no published recommendations for residual basal area for upland central hardwoods, but the general practice is to maintain a residual basal area of 65-75 ft²/acre for trees 2 inches in d.b.h. and larger.

This density is consistent with the B-level line for even-aged upland hardwood stands averaging 9 to 15 inches in d.b.h. (see figure) (Roach and Gingrich 1968, Mills and others 1987).

Another step in uneven-aged silviculture is selection of the cutting cycle length. Common cycles range from 5 to 25 years depending on site quality, species composition, management objectives, and the financial requirements of the logging contractor and landowner. This decision is generally dominated by practical considerations such as how much volume per acre must be available for a profitable commercial harvest. Cutting cycles are usually set so that the stands are harvested when residual growing stock has increased enough to support commercial harvests (Davis and Johnson 1987, Mills and others 1987).

The number of age-class groups in an uneven-aged stand will approximately equal the number of years required to grow a mature tree divided by the years in the cutting cycle. Since each entry for selection cutting recruits a new cohort, foresters can represent the numbers of age classes in selection system stands by: NAC = r/cc

where:
NAC = number of age classes
r = the planned age of a class at maturity
cc = the cutting cycle, or period of years between successive entries to a stand

To facilitate management applications and offer regular and consistent yields (or other values) over time, each selection system stand should have at least three age classes (Smith 1986, Nyland 1996).

One of the last steps in applying the selection system is preparing a marking guide. Before cuttings are made, a marking guide must be made that identifies how many trees in each size class to remove. In theory, cuttings are made to achieve the idealized diameter distribution.

Marking guides are developed by comparing the current stand structure to an idealized stand structure. Smith and Lamson (1982) recommend that the forester determine the fraction of each size class to be removed to create a balanced uneven-aged stand (Hicks 1998). While foresters normally need fairly detailed inventory information to assess the stand structure adequately, the final field marking guide consolidates the 1- or 2-inch d.b.h. classes from the inventory into broader classes to simplify implementation. To make a cutting plan useful to marking crews, silviculturists must reduce the prescription to no more than three or four simple instructions. These need include only the proportions of trees in various size classes that must be cut.

(Table: Marking Guide)

Marking to Enhance Stand Quality and Value

In marking for timber management goals, most foresters use a priority system for choosing trees to leave or cut among the immature classes (Nyland 1996):

1. Risk - Keep trees that have the greatest vigor and the least chance of dying or declining during the ensuing cutting cycle.
2. Vigor - Keep full- and deep-crowned (high-vigor) trees of each age class, and regulate intertree spacing to enhance crown volume and diameter increment.
3. Soundness - Focus the growth on stems lacking unacceptable internal decay, or structural weakness.
4. Stem Form, Crown Size, and Branching Habit - Keep trees with clear straight boles, deep crowns that encircle the main stems; and evidence of a uniform diameter growth rate around the stem.
5. Species - Retain species that optimize values of interest, and have the potential for long-term growth and development.
6. Crown Position - Ensure adequate spatial interspersion of high-vigor trees of different age classes, and maintain a discontinuity of canopy closure to ensure a deep vertical distribution of live foliage among trees growing near one another.
7. Maturity - Adhere to financial or other maturity criteria to open sufficient space for a new age class and to concentrate the growth potential onto younger trees with desirable attributes.

By adhering to these standards over repeated cutting cycles, foresters can progressively improve stand quality and maintain consistently desirable attributes. In the precutting inventory, foresters can note whether each sample tree has acceptable or unacceptable characteristics. This assessment may show that some stands lack adequate acceptable growing stock for meaningful management under a selection system. Some alternative prescriptions for stands with high percentages of unacceptable growing stock are:

1. Reducing residual density, and accepting a longer cutting cycle to match;
2. Lowering the maximum residual diameter when the largest trees have poor vigor, high risk, and little potential for future value growth;
3. Keeping some healthy immature trees of suboptimal grade, form, or species to fill in the structure for one or two additional cutting cycles (Nyland 1996).

