Nitrogen-concentration-and-d15N-signature

**Department of Earth Sciences, University of Parma, Parco Area delle Scienze 157, I-43100 Parma, Italy, wwSlovenian Forestry Institute, Department of Forest Ecology, Vecˇna pot 2, SI-1000 Ljubljana, Slovenia, zzDepartment of Plant Ecology,Evolutionary Biology Centre, Uppsala University, Villava¨gen 14, SE-75236 Uppsala, Sweden, §§Department of Biology, Universityof Joensuu, PO Box 111, FIN-80101 Joensuu, Finland