1. School of Forestry & Environmental Studies, Yale University, 370 Prospect Street, New Haven, CT 06511 USA;
2. Present address: Complex Systems Research Center, Morse Hall, 39 College Road, University of New Hampshire, Durham, NH 03824 USA
3. AMBL, ANRI, ARS, U.S. Department of Agriculture, Beltsville, MD 20705 USA;