1. Department of Biological Sciences, University of Essex, Colchester, Essex, CO4 3SQ, UK,
2. Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton, Huntingdon, Cambs., PE17 2LS, UK, Present addresses: ‡Agricultural Research Service, Photosynthesis Research Unit, Urbana, Illinois 61801–3838, USA,
3. Centre for the Study of Environmental Change and Sustainability, University of Edinburgh, Edinburgh, EH9 3JL, UK