Forest Investment Account (FIA) - Land Base Investment Program - Innovative
FIA Project 6353007

    5-year Summary on the effects of group retention on forest songbirds in Coastal British Columbia: progress report
Project lead: Weyerhaeuser Company Ltd.
Contributing Authors: Preston, Michael I.; Harestad, Alton S.
Imprint: Nanaimo, B.C. : Weyerhaeuser Company Ltd., 2005
Subject: Forest Investment Account (FIA), Songbirds, British Columbia
Series: Forest Investment Account (FIA) - Land Base Investment Program - Innovative
Weyerhaeuser's coastal tenure in British Columbia is now predominantly managed using a variable retention silvicultural system, following an approximate 20 % per year phase-out of clearcutting over the period 1999-2003. The resulting retention, defined primarily as 'grouped' and 'dispersed', is meant to serve two primary functions: 1) to retain late succession structure for enhancing second-growth diversity, maintaining and increasing connectivity across the landscape, and providing refugia for survival and dispersal of various organisms; and 2) to match a wide range of retention alternatives with site-specific stand and wildlife needs. In this progress report, we summarize broad patterns of species richness, abundance, and diversity as it pertains to the effects of group retention on forest songbirds in TFL 39 on Vancouver Island, British Columbia. We summarize briefly the preliminary stages of the project (2000 - 2001) and focus mainly on the final three years (2002-2004) where specific hypotheses were defined and adequate study sites were available for surveying. Because the variable retention system was relatively new in 2000, initially too few sites were available for a rigorous study design, thus leaving the study to evolve with the harvest schedule. Therefore, during the first three years of this study (2000-2002), methodologies and sample plots were adjusted accordingly. In 2000 and 2001, only a few retention sites were available for monitoring, and at that time, only group patches were of concern, not overall treatment effects. Thus, in 2000, 33 patches from 15 group retention sites were monitored and compared to 6 remnant and 8 forested control sites. In total, 133 surveys were completed, yielding 31 species from group retention patches, and 21 species in each of the remnant and control sites. Both remnant and control sites supported more forest-dependent birds, in terms of abundance, than group retention sites. However, within group retention sites, richness, abundance, and diversity all increased with increasing patch size. In 2001, 16 group retention sites with 72 plots, and 6 uncut stands with 25 plots were monitored four times each. Remnant sites were not included as comparisons from this point forward. Once again, group sites supported more species than control sites, but for forest-dependent species, they usually occurred at lower abundance. In 2002, it was evident that too few sites were available to: a) control for confounding factors (e.g., harvest methods, slope, elevation); b) have a rigorous analysis with an unbalanced study design; and c) make any comparison with traditional clearcuts. Thus, the study was refined to: a) limit sites to lower elevation and non-heli-logged sites; b) equalize the number of sites in each treatment; and c) add clearcuts as a comparison. These adjustments resulted in bird surveys for 2002 and 2003 in 12 group retention stands, 12 clearcuts, and 12 uncut control stands, each containing five monitoring stations, and each surveyed three times each year. In total 1,065 surveys were completed for these two years, yielding 3,259 songbird observations. Additionally, structural plots were obtained for 202 of the 208 plots, using methods similar to Weyerhaeuser's structural monitoring group. Preliminary results showed that all of the 18 most frequently detected species occurring in uncut stands were represented in group retention stands, but most (66%) occurred in lower abundance. Compared to clearcuts, group retention sites supported more species and, unlike clearcuts, were not dominated by Dark-eyed Junco and Winter Wren. The response of most forest-dependent species to percent retention appears relatively linear, although the slope (and possibly the shape) of the line varies among species. That assessment is ongoing. Furthermore, considerable variation among stands, but within treatments, is noticeable for some species (particularly in uncut stands). We anticipate that some of this underlying variation will be explained through an ongoing multivariate analysis of spatial and structural attributes. In 2004, 4 of the 12 group retention stands used in 2002/2003 were combined with 6 new group retention stands to assess fine-scale habitat selection among forest songbirds. In total, 352 monitoring stations were censused 4 times each, yielding 4,522 bird observations. Additionally, structural attributes were measured for 70 census units, and estimated for 135 census units. Data analysis for this component is ongoing, although based on preliminary plots, it appears evident that some forest-dependent species occur only in some kinds of patches (e.g., > 0.5 ha), while other species occur in most patches. These differences may be related to spatial attributes (e.g., patch size, distance to next patch), structural attributes, or both. Extension work from this study thus far has included six poster presentations, of which three have involved collaborators using the same data to answer different types of questions. Abstracts from those posters have also appeared in Northwest Naturalist, Murreletter, and conference proceedings. Results of this study have also been summarized in No. 9 of Weyerhaeuser's 'report' series (in press). The field component of this project is complete. What remains is a comprehensive analysis of data and a final write-up. This is the objective of the M.Sc. thesis, and upon completion, will be provided to Weyerhaeuser and the Adaptive Management Working Group as a separate document. Peer-reviewed papers in scientific journals are also anticipated once the thesis is complete.
prepared by M.I. Preston and A.S. Harestad.


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Updated August 16, 2010 

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