Evaluating aspen responses to changes in elk abundance, distribution and behavior following wolf reestablishment in West-Central Yellowstone National Park
Shafer, Timothy Lee.
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The reintroduction of wolves to Yellowstone National Park (YNP) in the mid-1990's has created a unique natural experiment for the investigation of trophic cascades operating at large spatial scales and involving large terrestrial mammals. Wolves have been directly linked to changes in elk density/behavior and have been hypothesized to be the driving force behind observed changes in woody plant growth in the system. The primary objectives of this study were to investigate the occurrence of a trophic cascade among wolves, elk and aspen in an area of YNP where elk abundance and distribution changed dramatically as a direct result of wolf reestablishment in the system. In Chapter 2, I determined the distribution and demographic characteristics of aspen in the Madison headwaters study area (MHSA) and identified the environmental attributes associated with its distribution on the landscape. Additionally, I evaluated the morphology, productivity, and persistence of aspen in both clonal and seedling-established. In Chapter 3, I established a climate-growth relationship for aspen to investigate the occurrence of a shift in productivity related to climate coincident with the timing of wolf reestablishment. I used standard dendrochronology techniques to investigate growth trends and identified which climate variables are most important to aspen productivity in this region. Additionally, I established the timing of historic aspen recruitment in the MHSA using age of mature trees. In Chapter 4, I investigated a trophic cascade among wolves, elk and aspen. I reconstructed historical browse conditions for aspen to look for a shift in browse regimes that occurred concurrently with the changes in elk abundance/distribution by performing a dendrochronological analysis of aspen architectural morphology. I also evaluated plant height, productivity, and longevity of aspen where elk densities had declined dramatically in order to capture the expected growth response. I used ANOVA's and multiple comparison procedures to evaluate browse conditions and aspen growth among sites where elk densities have declined dramatically and those where elk densities have remained relatively constant.