1. Department of Forest Sciences, P.O. Box 27, University of Helsinki, FIN-00014 Helsinki, Finland.
2. School of Agriculture and Biology, Shanghai Jiao Tong University, Dongchuan Rd. 800, Shanghai 200240, China; Key Laboratory of Urban Agriculture (South), Ministry of Agriculture, People's Republic of China, Dongchuan Rd. 800, Shanghai 200240, China.
3. Institute of Environmental Sciences, Jagiellonian University, Gronostajowa 7, PL-30-387 Krakow, Poland.
4. Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, UK.