1. Department of Ecology and Evolutionary Biology, Graduate School of Life Sciences, Tohoku University, Aoba, Sendai 980–8578;
2. Chair Group of Production Systems, Wageningen University, Haarweg 333 6709 R2, The Netherlands;
3. Present address: Department of Plant Sciences, Utrecht University, PO Box 800.84 NL‐3508, TB Utrecht, The Netherlands
4. Japan Science and Technology Cooperation, Tohoku National Agricultural Station, 4 Akahira, Shimikuriyagawa, Morioka, 020‐0198;
5. National Institute of Agro‐Environmental Sciences, 3‐1‐1 Kannondai, Tsukuba, 305‐8604, Japan;