UBC Theses and Dissertations
The effects of thinning on forest bird communities in dry interior Douglas-fir forests Booth, Barry P.
I studied the effects of the Thompson/Nicola Mule Deer Forage and Slashing Project on vegetation structure and bird species abundance in the summers of 1990 and 1991. I sampled stand structure and bird abundance on three thinned and three unthinned study sites , each 25 ha in size. Thinning was targeted at smaller size classes , and as a result, there were significantly fewer small Douglas-fir trees (<10 cm dbh) in treatment plots. Individual tree canopy volumes were not significantly different between treatment and control sites. Canopy volumes per hectare (m3/ha) of Douglas-fir trees <10 cm dbh were significantly lower (60% lower) in treated sites. There was no significant increase in herb and shrub cover in treatment sites. Percent cover of down and dead woody debris was significantly higher in treated sites. This thinning trial had little effect on the forest bird community. Poor understory response, either because of the effects of cattle grazing, insufficient thinning, leaving slash on site, or an insufficient amount of time since treatment, or all four, may explain the failure of ground-and shrub-feeding bird species to increase as predicted. Elevated levels of spruce budworm and Douglas fir tussock moth, and potential foraging habitat in other forest strata may have prevented the predicted reduction in abundance of foliage-feeding species. The increased amount of down and dead wood likely accounted for the modest increase in woodpecker use of thinned sites. Chi-square and discriminant function analysis suggested that several within-site bird/habitat associations exist. Northern flickers , Vesper sparrows and Chipping sparrows were associated with open forest habitats in control sites. Yellow-rumped warblers, and Dusky flycatchers were associated with dense, unthinned habitat in treatment sites. Ruby-crowned kinglets, Orange-crowned warblers and Dusky flycatchers were associated with riparian habitats in control sites. I recommend that grazing regimes be modified to assess the effect of grazing on vegetation response. Slash could be piled to benefit ground foraging/nesting species instead of allowing it to lie where it was felled. Even if not required for snag management, unthinned patches should continue to be left as part of the treatment to maintain spatial habitat heterogeneity. Long-term monitoring of both vegetation and bird communities is recommended particularly if the Thompson/Nicola Mule Deer Forage and Slashing Project becomes a model for thinning projects in these forest types.
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