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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

Selection System Variations

We will discuss three major variations of the selection system:

arrow Selection system definition

Single Tree Selection System

Single tree selection removes individual trees of all size classes more or less uniformly throughout the stand to maintain an uneven-aged stand and achieve other stand structural objectives. While it is easier to apply such a system to a stand that is naturally close to the uneven-aged condition, single tree selection systems are prescribed for even-aged stands, although numerous preparatory cuttings must be made to create a stand structure where the system can truly be applied.

Once the uneven-aged structure approximates the balanced condition, the single tree selection system generally produces a complex mixture of small, even-aged clumps which are thinned over time to theoretically produce one mature tree. In theory these clumps should yield at least one mature tree of the specified maximum diameter, although in practice these clumps are often larger.

New regeneration develops in small scattered openings created theoretically in small gaps with an area equivalent to the crown spread of a single mature tree. In practice these gaps are often larger, created through the removal of several mature trees. Since regeneration is always being recruited and larger mature trees are scattered, or in very small groups, these stands appear quite open, with many gaps. The system is generally used for the most tolerant species in an area. Using the single tree selection system to encourage species mixtures requires effort, especially where some less tolerant seral species aredesired. Such stands must be opened considerably for this system to work.

Since these stands are a confusing jumble of age classes, regulation of these standstends to be complex. Usually guidelines for residual stocking, maximumdiameter, diameter distribution (usually expressed by a "q-ratio"), and cutting cycle are used during each entry. Smith (1993) warns against attempting to create mini, sustained yield units at the stand level. He suggests that foresters should instead focus on maintaining a continuous stock of larger trees, without excessive concern about perfectly balancing age classes. Nyland (1996) also warned about relying on the pure mathematical relationship defined by a q-ratio for diameter distributions, pointing out that q-distributions never remain stable even through one cutting cycle.

Diagram of single tree selection over time.
Single tree selection over time

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However, Nyland does recommend using all of the classic selection parameters, including q-ratios, as interim guidelines. Both Smith and Nyland suggest close monitoring and periodic remeasurement to follow selection stands through their cutting cycles such that parameters can be adjusted to fit the biological reality with the management objectives. All authors encourage foresters to never forget the basic tenets for successful single tree selection: to provide sufficient gaps for regeneration and to maintain vigour throughout the stand.

For more information on these complex systems, please refer to Chapter 5 and Appendix 2.

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Group Selection Systems

Group selection systems also promote uneven-aged stands with clumps of even-aged trees well distributed throughout the cutting unit. Unlike single tree selection, however, these even-aged groups are large enough to accommodate some shade-intolerant seral species in addition to more tolerant climax species. Small gaps or openings are created on short intervals to develop into a mosaic of at least three or more age classes throughout the stand (see the diagram).

Illustration of group selection systems.

Remember that the choice of group or single tree selection must consider the resource management objectives at all levels and the existing standand site conditions. Because of stand-level advantages, group selection or any other system cannot be viewed as a panacea in areas with many conflicting management objectives. The implications of using a broad application of one silvicultural system over a large area could be serious for one or more management objectives.


Advantages of group selection system over single tree selection

Basically the group selection system is easier to administer and treat than single tree selection. The simplest types of group selection systems create definite gaps in the forest canopy. These systems fit ecologically with "gap-regenerated" stands, which tend to be common in some unmanaged stand types. If gaps are large enough, the entire spectrum of local vegetation may regenerate within them. This may encourage a diverse habitat for wildlife and promote biodiversity.

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In the Vernon Forest District during 1994-1995, 68% of the annual cut for the Small Business Forest Enterprise Program was harvested using shelterwood, single tree selection, group selection, or strip selection systems. The remaining 32% was harvested using seed tree systems and clearcut with reserves.
BC Ministry of Forests

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Such a stand can be regulated using simple area management techniques, although diligent stratification and mapping would be required. However, if groups are quite small and the age classes numerous, the stand may best be regulated as a single tree selection with intensive marking and the full range of uneven-aged parameters (q-ratio, maximum diameter, etc.).

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Size of group openings

Some authors suggest that group selection systems may have openings as small as that created through the removal of two or three trees, up to as large as several tree lengths across (Nyland 1996). However, single tree selection system, where opening size approximates that created through the removal of a single mature tree, is rarely applied on the ground. Instead, single tree selection often removes clumps of several mature trees, potentially confusing it with this definition of group selection. Also, some authors speak of group/single tree selection combinations and patch selection systems, where single tree selection iscombined with small fixed-area patches at widely scattered locations (Nyland 1996).

As you can see, the array of terms used for uneven-aged systems can become quite confusing. Therefore, it is perhaps simpler to recognize a continuum of group or clump sizes between single tree selection and group selection. Use of the terms may vary among managers depending on the species they manage and the ecology of their sites. At the margin between the two systems the name chosen to describe them probably doesn't matter too much. It is appropriate to call selection systems single tree selection when the group openings created are so tiny that simple areabased regulation is impractical and the classic uneven-aged parameters (q-ratios,etc.) must be used. Some managers may still call such systems group selection or small-group selection, although this is probably not a huge issue.

At the other end of the group-size question, a group selection with 5 ha openings to harvest a 200 ha stand stretches the definition of group selection too far. Authors worldwide agree that a group becomes a clearcut ecologically when most of the opening (greater than 50%) starts to have the same environmental regime as a large clearcut. The opening size will depend on the biological requirement of the preferred tree speciesand other resource objectives. However, if the openings become larger then several tree lengths, they may approach the clearcut environment. The Ministry of Forests therefore defines group selection as having openings of two tree lengths or less in width. In spite of this administrative definition, ecological considerations related to the influence of the stand edges would depend on aspect, slope, and other terrain features which may influence the angle of solar radiation and windflow.

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Clearcutting: The harvesting of all trees in a single cut from an area of forest large enough so that the "forest influence" is removed from the majority of the harvested area.
Hamish Kimmins (1992)


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