Even-Aged Systems
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.
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).
Encyclopedia ID: p1785
