The theory and practice of controlling the establishment, composition, and growth of stands of trees for any of the goods (including timber, pulp, energy, fruits, and fodder) and benefits (water, wildlife habitat, microclimate amelioration, and carbon sequestration) that they may be called upon to produce. In practicing silviculture, the forester draws upon knowledge of all natural factors that affect trees growing upon a particular site, and guides the development of the vegetation, which is either essentially natural or only slightly domesticated, to best meet the demands of society in general and ownership in particular. Based on the principles of forest ecology and ecosystem management, silviculture is more the imitation of natural processes of forest growth and development than a substitution for them.
The spatial patterns in which old trees are removed and the species that replace them determine the structure and developmental processes of the new stands. If all the trees are replaced at once with a single species, the result is so-called pure stand or monoculture in which all of the trees form a single canopy of foliage that is lifted ever higher as the stand develops. If several species start together from seeds, from small trees already present, or from sprouts, as a single cohort, the different species usually tend to grow at different rates in height. Some are adapted to develop in the shade of their sun-loving neighbors. The result is a stratified mixture. Such stands grow best on soils or in climates, such as in tropical moist forests, where water is not a limiting factor and the vegetation collectively uses most of the photosynthetically active light. If trees are replaced in patches or strips, the result is an uneven-aged stand which may be of one species or many.
These different spatial and temporal patterns of stand structure are created by different methods of reproduction. The simplest is the clear-cutting method, in which virtually all of the vegetation is removed. Although it is sometimes possible to rely on adjacent uncut stands as sources of seed, it is usually necessary to reestablish the new stand by artificial seeding or planting after clear-cutting. The seed tree method differs only in that a limited number of trees are temporarily left on the area to provide seed.
In the shelterwood method, enough trees are left on the cutting area to reduce the degree of exposure significantly and to provide a substantial source of seed. In this method the growth of a major portion of the preexisting crop continues, and the old trees are not entirely removed until the new stand is well established. The three methods just described lead to the creation of essentially even-aged stands.
The choice of methods of regeneration cutting depends on the ecological status of the species and stands desired. Species that characterize the early stages of succession, the so-called pioneers, will endure, and usually require the kind of exposure to sunlight resulting from heavy cutting or, in nature, severe fire, catastrophic windstorms, floods, and landslides. These pioneer species usually grow rapidly in youth but are short-lived and seldom attain large size. The species that attain greatest age and largest size are ordinarily those which are intermediate in successional position and tolerance of shade. Some of them will become established after the severe exposure of clear-cutting, but they often start best with light initial shade such as that created by shelterwood cutting. Their longevity and large size result from the fact that they are naturally adapted to reproduce after disturbances occurring at relatively long intervals. The shade-tolerant species representing late or climax stages in the succession are adapted to reestablish themselves in their own shade. These species represent natural adaptations to the kinds of fatal disturbance caused by insects, disease, and atmospheric agencies rather than to the more complete disturbance caused by fire. Ecological succession Forest ecosystem
Much silvicultural practice is aimed at the creation and maintenance of pure, even-aged stands of single species. This approach is analogous to that of agriculture and simplifies administration, harvesting, and other operations. The analogy is often carried to the extent of clear-cutting and planting of large tracts with intensive site preparation, especially with species representative of the early or intermediate stages of succession. Mixed stands are more difficult to handle from the operational standpoint but are more resistant to injury from insects, fungi, and other damaging agencies which usually tend to attack single species. They are also more attractive than pure stands and make more complete use of the light, water, and nutrients available on the site. They usually do not develop unless these site factors are comparatively favorable; where soil moisture or some other factor is limiting, it may be possible for only a single well-adapted species to grow. If the site is highly favorable, as in tropical rainforest, river floodplains, or moist ravines, it is so difficult to maintain pure stands that mixed stands are inevitable.
The application of silviculture involves a number of accessory practices other than cutting. In localities of high fire risks, it may be desirable to burn the slash (logging debris) after cutting. Not only does this reduce the potential fuel, but it may also help the establishment of seedlings by baring the mineral soil or reducing the physical barrier represented by the slash. Slash disposal is most often necessary where the cutting has been very heavy or where the climate is so cold or dry that decay is slow. Deliberate prescribed burning of the litter beneath existing stands of fire-resistant species is sometimes carried out even in the absence of cutting to reduce the fuel for wild fires, to kill undesirable understory species, to enhance the production of forage for wild and domestic animals, and to improve seedbed conditions. Forest fire
Integrated schedules of treatment for stands are called silvicultural systems. They cover both intermediate and reproduction treatments but are classified and named in terms of the general method of reproduction cutting contemplated. Such programs are evolved for particular situations and kinds of stands with due regard for all the significant biological and economic considerations involved. These considerations include the desired uses of the land, kinds of products and services sought, prospective costs and returns of the enterprise presented by management of the stand, funds available for long-term investment in stand treatments, harvesting techniques and equipment employed, reduction of losses from damaging agencies, and the natural requirements that must be met in reproducing the stand and fostering its growth. Forest and forestry Forest management