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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.〔Franklin, J.F., Berg, D.R., Thornburgh, D.A. & Tappeiner, J.C. (1997). Alternative silvicultural approaches to timber harvesting: Variable retention harvest systems. In textbook: Creating a Forestry for the 21st Century: The Science of Ecosystem Management. (eds K.A. Kohn & J.F. Franklin), pp. 111. Island Press, Washington, D.C.〕 Some examples for environmental values are forest cover connectivity, soil stabilization, microclimate under retained trees, habitats 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 == Value of variable retention == Variable retention is gradually becoming a popularly promoted tool for helping resolve the dilemma between demands for wood 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 by leaving biological legacies such as coarse woody debris(nurse logs and snags). 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: * 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 habitat availability, such as large trees, very young trees, snags, and 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 dynamics, that can serve as stepping stones for the dispersal of newly produced offspring, seeds, and spores * By increasing the structural diversity of managed stands 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Variable retention」の詳細全文を読む スポンサード リンク
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