1. Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE‐901 83 Umeå, Sweden;
2. Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99775‐0180, USA;
3. Department of Forest Genetics and Plant Physiology, Umeå Plant Science Centre, Swedish University of Agricultural Sciences, SE‐901 83 Umeå, Sweden