1. USDA Forest Service, Northern Research Station, Forestry Sciences Laboratory, 410 MacInnes Drive, Houghton, MI 49931, USA;
2. Department of Plant and Microbial Biology, 111 Koshland Hall, University of California, Berkeley, CA 94720, USA;
3. Center for Stable Isotope Biogeochemistry, Department of Integrative Biology, University of California, Berkeley, CA 94720, USA