| Summary: | © 2018 The Author(s). Published by IOP Publishing Ltd. Recent observed shifts in Arctic tundra shrub cover have uncertain impacts on 21st century net ecosystem carbon exchanges. Here we applied a well-tested ecosystem model, ecosys, to examine the effects of North America Arctic tundra plant dynamics on ecosystem carbon balances from 1980-2100 under the RCP8.5 scenario. Tundra productivity was modeled to increase from enhanced carbon fixation and N mineralization under recent and future climates. Between 1982 and 2100 and averaged across the region, predicted increases in relative dominance of woody versus non-woody plants increased ecosystem annual net primary productivity by 244 g C m-2that offset concurrent increases in annual heterotrophic respiration (139 g C m-2), resulting in an increasing net carbon sink over the 21st century. However, smaller increases in seasonal carbon uptake during winter (1 g C m-2) and autumn (22 g C m-2) and greater increases in ecosystem respiration (winter (23 g C m-2) and autumn (47 g C m-2)) by 2100 versus 1982 resulted in larger carbon losses during these seasons that completely offset the gains in spring (13 g C m-2) and 25% of the gains in summer (140 g C m-2). Modeled soil temperatures were predicted to increase more slowly than air temperatures (∼0.6 °C for every 1 °C increase in air temperature over the 21st century). This slower soil versus air warming, and thus greater increases in CO2fixation versus soil respiration rates, also contributed to the tundra remaining a carbon sink through 2100. However, these higher gains versus losses of carbon may be a transient response and not sustainable under further soil warming beyond 2100. Our modeling analysis allows us to extend beyond results from short-term warming experiments, which cannot characterize effects associated with decadal-scale changes in plant communities.
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