1. College Gate, University of St. Andrews, St. Andrews KY18 9LB, UK.
2. Forest Research, Alice Holt Lodge, Farnham, Surrey, GU10 4LH, UK.
3. Department of Plant Science, University of Cambridge, Cambridge, CB2 3EA, UK.
4. Department of Zoology, University of Oxford, Oxford, OX1 3PS, UK.