1. a Unit for Field-based Forest Research , Vindeln Experimental Forests, Swedish University of Agricultural Sciences , SE-922 91, Vindeln, Sweden
2. b Department of Forest Ecology and Management , Swedish University of Agricultural Sciences , SE-901 83, Umeå, Sweden
3. c Metsähallitus , Forestry, Hermanninaukio 3 , P.O. Box 1058, FI-70101, Kuopio, Finland
4. d Southern Swedish Forest Research Centre , Swedish University of Agricultural Sciences , SE-230 53, Alnarp, Sweden