Selection systems can be modified to fit different management goals. Advances in biometrics and computer modeling fostered several recent studies of uneven-aged silviculture using Q structures. These studies explored effects of altering maximum diameter, residual density, cutting cycle length, and structural type (the value of Q) to address different economic objectives (Moser 1976, Adams and Ek 1974, Buongiorno and Michie 1980, Haight and others 1985, Hansen 1987, Hansen and Nyland 1987). Findings emphasize that different stand structures result in different patterns of stand development, which optimize different sets of production goals. These advances have increased opportunities for foresters to evaluate various management options. Options might include management to maximize total volume growth, total large sawtimber production, composite stand value increment, or compound rate of return. One also can adjust for the effects of site quality, product potential, or variations in operating costs. Foresters could use a similar approach in assessing treatments for nonmarket values, such as managing the habitat for different wildlife or plants and influencing visual qualities to enhance recreational values (Nyland 1996).

A third component of an idealized diameter distribution is the Q-structure, which defines the ratio between the numbers of trees in adjacent diameter classes in the stand. Foresters calculate the density of trees for the next-to-largest diameter by multiplying the numbers in the largest class by the value Q. In turn, they multiply the result by Q to determine numbers for the next smaller diameter, and repeat the process through each progressively smaller size class (commonly by 2-inch or 5-cm classes). By adjusting the maximum diameter and/or the number of trees in the maximum size class, they can keep total basal area at an acceptable level (Nyland 1996).

Typical Q-factors for 2-inch diameter classes are in the range of 1.7 for young stands on poor sites to 1.3 for regulated stands on good or better sites (Leak and Gottsacker 1985). The lower the Q-factor the greater the percentage of basal area that will be in sawtimber trees. No research-supported standards were found in the literature for upland central hardwoods; however, the accepted practice is to manage for Q-factors between the 1.3 and 1.5 for 2-inch diameter classes (Mills and others 1987).

See also: Limitations of Using Q-Structures

Q-structures define the ratio between the numbers of trees in adjacent diameter classes in the stand. The numbers of trees among progressive age classes in stands managed under the selection system are shown to follow a geometric pattern:

m, mq, mq².... mq^n-1

where m indicates the numbers of trees in the largest of n diameter class. Foresters have referred to such distributions as Q-structures, after the Q-value that defines the regular change of numbers across consecutive diameter classes. A logarithmic equation to describe such a structure has the form (Meyer and others 1961, Davis and Johnson 1987):

log N = log k - qD log e

where:
N = the number of trees in a diameter class
D = the diameter class, or d.b.h.
e = base of natural logarithm
k = the number of trees in the smallest diameter class
accounted for in the inventory
q = the slope of the line, or rate that numbers decline across progressive diameter classes

Plotted on semilog paper, such a structure describes a straight line, with the change between adjacent classes (the slope) equivalent to q (Nyland 1996).

There are several challenges and problems in implementing selection systems. Determining an idealized stand structure is complex and requires a great deal of knowledge, skill, and effort. Maximum tree diameter, residual stocking, and Q-values should be integrated, rather than chosen independently. Moreover, several lines of evidence suggest that Q-structures may not appropriately control stand structure. For example, Nyland (1996) states that in most cases, using Q-values to generate structures by 1-inch (2-3 cm) classes gives unrealistically high values for total tree numbers and basal area, except when very low values of Q (e.g., Q = 1.2) are used. Yet, small Q-values underestimate the numbers of trees needed to regenerate and grow into the sapling and small-pole classes in managed stands (Nyland 1996).

Evidence also suggests that Q-type distributions may have little biological relevance in depicting real patterns of recruitment or upgrowth among uneven-aged selection-system stands (Davis 1966, Leak and Filip 1977). Some observations show that sustainable structures do not have a Q-type distribution at all, and that managed stands have different Q-values for different parts of the diameter distribution (Leak 1978). Computer simulations with northern hardwoods indicate that for structural stability over repeated cycles, managed stands may need a reverse-J distribution based upon unique Q-values for sawtimber (Q=1.2), pole timber (Q=1.5), and sapling classes (Q=1.8) (Hansen 1987, Hansen and Nyland 1987, Nyland 1996).

