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1.2. Introduction: Silvicultural Systems

Definition

A silvicultural system is a planned process whereby a stand is tended, harvested, and reestablished (Adams et al. 1994). It includes every management activity that occurs on that stand over an entire rotation. This is a useful scale for thinking about silviculture, both in spatial (one stand) and temporal (one rotation) terms.

While silviculture is typically practiced at the stand scale, forest management often occurs over multiple stands at the same time within a single land ownership. Silvicultural plans may be written for either shorter (5-10 years) or longer (100 years) time periods, depending on landowner objectives. For the purposes of this online textbook, we will primarily focus on single stands managed for one rotation.

Silvicultural systems are commonly categorized based on their age class structure. Age class structure describes how many cohorts can be found in a forest.

  • Even Age: All the trees are the same age. There is one cohort.
  • Two Age: There are two distinct age classes of trees, one older, and one younger. There are two cohorts.
  • Uneven age: There are three or more age classes of trees. There are at least three cohorts.

Management activities, or treatments, that together compose a silvicultural system can be broken down into several categories. These are regeneration treatments, stand establishment treatments, and intermediate treatments.

The silvicultural process shows how regeneration treatments are followed by establishment treatments, then intermediate treatments. The cycle then repeats.

Figure 1.2.1. The three phases of a silvicultural system.

Regeneration Treatments

Regeneration treatments are harvest techniques that end one rotation and allow a new rotation to begin. A new rotation begins when the entire stand is established with one or more new cohorts. These are thus essentially harvest techniques. We call these harvest techniques regeneration treatments because the silviculturist’s primary objective in harvesting a forest is to successfully establish the next rotation, which regenerates the forest. Regeneration methods are also categorized based on the age class structure of the newly established forest. This text will focus on eight distinct regeneration treatments:

  • Even Age
    • Clearcut
    • Seed-Tree
    • Shelterwood
  • Two Age
    • With Reserves
    • Plus Deferment
  • Uneven Age
    • Patch Selection
    • Group Selection
    • Single Tree Selection

It is important to note that silvicultural systems are most commonly named after the regeneration method. This can cause confusion if you are uncertain whether a regeneration method alone is being referenced, or a whole silvicultural system that includes the regeneration method, establishment treatments, and intermediate treatments.

These regeneration treatments (and the silvicultural systems named after them) vary based on the age class structures they create, and on the amount of light, water, and nutrients that they make available to newly establishing cohorts.

Table 1.1.1. Eight methods of regeneration sorted by the age class structure they create and the shade tolerance of the species that they favor.

The eight silvicultural systems described aboved are listed in a table sorted by the age class structures they create (rows) and the shade tolerance of the species most suitable to be regenerated by them (columns).

Even Aged Regeneration Treatments

Clearcut: A method of regenerating an even aged stand in which a new age class develops in a fully exposed microclimate after removal, in a single cutting, of all trees in the previous stand. Regeneration is from natural seeding, direct seeding, planted seedlings, and/or advance reproduction. The management unit or stand in which regeneration, growth, and yield are regulated consists of the individual clearcut stand (Adams et al. 1994).

Figure 1.2.2. A 120 acre clearcut on a xeric ridgetop in East Texas prior to site preparation and replanting. The previous and next rotations are both loblolly pine plantations. Notice that there is little slash or debris apparent on the site, a common feature of clearcut plantations of coniferous species. Photo Credit: Jeremy Stovall

A diagram depicting the single cut in the clearcut regeneration method described above.

Figure 1.2.3. An example of the clearcut regeneration method. Note that this is a one-harvest system that regenerates an even-aged stand.

Seed-Tree: An even aged regeneration method in which a new age class develops from seeds that germinate in fully exposed microenvironments after removal of all the previous stand except a small number of trees left to provide seed. Seed trees are removed after regeneration is established (Adams et al. 1994).

Figure 1.2.4. A slash pine stand, with half the original stand still intact (right side) and the other half following the seed-tree cut (left side). Once the new cohort is established, the seed-trees will be removed. Photo Credit: David Moorhead, UGA, Bugwood.org

A diagram depicting the two cuts in the seed-tree regeneration method described above.

Figure 1.2.5. An example of the seed-tree regeneration method. Note that this is a two-harvest system that regenerates an even-aged stand.

