1. Joint Nature Conservation Committee, Monkstone House, Peterborough PE1 1JY, UK
2. Forest Research, Northern Research Station, Roslin, Midlothian EH25 9SY, UK
3. Royal Botanic Gardens Kew, Richmond, Surrey TW9 3AB, UK
4. School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK