Hardwood Silviculture: Even-aged and Uneven-aged Systems

Hardwood silviculture is extremely complex. More than 40 commercial hardwood species make up the southern hardwood forest. These commercial species grow in association with one another and are mixed with an additional 250 species.

Silvicultural systems produce healthy forest, regenerate new forests and stabilize the environment. Silviculture is implemented, to a large degree, with timber harvesting, which creates openings, removes competition, controls species and regulates stand density.

There are broad silvicultural systems based on tree ages. Even-aged silviculture has trees within a stand as one or two age classes and uneven-aged silviculture with three or more age classes. In even-aged forests the upper canopy will appear smooth because the trees are generally all in one height class. In a similar way the diameters of the trees in an even-aged stand will be uniformly grouped around an average diameter. Conversely, in uneven-aged stands trees will be distributed with many heights, so several canopy layers are present. Tree diameters will vary greatly with many trees in the small diameters and continuous decreasing numbers of trees in each successively larger diameter class.

Within the two broad classes of silviculture there are five methods of forest regeneration. The variety of methods offers owners of hardwood timber flexibility in management of their forest. Each system is distinctly designed to carry out a specific purpose and must be based on tree biology (silivcs). Each hardwood species has unique silvicultural characteristics that must be considered in management.

Even-Aged System

A stand is even-aged if the age difference between the youngest and oldest trees is less than 20 percent of the rotation. For example, if an even-aged stand is managed on a 50-year rotation (trees will be no older than 50 years at harvest), the oldest trees are no more than 10 years older than the youngest trees. The upper canopy of tree crowns is smooth in an even-aged because diameter is affected by competitive conditions.

Methods used to produce even-aged hardwood stands include clearcuts, seed tree cuts and shelterwood cuts.

Clearcuts

In a clearcut all trees are cut this includes all trees >2" in diameter when managing for hardwood regeneration. This method is useful for those species intolerant of shade. Regeneration from clearcuts normally depends on stump sprouts and seeds in place. Densities as high as 30,000 seedlings per acre have been produced in hardwood clearcuts designed for natural hardwood regeneration. Clearcuts can range considerably in size. Preferably, Clearcuts should be irregularly shaped and be no larger than 40 acres, depending on terrain, age of adjacent stands and nearness to streams.

Shelterwood

In a shelterwood, the overstory is gradually removed in a series of harvest to provide shelter for regeneration. The initial harvest will remove all but 20 to 50 trees per acre. The number of trees left depends upon advance regeneration already present and species desired.

A second harvest is required after the new stand has been regenerated to remove the overstory, releasing the newly established even-age hardwood stand. Species that regenerate well in a shelterwood are intermediate in tolerance to shade.

Seed Tree

Hardwood regeneration with a seed tree cut requires that 6 to 15 trees per acre, evenly spaced, are left to produce seed to regenerate the new even-age stand. After establishment of the new stand, the seed trees must be removed or killed. This method is useful to regenerate light seeded species that are intolerant to shade.

Uneven-Aged System

Hardwood forests in the Eastern United States have been cut heavily, by removing the largest trees (often referred to as diameter limit cut). Diameter-limit cuts are wrong, and should never be practiced in hardwood selection silviculture. Hardwood stands that have always had only the larger trees cut produce stands that are out of balance. Out of balance stands need to be either brought back into a manageable state with single-tree selection, or completely regenerated with even-aged silviculture.

Single-tree Selection

This is one of two selective methods to regulate an uneven-aged hardwood forest. Single-tree selection removes trees from all diameter classes. The goal is to produce a stand with a balanced distribution of trees in all diameter classes with many small trees and a decreasing number of trees in each next larger diameter class. This process requires several harvests to bring the stand under management. Choosing a stewardship logger is important as damages to residual trees must be keep to minimum. Single tree selection also requires detailed record keeping and patience. All hardwood species can be managed under this method, but those trees tolerant to shade will be favored in regeneration.

Group Selection

This selection method produces a balance forest of stands of several ages. Group selection develops patches of even-aged stands scattered throughout the forest as groups. In group selection, groups of trees are harvested to create openings of less than two acres. These openings then regenerate to intolerant species, like small clearcuts, but a forest of many different-aged stands results. Group selection relies on maintenance of an uneven-aged, balanced forest.