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Silviculture

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Seedling trees are replanted in a cutblock after harvesting.
Silviculture is the science of renewing a forest crop through controlling the stock, density, composition, growth, health, and overall quality of a forest stand throughout its lifetime. As a practice, it involves a range of activities that include stand tending, tree harvesting, site preparation, and reforestation. The science of silviculture also warrants an understanding of silvics. Silvics is the study of how trees grow, reproduce, and respond to environmental and ecological change.[1] The silvics of a particular tree species is therefore used as the foundation to develop a silviculture system that aims to meet forestry management objectives. Silviculture should not be confused with forestry management however. Forestry managment differs in that it addresses how a forest is to be managed and for what reasons.[2]

Contents

[edit] History

The first application of silviculture was carried out in tree planting operations on prairie farms and abandoned farmlands in eastern Canada between 1910 and 1950. During the same period, logging relied primarily on natural regeneration for new forest growth. The intervention of silvicultural techniques to speed up the regeneration process was explored after 1950. A number of tree improvements and tree planting programs also began to emerge. The 1970s witnessed a shift towards more quality tree planning especially with the restocking of NSR lands. This also led to the greater use of herbicides.

By the 1980s, most of the silvicultural programs in Canada were being implemented and administered by governement agencies.  The contracting out of silvicultural programs grew government privatized programs. Today, the emphasis on silviculture practices is more concerned with developing and tailoring silvicultural and forest management programs that produce forest-age class structures that ensure continous forest cover, particularily as Canada's supply of old-growth virgin timber becomes depleted.[3]

[edit] Process

Reforestation of harvested stands is a common practice of silviculture
After a stand of forest has been harvested, it must be renewed through the implementation of a silvicultural practice or method. The silvicultural method can involve all or a combination of: preparing the stand site, planting, seeding, and tending. Silviculture systems can be either intrusive or non-intrusive and are devised to guide the evolution of a forest stand into new growth forest. This process is known as forest regeneration or reforestation. After a stand or cutblock of trees has been harvested, regeneration of the forest can occur naturally or artificially. For these reasons, silviculture systems typically fall into one of three categories. With extensive silviculture, the natural regeneration of trees is relied upon for forest renewal. In basic silviculture, mechanical site preparation in undertaken followed by seeding or planting. Intensive silvilculture depends on some form of artificial regeneration such as planting or seeding to re-establish the next forest stand.[4] Another way of classifying silvicultural methods is to divide them into those systems that are even-aged and those that are uneven-aged. Even-aged systems include clear-cut, patch-cut, coppice, seed tree, and shelterwood silviculture systems. Uneven-aged systems are called selection systems.[5]

[edit] The Role of Thinning

Pre-commercial thinning (PCT) is a silvicultural technique used in natural forest stands aged 10 to 15 years to reduce density and provide trees with optimal space to grow.[6] 

Commerical thinning (CT) is also a widely used silvicultural technique. The objective of commercial thinning is to literally “thin” out an overstocked forest stand by removing trees that are big enough to be sold and have product value.[7] Thinning out the stand this way ensures the remaining trees grow and stay healthy.

Pre-commercial and commercial thinning and longer rotations between actual harvests are becoming more frequent. The use of thinning is also taking into account a growing awareness of biodiversity and has even been adapted to increase both horizontal and vertical structures. Many European forest stands that were once even-aged are being converted to uneven-aged stands. This type of change suggests a more holistic view of forestry practices that essentially support “close-to-nature” silviculture methods.[8]

[edit] Global Use of Silviculture

Current silvicultural trends and practices vary in different parts of the world. In recent years, the application of silviculture methods globally has focused on a push towards continuous cover forestry. In B.C., this is evident in the growing use of variable retention methods along the coast where logging old-growth forests continues to spark much debate. Organizations such as the Forest Stewardship Council have also evolved to address and advocate the use of silvicultural methods within the forest industry. The type of silvilcultural system used will depend on the type of forest stand (even-aged or uneven-aged) and tree species. In Europe, forestry practices are aimed at the multiple use of products; therefore, uneven-aged silvicultural systems that include selective, shelterwood, and coppicing are more dominant in that part of the world. In North America, production of timber is still commodity driven. Even though this is slowly beginning to change, silvicultural practices in North America and other parts of the world such as New Zealand, northern Sweden, and southern USA are designed to produce saw logs devoid of defects and stripped of debris. This involves producing a homogenous forest stand through even-aged silvicultural methods such as clearcutting. [9]

[edit] Types

A clear-cut system refers to a situation whereby all the trees in an entire stand are harvested and new even-aged stands are then regenerated after harvesting within the cleared block by planting, natural regeneration, retention, or advanced regeneration.

A seed tree system is managed similarly to a clear-cut system, except a number of trees are left unharvested to supply seed for the next crop. In some cases, natural regeneration may be supplemented with some planting under the seed trees.

A shelterwood system is when an old stand of trees is removed in a series of cuttings designed to foster the growth of an even-aged new stand of trees under shelter of the existing one. The old stand of trees serves as a protective shelter for the natural regeneration of a new forest stand.

A coppice system is an even-aged silvicultural approach where forest regeneration is carried out through vegetative sprouting of either the existing root system of cut trees (called suckers) or from cut stumps known as shoots. Coppice systems are limited in use to hardwood tree species.

A patch-cut system involves removal of an entire stand of trees less than 2.5 acres (1 ha) in size, with each patch being managed separately as an even-aged opening. Forest regeneration is accomplished either naturally, artificially, or a combination of the two.

A retention system is when individual trees or a group of trees are retained to encourage structural diversity over the cutblock area for at least one rotation or cutting cycle.

A selection system is used to produce uneven-aged stands. Mature timber is harvested in small patches to make room for younger trees and new growth. Trees are harvested on the basis of size and proximity to other trees. This method works well for trees that thrive in the shade.

[edit] References

  1. Silvic and Silviculture--The Agriculture of Trees. West Virgina University. 2008-11-21.
  2. Silviculture. Grand River Forest Plan. 2008-11-21.
  3. http://nfdp.ccfm.org/silviterm/silvi_e/silvitermintroe.htm#pct
  4. Silvicultural Intensities. The Legacy Forest. Faculty of Forestry, Lakehead University. 2008-11-21.
  5. Silvicultural Systems Guidebook. Government of British Columbia. 2008-11-21.
  6. Pre-commercial Thinning. Kruger Publication Papers. 2008-11-21.
  7. Commercial Thinning. Mongabay. 2008-11-21.
  8. Recent Developments in Silviculture. John Innes. Canadian Silviculture. 2008-11-21.
  9. Recent Developments in Silviculture. John Innes. Canadian Silviculture. 2008-11-21.