| Summary: | Thesis (Ph.D.)--University of Washington, 2015-12 Although the direct effects of climate change have been studied though observational and experimental methods in alpine treeline ecotones (ATEs), indirect effects due to shifts in disturbance regimes have received less attention, despite evidence that the frequency and extent of large disturbances are increasing in many other ecosystems. At a regional scale, I analyzed wildfires occurring over a 29-year period (1984-2012) in ATEs in eight mountainous ecoregions of the Pacific Northwest and Northern Rocky Mountains. I focused on two components of the ATE: (1) subalpine parkland, which extends from closed subalpine forest through a fine-scale mosaic of forests and non-forest, and (2) alpine vegetation, which includes meadow, shrubland, and alpine tundra. I expected that subalpine parkland and alpine vegetation would burn less, proportionally, than the entire ecoregion. In four of eight ecoregions—three in Rocky Mountains and one in the Cascades—the proportion of subalpine parkland burned was comparable or greater than the proportion of the entire ecoregion that burned. In alpine ecosystems little of the area (<7%) burned during the 29-year study period. At a local scale, I examined variability in fire severity and changes in plant structure, using data from >500 plots within four alpine treeline ecotones sites in the Cascade Range and Northern Rocky Mountains, which had burned 18-27 years prior. I assessed the likelihood of different pre-fire canopy-cover structural classes—closed forest (>40% tree cover), open forest (10%-40%), parkland (<10%), and unforested areas (alpine, meadow, and Krummholz)—to burn and to change to a different structural class after fire. I also evaluated changes in forest structure—specifically the abundance of live trees within five diameter at breast height (DBH) classes—using non-metric multidimensional scaling (NMDS) to visualize differences and Permutational Multivariate Analysis of Variance (PERMANOVA) to test ...
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