Silviculture is the science and art of growing and tending forest crops. More particularly, the term silviculture means the theory and practice of controlling the establishment, composition, character, and growth of forest stands to satisfy specific objectives (Broun 1912, Kostler 1956, Helms 1998, Daniel and others 1979, Smith 1986). The principal focus of silviculture, particularly in managing upland central hardwoods, has been timber production (Mills and others 1987). However, silviculture also involves the manipulation of forest stands to achieve a variety of benefits other than timber. These benefits are defined by forest owners and may include aesthetics, wildlife habitat, watershed protection, or recreation, to name a few. Silviculture, then, is used to provide an effective and appropriate means for managing a stand to provide desired benefits, whatever they may be, and to sustain them in perpetuity (Nyland 1996).

To serve these purposes, silviculturists draw upon principles and theory amassed through scientific inquiry in botany, zoology, soil science, physical sciences, ecology, silvics, managerial science, economics, and quantitative methods. All contribute techniques and information that foresters can use in identifying and evaluating the options for practical management, and in forecasting the likely outcomes from a given set of silvicultural treatments. In both theory and practice, silviculture draws heavily upon silvics. By definition, silvics deals with principles underlying the growth and development of single trees and of the forest as a biological unit.

Silvicultural systems provide the conceptual framework for forest management by describing the long-term plan for managing an individual stand to sustain a particular set of values (see figure; Champion and Trevor 1938, Helms 1998, Van Lear 1981, Smith 1986). The three main functions of a silvicultural system are (1) regeneration, (2) tending, and (3) harvesting.

Cycle of an Even-Aged Silvicultural System

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-age systems, uneven-age systems, and two-age systems. Even-age systems use regeneration harvest methods that 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 clearcuttting, shelterwood, and seed-tree methods. Uneven-age systems use regeneration harvest methods that 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 methods. Two-age systems provide for regeneration of shade-intolerant species while carrying a variable density overstory of mature trees.

Additional site preparation treatments may be a necessary component of a silvicultural system. Site preparation includes any treatment that modifies existing vegetative or physical site conditions to improve seed germination and survival or subsequent growth of desired seedlings. Active site preparation can be accomplished by prescribed burning, mechanical, or chemical methods (herbicides).

The other major component of a silvicultural system is the tending of developing or mature forest stands; its techniques that collectively called intermediate management. Although intermediate management can involve practices that improve the site (fertilization, etc.) or manage pests (pesticides, etc.), it often involves intermediate cuttings. The several types of intermediate cuttings are distinguished by the stage of stand development during which cuttings are made and by the criteria used for targeting tree removal. Thinnings are intermediate cuttings made in immature stands to control stand density. Release operations are intermediate treatments designed to free young stands from undesirable vegetation that threatens to suppress them. Improvement, sanitation, and salvage cuttings are intermediate treatments aimed at removing poor quality, diseased, or dead and dying trees. In addition to the above treatments, which tend to focus on the condition of the stand, crop-tree management is a type of intermediate management that focuses on individual trees of potentially high value to the landowner.

Different forest ecosystems managed for different goals will usually require different silvicultural systems. Silvicultural systems consist of a planned program of silvicultural treatments during the whole life of the stand; silvicultural systems include not only reproduction cuttings but also any tending operations or intermediate cuttings (Nyland 1996).  A comparative analysis of silvicultural systems appropriate for southern Appalachian ecosystem management reveals several interesting findings. 

See: Adaptive silviculture

This section draws from Miller and Kochenderfers (1998) discussion of appropriate silvicultural techniques for the southern Appalachians. They addressed two questions in their comparison of silvicultural alternatives:

1. What silvicultural strategies will satisfy a broad array of forest management goals?

2. What stand structures and species compositions are appropriate for sustaining the production of desired woodland benefits (Miller and Kochenderfer 1998)?

