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Dr. Monica G.Turner
Department of Integrative Biology
University of Wisconsin
430 Lincoln Dr.
Madison, WI 53706
Ecosystem and
Landscape Ecology Lab
 

Hansen, Winslow D. 2018. Resilience to changing climate and wildfire in subalpine forests of Greater Yellowstone. PhD Dissertation, Dept. of Zoology, University of Wisconsin-Madison.

Climate and disturbance regimes are rapidly changing in earth’s forests, and these trends are expected to continue through the 21st century. It remains unresolved whether and where forests will absorb increased perturbations without changing qualitatively and where forest resilience might erode. This dissertation provides a foundation to begin addressing these uncertainties. I combined field observation, experiments, and process-based simulation to study effects of changing climate and wildfire on postfire tree regeneration and forest resilience in Yellowstone National Park, the largest intact wildland area of the contiguous United States. Chapter 1 quantifies effects of ecological filters on a colonizing cohort of aspen. These aspen trees established from seed after the 1988 fires and survived at higher elevations than their prefire distribution. I then conducted a long-term field experiment and shorter controlled-environment experiment to determine how temperature and soil moisture consistent with 21st-century projections may alter postfire seedling establishment of two widespread conifers (Chapter 2). In chapters 3 and 4, I used a forest simulation model to test multiple mechanisms of regeneration failure and to explore how suppression may alter 21st-century fire and forests. Long-term study of colonizing aspen demonstrated how wildfire can catalyze rapid shifts in tree-species distributions. Aspen seedlings were initially favored at lower elevations close to their prefire distribution. By 25yrs postfire, aspen was favored to survive at higher elevations, likely due to warming. From the experiments, it appears postfire drought may be a powerful force for change in subalpine forests because regeneration was drastically reduced under hotter-drier conditions. Simulations, where multiple climate-fire drivers could be considered over longer periods, however, indicate the potential for remarkable resilience. Regeneration failure was the exception, not the rule. Suppression of fire also had little impact on 21st-century fire or forests. Collectively, this research demonstrates that multiple streams of quantitative inquiry are necessary to better resolve how changing climate and disturbance will alter forests. Management steps could be taken to bolster vulnerable forests (e.g. reseeding after fires), if mechanisms of change are understood. However, forest-management strategies should not discount the inherent resilience of the system.