Because of these complications, many foresters just refer to published tables showing precalculated structures by 2-inch (5 cm) classes. These normally give distributions for several separate Q-values, levels of residual density, and maximum diameter (Smith and Lamson 1982). However, for community types where research has not yet identified a sustainable structure through cutting trials, Q-values must suffice as a convenient interim means for controlling selection cutting. Periodic remeasurement will eventually indicate if too many or too few trees develop in the smaller size classes. Simulation approaches have been used to calculate optimal and sustainable diameter distributions, but  significant computer time and expertise is required to apply these procedures (Mills and others 1987, Nyland 1996).

Two-aged silvicultural systems involve a planned sequence of treatments designed to maintain and regenerate a stand with two age classes. Two-age methods generally involve removal of all but 10 to 40 ft²/acre of basal area. The residual overstory trees are termed reserves or standards. The limited number of reserve trees allows abundant light to reach the forest floor and provides for the rapid growth of the understory. In contrast to the shelterwood method, the reserve trees will be left standing for a second rotation, thus maintaining two predominant age classes. Generally, the treated area is subjected to site preparation treatments similar to those used after clearcutting (Stringer 2000).

In two-aged systems, stands continue to develop through all successional stages with residual trees in place. In later successional stages, the younger age class will begin to merge vertically with the older age class. Depending on the kinds of trees initially retained, later successional stages may contain trees much larger than would normally be found in mid- or late-successional stands. Therefore, at least some of the attributes of much older stands can be provided in stands managed with this system (SAMAB 1996).

After 40 to 60 years, several options exist for future treatment of residual trees in two-aged systems, depending on management objectives: (1) the older trees can be retained into the future along with the younger age class, (2) the older age class can be removed, leaving the younger age class as an even-aged stand, or (3) the regeneration process can be initiated again by removing the older age class and some of the younger age class, perpetuating the two-aged stand condition (SAMAB 1996).

• Clearcutting with Reserves: A clearcutting method in which varying numbers of reserve trees are left standing to attain goals other than regeneration. The overstory trees retained, called reserve trees, may be small or large trees, or combinations of small and large trees. They may be retained for future growth, certain species components, current or future den trees, future sources of snags or coarse woody debris, or some level of visual quality.
• Coppice with Reserves: A method of regenerating a stand in which the majority of regeneration is from stump sprouts or root suckers, and in which reserve trees are retained to attain goals other than regeneration. The conditions created with coppice with reserves are the same as with clearcutting with reserves or shelterwood with reserves. Conditions depend on the number of reserve trees retained.
• Seed-Tree with Reserves: A seed-tree method in which some or all of the seed trees are retained after regeneration is established to attain goals other than regeneration. The conditions created in a seed tree with reserves are identical to those created by clearcutting with reserves. The only difference between the two systems is that in the regeneration period, the trees retained have the specific function of producing seed to regenerate the stand.
• Shelterwood with Reserves: A variant of the shelterwood method in which some or all of the shelter trees are retained well beyond the period of normal retention to attain goals other than regeneration. Initial conditions created are identical to those for the even-aged variant of this method-- a micro-environment moderated by retention of residual trees. However, retaining overstory trees beyond 20 percent of the rotation creates a distinct two-storied condition that persists for 20 to 40 years (SAMAB 1996).

As in other systems, the choice of residual trees in the two-aged system is dictated by management objectives. Choosing residual trees for cavity trees, mast producers, growth, future snags or coarse woody debris provides the values associated with those trees (Stringer 2000).

Coppice methods use vegetative means to regenerate species; tree crops originate mainly from shoots and suckers, and are grown in relatively short rotations (Helms 1998, Society of American Foresters 1989). Coppice methods are applicable only to the species and individual trees that have a capacity to sprout or sucker. That requirement limits coppice systems mainly to stands of broadleaved species, and to trees of young to moderate ages.

