1. Cary Institute of Ecosystems Studies, Millbrook, NY 12545, USA.
2. Department of Biology, Duke University, Durham, NC 27708, USA.
3. Institute of the Environment, University of Connecticut, Storrs, CT 06269, USA.
4. Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH 03824, USA.
5. Department of Physical and Environmental Sciences, University of Toronto, Toronto, ON M1C1A4, Canada.