Local reindeer grazing history is an important determinant in the response of an ecosystem's carbon sink to climate warming, say researchers at the Arctic Centre of the University of Lapland. Their study was published in the journal Nature Climate-Change'>Climate Change on 16 March 2014. The research project has been funded by the Academy of Finland.
The consequences of global climate warming on ecosystem carbon sink in tundra are of great interest, because carbon that is currently stored in tundra soils may be released to the atmosphere in a warmer climate. This could contribute to atmospheric carbon dioxide concentration, and thus create a positive feedback that intensifies global change.
A major portion of the Arctic is grazed by reindeer. In northernmost Europe, the reindeer was domesticated a few centuries ago. In a field experiment in northern Norway, the effects of experimental warming were compared between lightly and heavily grazed tundra. The grazing history between these areas had varied for the past 50 years. Carbon balances showed that under the current climate, lightly grazed, dwarf-shrub-dominated tundra were a stronger carbon sink than heavily grazed, graminoid-dominated tundra. However, warming decreased the carbon sink in lightly grazed tundra, but had no effect in heavily grazed tundra. Thus, tundra with a long history of intensive grazing showed a weak response to climate warming.
The main reason for this grazer-induced difference was that in heavily grazed tundra, graminoids with rapid growth rates were able to increase their photosynthesis and carbon fixation under increased temperatures. A similar phenomenon did not occur in tundra under light grazing, where nutrient availability limits plant production. Increased carbon fixation in heavily grazed tundra compensated the warming-induced increase in the carbon dioxide release from the ecosystem.
"Grazing alters several ecosystem properties, such as plant species composition and soil nutrient availability, which in turn alter ecosystem responses to climate warming," says Academy Research Fellow Sari Stark from the Arctic Centre.
The significance of reindeer grazing history to tundra carbon balances has not been previously studied. The present results may modify climate models that predict the effects of global warming on global carbon cycles. The study shows that it is critical to know the grazing history before the responses of tundra carbon balances to climate warming can be understood. Different tundra systems possess highly varying grazing histories as a result of past and present reindeer management practices.
The study is part of a doctoral thesis by Maria Väisänen at the University of Oulu. The other members of the research team are Henni Ylänne from the University of Lapland, Johan Olofsson and Elina Kaarlejärvi from Umeå University, Sweden, and Sofie Sjögersten and Neil Crout from the University of Nottingham, UK. The research project is headed by Academy Research Fellow Sari Stark at the University of Lapland.
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