The coppice method involves clearfelling all trees in the original stand in blocks, strips, or patches. Clearfelling allocates total space to the new age class, and makes maximum resources available to the sprouts and suckers. Because the new shoots live off well-established and large root systems of the parent trees, with many absorbing and actively growing tips, new coppice grows rapidly and forms a closed canopy sooner than in even-aged seedling stands (Nyland 1996).

Two variants of coppice methods are distinguished by whether they include trees of seed origin. Simple coppice methods retain only trees of sprout origin, while the coppice-with-standards method rentains both sprout- and seed-origin trees (Nyland 1996).

Simple coppice methods retain only trees of sprout origin, and vary according to the location of sprouts (from stumps or roots) and the length of rotation (Nyland 1996).

Coppice Methods Based on Stump Sprouting

There are several considerations in using coppice systems with stump sprouts. Sprouts that emerge close to the ground or from the root collar are less likely to develop decay than sprouts off the tops or upper parts of tall stumps. Sprouts attached to a large mass of decaying stump also are more likely to develop rot. Perhaps for that reason alone, sprouts from younger trees (less than 35-40 years old) do not develop decay as readily as sprouts from older trees. Foresters can control these problems by having the logging crews cut the trees close to the ground, and by limiting the rotation length (Nyland 1996).

Stools repeatedly coppiced decrease in sprouting capacity after several generations, so that after about three rotations landowners must replace the old stools. In studies, average shoot height and diameter from surviving stools decreased with each successive cycle. With sycamore, trials have shown that annual coppicing reduces the number of sprouts, their basal diameter, their total height, and their green weight compared with rotations of 3-4 years (Kennedy 1975, Schmeckpeper and Belanger 1985). Additionally, as many as 15 percent of the stumps may not sprout due to logging damage (Zobel and others 1987). With alder and locust, only stools with moderate to heavy damage decline in sucker productivity (Nyland 1996).

While coppicing via stump sprouts normally produces a multistem clump, the weaker sprouts die as a clump develops. Numbers per clump decline fairly rapidly during early stages of stand development, leaving only one or two by large pole or sawtimber sized stems (Stroempl 1984). With long rotations, this self-thinning leads to a normal-looking community of trees that pose no particular problems during eventual harvesting. In short rotations, logging crews must use equipment suited to cutting multistem stumps close to the ground. Otherwise they leave tall stumps that give rise to poor-quality sprouts (Nyland 1996).

Short-Rotation Coppice Systems

Interest in using wood as an energy source led to several innovative coppicing systems. Generally, species that reproduce vegetatively by stump sprouts were used in plantations. Short- and mini-rotation coppice crops appeared to offer considerable potential for producing high volumes of wood fiber on a relatively limited land area. These schemes usually include production of woody biomass in rotations of 1 to a few years. Species of Alnus, Platanus, Populus, and Salix have shown great promise for short-term fiber crops in temperate regions. One early scheme tested in the Southeast became known as silage sycamore. Sycamore planted at 1- by 4- to 4- by 4-foot spacing on 3-year rotations yielded 13-14 green tons/acre/year (32.1-34.6 green tons/hectare/year) (Nyland 1996).

With short rotation coppice systems, managers must manage soil nutrients to maintain high levels of fertility over repeated cutting cycles. Since nutrients are concentrated in living tissues, harvesting on short cycles eventually reduces available nutrients below the levels required for continued vigorous growth. Unless managers lengthen the rotation or supplement the nutrient losses by applying fertilizer, productivity will decline (Nyland 1996).

Coppice Methods Based on Root Suckers

Coppice systems based on root suckers differ in several ways from coppice based on stump sprout. Compared with stump sprouts, suckers come up singly, develop independent root systems, fill the area more evenly, and do not develop decay from the parent tree. Furthermore, suckering potential does not decline over repeated rotations. The new age class usually has a greater stem density than the parent stand, and the spatial distribution resembles that of a well-stocked seed-origin community (Nyland 1996).

