1. Department of Fruit Tree and Woody Plant Science, University of Bologna, Bologna I-40127, Italy
2. School of GeoSciences, University of Edinburgh, Edinburgh EH93JU, UK
3. Department of Crop Systems, Forestry and Environmental Sciences, University of Basilicata, Potenza I-85100, Italy
4. INRA, UR1263 EPHYSE, Villenave d'Ornon F-33883, France
5. Departement des Sciences Biologiques, University of Québec à Montréal, Montréal, Quebec, H3C 3P8, Canada
6. Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, SE-75007 Uppsala, Sweden
7. Department of Forest Ecology, University of Helsinki, FIN-00014 Helsinki, Finland
8. Department of Physical Geography and Ecosystems Analysis, Lund University, S-223 62 Lund, Sweden
9. College of Forestry, Oregon State University, Corvallis, OR 97331, USA
10. Department of Forest Resources and Environment, University of Tuscia, Viterbo I-01100 Italy
11. Present address: Institute for Environment and Sustainability—Climate Change Unit, Joint Research Center, European Commission, I-21020 Ispra, Italy.

Correspondence to: Federico Magnani¹ Correspondence and requests for materials should be addressed to F.M. (Email: federico.magnani@unibo.it).

Abstract

Temperate and boreal forests in the Northern Hemisphere cover an area of about 2  10⁷ square kilometres and act as a substantial carbon sink (0.6–0.7 petagrams of carbon per year)¹. Although forest expansion following agricultural abandonment is certainly responsible for an important fraction of this carbon sink activity, the additional effects on the carbon balance of established forests of increased atmospheric carbon dioxide, increasing temperatures, changes in management practices and nitrogen deposition are difficult to disentangle, despite an extensive network of measurement stations^{2, 3}. The relevance of this measurement effort has also been questioned⁴, because spot measurements fail to take into account the role of disturbances, either natural (fire, pests, windstorms) or anthropogenic (forest harvesting). Here we show that the temporal dynamics following stand-replacing disturbances do indeed account for a very large fraction of the overall variability in forest carbon sequestration. After the confounding effects of disturbance have been factored out, however, forest net carbon sequestration is found to be overwhelmingly driven by nitrogen deposition, largely the result of anthropogenic activities⁵. The effect is always positive over the range of nitrogen deposition covered by currently available data sets, casting doubts on the risk of widespread ecosystem nitrogen saturation⁶ under natural conditions. The results demonstrate that mankind is ultimately controlling the carbon balance of temperate and boreal forests, either directly (through forest management) or indirectly (through nitrogen deposition).

1. Department of Fruit Tree and Woody Plant Science, University of Bologna, Bologna I-40127, Italy
2. School of GeoSciences, University of Edinburgh, Edinburgh EH93JU, UK
3. Department of Crop Systems, Forestry and Environmental Sciences, University of Basilicata, Potenza I-85100, Italy
4. INRA, UR1263 EPHYSE, Villenave d'Ornon F-33883, France
5. Departement des Sciences Biologiques, University of Québec à Montréal, Montréal, Quebec, H3C 3P8, Canada
6. Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, SE-75007 Uppsala, Sweden
7. Department of Forest Ecology, University of Helsinki, FIN-00014 Helsinki, Finland
8. Department of Physical Geography and Ecosystems Analysis, Lund University, S-223 62 Lund, Sweden
9. College of Forestry, Oregon State University, Corvallis, OR 97331, USA
10. Department of Forest Resources and Environment, University of Tuscia, Viterbo I-01100 Italy
11. Present address: Institute for Environment and Sustainability—Climate Change Unit, Joint Research Center, European Commission, I-21020 Ispra, Italy.

Correspondence to: Federico Magnani¹ Correspondence and requests for materials should be addressed to F.M. (Email: federico.magnani@unibo.it).

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