Regeneration Harvest

Several systems are used to harvest and regenerate forest stands. Selection of the system influences the character of the “new” stand. Stands may develop that are even-aged (all trees are essentially the same age), or uneven-aged (trees are many ages, from young seedlings to mature trees and everything in between). Regeneration methods that produce even-aged stands are Clearcut, Seed Tree, and Shelterwood. Those that produce uneven-aged stands are Single Tree Selection and Group Selection. Even-aged methods should be used for reproducing shade intolerant species. Uneven-aged methods can be used where favoring shade tolerant species is desirable. A Two-Aged Stand may be a good compromise for species of intermediate shade tolerance, or where leaving some standing timber between harvests is appropriate. Each method has advantages and disadvantages. Each is briefly described.

Clearcut Method

The main objective of Clearcutting is to reproduce the forest with shade intolerant species that require full sunlight, benefit from even-competition, and are best managed biologically and economically in even-aged stands. The method is used equally well with shade intolerant early successional pines, or with shade intolerant hardwoods. It is the method used for establishing new species (introduced species) from those harvested. But, the method is technically sound for reproducing many native species of pines and hardwoods. Release of advanced reproduction, germination of new seed, and creation of abundant sprouts and root suckers are all benefits of clearcutting. Many species of forest trees and individuals of the same species benefit from even-competition. Clearcutting can be used effectively with one species (pine) or with mixtures of species as with hardwoods.

The method involves harvest or otherwise felling all trees above 1 1/2 inches in diameter or about six feet tall to remove overhead shade and competition. Clearcutting has considerable biological and economical advantages in managing numerous Virginia tree species.

Seed Tree Method

The main objective of the Seed Tree Method is to reproduce the forest with “light-seeded” species presently occupying the site. These are usually early successional pine species that are shade intolerant and managed in even-aged stands. Species must be wind-firm enough to withstand the elements as they stand exposed. Trees left as seed trees must have seed bearing capacity, or must be developed as seed bearers prior to timber harvest. In Virginia, loblolly pine is readily reproduced with seed trees in the Coastal Plain, and white pine in the Mountains. Both loblolly pine and white pine are covered by the Virginia Seed Tree Law. Virginia landowners must abide by the provisions of this law, or other suitable regeneration provisions signed as alternative options in an Alternate Management Plan. The method involves “clearcutting” all timber except the designated seed trees which are retained as a seed source. Generally, eight seed trees per acre are left after the harvest. All other trees above 1 1/2 inches in diameter or about six feet tall are removed (harvested commercially or otherwise felled to remove shade and competition).

Seed trees are usually of a single species. They are selected prior to the harvesting process and should be the most dominant, well-formed, genetically superior, and most prolific cone bearing trees in the stand.

Once a successful crop of young trees has been attained, the seed trees should be removed in a second harvest. Another option is to retain them as insurance.

Disadvantages to leaving seed trees are

  1. potential loss or damage by wind, lightning strike or insects, and
  2. limited timber volume per acre in seed trees may make the area uneconomical to harvest.

Shelterwood Method

Shelterwood cutting leads to conditions suitable for natural reproduction under the protection of the older stand and is finally released when it becomes desirable to give the new crop full use of the growing space. Managing shade also gives opportunity to regulate species composition in the new stand. To assure regeneration success, site preparation may be necessary. As with clearcutting and seed trees, Shelterwood gives rise to even-aged stands.

The Shelterwood System usually involves a three phase sequence of cuttings over a 5 to 10 year period, but not to exceed 1/5 the stand rotation age.

The three cuts are:

  1. Preparation Cuttings - those that set the stage for regeneration,
  2. Establishment or Seeding Cuttings - those to induce actual establishment of seedlings,
  3. Removal Cuttings - those to release the established seedlings for development.

The cutting sequence of shelterwood generally involves a prescription for cutting the worst first. This leaves for the final cutting those most vigorous, best formed individuals of desirable species - (BEST trees grow best, and are left to provide seed for the new crop. Also, they continue to lay down wood of high quality at a rapid rate).

Natural conditions may negate the need for cuts prior to the removal cut. Reproduction may have been established so well that neither preparation nor establishment cuttings are needed. Thus, the first reproduction cutting will be a removal cutting. New regeneration may be almost pure, or of mixed species, but some control over composition of reproduction can be maintained through managing the intensity of sunlight.

There are several advantages to shelterwood methods. Some are:

  1. control of site conditions for the regeneration of even-age stands,
  2. effective for distribution of heavy-seeded species, i.e. oaks, hickories,
  3. control over regeneration composition, amount and distribution,
  4. flexibility-applicable to tolerant and intolerant species,
  5. good soil and site protection,
  6. high aesthetic qualities, and
  7. short period of time between income producing harvests.

Single Tree Selection

Under the single tree system of forest harvest/regeneration, individual trees are marked and cut. The objective of the method is to maintain a self-sustaining forest of multiple age/size classes. Stand structure is regulated by harvesting a specific number of trees in each size class. The cutting pattern is repeated at regular intervals throughout the stand rotation. The single tree selection method maintains continuous forest cover and provides frequent entries for harvest of forest products. It leads to the creation of uneven-aged stands.

Single tree selection favors the reproduction of shade tolerant Virginia species such as American beech, blackgum, flowering dogwood, elms, hemlock, eastern hophornbeam, red maple, and sugar maple. Where these species are not desired, single tree selection should be avoided. It is not a good system for the long-term management of oak forests.

Group Selection

Group Selection as the term implies is similar to single tree selection in design, stand attributes, and benefits, but differs in that groups of trees together are removed to create small openings. These openings are actually small “clearcuts” which technically do not exceed in width approximately twice the height of the surrounding timber. Group selection provides for continuous forest cover and frequent entries for harvesting. Uneven-aged stands of small groups are created.

As with single tree, group selection favors the reproduction of shade tolerant species, although those of intermediate tolerance can be maintained near the center of the group.

With both single tree and group selection, timber harvest is both tedious and time consuming if damage to residual growing stock is to be minimized. Minimizing damage is very important in the growing of high value quality timber.

Diameter limit cutting is often used synonymously with selection cutting. This is inaccurate. Diameter limit cutting is restricted to harvesting trees above a certain specified stump or stem size rather than by design within all size groups. This results in cutting only the largest trees which may or may not be the most mature trees. Actually, they often are the best genetically constituted trees, the best inherent competitors, the fastest growers, the best quality, the less mature, and the best future investment. Diameter limit cutting can more accurately be referred to as high-grading, or cutting the best and leaving the rest. Ultimately, this method of harvest, although favoring short-term economic advantages, leads to devaluation of timber quality, value, and other benefits.

The “Two-Age” Stand

A two-aged stand alternative, sometimes called a deferment cut, is a good compromise for regenerating shade intolerant species which benefit from full sunlight and even-aged conditions. A form of shelterwood, two-aged management is an improved esthetics method which retains the benefits from clearcutting while maintaining some high forest cover. In its most desirable form, approximately 12 trees of the dominant crown position and having quality potential are retained per acre. All other trees above 1 1/2 inches in diameter or about six feet tall are removed or felled. The two tree levels (understory of even-aged reproduction, and overstory of large trees) are then allowed to develop together until the end of the next rotation. Then during harvesting, other trees are retained for high forest cover. The method allows development of potentially very large valuable trees, the establishment and growth of the new regeneration, and the retention of some benefits associated with continuous cover.