Although treatments, such as disking, that sever or break root systems will reduce suckering, small injuries that break the bark and promote callus formation may enhance sprouting in some species (Jones and Raynal 1986). Prescribed burning has promoted suckering at sites with a fairly thick organic layer, and in clearfelled and burned areas suckers are produced from deeper roots, perhaps due to added heat absorption by the blackened surface (Nyland 1996). Aspen and American beech are two tree species that lend themselves well to coppice methods based on root suckers.

Setting Rotation Length in Coppice Systems

In sucker- and sprout-origin stands, the crop should be harvested when mean annual increment peaks. Because landowners use coppice systems primarily to produce fiber products, they do not thin the stand. Intermediate treatments may include a release cutting to eliminate undesirable tree or shrub species, and protection measures to ensure stand health and safety. Harvesting is done by clearfelling (Nyland 1996).

Conversion from Coppice to High-Forest Systems

Conversion from coppice to high-forest systems usually takes a long time. This transition will include: (1) holding the coppice growth to an advanced age to weaken its sprouting capacity and reduce its crown density, (2) increasing the numbers of trees of desired species and growing them to seed-bearing age to serve as parents for seed-origin trees and to increasingly shade the coppice undergrowth, (3) thinning the coppice growth to maintain its vigor to an extended age, and (4) gradually removing the sprout-origin trees to make space for seedlings to develop (Nyland 1996).

See: Advantages and Disadvantages of Simple Coppice Systems

Coppicing offers some general advantages over high-forest methods (Nyland 1996):

1. It involves relatively simple harvesting methods (clearfelling), and provides prompt and certain regeneration.
2. It effectively regenerates areas of any size and shape without concern for advance regeneration or seed source.
3. It produces abundant coppice shoots that grow rapidly in height and diameter, with high annual production per unit of area.
4. It allows landowners to produce high volumes of fiber crops, fuel wood, and forage over relatively short rotations.
5. It minimizes the health and disease problems associated with long rotations and old trees.
6. It produces stands of great uniformity, well suited to mechanized harvesting.
7. It provides the potential for maintaining a variety of developmental stages between adjacent stands, thereby diversifying the habitats for wildlife and herbs.

Coppice methods have some distinct shortcomings as well (Nyland 1996):

1. Financial success depends upon access to markets for small-diameter pieces and wood chips rather than saw logs.
2. Coppice systems serve a limited set of management goals, and landowners have traditionally used them with only a few species.
3. Frequent entry for harvesting requires extra caution to minimize soil disturbance on slopes, and soil nutrients may be lost after repeated rotations.
4. Succulent coppice shoots suffer damage from early- or late- growing-season freezing temperatures, and from browsing by wild animals.
5. To maintain sprouting capacity, landowners must eventually replace the old stools with new rootstocks or seedling-origin trees.
6. Replacing existing coppice stands with new trees often proves difficult and costly.
7. Coppice stands have limited amenity and other nonmarket values due to the small trees, and the uniformity of size classes.

A coppice-with-standards method maintains seed-origin trees at wide spacings for long periods interspersed with coppice crops managed on short rotations. The standards serve as parents for seedlings to renew the coppice growth as sprouting vigor declines, and to produce large-diameter sawtimber. Foresters can use coppice-with-standards systems to grow mixed-species communities, and maintain species that do not reproduce vegetatively. Most landowners use a coppice-with-standards system where markets take both small- and large-diameter products. This system may also enhance recreational uses, maintain a more favorable habitat for some wild creatures, or have some other special value to a landowner. Foresters may also use a system called "compound coppice," in which some old standards are harvested and the remainder are left to grow for additional rotations (Nyland 1996).