The property rights of forest owners entitle them to define the forest benefits they desire. Owners in the Appalachian region desire a wide variety of outputs, including timber, wildlife, recreation, aesthetics, and ecosystem services. Miller and Kochenderfer (1998) maintain that species composition is the most important factor that determines the output of benefits from Appalachian forests. Throughout the region, there are more than 20 commercial timber species that form complex local forest communities. Species vary widely in their market value for wood products, their ecological value for supporting associated plant and animal communities, and the sociological value of aesthetic benefits and recreational opportunities. Therefore, maintaining species diversity is the key to sustaining the productivity of Appalachian forests (Miller and Kochenderfer 1998).

The regions extremely diverse forests resulted from major disturbances: clearing of land for agriculture in the 1800s, heavy cutting and wildfires in the early 1900s, and the death of the American chestnut (Castanea dentata) in the 1930s (Carvell 1986). Infrequent local disturbances also shaped the composition of these forests. Using their understanding of forest ecosystems, managers can prescribe innovative silvicultural practices to mimic disturbances that are needed to sustain forest productivity and achieve multiple long-term goals (Miller and Kochenderfer 1998).

OHara (1998) suggested that traditional uneven-aged silviculture based on maintaining a reverse-J diameter distribution has serious shortcomings. Detailed examination of such practices spanning nearly 50 years in the central Appalachians confirms that management resulting in single-tree canopy openings promotes the regeneration and recruitment of shade-tolerant species. Shade-tolerant species generally have slower growth rates and lower commercial value than shade-intolerant species (Trimble 1967, Smith and Debald 1975). Consequently, practices that result in single-tree canopy gaps eventually reduce economic returns from commodity products as the proportion of shade-intolerant species declines. Reduction in tree species diversity also has an adverse impact on food production and habitat quality for wildlife (DeGraaf and others 1992). The best trees in the region for producing mast and other wildlife foods are shade-intolerant species that require sizable canopy openings to regenerate (Miller and Kochenderfer 1998).

Long-term research confirms that silvicultural practices that create canopy openings larger  than 0.5 acre at the appropriate times can be used to regenerate virtually all desirable tree species in the region. Large openings also support abundant populations of blackberries (Rubus spp.), another excellent wildlife food, for several years before canopy closure (Core 1974). Practices that create openings of sufficient size include clearcutting, group selection cutting, and shelterwood cutting with reserves. These methods promote even-aged, uneven-aged, and two-aged stand structures, respectively.

Group selection is most applicable on non-industrial private forests, where average tract size is relatively small or where owners are averse to harvesting their entire property at once. On large public or industrial forests, it is probably more practical to maintain uneven-aged landscapes comprised of  even-aged or multiaged stands resulting from clearcutting or shelterwood cutting with reserves. Shelterwood with-reserves practices provide the opportunity to control the species, size, and density of residual trees to promote two-aged stand structures (Miller and others 1997). Long-term field trials indicated that species composition of regeneration in two-aged stands was as diverse as that observed after clearcutting (Miller and Schuler 1995). Similarly, patch or strip clearcutting can be used to maintain uneven-aged landscapes comprised of small, even-aged management units.

Silvicultural alternatives differ in the frequency and degree of site disturbance, but they need not differ in their impact on soil and water resources. Adverse impacts on soil and water resources can be controlled, regardless of the silvicultural system, if responsible logging practices are used during periodic harvests (Patric 1980; Martin and Hornbeck 1994, Kochenderfer and others 1997).

Finally, visual impacts of silvicultural treatments are related to the relative reduction in stand density associated with harvest operations (Pings and Hollenhorst 1993). For aesthetic purposes, uneven-aged systems maintain forest structure more than other systems, and therefore impact visual quality of managed stands the least. However, Miller and Kochenderfer (1998) argue that, although visual impact may vary by silvicultural treatment immediately after logging, in the southern Appalachians the effect is short term, and aesthetic quality improves rapidly after harvest.

An even-aged silvicultural system is a planned sequence of treatments designed to maintain and regenerate a stand with one age class. The range of tree ages is usually less than 20 percent of the rotation length. The following sections discuss the three basic methods of even-aged silviculture: the clearcutting, seed-tree, and shelterwood methods.

• Even-Aged Systems: An Overview
• The Clearcutting Method
• The Seed-Tree Method
• The Shelterwood Method