School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario, Canada

Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, SE-750 07, Uppsala, Sweden;4. Department of Biology, University of Eastern Finland, P.O. Box 111, 80101 Joensuu, Finland;5. Department of Plant Sciences and Northern Research Center (CEN), Laval University Quebec, Canada;6. Department of Natural History, Norwegian University of Science and Technology University Museum, Trondheim, Norway;7. Department of Plant Ecology and Evolution, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18D, SE-752 36 Uppsala, Sweden