See: Advantages and Disadvantages of Coppice-with-Standards

A coppice-with-standards system offers some distinct advantages over simple coppice systems (Nyland 1996):

1. It yields materials of several different sizes, with some large-diameter trees of high value.
2. It provides regular returns from each stand at short intervals.
3. It retains only a relatively low residual value per unit of area, benefitting landowners who require high compound rates of return and who also wish to grow large and high-value trees.
4. The standards grow rapidly, and increase in volume and value at above-average rates.
5. The standards eventually produce viable seeds, allowing landowners to establish a seed-origin stand, or to maintain both vegetative and seed-origin growing stock.
6. The continuous partial cover of tall trees and dense understory of coppice growth protect the soil better than simple coppice systems.
7. The dense coppice between the standards prevents site occupancy by undesirable trees and other woody growth.
8. The standards enhance the appearance of a stand, both immediately after a felling and during the interim between successive entries.
9. Landowners can maintain a more diverse array of species and age classes, and provide habitat for a broader community of wildlife.

A coppice-with-standards also system has some important limitations (Nyland 1996):

1. Its complexity makes the system difficult to apply. It is particularly difficult to maintain an appropriate balance of growing space between the coppice growth and the standards.
2. The dense growth of coppice often obscures crowns of the taller trees, making reserve tree selection difficult during marking.
3. Foresters must develop regular markets for large volumes of small-diameter pieces from the coppice growth, as well as for saw logs.
4. Exposure increases the likelihood of epicormic branching and sunscald on the standards, potentially degrading their main trunks.
5. Shading by the standards may suppress coppice growth beneath reducing its development and yield, particularly in stands with multiple age classes.
6. With compound coppice systems, shading may inhibit most shade-intolerant species in the coppice growth.
7. The large machines and high-production logging systems that offer the greatest cost effectiveness for harvesting fiber crops may injure the standards.
8. Standards freed by heavy cutting may suffer wind and snow damage on exposed sites, and blowdown in areas with shallow soils.
9. When harvested, the standards may not sprout, necessitating replacement by planting or regeneration from seed.
10. Young trees intended as future standards require early release (cleaning) to ensure adequate rates of development.
11. Landowners may need to prune the lower boles of the standards to produce high-quality saw logs.
12. Standards often develop poor form and heavy branching, making them more susceptible to ice and snow loading, breakage, and blowdown.

Natural regeneration methods rely on dispersed seeds, seeds stored in the soil, stump sprouts, or advance reproduction to form regeneration after harvests. Assessing the regeneration potential, the capacity of these sources of reproduction to capture new growing space, is essential for the success of natural regeneration methods. Where natural regeneration potential is inadequate, artificial regeneration methods are used before or after a regeneration harvest. Artificial regeneration methods include planting seedlings and sowing seeds. Enrichment plantings and reinforcement plantings are often used to supplement natural regeneration. Enrichment plantings add a limited complement of species or genotypes that would not otherwise occur in a community. Reinforcement plantings supplement natural regeneration on sites lacking sufficient regeneration for full utilization of the site (Nyland 1996).

Natural regeneration methods are usually less expensive than artificial methods and almost always result in species and trees well adapted to sites. However there are several disadvantages. Natural methods are limited to indigenous genotypes and species, subject to natural disasters, and dependent upon good seed crops. They often produce stands with variable species composition, stocking, and time of establishment (Boyette 1989).

Many factors should be considered with natural methods that rely on regeneration from seed. Nyland (1996) points out that creating adequate regeneration from seed is a process, rather than an event. Managers must: (1) ensure an adequate and suitable natural supply of seed; (2) create suitable establishment conditions by manipulating the overstory and improving seedbeds; (3) protect the newly established seedlings; and, (4) enhance long-term development of the new crop. More management options exist for ensuring germination and seedling survival than for ensuring seed production. Therefore, species that have historically produced abundant seed only at long or irregular intervals present a special challenge in planning a reproduction method. Managers must delay cutting until good seed years, or retain a seed source either on site or within a close distance. However, it is difficult to predict years of good seed crops. Species that colonize from dormant seeds stored in the soil eliminate the need to schedule cutting to coincide with a heavy seed crop. When these methods fail, managers can opt for sowing seeds or planting seedlings (Kelty 1988).

