Northern terrestrial ecosystems hold vast stores of carbon (C) in permafrost, with potentially strong positive feedbacks to climate warming as permafrost thaws. However, how vulnerable this C is, and whether increased vegetation growth may offset C losses from permafrost soil, is still unclear. We have observed year-round C loss and uptake from three arctic tundra ecosystems (wet sedge, tussock tundra, and heath) in northern Alaska since 2008. Over this 17-year period, our record shows that two ecosystems are a net C source to the atmosphere due to overwinter C losses, while the third is roughly in balance. We measured vegetation and active layer soil C and N stores in these ecosystems in 2013 and 2023. Overall, vegetation changed little. In contrast, soil C and N pools in the active layer increased substantially in the two ecosystems that have lost the most C, likely due to deeper thaw making more soil C available for decomposition. Our results suggest that sustained C losses from these ecosystems are being fueled by permafrost thaw, and that vegetation changes that might offset some of these losses are slow to develop, and likely will not change the trajectory of C loss on a decadal scale.