1. D. F. R. P. Burslem is in the Department of Plant and Soil Science, University of Aberdeen, Aberdeen AB24 3UU, UK.
2. N. C. Garwood is in the Department of Botany, Natural History Museum, London SW7 5BD, UK.
3. S. C. Thomas is in the Faculty of Forestry, University of Toronto, Toronto M5S 3B3, Canada.