Variable-retention (VR) harvesting is a promising silvicultural technique for maintaining biodiversity while allowing timber extraction (Lindenmayer and Franklin 2002). However, many questions remain regarding its effectiveness in sustaining biodiversity, and in how it should be implemented to maximize its effectiveness (Lindenmayer and Franklin 2002). The amounts, types, and spatial arrangement of both live and dead residual structures vary widely from one form of variable-retention harvesting to another. In dispersed retention, single residual structures are distributed uniformly over the harvested area, but in aggregated retention, residual structures are concentrated in small intact areas of forest within the harvested matrix. There are many tradeoffs between each approach, with for example, dispersed retention accommodating territorial behavior in animals but lacking the niches provided by the intact soil, understory and overstory layer in aggregated retention. The variability of structural retention harvesting, coupled with its short implementation history, mean that the key issues of what structure to retain, how much to retain, and how to distribute the structures across space have received limited empirical assessment despite its widespread application.
The overall objective of this study is to evaluate the effects of dispersion pattern, retention level, patch size, and size of opening on avian communities, using an experimental approach that specifically addresses the knowledge gaps and limitations of the existing database on VR effects, and that exploit existing experimental sites and data. The study treatments and study sites for this research are part of a series of replicated, experimental harvesting blocks that test different types, amounts and spatial patterns of retention. These Variable Retention Adaptive Management (VRAM) experimental sites were previously the foundation of Weyerhaeuser?s Adaptive Management program; this program is being continued by Western Forest Products Inc with the change in company ownership. Each VRAM site was installed with random allocation of treatments, and was chosen to be as uniform as possible in timber type, site series and topographic features.
This proposal seeks to exploit the colossal investment in funding and effort that has gone into establishing these sites and in the availability of baseline data for various vertebrate and invertebrate taxa. The effectiveness of VR methods in sustaining biodiversity may best be addressed at these experimental sites with new research on avian response because: 1) Birds are by far the most species rich group of vertebrates, and represent the full spectrum of wildlife habitat requirements; 2) Birds are significant biodiversity indicators of other taxonomic groups, including woody plants and aquatic herpetofauna (Kati et al. 2004); 3) There are tight and well-defined relationships between bird communities and stand-level structure and habitat; and 4) Many existing studies have shown that birds do respond positively to green tree retention - residual tree patches such as those found in VR cutblocks retained avian species that would normally disappear after clearcut logging and that are characteristically associated with old-growth forests (Beese and Bryant 1999; Schieck et al. 2000; Tittler et al. 2001). Moreover, there is evidence to indicate that different forms of variable-retention harvesting, created by varying such factors as retention level, will differ in their effectiveness in sustaining biodiversity (e.g., Tittler et al. 2001, Chan-McLeod and Vernier 2004,Stuart-Smith et al. 2006). Existing data on VR effects on birds are, however, limited by various study constraints including: 1) the evaluation, to date, of only a small subset of dispersion patterns and retention levels; 2) generally snap-shot assessments of very short time windows following VR harvesting; and 3) rare simultaneous analyses of multiple VR factors such as ...
Chan-McLeod, Ann. 2008. An experimental study of variable-retention harvest methods on forest birds. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2008MR065
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), British, Columbia
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