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Variable Retention

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Forestry Processes

Variable retention (VR) is a relatively new form of silviculture practice designed to retain some of the natural forest stand after harvesting. The name derives from the fact that various levels of retention are used with different types, amounts, and patterns of leave-trees being established in a cutblock. The focus of variable retention as a silvicultural system is to preserve forest biodiversity and ecological integrity.


[edit] History

Variable retention evolved in response to massive protesting over the logging of old growth timber forests in Clayoquot Sound during the early 1990s.[1] Due to public outcry, The Scientific Panel for Sustainable Forest Practices in Clayoquot Sound (CSSP) was established by the government of British Columbia to develop sustainable forestry practices for coastal temperate rain forest ecosystems characteristic of the Clayoquot Sound and other coastal B.C. regions.

In 1994 CSSP published a report that pinpointed the economic significance of logging in the Clayoquot area but not at the expense of removing all the trees. What resulted was a compromise between forestry companies and public concern to preserve the region’s old growth forests by logging only some of the trees and leaving the rest. The problem was deciding which trees to log and which ones to leave behind. Such factors in the decision making process led to the development of a new method that stressed non-timber specific objectives—variable retention. The new approach outlined three primary goals:

  • To provide, after harvest, habitat important to the survival of organisms and processes that may otherwise be lost temporarily or permanently from a harvested area
  • To enrich current and future forests by maintaining some existing structure features and organisms from previous stands that might otherwise become absent from a harvested area even decades after logging
  • To improve "connectivity" between cutting cycles and forest areas by facilitating the migration of life throughout the cutblock[2]

Since the report was published, a number B.C. forestry product companies that operate on the coast have gradually adopted variable retention in the harvesting and planning of forest stands.[3]

[edit] Process

Variable retention is designed to retain and preserve the natural structures and processes of a forest. It differs greatly from clearcutting because science is used to determine which trees actually get cut down and which ones get left behind. While clearcutting focuses on removing all the trees from within a stand, variable retention balances the removal of timber with the retaining of timber in a stand. This usually involves leaving behind trees dispersed or clumped together in groups in an opening after harvesting. These trees may include healthy trees, dead trees, snag trees, and woody fallen debris that combined together, maintain old growth structure, provide natural habitat protection and enhance the visual quality of a stand. In this way, variable retention closely mimics the end-effect created by natural disturbances such as forest fires, wind, or disease.[4] A portion of the original forest stand always remains afterward.

Forest influence is also used to qualify variable retention from conventional silvicultural systems. What forest influence proposes is that the edge effect of the forest doesn’t necessarily stop at its edge. The effects the forest has on climate, soil, and water can extend well beyond its natural border. Everything within one tree length of the cutblock is still considered to be part of the forest.[5] With variable retention, over half of the cutblock must be within the forest's influence or one tree length from the edge.[6] What this means is that openings or clearcuts can be no more than four tree lengths across. These openings are smaller than traditional clearcuts but still provide ample space to log a cutblock. Another practice in using variable retention is to place mandatory buffers around all fish streams.[7]

Variable retention can also be combined with conventional silvicultural systems to describe a cutting and regeneration strategy. An example may be uniform shelterwood with group retention.[8]

[edit] Types

There are two types of variable retention:

Dispersed retention is the retaining of individual trees throughout a cutblock. This method allows for the preservation of a greater variety of species.[9]

Group or aggregate retention is the retaining of groups or clusters of trees throughout a cutblock. These patches of forest create what are called "lifeboats" for a variety of life forms associated with more mature forests.[10]

[edit] References

  1. Truck Logger. Greenspirit. 2008-10-28.
  2. The Retention System. NRC-CNRC Journals. 2008-10-28.
  3. The Retention System. NRC-CNRC Journals. 2008-10-28.
  4. The Retention System. NRC-CNRC Journals. 2008-10-28.
  5. Tree. Spruceroots. 2008-10-28.
  6. Past Issues. The Working Forest. 2008-10-28.
  7. Truck Logger. Greenspirit. 2008-10-28.
  8. Forest Strategy. Forest Bio Diversity in BC. 2008-10-28.
  9. Past Issues. The Working Forest. 2008-10-28.
  10. Article. The Working Forest. 2008-10-28.