1 Definition of a Silvicultural System 4 The Decision Process Appendix 1 Answer Key
2 Major Types of Systems 5 There's More to Learn Appendix 2 Advantages and Disadvantages
3 Variations of Major Types 6 Implementation Appendix 3 References

Shelterwood System Variations

Many types of shelterwoods are used worldwide; we will discuss the major variations.

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A good understanding of stand dynamics of old forests is required for shelterwood prescriptions in over mature forests.
Gordon Weetman (1996)

Uniform Shelterwood System

In uniform shelterwood systems, treatments are applied uniformly over the same stand: this is the standard type of shelterwood. The uniform system is the system that most often relies on a series of preparatory cuttings (thinnings) to ready the stand for the regeneration/establishment cutting by encouraging crown expansion, and promoting windfirmness and cone production.

Mathews (1989) notes that in Europe the practice of making preparatory cuttings 2-10 years before the regeneration cutting has generally been replaced by a schedule of preparation thinnings throughout the life of the stand to better encourage the crown expansion and windfirmness required for the establishment cutting.

Illustration of uniform treatments.

In BC we are faced with the prospect of entering mature to overmature natural stands for the first time to attempt natural shelterwoods. If we do not have well-established dominant trees to work with, we may face problems in areas of high wind exposure.

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The uniform shelterwood systemin its modern application will always be associated with Georges-Ludwig Hartig, Director-General of the Prussian State Forest Service from 1811 to1837.

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Strip Shelterwood System

Harvesting entries in a strip shelterwood are made in relatively narrow strips that advance progressively through a portion of the block over the regeneration period. In this way initial harvesting occurs in the stand as uniformly staggered linear strips. Future harvesting strips are added beside the initial strips and progress into the wind until the entire block is harvested, usually within a normal even-aged regeneration period (10-25 years). Harvesting in each strip may occur gradually and include a preparatory, regeneration and removal cut, following in sequence, or strips may be oriented to use the side shade from adjacent timber.

Strip shelterwoods evolved in Europe to provide some protection from windthrow. Establishment cuts (preceded possibly by preparatory cuts) and removal cuts are made in narrow strips running perpendicular to, and advancing progressively against, the prevailing wind direction.

Illustration of strip treatments.

This system contains many possible modifications, including advancing narrow clear-felled strips, which use side shade for protection, and the wedge shelterwood, which orients strips into a wedge shape with the apex into the wind for further protection from windthrow.

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Strip shelterwood does not necessarily cause any greater range of age between the individuals of the new stand than does application uniformly over the entire area since the period of regeneration is the same in the two cases.
Ralph Hawley (1929)

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Group Shelterwood System

In the simplest form of the group shelterwood system, small gaps (one to two tree lengths in diameter) are created in the stand. The adjacent trees can shelter the new regeneration growing in the gaps. For example, these openings may comprise 20-40% of the stand area during a given entry. Further cuts expand existing openings or create new openings (see diagram below).

The regeneration period for the cutting unit is still concentrated at the beginning of the rotation over 20-30 years, creating an even-aged stand.

Illustration of group shelterwoods.

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The beech forest of Sihlwald in Zurich, Switzerland, was classically managed for over 400 years under a group shelterwood system. In the early 1900s this forest inspired Gifford Pinchot, the first chief forester of the US Forest Service. In 1989, public pressure forced city officials to limit cutting to commercial thinning.

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Variations on the simple group shelterwood

A German variation (Gruppenschirmschlag) mixes the simple group and the strip shelterwood systems. This system uses existing gaps with advance regeneration by initiating a regeneration cutting in a ring around the existing gaps. Additional gaps may be created, each followed by a regeneration-cut ring, once regeneration is established. The regeneration-cut ring continues to expand in successive passes, with removal cuttings following it, once regeneration is established. Eventually, the shelterwood expands to occupy the whole stand.

As you may imagine, woodflow and skid trail networks must be laid out carefully ahead of time to minimize damage to developing regeneration. In windy areas, exposed stand edges should be anchored by windfirm trees and cutting should be modified to progress into the wind as much as possible.

