Top‐Down Regulation by a Reindeer Herding System Limits Climate‐Driven Arctic Vegetation Change at a Regional Scale.

Warming‐driven growth of tall woody vegetation in the Arctic has the potential to accelerate climate change through multiple positive feedbacks. Local‐scale evidence suggests that large herbivores limit this vegetation shift, but there is uncertainty at larger, regional scales whether current herbivory pressure is a major top‐down control on ecosystem structure and functioning. Across a 67,000 km2 region of the Yamal Peninsula in West Siberia, we integrated satellite remote sensing with a novel data set mapping the migrations of herds comprising 151,000 domesticated reindeer. Where reindeer numbers varied over space, higher reindeer herbivory pressure was consistently linked with lower coverage of tall woody vegetation. Within areas dominated by this vegetation type, productivity and climate were increasingly decoupled where reindeer density was higher. Our spaceborne fingerprint detection suggests that large herbivores, at current population densities, counteract Arctic vegetation responses to climate change over large spatial scales. Plain Language Summary: Large herbivores influence tundra vegetation via their grazing, trampling, and nutrient inputs from feces and urine. However, the extent of these impacts remains unclear as we seek to understand and predict the responses of Arctic ecosystems to climate change. Using maps that we developed that record the annual summertime movements of herds comprising 151,000 domesticated reindeer managed by indigenous Nenets herders, we assessed whether reindeer had an effect on vegetation productivity and composition across a 67,000 km2 region of the Yamal Peninsula in West Siberia, an area about the size of the Republic of Ireland. Analyses of satellite data comparing areas with varying reindeer density consistently showed that higher reindeer herbivory pressure was limiting the coverage of tall woody vegetation and its expansion in response to warming. This effect had not been observed previously at such a large spatial scale and was significant enough to be detected from space. Our findings demonstrate that reindeer have a substantial impact on ecosystem structure and functioning, which likely has implications for the carbon cycle and energy balance within the Arctic and beyond. Key Points: Integrating maps of migrations of indigenous‐managed reindeer herds with remote sensing enabled analysis of large‐scale herbivory impactsAreas on the Yamal Peninsula with higher reindeer herbivory pressure consistently had lower coverage of tall woody vegetationThe productivity of tall woody vegetation was less sensitive to climate variability where reindeer density was higher [ABSTRACT FROM AUTHOR]