Date of Award
Doctor of Philosophy (PhD)
Thomas T. Veblen
Victoria A. Saab
William R. Travis
The impacts of widespread forest disturbances on wildlife communities must be understood in order to effectively preserve biodiversity into the future. We quantified relationships between occupancy of avian species and forest structure characteristics within spruce-fir forests of southern Colorado that had been logged and infested by bark beetles.
We used a novel geographic information system (GIS) approach to explore American Three-toed Woodpecker (Picoides dorsalis; ATTW) relationships with bark beetle outbreaks across its range in the U.S. Rocky Mountains over the period 2010 to 2015. Results show that ATTWs are strongly associated with higher elevations (1.04, 95% CL [0.87,1.20]), decreasing distance to bark beetles (-1.29, 95% CL [-1.53,-1.04]), and increasing spruce-fir cover (0.25, 95% CL [0.18,0.31]) in the southern Rocky Mountain Ecoregion. In the U.S. Northern Rocky Mountain Ecoregion, our data suggested that ATTW abundance was most tied to higher elevations (0.22, 95% CL [0.05,0.39]), increased cover of mixed conifer (0.23, 95% CL [0.04,0.41]), spruce-fir (0.35, 95% CL [0.17,0.53]), and lodgepole pine (0.19, 95% CL [0.07,0.32]) forests, and decreased cover of ponderosa pine forest (-0.52, 95% CL [-1.04,-0.01]. In the northern region of our study, we did not find evidence that ATTW abundance was related to proximity to bark beetle species identified by U.S. Forest Service aerial detection survey data.
Our next objective was to determine if avian occupancy and species richness was significantly different in Engelmann spruce-subalpine fir forests that were logged > 20 years ago. We did not find any evidence of changes in species richness between sites as a result of selective thinning (90% of all BCIs for logging effects overlapped zero). However, occupancy of certain species changed in relation to a spruce beetle outbreak that began in 2015. We provide evidence that thinning does not negatively impact bird species occupancy in subalpine forests in the long term - an important contribution for assessing the impacts of management treatments on a key ecosystem service of spruce-fir forests.
The third research objective was to determine which forest structure attributes best predicted ATTW occupancy in spruce-fir forests of Colorado’s San Juan Mountains. We used an occupancy model to quantify relationships between woodpecker presence and forest structural attributes. ATTW occupancy probability jumped from 0.56 when there were zero infested spruce to 0.99 when there were 9 or more infested spruce per 20 m x 20 m plot. We did not find a relationship between ATTW occupancy and trees that were infested for more than five years. The number of snags per plot and increasing quadratic mean diameter of trees also did not have a supported effect on occupancy.
Taken together, our findings show that selective logging activities in spruce-fir forests that took place > 20 years prior to our study did not affect avian species richness. Presence of ATTWs at both broad and fine scales was largely related to the presence of beetle-infested trees. Our results suggest regional variation in ATTW associations with infestations of spruce beetles and mountain pine beetles, and provide evidence of habitat specialization in the southern Rockies and a tendency toward relative habitat generalization in the northern Rockies. As the primary cavity excavator in spruce-fir forests, conservation of ATTWs may stimulate an umbrella effect by protecting other cavity nester’s habitat in subalpine forests of Colorado. More importantly, spruce-fir forests are extensively managed for resource output and to maximize resistance to ecological disturbances such as fire and bark beetle outbreaks. Because logging initiatives will continue, we argue that selective thinning activities in forests unaffected by spruce beetles should not harm avian species richness in the long term. However, thinning treatments in stands infe
Kelly, Julia Jean, "Influences of Anthropogenic and Bark Beetle Disturbances on Breeding Bird Populations in the U.s. Rocky Mountains: Lessons from the American Three-Toed Woodpecker" (2016). Geography Graduate Theses & Dissertations. 98.