1. Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83 Umeå, Sweden, Department of Aquatic Sciences and Assesment, Swedish University of Agricultural Sciences (SLU), SE-750 07 Uppsala, Sweden, Department of Geography, University of Zurich, CH-8057 Zurich, Switzerland, and Department of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91 Stockholm, Sweden