1. Grassland and Landscape Division, Norwegian Institute for Agricultural and Environmental Research, Postvegen 213, NO-4353 Klepp Stasjon, Norway
2. Department of Mathematical Sciences and Technology, Norwegian University of Life Sciences, PO Box 5003, NO-1432 Ås, Norway
3. Plant Health and Plant Protection Division, Norwegian Institute for Agricultural and Environmental Research, Høgskoleveien 7, NO-1432, Norway
4. Centre for Ecology and Hydrology, Edinburgh, Natural Environment Research Council, Bush Estate, Penicuik, EH26 0QB, UK