Application of variable retention for biodiversity conservation in British Columbia

Background/Question/Methods

Research in natural forests in the Pacific Northwest over the past several decades has verified the importance of structural and other biological legacies that remain following natural disturbances and the role they play in facilitating ecosystem recovery and maintaining biodiversity. The concept of retaining trees during harvesting for purposes other than silviculture was promoted in the early 1980s by Dr. J.F. Franklin using the terms “green tree retention” and “new forestry.” The term “variable retention” was first introduced by the Clayoquot Scientific Panel in 1995. Marketplace pressure over clearcutting led a BC forest company to begin a five-year phase-in of variable retention in 1998. In 1999, the BC government officially recognized the “retention silvicultural system” in its forestry regulations. The retention system uses the concept of “forest influence” to distinguish the system from clearcuts, and requires that retention is distributed over the cutting unit and remains for an entire forest rotation. This paper reviews studies focused on this key question: What is the efficacy of different types, amounts and spatial patterns of retention for conserving different organisms.

Results/Conclusions

For coastal BC, 40% of harvesting in recent years used the retention silvicultural system. Group (aggregated) retention was the most common, with groups typically 0.25 to 1 ha, anchored on riparian or other ecological features. Retention averaged over 20%, not including landscape-level reserves. Wind damage to retained trees is a key concern. Studies showed regional differences in wind damage. Small patches and strips had higher rates of wind damage than external edges and large patch edges, although over half of total windthrow was due to cutblock edges. Group retention was beneficial for birds typical of uncut forests, and bird occurrence was positively correlated with percent retention. Species that are sensitive to microclimatic changes and are less mobile (e.g., gastropods, carabid beetles and amphibians) may be important indicators of forest influence.  Ten of twelve species of gastropods showed significant positive or negative treatment effects. Studies of beetles suggest that there may be some life-boating benefit of group retention for forest specialist beetles in drier climates, and a positive response to patch size. Abundance of ectomycorrhizal (EM) fungi decreased with distance from forest edges and EM diversity increased with increasing retention of live trees. Initial findings have been used to examine how management practices can be improved.