1. Institute of Botany, Dendrology and Forest Genetics, the Arboretum, The Royal Veterinary & Agricultural University, Kirkegaardsvej 3A, DK‐2970 Hoersholm, Denmark
2. To whom correspondence should be addressed. E‐mail:
3. Environmental Biology and Instrumentation Division, Brookhaven National Laboratory, Upton, NY 11973–5000, USA
4. Institute of Environmental and Natural Sciences, Lancaster University, Lancaster LA1 4YQ, UK