Rapid Assessment of Ecological Resilience in Aspen Communities
In some regions of the West, quaking aspen (Populus tremuloides) has been declining after more than a century of changing human land-use patterns associated with urbanization, fire suppression, predator extirpation, and agriculture. More recently, episodes of large-scale decline, such as Sudden Aspen Decline (SAD), have been identified in association with drought and other inciting and predisposing factors. Although in many instances decline has been overstated, aspen ecosystems are expected to continue to be vulnerable to climate change and perturbing trends in human land-use patterns. Restoring processes and structures associated with resilience to aspen communities would aid in making them more resilient to future SAD-inciting factors. In this thesis I review the literature on aspen decline, explore the current use of rapid assessment tools in aspen research, identify key indicators of resilience in aspen, and propose a Community-level Aspen Resilience Assessment Tool (CARAT). Aspen decline is a complex and context-dependent phenomenon with multiple inciting and predisposing factors. Climate change-induced drought is a major factor in aspen decline, in addition to chronic ungulate herbivory, habitat fragmentation, fire suppression, and other alterations to stand ecology. Future increases in environmental stressors, such as climate change-induced drought (Worrall et al. 2013) and human land-use patterns associated with urban development and agricultural expansion (Hilty et al. 2006), may require efforts aimed at the conservation and restoration of processes, functions, and structures that will maintain the resilience of aspen communities. An aspen conservation approach that focuses on restoring resilience may prove to be the most robust strategy to maintain aspen communities in the face of uncertain future environmental stressors. Restoring fire regimes and taking action to ease the stress of herbivory on aspen can be instituted through a number of management strategies in both wilderness areas and working landscapes. I present recommendations, such as restoring ecologically effective populations of wolves (Canis lupus) where possible, and implementing prescribed burns, coppicing, and strategically placed exclosures. A rapid assessment tool of resilience in aspen communities would be valuable to conservation and restoration work. Landscape-level, coarse-scale assessments are commonly used to assess cover changes in aspen research to identify decline. I present a review of the literature pertaining to the use of satellite imaging, aerial photography and surveys, and repeat photography in aspen research. While these are ideal tools for detecting cover changes over large areas, they are limited in scale and to documenting exterior stand structures (i.e., canopy and vertical edge structures), and may be inadequate to assess aspen resilience. In order to develop a community-level rapid assessment tool for aspen resilience, I review 6 factors associated with aspen resilience: climate, disturbance, predation, ecophysiology, genetics, and connectivity. These factors are synergistic and interactive. The criteria I use to determine whether they are suitable resilience indicators for use in a rapid assessment tool are 1) is it feasible to rapidly assess the indicator at the community-level? and 2) is the indicator manageable in the context of conservation or restoration goals? I found that the most feasible and effective indicators were predation, disturbance, and connectivity. However, due to the importance of climate, it is essential that it be taken into account in both the application and interpretation of the results of this tool. I propose a resilience index that measures resilience on a 3-point scale, where 1 is the least resilient and 3 is the most. For each indicator, I identify a set of measurable variables. For predation I propose to use wolf population, elk (Cervus elaphus) population density, and browse intensity. For disturbance, I propose to use fire history. I propose to measure connectivity using road density. Finally, I propose to measure general aspen ecology using aspen recruitment ratio, aspen regeneration, and conifer canopy cover. I was unable to validate my rapid assessment tool within the scope of this thesis. Validation of this tool would be the subject of further research, in the form of PhD studies, to determine its effectiveness in measuring resilience. If effective, this tool could be valuable in guiding conservation actions to help make aspen communities more resilient to stressors.
Kirkey, J. 2014. Rapid Assessment of Ecological Resilience in Aspen Communities. Prescott College. [Thesis] 194 p.