| Summary: | Dissertation (Ph.D.) University of Alaska Fairbanks, 2020 Since the 1970s, Arctic temperatures have risen by 2.7 °C, more than twice that of lower latitudes. Productivity of tundra vegetation is historically nutrient-limited, largely due to low rates of decomposition in soils underlain by permafrost, where cold temperatures limit nutrient uptake by plants. However, climate warming is implicated in the recent expansion of tall (≥ 0.5 m) deciduous woody shrubs across the Arctic. Among the largest tundra plants, deciduous shrubs exert strong controls on hydrology, heat balance, nutrient cycling, and food webs. These shrubs may be key players in carbon storage and re-stabilization of thaw-deformed permafrost landscapes (thermokarst), however, shrub-climate feedbacks are complex and their magnitude remains uncertain. Warming associated with recent thermokarst activity includes large (≥ 1 ha) de-vegetated depressions on hillslopes caused by mass soil thaw, known as retrogressive thaw slumps (RTS). RTS have increased on Alaska's North Slope by two-thirds since the 1980s. Within a few decades, some RTS near Toolik Lake support tall willow (Salix spp.) and dwarf birch (Betula nana) colonies. This study quantified three aspects of plant response in RTS of different ages (chronosequences) at two North Slope lakes: 1) recruitment (seedlings m⁻² and percent germination of soil seedbanks), 2) clonal (asexual) growth of dominant vegetation (willow), and 3) secondary growth (annual rings) of dwarf birch and willow. I hypothesized that conditions in RTS support greater recruitment, genetic diversity, and growth than conditions in undisturbed moist acidic tussock tundra, and that the climate signal (June mean temperature) is amplified in RTS shrub ring widths. The study found higher seedling density and seedbank viability associated with warm, nutrient-rich bare soil in recent RTS. Willow species richness was higher in RTS than in undisturbed tundra, but all willows showed high heterozygosity and low clonal spread regardless of ...
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