Year of Award

2016

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Systems Ecology

Department or School/College

Division of Biological Sciences

Committee Chair

H. Maurice Valett

Commitee Members

Aaron Thomas, Brady Allred

Keywords

browsing, GLMM, riparian, snow characteristics, topography, willow

Publisher

University of Montana

Subject Categories

Terrestrial and Aquatic Ecology

Abstract

Variation in ungulate browsing intensity on willow (Salix spp.) communities across Yellowstone’s northern range has been attributed to various biophysical factors including climate, topography, and predators. These studies were conducted without a long-term browsing dataset; they lacked a spatially extensive sampling design, and did not include dynamic snow characteristics known to affect ungulate mobility, habitat selection, and foraging strategies. We therefore focused on snow, an extreme landscape level disturbance that places nutritional stress on ungulates during the winter when most browsing on shrubs occurs. We applied a non-destructive sampling method by consecutively recording whether stem leaders were browsed between terminal growth scars in order to reconstruct annual winter browsing intensity from 1995 through 2014 with measurements made on an average of 66 plots that spanned Yellowstone’s northern range. Our best-fit generalized linear mixed model selected according to lowest Akaike information criterion identified March snow depth as negatively associated with browsing and as the singularly most influential variable related to browsing intensity. Our model also included a negative association between browsing and March snow density, a negative association between browsing and terrain slope, and a positive association between browsing and the magnitude of precipitation during the past growing season. Past studies have argued for the primacy of top-down influences like predation and fear-induced behavioral change, our work suggests that bottom-up forces including the spatial and temporal distribution of snow and its characteristics are also strong determinants of browsing intensity on willow communities across Yellowstone’s northern range.

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© Copyright 2016 Donald L. Belile Jr.