Sammendrag
Themigratory tundra caribou herds in North America follow decadal population cycles, and browsing from abundant
caribou could be expected to counteract the current climate-driven expansion of shrubs in the circumpolar tundra
biome. We demonstrate that the sea ice cover in the Arctic Ocean has provided a strong signal for climate-induced
changes on the adjacent caribou summer ranges, outperforming other climate indices in explaining the caribou-plant
dynamics. We found no evidence of a negative effect of caribou abundance on vegetation biomass. On the contrary,
we found a strong bottom-up effect in which a warmer climate related to diminishing sea ice has increased the plant
biomass on the summer pastures, along with a paradoxical decline in caribou populations. This result suggests that
this climate-induced greening has been accompanied by a deterioration of pasture quality. The shrub expansion in
Arctic North America involves plant species with strong antibrowsing defenses.Our resultsmight therefore be an early
signal of a climate-driven shift in the caribou-plant interaction from a system with low plant biomass modulated by
cyclic caribou populations to a system dominated by nonedible shrubs and diminishing herds of migratory caribou.
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