Variable retention
Encyclopedia
Variable retention is a relatively new silvicultural
system that retains forest
structural elements for at least one rotation in order to preserve environmental values associated with structurally complex forests. .
Some examples for environmental values are forest cover connectivity, soil
stabilization, microclimate
under retained tree
s, habitat
s associated with live or dead trees and species diversity due to habitat preservation, and wildlife corridor
preservation. Variable retention also aids in emulation of natural disturbances by leaving behind some residual structure from previous stand which is typical for stand replacing disturbances Traditional silviculture systems such as clearcut, patch cut, shelterwood, etc. are focused on maximizing timber production and future regeneration of the trees. Variable retention on the other hand is focused on what is retained
and demands to maintain habitat
and ecology
biodiversity
and structural diversity 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 minimize the impact of logging operation better than other harvesting options. Either few trees or many trees can be retained under the variable retention system, and trees can be retained in patches (aggregated retention) or left uniformly throughout a stand (dispersed retention); hence the name "variable retention". It is a technique for retaining trees as key structural elements of a harvested stand for at least until next harvest rotation in an effort to maintain species, habitat diversity and forest-related processes.
There are four key mechanisms through which variable retention is presumed to maintain biodiversity
:
. In some cases helicopters
take the logs out. The use of the helicopters avoid the need to build logging roads inside the forest and hence avoid the disturbance of the forest floor. Yet, helicopter logging is less fuel efficient than truck use. Some logging operations that use variable retention try to emulate forest disturbances. In those cases, deadwood in snags and logs is left behind. Such dead or dying trees provide a growth medium for fungi and epiphytes, as well as food and shelter for insect
s and the bird
s. Loggers also leave younger trees that will continue to grow.
Variable retention harvesting retains more than 15% of the original stand in both rolling and permanent pockets of untouched trees and refugia
patches. Retained forest structure is composed of live and dead trees that enhence structural diversity, and provide a hospitable habitat
for variety of living organisms that prevail in the unlogged forest. This silviculture regime provides post-harvest ecological structure while creating sufficient opportunity to plant and naturally regenerate valuable tree species for timber
, as well as restore historical coniferous and/or deciduous
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.
s 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 clearcutting.
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
for green building
. Many 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, and even MacMillan Bloedel had 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.
Silviculture
Silviculture is the practice of controlling the establishment, growth, composition, health, and quality of forests to meet diverse needs and values. The name comes from the Latin silvi- + culture...
system that retains forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
structural elements for at least one rotation in order to preserve environmental values associated with structurally complex forests. .
Some examples for environmental values are forest cover connectivity, soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
stabilization, microclimate
Microclimate
A microclimate is a local atmospheric zone where the climate differs from the surrounding area. The term may refer to areas as small as a few square feet or as large as many square miles...
under retained tree
Tree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
s, habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
s associated with live or dead trees and species diversity due to habitat preservation, and wildlife corridor
Wildlife corridor
A wildlife corridor or green corridor is an area of habitat connecting wildlife populations separated by human activities . This allows an exchange of individuals between populations, which may help prevent the negative effects of inbreeding and reduced genetic diversity that often occur within...
preservation. Variable retention also aids in emulation of natural disturbances by leaving behind some residual structure from previous stand which is typical for stand replacing disturbances Traditional silviculture systems such as clearcut, patch cut, shelterwood, etc. are focused on maximizing timber production and future regeneration of the trees. Variable retention on the other hand is focused on what is retained
Value of variable retention
Variable retention is gradually becoming a popularly promoted tool for helping resolve the dilemma between demands for woodWood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...
and demands to maintain habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
and ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
and structural diversity 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 minimize the impact of logging operation better than other harvesting options. Either few trees or many trees can be retained under the variable retention system, and trees can be retained in patches (aggregated retention) or left uniformly throughout a stand (dispersed retention); hence the name "variable retention". It is a technique for retaining trees as key structural elements of a harvested stand for at least until next harvest rotation in an effort to maintain species, habitat diversity and forest-related processes.
There are four key mechanisms through which variable retention is presumed to maintain biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
:
- 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 habitatHabitat* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
availability, such as large trees, very young trees, snagSnagIn forest ecology, a snag refers to a standing, partly or completely dead tree, often missing a top or most of the smaller branches. In freshwater ecology it refers to trees, branches, and other pieces of naturally occurring wood found sunken in rivers and streams; it is also known as coarse woody...
s, and coarse woody debrisCoarse woody debrisCoarse woody debris is a term used in English-speaking countries for fallen dead trees and the remains of large branches on the ground in forests. Some prefer the term coarse woody habitat . A dead standing tree is known as a snag and provides many of the same functions as coarse 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, Patch dynamicsPatch dynamicsPatch dynamics is a conceptual approach to ecosystem and habitat analysis that emphasizes dynamics of heterogeneity within a system ....
, 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 - Harvesting methods
The method of variable retention is a more deliberate process than clearcuttingClearcutting
Clearcutting, or clearfelling, is a controversial forestry/logging practice in which most or all trees in an area are uniformly cut down. Clearcutting, along with shelterwood and seed tree harvests, is used by foresters to create certain types of forest ecosystems and to promote select species that...
