Silvicultural Systems

In simple terms, silviculture manages the growing space of individual trees through treatments that regenerate and space trees. Trees can be spaced farther apart to create open stands that favor more understory plant develop ment and larger trees with large branches, or they can be spaced closer together to create dense stands with little understory and smaller trees with smaller branches, but more overall tree volume production. Spacing can be managed when seedlings first establish or over time with intermediate thinning treatments.

A silvicultural system is a series of treatments applied to a forest stand to create target conditions. A given stand can be directed on many pathways, depending on management objectives. This is accomplished by altering the timing and intensity of silvicultural treatments to control light condi tions and species composition. Traditional silvicultural systems represent a gradient of tree removal; the heaviest removals typically promote development of structurally and compositionally uniform conditions while the lightest removals may have little effect on diversity. Treatments between these extremes often promote the most complexity.

Even aged silvicultural systems produce a single cohort of regeneration, which means the newly establishing trees are close in age. Clearcutting is an even aged system that removes almost all trees, creating a fully exposed site. It is sometimes referred to as a monocyclic system, particularly in tropical forests. Seed tree systems also result in a single age class of regeneration. In this system, almost all trees are removed at harvest, but a few are retained so their seeds can produce the next generation. In contrast, a shelterwood system regenerates a stand with two age classes. It is dis tinguished from a seed tree system by using shade intentionally to give the seedlings of desired species a competitive advantage over other vegetation during their germination and early growth. If the shelter trees are removed after regeneration, the stand is even aged.

Uneven aged silvicultural systems regenerate a stand with three or more age classes. The objective is a forest with trees of different ages or sizes intermingled, typically accomplished with some form of selection system. In these systems, mature and immature trees are felled to create or maintain uneven aged stands. Single tree selec tion fells individual trees and generally tends to increase the proportion of shade tolerant species in mixed species stands. Group selection creates small openings and, there fore, maintains a higher proportion of shade intolerant species in mixed species stands than does single tree selection. Partial cutting is a general term denoting some thing other than a clearcut and can include selection systems. In tropical regions, selection systems are also referred to as polycyclic systems.

The methods used to manage structural and composi tional diversity depend on site specific conditions as well as on management objectives. For example, thinning is one silvicultural technique applied in various ways and for various objectives. It is a treatment to reduce tree density and alter spacing. Thinning can also be used to modify the stand structure to achieve greater vertical structural diversity, modify species composition, or encourage the development of new cohorts of trees or other plants. Precommercial thinning implies that trees cut are not merchantable, whereas in commercial thin ning the trees are valuable enough to recover some of the costs of their harvest. When trees are cut from the lower crown classes to favor those in the upper classes, the term used is thinning from below. In contrast, thinning from above involves cutting dominant and co dominant trees to favor the best trees of these classes. By targeting spe cific diameter classes or strata, thinning can be used to alter wildfire behavior, to recruit specific understory plants, to provide thermal and hiding cover for animals, and to grow trees with specific characteristics for wildlife habitat. For example, spatial patterns of residual trees can be altered to leave random or clumpy patterns depending on management objectives (see Figure 8). Although con temporary practices like variable density thinning do not have the benefit of long term data supporting their inten sity or design, they do rely on established theories of plant density and tree growth. Further, extensive silvical infor mation on the shade tolerance and growth form of various species can be used to predict responses of mixed species stands to various types and intensities of thinning.

Figure 8 Variable retention harvesting in Pacific Northwest of North America. Some trees are left within the cutting unit in clumps.

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