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Consequences of warming on tundra carbon balance determined by reindeer grazing history


Arctic tundra currently stores half of the global soil carbon (C) stock1. Climate warming in the Arctic may lead to accelerated CO2 release through enhanced decomposition and turn Arctic ecosystems from a net C sink into a net C source, if warming enhances decomposition more than plant photosynthesis2. A large portion of the circumpolar Arctic is grazed by reindeer/caribou, and grazing causes important vegetation shifts in the long-term. Using a unique experimental set-up, where areas experiencing more than 50 years of either light (LG) or heavy (HG) grazing were warmed and/or fertilized, we show that under ambient conditions areas under LG were a 70% stronger C sink than HG areas. Although warming decreased the C sink by 38% under LG, it had no effect under HG. Grazing history will thus be an important determinant in the response of ecosystem C balance to climate warming, which at present is not taken into account in climate change models.

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Figure 1: Midday (08:00–18:00) CO2-C flux rates averaged over growing seasons 2011–2012 in a subarctic tundra heath under light grazing (LG), short-term reindeer exclusion (HGexc) and heavy grazing (HG) by the reindeer.
Figure 2: The total abundance of vegetation and litter in a subarctic tundra heath under light grazing (LG), short-term reindeer exclusion (HGexc) and heavy grazing (HG) by the reindeer.
Figure 3: Seasonal patterns in Re in a subarctic tundra heath under light grazing (LG), short-term reindeer exclusion (HGexc) and heavy grazing (HG) by the reindeer.
Figure 4: Normalized difference vegetation index in a subarctic tundra heath under light grazing (LG), short-term reindeer exclusion (HGexc) and heavy grazing (HG) by the reindeer.


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We thank A. Niva and S. Aakkonen for their valuable help with the field experiments. We thank Su. Katves and Si. Katves for assisting with vegetation recording and J. Hyvönen for helping with the statistical analysis. This study was funded by the Academy of Finland (project numbers 218121 and 130507).

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Authors and Affiliations



S.St. and M.V. initiated and managed the field experiment. J.O. and S.Sj contributed to the planning of the experiment. M.V. and H.Y. carried out C flux measurements with contributions from S.St. and S.Sj. E.K. and J.O. were responsible for plant and NDVI analyses. N.C. was responsible for modelling C fluxes. M.V. carried out the statistical analyses. M.V. and S.St. wrote the manuscript, to which all other authors contributed with discussions and text.

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Correspondence to Maria Väisänen.

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The authors declare no competing financial interests.

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Väisänen, M., Ylänne, H., Kaarlejärvi, E. et al. Consequences of warming on tundra carbon balance determined by reindeer grazing history. Nature Clim Change 4, 384–388 (2014).

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