Seedling recruitment, genetic diversity, and secondary growth of deciduous shrubs in Arctic tundra disturbed by retrogressive thaw slump thermokarst on Alaska's North Slope

Abstract

Thesis (Ph.D.) University of Alaska Fairbanks, 2020Since 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 disturbance. Ramets (branches) within clones were more widely spaced in RTS, suggesting that RTS can fragment and disperse asexual propagules. Shrub rings in RTS were wider than in undisturbed tundra, but climate sensitivity to warmer temperatures was not amplified in the growth rings of most RTS shrubs. Most RTS shrubs had wider rings associated with greater September precipitation in the previous year, while shrubs growing outside of RTS did not, which suggests protective effects of early snow accumulations in RTS depressions. These results demonstrate that some North Slope RTS support greater seedling recruitment and shrub growth than undisturbed tundra and may enhance tundra shrub growth.the Arctic Institute of North America (DCH), the National Science Foundation (DEB 1556481, DEB 1637459 and PLR 1623461 to MSBH), the University of Alaska Fairbanks Center for Global Change/Alaska Climate Center Student Research Grant (DCH), the University of Alaska Fairbanks Dissertation Completion Grant (DCH), the University of Alaska Fairbanks Institute of Arctic Biology Director’s Office (DCH), the University of Alaska Fairbanks Institute of Arctic Biology Graduate Research Fellowship (DCH), the University of Alaska Fairbanks Office of the Vice Chancellor for Research (DCH), and the University of Alaska Fairbanks Randy Howenstein Memorial Field Research Fund (DCH).Chapter 1. General introduction -- Chapter 2. Microsite conditions in retrogressive thaw slumps may facilitate increased seedling recruitment in the Alaskan Low Arctic -- Chapter 3. Recruitment dynamics and population structure of willows in tundra disturbed by retrogressive thaw slump thermokarst on Alaska's North Slope -- Chapter 4. Secondary growth responses of deciduous shrubs in retrogressive thaw slump thermokarsts in the Alaskan Low Arctic -- Chapter 5. General Conclusion