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 t ...
Preston, Michael I., Harestad, Alton S.. 2004. 5-year Summary on the effects of group retention on forest songbirds in Coastal British Columbia: progress report. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2005MR116
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), Songbirds, British, Columbia
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