Several management options exist for enhancing seed germination and survival. In fact, regeneration harvest systems are designed for this very purpose. Managers can combine appropriate combinations of tree cutting and site preparation treatments to alter microenvironments and optimize conditions for the target species. These treatments are aimed at improving seedbed conditions, minimizing competition, and increasing light, soil moisture, and nutrients. Measures to protect seedlings and contain destructive agents may include fencing, fuel reduction, or pest control. Of course, macroscale climatological events or limitations of soil conditions are not controllable. These factors are best addressed by managing species well-adapted to a sites soil and physical environment (Nyland 1996).

Artificial regeneration methods permit direct control over the genotypes, species, and placement of trees in the new stand, as well as complete control over the time and duration of establishment. However, artificial regeneration methods are expensive and impractical to use in remote areas, and they require a major logistical effort. Successful artificial establishment requires attention to site quality, site preparation, quality of planting stock, planting techniques, and competition control (Nyland 1996).

Artificial hardwood establishment is possible, but it is not widely practiced in the southern Appalachians. Although direct-seeding has proven successful for some species, such as yellow-poplar (Russell 1970), the success of direct-seeding is limited for other hardwood species (black walnut, black cherry, white ash) by high losses to animal predation. For many hardwood species, such as the oaks, the lack of seed production and dispersal often is not the limiting problem. These species require advance regeneration of sufficient size. Direct-seeding has been used primarily with conifer species in the Appalachians (Kelty 1988), but changes in social, political, and environmental factors may create more opportunities and demand for artificial establishment of hardwood species (Kelty 1988).

See also: Artificial Regeneration of Oak, Artificial Regeneration of Yellow-Poplar

The regeneration potential of a species or species group is the capacity of its various sources of reproduction to capture new growing space when it becomes available. At any given time the regeneration potential of a stand depends on the presence of one or more sources of reproduction, including sources arising from seed (either from the current seed crop or seeds stored in the forest floor) or advanced reproduction (seedlings, seedling-sprouts, root suckers, and stump sprouts) (Nyland 1996).

Regeneration harvest methods are designed to ensure the prompt establishment of a new community or age class. However, harvesting alone often does not guarantee the establishment of desired species. A substantial proportion of the variation in the natural regeneration process is uncontrollable or not understood. Despite the uncertainties, there are at several factors forest managers can manipulate. First and foremost, forest managers must choose species that are well-suited to a site. Second, forester managers should determine if the regeneration potential of these species is adequate (Hicks 1998).

The relative importance of a given reproduction source varies by species. For example, the primary reproduction source for yellow-poplar is seed stored in the forest floor. In contrast, reproduction of eastern cottonwood originates primarily from wind-dispersed seed from the current seed crop. Most hardwoods in the southern Appalachians rely on advance reproduction. Therefore, it is possible to predict the regenerative potential of many species before harvesting a particular stand, based upon their occurrence either as advance-growth seedlings of sufficient size and vigor, or as overstory trees that will produce stump sprouts after cutting. The conditions required for the initial establishment and early growth of the desired species largely determine what regeneration method should be used and any supplemental treatments that are needed to enhance regeneration (Johnson 1989). Silviculturists must be aware of these "reproductive strategies" in order to design treatments that take advantage of these adaptations rather than try to succeed in spite of them (Kelty 1988).

While most southern Appalachians species will grow on a wide range of sites, each species will regenerate and/or grow best over a more limited site range. Often, the relative abundance of the different sources of reproduction for a given species varies by site quality. For example, advance oak reproduction is often absent or deficient under oak stands on good sites but may be abundant on medium and poor sites. Thus, abundant regeneration does not always equate with site conditions that are best for subsequent stand growth.

Loftis (1989) attributes successful regeneration of hardwoods to initial floristic composition (Egler 1954) and vital ecological attributes. Unlike the classical successional concept of "relay floristics," which assumes that sites undergo a series of vegetational changes, the initial floristics model analyzes sources of regeneration of the currently existing species (seed, advance seedlings, sprouting potential). The vital ecological attributes are the conditions that promote growth and survival of the propagules (site conditions, sunlight, etc.).

See also: Regeneration, Oak Regeneration Potential