Publication Date

8-2014

Date of Final Oral Examination (Defense)

4-22-2014

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Biology

Department

Biology

Supervisory Committee Chair

Ian C. Robertson, Ph.D.

Supervisory Committee Member

James R. Belthoff, Ph.D.

Supervisory Committee Member

Susan L. Earnst, Ph.D.

Abstract

Quaking aspen (Populus tremuloides) woodlands are expected to be sensitive to climate change, and have declined in parts of the West. Great Basin mountain ranges may be near the limits of aspen’s climatic threshold, in terms of temperature and aridity, and thus are particularly vulnerable to climate change. Birds associating with aspen are likely to undergo regional population fluctuations and changes in distribution as a result of changes in aspen availability or distribution. Thus, understanding the habitat relationships of avian communities in aspen and other montane cover types is important for tracking the impacts of future landscape change. The mountainous terrain of the Humboldt-Toiyabe National Forest in northern Elko County, NV, supports a patchy array of aspen and conifer forest and shrubland distributed across 2,755 km2. I quantified avian abundance using point count sampling over two breeding seasons (2010 – 2011) at 389 point locations in or near systematically selected stands of aspen (n = 135). For common species, I compared the mean abundance per stand in aspen to that in conifer, mixed aspen-conifer, and montane sage, using both paired and partially-paired t-tests. Most focal species were significantly more abundant in aspen than conifer (22 of 37 species) or montane sage (30 of 39 species) in the partially-paired comparisons. In paired comparisons, 4 of 15 species were significantly more common in aspen than conifer, and 19 of 24 were significantly more common in aspen than montane sage, but most comparisons with both conifer and mixed aspen-conifer were non-significant due to small sample sizes. I then used mixed-effects multiple regression, with stand and observer crew as random effects, to identify key habitat and physiographic parameters driving species’ abundance. For 8 of 11 aspen associates in mixed-effects models, abundance increased with an increase in aspen within 75 m; abundance also decreased with an increase in conifer for three aspen associates. For three of five conifer associates, abundance increased with an increase in 75-m conifer; abundance also increased with an increase in mixed aspen-conifer for three conifer associates. The results of my study underscore the ecological importance of aspen for montane passerine communities–aspen is preferred by most species over other available habitats, and abundance of most aspen-associated species increased as the proportion of aspen within 75 m increased. A reduction in aspen distribution in the Great Basin is likely to result in a decrease in abundance and distribution of a host of aspen-associated species and could have long-term effects on montane passerine communities. I suggest that management activities promoting aspen’s persistence and resilience to climate change be considered as a means for maintaining abundance and species composition of montane bird communities in the island ranges of the Great Basin.

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