Response of Birds to Thinning Young Douglas‐fir Forests

Abstract

Silvicultural practices, such as thinning, are increasingly performed both for commodity production and to help achieve biodiversity goals and promote ecological restoration. However, relatively little research has examined effects of thinning conifer forests on vertebrates. We experimentally manipulated stands using a randomized block design to evaluate influences of two thinning intensities on populations of diurnal breeding birds in western Oregon. We conducted point counts of birds seven times each year in 1994 (prior to treatment) and from 1995 through 2000 (subsequent to treatment). We analyzed data using multiple linear regression and information‐theoretic approaches to model selection. Of the 22 species for which we had sufficient data for analysis, detections of nine species decreased and eight species increased in thinned stands relative to controls, and there was no strong evidence that thinning influenced numbers of five species. Of the 17 species that responded to thinning, the magnitude of response of eight species varied with thinning intensity; for each of these species, response was greatest in the more heavily thinned stands. Although no species was extirpated from stands following thinning, detections of Hutton's Vireos (Vireo huttoni), Golden‐crowned Kinglets (Regulus satrapa), Brown Creepers (Certhia americana), Black‐throated Gray Warblers (Dendroica nigrescens), and Varied Thrushes (Ixoreus naevius) decreased to less than half of the detections in controls in one or more treatment types, suggesting thinning significantly reduces their numbers. In contrast, American Robins (Turdus migratorius), Townsend's Solitaires (Myadestes townsendi), and Hammond's Flycatchers (Empidonax hammondii) were rare or absent in controls but regularly present in thinned stands, and detections of Western Tanagers (Piranga ludoviciana), Evening Grosbeaks (Coccothraustes vespertinus), and Hairy Woodpeckers (Picoides villosus) increased by threefold or more in thinned stands relative to controls. Only Pacific‐slope Flycatchers (Empidonax difficilis), Warbling Vireos (Vireo gilvus), and Western Tanagers showed strong evidence of temporal trends in response. For these species, differences between numbers in controls and treated stands became more extreme through time. Our findings suggest that thinning densely stocked conifer stands in landscapes dominated by younger stands enhances habitat suitability for several species of birds, but that some unthinned patches and stands should be retained to provide refugia for species that are impacted by thinning.

Influences of forest type and forest structure on bird communities in oak and pine woodlands in Spain

“…Thinning may negatively affect foliage gleaners and troglodyte birds, since they reduce nesting sites and food supplies, also increasing bird conspicuousness (Hake, 1991;Christian et al, 1996;Tubelis and Cavalcanti, 2000;Hayes et al, 2003). However, most of the studies examining such topics emphasized the benefit that thinned patches usually involve for bird communities (DeGraaf et al, 1991;Dellasala et al, 1996;Easton and Martin, 2002;Haveri and Carey, 2000;Hayes et al, 2003;Hagar et al, 2004). When monotonous forests are considered, thinning provides for a more heterogeneous vegetation structure, allowing the entrance of bird species of more open wooded areas, and ground gleaners.…”

Long term thinning and logging in Australian cypress pine forest: Changes in habitat attributes and response of fauna

“…Thinning, involving the removal of competing small-diameter advanced regeneration, is extensively and intensively done as a silvicultural technique in densely stocked forests to accelerate growth rates and improve merchantability of retained trees. The direct effects of thinning on forest structure tend to be both short-and longterm, with short-term effects being the obvious reduction in the density of understorey vegetation and/or small trees and an increase in vertical canopy gaps (Hayes et al, 2003;Harrod et al, http://dx.doi.org/10.1016/j.biocon.2015.03.009 0006-3207/Ó 2015 Elsevier Ltd. All rights reserved. 2009).…”

“…In general, there is a negative response to intensive logging, but a neutral to positive response to thinning (see meta-analyses by Kalies et al, 2010;Verschuyl et al, 2011). The uniformity of thinning through space and thinning intensity can drive the magnitude and direction of responses , particularly for bird species (Hayes et al, 2003;Hagar et al, 2004). Currently, in most North American forests, large-scale thinning is promoted not so much to increase timber productivity, but more as a tool to restore habitat.…”

The influence of wildfire, logging and residual tree density on bird communities in the northern Rocky Mountains

“…Species negatively associated with increasing densities of residual overstory trees were generally those that feed or nest on or near the ground in open habitats, such as the Dark-eyed Junco, American Robin, and MacGillivray's Warbler. These species previously have been reported to be negatively associated with tree density in Pacific Northwest forests (Marcot, 1984;Hansen et al, 1995;Hayes et al, 2003). Species with the strongest positive responses included those preferring open forest stands with large trees in the canopy (Western Tanager and Cassin's Vireo) and those preferring older, closed-canopy forests (Townsend's Warbler and Golden-crowned Kinglet).…”