Bird community composition after mechanical mastication fuel treatments in southwest Oregon oak woodland and chaparral
Introduction
Biodiversity and ecosystem function may be closely linked to historical fire regimes. These regimes have been altered by fire suppression policies implemented in the 20th century (Agee, 1993). In an attempt to restore fuel conditions created by historical fire regimes, management agencies are using prescribed fire, mechanical fuels treatments, and forest thinning to mimic the effects of natural fire (Stephens, 1998). The ability of these management activities to mimic the effects of natural fire on habitat structure and animal populations is not well understood (Tiedemann et al., 2000, Huff et al., 2005). In some cases, these treatments appear to have the desired effect of increasing the abundance of bird species that are associated with post-fire habitat conditions (Siegel and DeSante, 2003, Alexander et al., 2007). However, in other cases such treatments may fail to create the range of habitat conditions used by birds after naturally occurring wildfires (Smucker et al., 2005, Seavy and Alexander, 2006).
In oak woodlands and chaparral of southwest Oregon and northern California, fires are believed to have been common and to have played an important role in the maintenance of these communities (Agee, 1993). Because fires in these habitats may damage homes, property, and natural resources, fires have been effectively suppressed over the last 50 years. As a result of fire suppression, these habitats are believed to be changing or disappearing (Huff et al., 2005). In an attempt to reduce the risk of severe fire, while maintaining oak woodland and chaparral communities, managers are increasingly using mechanical fuels reduction in these habitats. By reducing canopy cover of shrubs and creating open areas without vegetation, these treatments are primarily designed to slow the rate at which fires spread, reduce the intensity with which they burn, and increase firefighter safety. The degree to which these treatments can help restore desired ecological conditions remains uncertain (Purcell and Stephens, 2005, Perchemlides et al., 2008).
In a previous study (Alexander et al., 2007), we compared bird abundance in areas where shrub cover had been reduced by hand on relatively small plots (7–42 ha) and untreated areas. In this study, six bird species were more abundant on the treated plots. These species were mostly those associated with open conditions or forest edges. Surprisingly, there was little evidence that species associated with shrubs were less abundant in the treated areas. We hypothesized that their ability to persist in the treated areas was facilitated by the small size of the treatment areas and the maintenance of untreated areas within treatment stands (0.4–1.2 ha). Since this study was conducted, larger-scale shrub removal treatments using heavy equipment have been implemented. We hypothesized that because these treatments are larger and leave a smaller proportion of the area untreated, the effects on shrub-associated birds would be greater. To test this hypothesis, we compared vegetation structure and bird abundance over a 2-year period in treated and untreated stands. The objectives of this project were to (1) describe the differences in vegetation structure and bird community composition and (2) compare these differences with those that were described in the previous study of smaller-scale treatments in the same habitat.
Section snippets
Study site and fuels treatments
The Bureau of Land Management Medford District is responsible for over 14,000 ha of oak woodlands, shrublands, and grasslands on public lands in the Applegate Valley of southwestern Oregon. Collectively, we refer to these vegetation types as “oak woodland and chaparral”, a term that encompasses hardwood-dominated vegetation at more mesic sites and shrub or grass-dominated vegetation at more xeric sites. Common tree species include oaks (mostly Quercus garryana and Q. kelogii), Arbutus menziesii,
Vegetation structure
There was no evidence that treated and untreated stands differed in total tree (Wilcoxon's Z = −0.45, P = 0.653) or herb cover (one-tailed Wilcoxon's Z = 1.08, P = 0.139; Fig. 1). As expected, untreated stands had greater total shrub cover (one-tailed Wilcoxon's Z = 1.93, P = 0.026) and shrub cover index (one-tailed Wilcoxon's Z = 1.90, P = 0.029) than treated stands (Fig. 1).
Bird abundance
We detected 22 bird species with sufficient frequency for analysis (Table 2). Bewick's wren (Thryomanes bewickii) and wrentit (Chamaea
Vegetation structure
Differences and similarities in vegetation structure of treated and untreated plots were consistent with the desired effects of the fuels reduction prescriptions on vegetation; treated stands had less shrub cover but similar tree cover relative to untreated stands (Fig. 1). These results are generally consistent with a more detailed comparison of the vegetation at these sites (Perchemlides et al., 2008). However, in their comparison, Perchemlides et al. (2008) documented greater herbaceous
Acknowledgements
We thank A. Darlak, F. Lospalluto, J. Kellerman, and L. Sutherlin for conducting field work and V. Arthur, and M. Rasmussen for administrative support. This project was funded by the Joint Fire Sciences Program project 01B-3-2-10, National Fish and Wildlife Foundation and the Bureau of Land Management Medford District.
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