| Summary: | Amplified climate change and fire regime shifts in the northern high latitudes are posing growing threat to key properties and functions of tundra ecosystems, including soil carbon stock, permafrost stability and vegetation types. However, it remains poorly understood how tundra ecosystems will feedback to the combined forces of changing climate and fire disturbance. In this study, I integrated paleoecology, numerical modeling and remote sensing observation to address (1) the resilience and sensitivity of tundra carbon stocks to shifting fire regimes, (2) the consequences of climate change and fire disturbance on thermokarst disturbance (e.g. collapse of ground surface after permafrost thaw), and (3) the patterns of shrub expansion in heterogenous tundra landscape in response to accelerated warming and fire disturbance. My results indicate that fire disturbance has threshold effects on tundra carbon stocks. Variation in fire return intervals from 5000 to 900 years causes minimal carbon stock loss (<5%). However, increasing fire frequency beyond every 800 years is projected to trigger sustained mobilization of ancient soil organic matter that leads to irreversible carbon stock loss from permafrost. Multi-decade remote sensing observations revealed that tundra fires resulted in pervasive thermokarst formation, and that this impact lasted more than four decades. Nevertheless, substantial spatial heterogeneity exists regarding thermokarst formation and the greatest amount of thermokarst appears in severely burned tundra ecosystems in ice-rich areas. Although fire disturbance is a strong force exacerbating permafrost degradation, widespread warming surpasses sporadic burning as the primary driver responsible for ~90% of thermokarst growth in northern Alaskan tundra over the past ~70 yrs. Permafrost thawing strongly influences shrub cover dynamics in tundra ecosystems, but the net outcome is largely contingent on topographical positions. In poorly drained tundra lowlands, thermokarst-induced water impounding resulted in massive shrub cover loss throughout three decades following fire. In contrast, shrub expansion was significantly enhanced in well-drained upland tundra after fire disturbance, especially in area burned of high severity fire. In unburned tundra, a general increase of shrub cover was detected, driven primarily by warming temperature in the lowland but by increased precipitation in the upland. Overall my research yields new insights into the complex responses of tundra ecosystems to climate and fire-regime changes, and suggests the importance of incorporating such information into earth system models for improving our understanding of land-atmosphere feedback processes. Limited Author requested closed access (OA after 2yrs) in Vireo ETD system
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