1. School of Biological Sciences, Plant and Soil Science, University of Aberdeen, St Machar Drive, Aberdeen, AB24 3UU, UK
2. The Macaulay Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK
3. Present address: Ecosystem Science and Management, University of Northern British Columbia, 3333 University Way, Prince George, BC V2N 4Z9, Canada
4. Biomathematics and Statistics Scotland, Craigiebuckler, Aberdeen, AB15 8QH, UK
5. Centre for Plants and the Environment, University of Western Sydney, Locked Bag 1797, Penrith South DC NSW 1797, Australia