1. a Department of Forest Soils , Swedish University of Agricultural Sciences , P.O. Box 7001, Uppsala, S‐750 07, Sweden
2. b Department Biologia Vegetal, Facultat de Biologia , Universität de Barcelona , Avgda. Diagonal 645, Barcelona, Spain
3. c Dept. Ecologia , Univ. d'Alacant , Ap. 99, Alacant, E‐03080, Spain