Allison Welch Dissertation Defense

Title: Changing Arctic Vegetation: Examining Shrubification, Permafrost Vulnerability, and Land-Atmosphere Interactions

Abstract: The Arctic is an important piece of the global carbon cycle, storing half of the Earth's soil carbon in perennially frozen permafrost. As temperatures rise rapidly, the balance of Arctic carbon stocks among the soil, vegetation, and atmosphere is changing in ways that are challenging to predict, amplified by limited data. In this dissertation, I investigated how vegetation changes, particularly deciduous shrub expansion ("shrubification"), are affecting soil properties, permafrost thermal regimes, and emissions of biogenic volatile organic compounds and carbonaceous aerosols in Alaska using field observations, remote sensing, laboratory analyses, and modeling. I found that increasing growth of the deciduous shrub, alder (Alnus viridis (Chaix) DC.), in alpine tundra corresponded to reduced soil organic layer thickness and lower carbon and nitrogen stocks within the organic layer, potentially influenced by fire history. At one site in Arctic tundra—where alder is dominant and exhibits a documented growth response to warmer growing seasons (but has no recent fire activity)—alder presence was associated with a 25% reduction in organic layer thickness. Ground temperature modeling revealed that this reduced organic layer thickness resulted in a warmer soil active layer, with decreased thermal offsets and freezing degree days. Among the three deciduous shrub genera with documented expansion in Alaska (alder, birch (Betula spp.), and willow (Salix spp.)), willow species had the highest biogenic volatile organic compound emissions, though shrublands did not drive regional emissions. Carbonaceous aerosols measured at the surface in Arctic Alaska were primarily from contemporary sources like secondary organic aerosol formation and boreal forest fires, with concentrations twice as high inland as on the Arctic coast. These results provide insight into how Arctic vegetation change influences soil properties, permafrost thermal regimes, and land-atmosphere interactions through complex feedbacks, with implications for regional carbon cycling and radiative forcing in the rapidly warming Arctic.