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Alteration of local biodiversity by climate change may be especially impactful in the Arctic, where species richness is characteristically low and environmental conditions are changing rapidly. Two species of arctic large herbivores that survived rapid warming during the Pleistocene-Holocene transition, caribou and muskoxen, again face accelerated warming and changes in vegetation seasonality under contemporary climate change. While numerous studies have investigated the roles of weather and primary productivity in population dynamics of these species, opportunities for investigation of their dynamics in sympatric populations have been limited. We analyzed the population dynamics and offspring production of sympatric caribou and muskoxen at a study site in West Greenland over an 18-year period (2002–2019) during which the timing of spring green-up has advanced rapidly. In caribou, population growth rate and calf production were greater following warm winters but lower in years with earlier spring green-up. While calf production was highly variable, caribou abundance declined over the course of the study. In contrast, muskox population growth and calf production both increased following earlier springs. Moreover, muskox abundance increased over the course of the study, despite a negative association between their rate of increase and winter temperature, perhaps indicating that winter weather was less important than spring green-up timing in their dynamics. Hence, sympatric populations of these two herbivores displayed opposing demographic responses to winter weather and spring plant phenology at this shared location. These results emphasize the complexity of predicting large herbivore responses to climate change, and highlight the potential for impacts to species richness of large herbivore communities as the Arctic continues to warm.