is a relatively new silvicultural
system that follows nature's model by always retaining a significant part of the forest
after harvesting. Variable retention harvests serve to promote desired natural regeneration while preserving unique features in the landscape
such as aesthetics
, legacy trees, water quality, soil stabilization, species diversity, and so on.
Value of variable retention
Variable retention is gradually becoming a popularly promoted tool for helping resolve the dilemma between demands for wood
and desire to maintain biodiversity
in managed forests. What is at issue is how much a forest operation can log without adversely interfering with other ecological processes within the forest. Variable retention is believed by some forestry experts to resolve this issue. Either few trees or many trees can be retained under the variable retention system, and trees can be retained in clumps (patches) or left uniformly throughout a stand, hence the name "variable retention". It is a technique for retaining trees as key structural elements of a harvested stand for at least the next harvest rotation in an effort to maintain both species and forest processes. There are four key mechanisms through which variable retention is presumed to maintain biodiversity:
- By providing a constant supply of structural features that are at high risk to being lost due to modern forestry practices and that are known to be important to biodiversity, such as large trees, very young trees, snags, and woody debris
- By providing adequate refuge for sensitive species that will colonize the surrounding managed forest environment as it develops suitable conditions
- By establishing habitat patches that can serve as stepping stones for the dispersal of newly produced offspring, seeds, and spores
- By increasing the structural diversity of managed stands.
Variable retention methods
The traditional method of variable retention is a more painstaking process than clearcutting
fly the loggers in so they don't have to build roads that destroy the forest floor. When they cut down the valuable hardwood trees for timber, they try to mimic the way the forest itself works. The natural process of disturbance is one tree, which is only a small-scale disturbance. Keeping this fact in mind, the loggers cut smaller patches of forest. They take mostly trees valued in the market place (like red cedar
), and they work around trees that help the forest survive, such as dead or dying trees called snags. Such dead or dying trees provide a growth medium for fungi
, as well as food and shelter for insects
and the birds
that prey upon insects. Loggers also leave younger trees that will continue to grow. In addition, loggers avoid cutting down designated older trees revered by native people for cultural and religious reasons, such trees having been scarred when the ancestors of the native people made carvings and inscriptions or stripped bark. Variable retention harvesting retains between 10% and 40% of the original stand (in some cases, more) in both rolling and permanent pockets of untouched trees and critical refugia
. This 10% to 40% of the forest that is retained is composed of very young trees, very old trees and dead trees that maintain the forest ecosystem, and provide a hospitable habitat
and other animals. This silviculture regime provides post-harvest ecological structure while creating sufficient opportunity to plant and naturally regenerate highly valuable tree species for timber
, as well as restore historical coniferous
tree dominance to the forestland. Some timber companies have restricted the use of variable retention silviculture to only poorly stocked stands of rare but valuable tree species. This silviculture is seldom used in conifer-dominated or decidual-dominated stands of vast size.
Controversy over variable retention harvesting
regard the variable retention method of harvesting as being far less detrimental to the forest ecosystem
than clearcutting. However, opponents claim that variable retention is much more laborious, tedious, time-consuming and expensive than simple clearcutting, and, in turn, greatly increases the monetary cost of furniture and other wood products that consumers buy.
Forestry organizations, such as the Forest Stewardship Council (FSC), approve of variable retention and are now declaring timber obtained through such a harvesting method as certified wood. Virtually all forest operations in the Canadian province of British Columbia, including MacMillan Bloedel, use the variable retention method to harvest timber in the temperate rain forests along the Pacific coast. Many logging companies have not maintained a total commitment to phasing out clearcutting and embracing variable retention harvesting, even MacMillan Bloedel has occasionally fallen short of its commitment to phase out clearcutting. Although world markets are beginning to ask for certified wood obtained entirely through variable retention harvesting, it is not yet clear whether the general population of consumers will be willing to pay higher prices for such wood.