Shelterwood: A method of regenerating an even aged stand in which a new age class develops beneath the moderated microenvironment provided by the residual trees. The sequence of treatments can include three distinct types of cuttings:

  1. an optional preparatory cut to enhance conditions for seed production;
  2. an establishment cut to prepare the seed bed and to create a new age class; and
  3. a removal cut to release established regeneration from competition with the overwood (Adams et al. 1994).

Two Aged Regeneration Treatments

Plus Deferment: A two aged regeneration method in which a new age class develops from seeds that germinate under overwood that is not removed. This regeneration method modifies the seed-tree or shelterwood methods by skipping or indefinitely deferring the overwood removal or seed-tree removal cuts. The two cohorts are thus the newly established cohort, and the deferred seed trees or overwood. This is alternately known as the leave tree, reserve tree, or irregular shelterwood method.

Figure 1.2.6. A large eastern cottonwood apparent in a sugarberry, elm, cottonwood, sycamore stand regenerated with a deferment system in the US South. Photo Credit: Brian Lockhart, USDA Forest Service, Bugwood.org

A diagram depicting the one cutc in the shelterwood plus deferment regeneration method described above.

Figure 1.2.7. An example of the shelterwood plus deferment regeneration method. Note that this is a one-harvest system that regenerates a two-aged stand. This is a modification of the shelterwood regeneration method in that the overwood is not removed in a second harvest, called the removal cut.

With Reserves: A two aged regeneration method in which a new age class develops from natural seeding, direct seeding, planted seedlings, and/or advance reproduction. This method is a modification of any of the three even aged systems that involves the retention of live or dead trees that are called reserves. The reserves can be in one or more groups, or may be dispersed throughout the stand.

Uneven Aged Regeneration Treatments

Patch Selection: A method of regenerating uneven aged stands in which trees are removed, and new age classes are established, in large groups. The minimum width of groups is greater than twice the height of the mature trees, with large openings providing conditions suitable for intermediate to very intolerant regeneration. In the patch selection method, the management unit or stand in which regeneration, growth, and yield are regulated consists of a landscape containing an aggregation of patches.

Group Selection: A method of regenerating uneven aged stands in which trees are removed, and new age classes are established, in small groups. The maximum width of groups is less than twice the height of the mature trees, with small openings providing microenvironment suitable for tolerant regeneration and the larger openings providing conditions suitable for more intolerant regeneration. In the group selection method, the management unit or stand in which regeneration, growth, and yield are regulated consists of a landscape containing an aggregation of groups (Adams et al. 1994).

Single Tree Selection: A method of creating new age classes in uneven aged stands in which individual trees of all size classes are removed more-or-less uniformly throughout the stand to achieve desired stand structural characteristics (Adams et al. 1994).

Stand Establishment Treatments

Stand establishment treatments are any management activities that follow regeneration treatments and promote or facilitate the growth of new cohorts. Many traditional silvicultural texts consider regeneration treatments and stand establishment treatments as the same. This is for good reason, as often regeneration treatments will require no subsequent stand establishment treatments to successfully create a new stand. However, we will consider them separately to better inform our understanding of intensively managed forest plantations. This text will focus on three distinct stand establishment treatments:

  • Site Preparation: Activities intended to improve site access, increase survival and early growth, and reduce competition.
  • Natural Regeneration: Activities intended to foster the establishment of a new cohort from natural seed source or advanced regeneration.
  • Artificial Regeneration: Activities including tree improvement, direct seeding, and planting.

Intermediate Treatments

Intermediate treatments are any management activities that take place between the time of stand establishment and the next regeneration treatment. Intermediate treatments are intended to affect stand growth rates, composition, and product class distribution. This text will focus on four distinct intermediate treatments:

  • Competition Control: Application of chemical or mechanical methods to allocate a greater percentage of site resources to crop trees.
  • Nutrition Management: Addition or manipulation of materials that affect the availability of mineral nutrients to crop trees.
  • Pruning: Removing branches from crop trees to improve stem quality and product class.
  • Thinning: Removing trees from a stand to reallocate site resources to crop trees and allow them to more rapidly grow into higher valued product classes.

References

Adams, D. L., J. D. Hodges, D. L. Loftis, J. N. Long, R. S. Seymour, and J. A. Helms. 1994. Silviculture Terminology with Appendix of Draft Ecosystem Management Terms. Silviculture Instructors Subgroup of the Silviculture Working Group of the Society of American Foresters. https://www.bugwood.org/silviculture/terminology.html