. In some cases helicopters
Heli-logging
Heli-Logging, or Helicopter Logging, is a method of logging that uses helicopters to remove cut trees from forests by lifting them on cables attached to a helicopter. Helicopter logging is often used in inaccessible areas of forests...
take the logs out. The use of the helicopters avoid the need to build logging roads inside the forest and hence avoid the disturbance of the forest floor. Yet, helicopter logging is less fuel efficient than truck use. Some logging operations that use variable retention try to emulate forest disturbances. In those cases, deadwood in snags and logs is left behind. Such dead or dying trees provide a growth medium for fungi and epiphytes, as well as food and shelter for insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s and the bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s. Loggers also leave younger trees that will continue to grow.
Variable retention harvesting retains more than 15% of the original stand in both rolling and permanent pockets of untouched trees and refugia
Refugia
In biology a refugium , sometimes termed simply a refuge, is a location of an isolated or relict population of a once more widespread species. This isolation can be due to climatic changes, geography, or human activities such as deforestation and over-hunting...
patches. Retained forest structure is composed of live and dead trees that enhence structural diversity, and provide a hospitable habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
for variety of living organisms that prevail in the unlogged forest. This silviculture regime provides post-harvest ecological structure while creating sufficient opportunity to plant and naturally regenerate valuable tree species for timber
Timber
Timber may refer to:* Timber, a term common in the United Kingdom and Australia for wood materials * Timber, Oregon, an unincorporated community in the U.S...
, as well as restore historical coniferous and/or deciduous
Deciduous
Deciduous means "falling off at maturity" or "tending to fall off", and is typically used in reference to trees or shrubs that lose their leaves seasonally, and to the shedding of other plant structures such as petals after flowering or fruit when ripe...
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.
Controversy
EcologistEcology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
s regard the variable retention method of harvesting as being far less detrimental to the forest ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
than clearcutting
Clearcutting
Clearcutting, or clearfelling, is a controversial forestry/logging practice in which most or all trees in an area are uniformly cut down. Clearcutting, along with shelterwood and seed tree harvests, is used by foresters to create certain types of forest ecosystems and to promote select species that...
. However, opponents claim that variable retention is much more laborious, tedious, time-consuming and expensive than clearcutting.
Forestry organizations, such as the Forest Stewardship Council
Forest Stewardship Council
The Forest Stewardship Council is an international not-for-profit, multi-stakeholder organization established in 1993 to promote responsible management of the world’s forests. Its main tools for achieving this are standard setting, independent certification and labeling of forest products...
(FSC), approve of variable retention and are now declaring timber
Timber
Timber may refer to:* Timber, a term common in the United Kingdom and Australia for wood materials * Timber, Oregon, an unincorporated community in the U.S...
obtained through such a harvesting method as certified wood
Certified wood
Certified wood and paper products come from responsibly managed forests – as defined by a particular standard. With third-party forest certification, an independent organization develops standards of good forest management, and independent auditors issue certificates to forest operations that...
for green building
Green building
Green building refers to a structure and using process that is environmentally responsible and resource-efficient throughout a building's life-cycle: from siting to design, construction, operation, maintenance, renovation, and demolition...
. Many forest operations in the Canadian
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
province of British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, 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, and even MacMillan Bloedel had occasionally fallen short of its commitment to phase out clearcutting. Although world markets are beginning to ask for certified wood
Certified wood
Certified wood and paper products come from responsibly managed forests – as defined by a particular standard. With third-party forest certification, an independent organization develops standards of good forest management, and independent auditors issue certificates to forest operations that...
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.
See also
- Conservation biologyConservation biologyConservation biology is the scientific study of the nature and status of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction...
- DeforestationDeforestationDeforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....
- Forest farmingForest farmingForest farming is an agroforestry practice characterized by the four "I's"- Intentional, Integrated, Intensive and Interactive management of an existing forested ecosystem wherein forest health is of paramount concern. It is neither forestry nor farming in the traditional sense.Forest farming, or...
- Forest managementForest management200px|thumb|right|[[Sustainable development|Sustainable]] forest management carried out by [[Complejo Forestal y Maderero Panguipulli|Complejo Panguipulli]] has contributed to the preservation of the forested landscape around [[Neltume]], a sawmill town in Chile...
- Hardwood Timber ProductionHardwood Timber ProductionHardwood timber production is the process of managing stands of deciduous trees to maximize woody output. The production process is not linear because other factors must be considered, including marketable and non-marketable goods, financial benefits, management practices, and the environmental...
- LoggingLoggingLogging is the cutting, skidding, on-site processing, and loading of trees or logs onto trucks.In forestry, the term logging is sometimes used in a narrow sense concerning the logistics of moving wood from the stump to somewhere outside the forest, usually a sawmill or a lumber yard...
- Old Growth ForestOld growth forestAn old-growth forest is a forest that has attained great age , and thereby exhibits unique ecological features. An old growth forest has also usually reached a climax community...
- Selection cuttingSelection cuttingSelection cutting is the silvicultural practice of harvesting a proportion of the trees in a stand. Selection cutting is the practice of removing mature timber or the lessening of older trees to improve the timber stand. This system may be used to manage even or uneven-aged stands...
- SilvicultureSilvicultureSilviculture is the practice of controlling the establishment, growth, composition, health, and quality of forests to meet diverse needs and values. The name comes from the Latin silvi- + culture...
- Sustainable forest managementSustainable forest managementSustainable forest management is the management of forests according to the principles of sustainable development. Sustainable forest management uses very broad social, economic and environmental goals...
- Sustainable forestry