1. Department of Biology, California State University, Northridge, California 91330;
2. Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721
3. Department of Environmental Sciences, Aarhus University, DK 4000 Roskilde, Denmark
4. Center for Permafrost (CENPERM), University of Copenhagen, DK 1350 Copenhagen, Denmark
5. Earth and Environmental Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352
6. Ecology Department, Climate and Ecosystem Sciences, Lawrence Berkeley National Laboratory, Berkeley, California 94720