The strip and group system combines elements of the two approaches. First, small groups are opened within a strip to encourage advanced patches of regeneration. After the regeneration has started, the groups are then opened wider and a uniform regeneration felling is made between the groups. A third entry to further widen the groups may occur before the removal cut, providing for regeneration of species mixes, including shade-intolerant species. Mathews (1989) provides a good discussion of all variations of shelterwoods.

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The strip and group shelterwood system was developed chiefly in Bavaria by H. von Huber, chief of the Bavarian Forest Service. It is now widely used in central Europe.

There is likely to be more injury to reproduction from removal ofthe old timber in the group than in the strip shelterwood method, because of the more irregular way in which old timber is intermingled with young growth.
Ralph Hawley (1929)

Irregular Shelterwood System

Irregular shelterwoods are defined by timing of regeneration establishment not by spatial arrangement. The regeneration period for the stand is extended so long that the new stand is not really even-aged. Although the regeneration period may extend up to 50 years over the whole cutting unit, the stand does not have three or more age classes, as in an uneven-aged stand. Therefore, the stand structural objective is between the even-aged and uneven-aged structure.

This system may be executed the same way as a shelterwood with reserves if the overstorey is retained for the entire rotation. The difference is that with an irregular shelterwood, the seedbed is receptive to regeneration for a long time and the intent is to continue to procure regeneration for much longer than the normal regeneration period.

"Irregular" refers to the subsequent variation in tree heights in the new stand. This system tends to draw on elements from other systems, notably group and single tree selection. While our example shows retained leave-trees scattered individually through a block, felling in groups is common. The groups are expanded slowly outward until they coalesce at the end of the regeneration period (50 years or longer). The irregular shelterwood system is very versatile and can be applied inuniform, strip, or group spatial variations (i.e., irregular group or irregular strip shelterwood).

This irregular shelterwood system is usually used to promote structural diversity while maintaining the simplicity of even-aged management. Objectives for aesthetics, wildlife, biodiversity, or hydrological green-up may be compatible with this system.

Illustration of irregular shelterwood.

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Of all the classical silvicultural systems, some form of the group systems and the irregular shelterwood system have the best potential for geriatric silvicultural prescriptions in over-mature Canadian conifer ecosystems.
Gordon Weetman (1996)

The irregular shelterwood is the most recent silvicultural system to have been developed and has replaced all the others except the selection system in Switzerland.
John Mathews (1989)

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Natural Shelterwood System

Illustration of natural shelterwood.

Often associated with clearcutting and called "overstorey removal," this system may better be called a shelterwood because the regeneration is established naturally under the shelter of an overstorey.

Some foresters express concern over labelling what will look like a regenerated clearcut a type of shelterwood. This concern relates to the difference between harvesting patterns, which are short term, and silvicultural systems, which are longterm. All shelterwoods or seed tree systems will look like regenerated clearcuts after removal cutting, unless some "reserve trees" have been retained.

By definition, a natural shelterwood may only be used once when some unmanaged stands come under management. However, these types of stands may be quite suitable for continued management using a shelterwood system over subsequent rotations.

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Nurse-tree Shelterwood System

These systems encourage development of two stories in a stand, each containing a different species or mix of species. With these systems, intolerant, seral species tend to make up the overstorey with tolerant, climax species making up the understorey (see diagram).

Illustration of nurse-tree shelterwood.

Advantages of the nurse-tree shelterwood

The nurse-tree shelterwood system may be used to maintain a component of shade-tolerant species on sites where these species require protection, or on cleared sites where desired tolerant species grow too slowly to compete successfully with other vegetation in the open.

Establishment of a hardy, shade-intolerant, exposure-tolerant species as a nurse-crop may precede establishment of the more sensitive, shade-tolerant species. The understorey of shade-tolerant trees may be established either naturally or artificially. However, a proper understanding of natural succession dynamics and species requirements is essential for success.

The spatial arrangement of leave-trees

  • uniform shelterwood system
  • strip shelterwood system
  • group shelterwood system


The timing of overstorey removal

  • irregular shelterwood
  • natural shelterwood


The mangement of different species indifferent canopy layers

  • nurse-tree shelterwood


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