Inter-annual mobility of nitrogen between beech rings: a labelling experiment Sliman Elhani, Brígida Fernández Lema, Bernd Zeller, Claude Bréchet, Jean-Marc Guehl and Jean-Luc Dupouey Ann. For. Sci., 60 6 (2003) 503-508 Accepted: 25 March 2003 DOI: 10.1051/forest:2003043