Significant knowledge gaps in mercury (Hg) cycling challenge our ability to assess the effectiveness of the Minamata Convention on Mercury in reducing human and wildlife Hg exposure due to the complicated impacts of climate change. In this study, we comprehensively quantified the Hg spatial distribution and determined the Hg stable isotopes to understand the Hg sources and accumulation pathways in forests along the timberline ecotone, an ecosystem that provides an early bellwether of global warming effects. Our results show that although there were subtle variations in meteorological and environmental conditions among the <50 m elevation variation, a spruce to shrub vegetation structure shift led to a distinct decrease in vegetation and soil Hg pool sizes by 36%-56% along the timberline ecotone. Further Hg isotopic evidence verified that vegetation-induced atmospheric Hg-0 deposition was the main source for Hg accumulation in soils, and the source contribution of such input decreases from 73% to 45% along the timberline ecotone. Our results indicate that if global warming induced a spruce tree line upward shift, it would significantly promote the atmospheric Hg burden, thus resulting in a conspicuous "Hg hotspot". We recommend further studies to assess the Hg ecological risk promoted by global warming in alpine ecosystems.

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