1. Plant and Soil Science Section; Department of Plant and Environmental Sciences; Faculty of Science; University of Copenhagen; 40 Thorvaldsensvej DK-1871 Frederiksberg C Copenhagen Denmark
2. Unité Ecologie et Dynamique des Systèmes Anthropisés EDYSAN (FRE 3498 CNRS); Université de Picardie Jules Verne; 33 rue St Leu 80039 Amiens France