1. PhD, Dept. of Earth Sciences, Uppsala Univ., Villavägen 16, SE-752 36 Uppsala, Sweden.
2. PhD, Dept. of Biometry and Informatics, Swedish Univ. of Agricultural Sciences, Johan Brauners väg 3, P.O. Box 7013, SE-750 07 Uppsala, Sweden.
3. Professor, Dept. of Biometry and Informatics, Swedish Univ. of Agricultural Sciences, Johan Brauners väg 3, P.O. Box 7013, SE-750 07 Uppsala, Sweden (corresponding author).