Ecosystem stores of carbon are a key component in the global carbon cycle. Many studies have examined the impact of climate change on ecosystem carbon storage, but few have investigated the impact of land-use change and herbivory. However, land-use change is a major aspect of environmental change, and livestock grazing is the most extensive land use globally. In this study, we combine a grazing exclosure experiment and a natural experiment to test the impact of grazer exclusion on vegetation dynamics and ecosystem carbon stores in the short term (12-year exclosures), and the long term (islands inaccessible to livestock), in a heavily grazed mountain region in Norway. Following long-term absence of sheep, birch forest was present. The grazing-resistant grass Nardus stricta, dominated under long-term grazing, whilst the selected grass Deschampsia flexuosa and herb species dominated the vegetation layer in the long-term absence of sheep. The established birch forest led to vegetation carbon stocks being higher on the islands (0.56 kg C m−2 on the islands compared to 0.18 kg C m−2 where grazed) and no difference in soil carbon stocks. In the short-term exclusion of sheep, there were minor differences in carbon stocks reflecting the longer term changes. These results show that aboveground carbon stocks are higher in the long-term absence of sheep than in the continual presence of high sheep densities, associated with a vegetation state change between tundra and forest. The reduction of herbivore populations can facilitate forest establishment and increase aboveground carbon stocks, however, the sequestration rate is low.
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We are grateful to the Norwegian Research Council for funding through the Environment 2015 program (Project 212897). We also thank Magdalena Rygalska for help with sampling and laboratory analyses, and Christoffer Høyvik Hilde, and Odd Helge Tunheim for field assistance. Insightful and constructive comments from two anonymous reviewers greatly contributed to this manuscript.
JDMS, VM & GA carried out fieldwork and designed the study with input from AM, JM and ØH. GA and AM set up the exclosures. VM analyzed soil and vegetation samples. JDMS analyzed the data with input from VM. All authors contributed with interpretation of data and patterns. JDMS wrote the manuscript with input from all co-authors.
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Speed, J.D.M., Martinsen, V., Mysterud, A. et al. Long-Term Increase in Aboveground Carbon Stocks Following Exclusion of Grazers and Forest Establishment in an Alpine Ecosystem. Ecosystems 17, 1138–1150 (2014). https://doi.org/10.1007/s10021-014-9784-2