Textbook

3.1. Regeneration Methods: Introduction

Definition

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.

Regeneration Methods Versus Silvicultural Systems

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. In the next sections describing each regeneration method, both the method itself and the whole system will be referenced as appropriate.

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

Figure 3.1.1. The three phases of a silvicultural system.

Regeneration Method Classification Scheme

Regeneration methods are commonly classified as either high or low forest methods (Matthews 1989, Nyland 2005, Smith et al. 1997). High forest methods rely on regeneration primarily from seed, and are by far the most common methods in North America. Low forest methods, often referred to as coppice systems, rely on vegetative reproduction of sprouts exclusively for regeneration. While low forest methods have been used more extensively in Europe, they are currently uncommon in North America.

Low Forest Regeneration Methods

Because low forest methods are currently little applied in North America, they will not be covered in this text other than a brief mention here. Commonly called coppicing, these regeneration methods rely on the vegetative reproduction of trees by a variety of means. Sprouts may come from new stumps, older stumps, branches that root when they touch the ground, or root sprouts in species that can grow via suckering. Coppice systems commonly involve a short rotation intended to rapidly produce a large quantity of small diameter stems, usually of a hardwood species. Because coppice systems involve vegetative rather than sexual reproduction, they have inherent disadvantages in terms of lower genetic diversity and the reduced ability of populations to adapt to changing environmental conditions over time.

Low forest systems were more important historically in Europe as a means of producing firewood. There is evidence that Native Americans applied a number of different coppice systems in North America to produce wood for baskets, managing species such as willows, hazelnut, and redbud (Anderson 1999). No federal, state, or industrial forests are currently widely deploying operational coppice systems. It should be noted that if strong bioenergy markets do develop in the future, it is possible that short rotation woody crops reproduced via vegetative means may become a more common management practice (Tuskan 1998). Poplars, willows, eucalypts, and other fast growing species are a subject of research interest for this purpose, but are not being operationally deployed at any significant scale via coppicing.

The term coppice may be used in two distinct and sometimes contradictory contexts. In the silvicultural context, coppice refers to the low forest regeneration methods described above. However, in the ecological context coppice may simply refer to stump or root sprouts. Thus a forester may refer to coppice regeneration in a shelterwood, clearcut, or other high forest system. This text will avoid using coppice in the ecological context to avoid this source of potential confusion. Stump or root sprouts will be referred to as such, and not as coppice regeneration.

In high forest regeneration methods such as clearcuts, shelterwoods, or the selection methods it is entirely possible that some percentage of the regeneration may come from either stump or root sprouts. Depending on the species, this may even represent a relatively high percentage of surviving individuals in a new cohort. Despite this fact, these systems are still classified as high forest methods because of the probability that at least some members of the new cohort came from seed. As long as the forester remains aware of the genetic and ecological effects of stump or root sprouting may have on a stand, these largely semantic distinctions need not cause further confusion.

High Forest Regeneration Methods

This text will focus on eight distinct regeneration treatments described previously in the silvicultural systems section:

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

Review of Regeneration 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 3.1.1. 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).

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).

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.

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).

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

Anderson, M. 1999. The Fire, Pruning, and Coppice Management of Temperate Ecosystems for Basketry Material by California Indian Tribes. Human Ecology 27:79-113. http://dx.doi.org/10.1023/A:1018757317568

Matthews, J. D. 1989. Silvicultural Systems. Oxford University Press, Oxford, England. ISBN: 019854670X

Nyland, R. D. 2005. Diameter-limit cutting and silviculture in Northern Hardwoods. Pages 16-23 in Proceedings of the Conference on Diameter-Limit Cutting in Northeastern Forests. USDA Forest Service, Northeastern Research Station, General Technical Report NE-342, University of Massachusetts. ISBN: 1577665279

Smith, D. M., B. C. Larson, M. J. Kelty, and P. M. S. Ashton. 1997. The Practice of Silviculture: Applied Forest Ecology. Ninth edition. John Wiley & Sons, Inc., New York, New York. ISBN: 047110941X

Tuskan, G. A. 1998. Short-rotation woody crop supply systems in the United States: what do we know and what do we need to know? Biomass and Bioenergy 14:307-315. http://dx.doi.org/10.1016/s0961-9534(97)10065-4