1. Laboratory of Plant Ecology, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands;
2. Koeman and Bijkerk Ecological Research and Consultancy, P.O. Box 14, 9750 AA Haren, The Netherlands;
3. ALTERRA /Green World Research, Marine and Coastal Research Team, P.O. Box 167, 1790 AD Den Burg, The Netherlands;